1 Genetics Part 1: Inheritance of Traits Genetics is the study of how traits are passed from parents to offspring. Offspring usually show some traits of each parent. For a long time, scientists did not understand how this could happen. Later, they found that the traits of the parents were passed to offspring by sex cells. Chromosomes You probably recall from your study of cells, that every cell has a nucleus at some time in its life. The nucleus really has two jobs. Its main day-to-day job is to direct the actions of the other cell parts. Its other job is to allow the cell to reproduce. Inside the nucleus are long threadlike parts called chromosomes. Chromosomes can be seen best when a cell is ready to reproduce. During cell reproduction (mitosis), the chromosomes become short and thick. Suppose we look closer at two kinds of cells. One kind of cell is a body cell seen in the left side of the picture below. Remember that body cells are cells that make up most of the tissues and organs in your body. A body cell has two of each kind of chromosome. The chromosomes of body cells are paired. 1 Another type of cell is a sex cell. The cell can be a sperm cell or an egg cell. In sex cells (pictured to the right above), there is only one of each kind of chromosome present. If you imagine your sock draw as the nucleus of a cell, a body cell would have all of your socks neatly paired up. To show the nucleus of a sex cell, you would have to take one sock of each pair and put it somewhere else. Sex cell, you see, have half as many chromosomes as body cells. Genes on chromosomes All chromosomes contain genes. A gene is a small section of chromosome that determines a specific trait of an organism. Examples of traits are eye color, hair color, and shape of body parts such as ears. Chemical processes inside the body, which cannot be seen, are also traits. Organisms have thousands of different traits. Genetics is really a study of the genes that control all of these traits. Note that the word genetics contains the word gene. (HUH, imagine that.) Genes are lined up on a chromosome, one next to another, much like beads on a necklace. Each chromosome has different kinds of genes that control different traits. The picture to the left shows drawings of human chromosomes. The darker bands are the locations of different genes.
2 Remember that chromosomes in body cells are paired. The genes on chromosomes in body cells are paired, too. There is one gene of each gene pair on each chromosome of a chromosome pair. You can see this pairing in the picture of the body cell chromosomes on the previous page. Each trait we study will have one gene pair, or two genes that represent it. The two genes representing the trait are located on chromosomes that make a pair. Passing Traits to Offspring How are traits passed from parents to their offspring? To answer this question, let s use the traits of earlobe shape in humans as an example. A person can have attached earlobes or free earlobes. What genes would you expect to find in the body cells of a person with attached earlobes? How about in the body cells of a person with free earlobes? Suppose the mother shows the attached earlobe traits. Suppose the father shows the free earlobe trait. Which trait would appear in their children? The figure below shows what genes the parents can put in their sex cells. Remember the parent can only give one of the two genes. Sex Cells from Sex Cells from Father Mother 2 Attached earlobe Free earlobe Attached earlobe Free earlobe The mother has the gene pair for attached earlobes. She can make eggs that have is gene. The father has the gene pair for free earlobes. He can make sperm that have this gene. What genes for earlobe shape will their child have? The child will have one gene for each trait. In fertilization (union of egg and sperm), one sperm will join with one egg. Which sperm and egg will join? In natural circumstances, we can t control that. We can see that is would make no difference in this example. Any child produced will have a gene for attached earlobes and a gene for free earlobes as seen in the diagram to the right. Attached Earlobe Free earlobe
3 From which parent did the child receive the gene for attached earlobes? From which parent did the child receive the gene for free earlobes? 3 Dominant and Recessive Genes What does a child with one gene for attached earlobes and one of r free earlobes look like? Children born with both genes will have free earlobes. Why? Some genes keep other genes from showing their traits. Genes that keep other genes from showing their traits are called dominant genes. The genes that do not show their traits when dominant genes are present are called recessive genes. In this example, the gene for free earlobes is dominant and the attached earlobe gene is recessive. An organism with two identical genes for a given trait is said to be homozygous. The individual can either have two dominant genes or two recessive genes. It doesn t matter as long as they are the same. If an organism has 2 dominant genes for a trait is said to be homozygous dominant. They could also be called true breeding dominant. They are called true breeding because if you breed two true breeding dominant individuals you will always get a true breeding dominant individual. In our example, the father is homozygous dominant for free earlobes. An organism with two recessive genes for a trait is said to be true breeding recessive or homozygous recessive. In our example the mother is true breeding recessive for attached earlobes. Whether the dominant or recessive The child with a gene for attached earlobes and a gene for free earlobes has two different genes. The child is heterozygous. A heterozygous individual is one with a dominant and a recessive gene for a trait. Even though the heterozygous individual has the recessive gene, the recessive trait does not show. The trait of the dominant gene will always show. The table below gives some traits in plants and animals. These traits are passed to offspring in the same way as the earlobe trait. Traits of Plants and Animals Trait found in: Dominant gene Recessive gene Flies Long wings Short wings Pea plants Red flowers White flowers Humans Tongue rolling Non-tongue rolling Corn plants Normal height Dwarf Dogs Short hair Long hair When Both Parents are Heterozygous When one parent had both genes for attached earlobes and the other parent had both genes for free earlobes, all of their children had a combination of genes. The children had free earlobes. Now, let s suppose both parents have a combination of genes. What could their children look like? If the mother s body cells have both genes, she can make two different kinds of eggs. One type of egg could carry the attached earlobe trait and the other type of egg could carry the free earlobe trait. Each egg can carry on or the other gene but not both. Remember, the egg/sperm only carry one of each type of gene. If the father s body cells have both genes, he can make two kinds of sperm. Each sperm
4 will have the gene for attached earlobes or the gene for free earlobes, but no both. The figure on the next page shows the possible combinations when each of the two different kinds of egg combines with each of the two different kinds of sperm. There are four possible combinations. Gametes Offspring 4 Attached Attached Attached Free Free Attached Attached Free Attached Free Free attached The table below shows what a child with each combination would look like. There are three chances in four that a child would have free earlobes and one chance in four that a child would have attached earlobes. We call this a 3 to 1 ratio. If you let the dominant gene be represented by F and the recessive gene represented by f, you can make write what their genes are in a short hand way. Gene combinations for Earlobe Traits With these genes for the Child s Egg Sperm genes The child is: The child has: Free Free FF Homozygous dominant Free earlobes Free Attached Ff Heterozygous Free earlobes Attached Free Ff Heterozygous Free earlobes Attached Attached ff Homozygous recessive Attached earlobes Notice that when writing the genes with the shorthand way, the dominant trait is always written first. That s one of the perks of being the dominant trait.
5 Expected and Observed Results How can knowing the types of genes that each parents has be helpful. Yo can predict what traits their children could have. The science of predicting the outcome of events is called probability. Sometimes, however, the combination of genes that you expect does not appear in the offspring. Let s learn how people use probability to predict the results of breeding. The Punnett Square We have seen how an egg and a sperm may combine to form an offspring. Each time, we figured out all the possible combinations of egg and sperm cells. There is an easier way to do this. This easy way is called a Punnett square. The Punnett square is a way to show which genes can combine when egg and sperm join. To make things easier, letters are used in place of genes. A large letter such as F, is used for the dominant gene. The large F stands for free earlobe. The lower case form of the same letter will be used for the recessive gene. The lowercase f stands for attached earlobes. NOTE: you can choose any letter you wish but a word of caution. Try to avoid using letters that look the same in upper case and lower case. For example O, both upper and lower case letters are formed the same way, just one is bigger. It is hard to keep track of the size difference unless you make one different in some way like putting a line under it for the recessive gene. Sometimes it is easier to just use a different letter entirely like B for dominant and b for recessive. A person with FF genes is pure dominant and has free earlobes. A person with Ff genes is heterozygous and also has free earlobes. Remember it only takes on dominant gene for the dominant trait to show. A person with ff is homozygous recessive. What kind of earlobes does this person have? Follow these six steps to figure out the possible combinations of genes a child could have. In this example, both parents will be heterozygous. They will have a gene for free earlobes and a gene for attached earlobes. Use the letters Ff to stand for the gene pair in their body cells. 1. Draw a Punnett square as shown in the picture to the right. Each small box stands for one possible combination of genes that could show up in the offspring. Each combination of genes comes from a sperm cell fertilizing an egg cell. 2. Decide what kinds of genes will be in the sex cells each parent. Write the letters that stand for the genes in the mother s eggs across the top. Remember we said both parents were heterozygous so you need to write one capital letter in one spot and a lower case letter in the second spot. 3. Now write the letters that stand for the genes in the father s sperm along the side of the square. There are two possible genes for the sperm since the father is heterozygous as well. They are F and f. F f F F f f 5
6 4. Copy the letters that appear at the top of the square into the boxes below each letter. The figure to the right shows you how. F f F F f f F f 6 5. Copy the letters that appear at the left side of the square into the boxes next to each letter. Remember the capital letter is always written first. 6. Look at the small boxes in the Punnett square. They show the possible combinations of eggs and sperm. When egg and sperm combine, a new organism develops. The boxes also show what combinations of genes the organism could have. In our example, a child could have one of the following combinations: FF, Ff, or ff. The Punnett square to the right shows you how the child could look. Remember, F is dominant over f. There are two different combinations of genes that would result in a child with free earlobes FF and Ff. Notice that three out of four boxes contain either FF or Ff. Three out of four gene combinations of egg and sperm would result in a child with free earlobes. There is only one combination that would result in child with attached earlobe ff. Only one out of four boxes in the Punnett square has ff. On average, one out of four gene combinations would result in a child with attached earlobes. F f F FF Ff f Ff ff F FF Free earlobes Ff Free earlobes f Ff Free earlobes ff Attached earlobes Expected Results The Punnett square that you drew shows what kinds of traits offspring can have. It shows what to expect when the sperm and egg of two parents join. Expected results are what can be predicted in offspring based on the genetic traits of parents. We predicted that one out of four children would have attached earlobes. We could do this because we knew what genes the parents had. The type of chin a person has is also a genetic trait. A person with a cleft chin has a small indentation in the middle of the chin. A cleft chin is a dominant trait, a smooth chin is a recessive trait. Let I stand for cleft chin and i stand for smooth chin. It might be easier to remember the symbols if you think of I standing for indentation. What is the possibility that a child will have a smooth chin if his mother is Ii and his father is ii? Fill in the Punnett square below to get the answer. Two out of four gene combinations would be for children with smooth chins. Two out of 4 gene combinations would be Ii and the child would have a cleft chin. What is the gene combination that gives a child a smooth chin? Would you expect these parents to have a child that is homozygous dominant for cleft chin? Why or Why not? Observed Results We know that the results expected from the Punnett square do not always occur in every family. Look at the table at the top of the next page. These data are F f
7 from 10 different families that show the cleft chin trait. In each family, one parents has the genes Ii and has a cleft chin. The other parent has the genes ii and has a smooth chin. 7 Offspring and chin types Family # of children in # of children # of children family with cleft chin with smooth chin A 2 O 2 B C D E F G H I J Totals The table shows exactly what you would see if you looked at the children of these families. The traits actually seen in offspring are called the phenotype. The phenotype is the observed results. Using the Punnett square allows you to predict that half the children in these families could have cleft chins. Half the children could have smooth chins. Please note that even though two organisms may have the same phenotype, cleft chin, they may have different combinations of genes. A person with II and Ii both have cleft chins but they are not the same in their genes. II is a true breeding individual and Ii is not. What the genes are for a trait is called the genotype. The phenotypes, however, do not exactly match the expected results because you don t know which sperm and egg will join. A little more than half the children have cleft chins. Nearly half the children have smooth chins. Each family by itself may not show the results you expect to see. If you add the results together, they are close to what you expect to see in a Punnett square. In fact, the more results you observe the closer the observed results will be to the expected results. Think about what happens when you flip a coin. You expect the coin to come up heads or tails. If you flip a coin twice, does it come up heads once and tails once? It may, but it may also come up heads twice in a row or tails twice in a row. Two heads or two tails is not what you expect. In fact, if you flip the coin four times you may even see four heads or four tails. Why? Each time you flip a coin, you are starting over. If you flip a coin once and see heads, it does not mean that the next flip must come up tails. The coin can land either way each time you flip it. With just a few flips, you are less likely to see the exact same number of heads as tails. If you flip the coin many, many times, the number of heads and tails should be about equal. The observed results will be closer to the expected results.
8 8 Mendel s Work Let s go back in time to An Austrian monk named Gregor Mendel saw certain traits in the garden pea plants he grew in his garden. Mendel counted the recorded the traits he saw in the pea plans. He used a scientific method to do hundreds of experiments. After all he had no life. Using the data he gathered and his knowledge of mathematics, he was able to explain some basic laws of genetics. Mendel explained what dominant and recessive traits were. He also showed how these traits passed from parent to offspring. How did he do it? Mendel looked at several traits in the garden pea plant. You can see these different traits in the chart to the left. One of those traits was height of the plant. He noticed that tall parent plants mated with short parents plants always produced tall plants. Remember that if T stood for the tall gene and t stood for the short gene, the parents would be TT and tt. What would the offspring have for a genotype? Would the offspring be heterozygous or homozygous? Mendel then took the tall offspring and mated them with each other. He noticed that about ¾ of the offspring they produced were tall and about ¼ of them were short. Look at the figure below. Can you see that Mendel s results came very close to what we now expect to see when heterozygous plants are mated? Mendel concluded that his plants were heterozygous after he saw the results of his experiments. T t T TT Tall Tt Tall t Tt Tall tt short
9 9 Remember that heterozygous individual has a dominant gene and a recessive gene. Mendel s heterozygous plants would be Tt. Mendel studied the seven traits in pea plant you saw in the picture at the top of the page. As long as he began with true breeding parents, when he bred their offspring, he always got very close to ¾ of the offspring s offspring had the dominant trait and ¼ of them had the recessive trait. By the way, there is a shorter way of referring to these different generations. The original parent organisms are usually referred to as the P 1 generation, their offspring the F1 generation, and the offspring that result for the F 1 generation breeding are referred to as the F 2 generation. Part 2: Human Genetics All living things can pass their traits to their offspring. You learned in the first part of the genetics reading that these traits can be dominant or recessive. The genes that control the traits are found on sections of the chromosomes. Chromosome number There are three things you should know about chromosome numbers in living things. 1. In any given organism, the body cells have pairs of chromosomes therefore body cells have only even numbers of chromosomes. 2. The gametes (sperm or eggs) of an organism have half the chromosomes the body cells do. 3. Different organisms have different numbers of chromosomes. The chromosomes in most living things are paired. A carrot plant has 18 chromosomes in each of its body cells. That s equal to nine pairs per cell. The table below gives the numbers of chromosomes in some other living things as well as the number of chromosomes in the organism s gametes. Chromosome numbers of different organisms Animal or plant # of chromosomes in body cells # of chromosomes in gametes Red clover 12 6 Pea 14 7 Onion 16 8 Corn Human White pine Cat Rabbit Chicken 78 39
10 10 Doctors can tell if a child has the correct number of chromosomes even before it is born. They use a process called amniocentesis. Amniocentesis is a way of looking at the chromosomes of a fetus. Look at the picture to the left. The needle is inserted into the pregnant woman s abdomen. The needle enters the amniotic sac in which the fetus develops, and some liquid is removed through a needle. In this liquid are cells from the fetus s skin. The cells have rubbed off the fetus just as your skin cells do. These cells are grown for about 10 days to 4 weeks in a nutrient liquid makes them divide. Remember that chromosomes can be seen best when the cell is dividing. The cells are then studied with a microscope and an enlarged photo like the one in the picture to the left. The chromosomes are cut out and the ones that match are arranged in pairs. A large chart is made showing the chromosome pairs. The lab technician looks at the chromosomes to make sure there are 46 of them and there are none missing or added parts. Using a newer way to look at the chromosomes of a fetus, a doctor can use chonionic villus sampling. In this procedure, the doctor takes a small piece of the placenta surrounding the fetus. This area has cells with the same number and kind of chromosomes as the fetus. The chromosomes are counted and studied to see whether parts of missing or added. Sex A genetic trait Whether you were born male or female depends on your chromosomes. In humans, a special pair of chromosomes determines sex. When scientists pair up the chromosomes and line them up from largest to smallest it is called a karyotype. You can see a karyotype in the figure below. You can see the 23 pairs of chromosomes for a female. Notice that the chromosomes are numbered, except for those in the lower right corner. Those are marked XX. Each of the X chromosomes is a sex chromosome of the female. Sex chromosomes are chromosomes that determine sex. A human female has two X chromosomes in each body cell.
11 11 As you can see in the picture below, the male s sex chromosomes are different from those of a female. The larger sex chromosome is an X chromosome. The smaller one is called a Y chromosome. The Y chromosome is a sex chromosome found only in males. A human male has one X and one Y chromosome in each body cell. The numbered chromosomes in the drawings are called autosomes. Autosomes are chromosomes that don t determine the sex of a person. They are also called nonsex or body chromosomes. The autosomes of a male are like those of a female. What kinds of sex chromosomes, X or Y, are found in eggs or sperm? Because females have only one kind of sex chromosome, they can make only one kind of egg. Each egg of a female has one X-chromosome. Males have two different sex chromosomes, so they can make two different kinds of sperm. Each sperm of a male has either an X chromosome or a Y chromosome. A child can receive only an X from its mother but can get either an X or a Y from its father. So the father determines a baby s sex. If the child receives an X chromosome from its father it will be a girl. If it receives a Y chromosome, what sex will it be? Part 3: Weird types of Inheritance Have you ever gone people watching? The next time you are at a shopping center, movie theater, or football game, look at he people around you. They have many traits that you can see. These traits come from their parents. People also have many traits you can t see. These traits direct things that go on inside the body. Survey of Human Traits Do you know someone who has lots and lots of freckles? Do you know someone who has dimples on their cheeks? Dominant genes cause both traits. In both of these traits, one or both of the parents shows the dominant trait. Remember, only one dominant gene is needed to make a dominant trait show up. Two recessive genes are needed to make a recessive trait show up. What kinds of traits are recessive? Attached earlobes is a recessive trait. Neither of the parents needs to have attached earlobes for their child to show this trait. Other recessive traits are straight hair and not being able to roll the sides of your tongue. Can you guess what the dominant traits are? The ability to roll your tongue is dominant. Curly hair is dominant to straight hair. Short eyelashes are recessive. What is the dominant trait? Incomplete dominance
12 12 Some traits are neither totally dominant nor totally recessive. Scientists know of several genes that don t show total dominance. A case in which neither gene is totally dominant to the other is called incomplete dominance. Incompletely dominant genes result in a blending of the phenotypes. An example of this is when a red flower is bred with a yellow flower. The offspring are a blend of the two colors: they are orange. You can see this cross to the right. Although the phenotype is a blend of the parents phenotypes, it is important to know that the genes themselves do not combine. Whey they are separated, we see the original traits again. If two orange flowers are mated, they will produce red, yellow, and orange offspring. Some traits in humans show incomplete dominance. One of these is the shape of red blood cells. In most people, red blood cells are round. Some people have blood cells shaped like those in the picture to the left. These cells are called sickle cells. They are shaped like a sickle that is used to cut grass. Some people have both shapes of blood cells. Let the letter R stand for the gene for round blood cells. Let R stand for the gene for sickle cells. In incomplete dominance, two different letters or symbols are used for the genes controlling the trait. The R genes is not totally dominant over the R gene. The R gene is not totally dominant over the R gene. Capital letters can stand for both. A person with round cells has RR genes. A person with sickle cells has R R genes. A person with both kinds of cells has RR. Someone with RR genes usually does not have serious health problems. People who have some sickle cells, however, may not be as active as those who have all round cells. As you may know, red blood cells carry oxygen to body cells. Sickle cells do not carry oxygen as well as a round blood cell. People with R R genes have sickle cell anemia. Sickle-cell anemia is a genetic disorder in which all the red blood cells are shaped like sickles. This disorder is much more common among African-Americans than among people of other races. People with sickle-cell anemia have serious health problems. Their lives may be shortened because of the disorder. The sickle cells cannot carry enough oxygen. Body tissues can be damaged because they don t receive enough oxygen. Sickle cells do not move easily through the blood capillaries because their shape causes the capillaries to become clogged.
13 13 If two parents have RR genes, what would we expect the children to have? If you look at the Punnett square to the right, you will see one out of four children is expected to have all round cells. One child in four is expected to have all sickle cells and have sickle cell anemia. Two out of four children will have a mix of cells. R R RR Normal round cells R RR Carrier with mixed cells R RR Carrier with mixed cells Blood Type in Humans There are four blood types A, B, AB, and O. Human blood type is controlled in part by dominance of genes. Although three genes control blood types, each person has only two of them. The three genes that control blood type are I A, I B and i. If you carry the I A gene you will have a certain shape marker on your blood cells. If you carry the I B gene you will have a different shaped marker. These markers are part of the way your body knows its own cells. The i gene does not make any markers on your cells. People with the type A marker will have type A blood. People with the type B marker will have type B blood. People without any markers will have type O blood. I A and I B are both dominant to i. However, I A and I B are not dominant to each other. They have a relationship called codominance. When a person carries both the I A and I B genes, both markers will be on that person s blood cells. They will have type AB blood. In codominance, the traits for both genes are expressed at the same time. There is no blending as in incomplete dominance. There are two genotypes for type A blood. One of course is I A I A the other is I A i. Since the I A gene is dominant to the i gene, the person with I A i genotype will only have A markers on their blood cells. There are two genotypes for a person with type B blood. They are I B I B and I B i. How many genotypes are there for type AB blood? How about type O blood? The answer to these two questions is just one genotype is possible for each of the blood types. Type AB people have to have the genotype I A I B and people with type O blood have to have the genotype ii. Before the use of DNA testing to determine paternity cases, doctors would use blood typing to determine paternity. More often than not the test showed who was not the father rather than who was the father. For example, type A woman has a type O baby. She claims a Type AB man is the father. Can this be? Well, the woman can have I A I A or I A i. If she has the genotype I A I A she cannot have a type O baby. The baby has to have the genotype ii. The baby has to get an i from its mother show the mom must be I A i. The man she claims is the father has to have a genotype of I A I B. To have type AB blood, there is no other possibility. Since the baby has the R R All sickle cells: Sickle cell anemia
14 14 genotype ii, the baby has to get an i from his father. The type AB man does not have the gene to give therefore he cannot be the baby s father. Genes on the X chromosome Like other chromosomes, the sex chromosomes carry genes. Some of the traits controlled by the genes on the sex chromosomes are blood clotting, color vision, tooth color, and skin dryness. These genes are only found on the x chromosome and not on the Y. They are called x-linked or sex linked traits. Females have two genes for these traits, males have only one. The Y chromosome does not have the genes that are on the X chromosome. When the X and Y chromosomes are together, genes on the X chromosome control the traits. Let s look at color blindness. Color blindness is a problem in which red and green look like shades of gray or other colors. Being able to see red and green as two separate colors is a dominant trait. Let s use C for this gene. Not being able to see red and green is a recessive trait. A c will be used for this gene. A female who has CC genes will be able to see red and green. If she has Cc genes, she will be able to see red and green. If she has cc genes, she will be colorblind. Two of these three gene combinations for female allow her to see red and green. A male is more likely to be color blind. Why? Think about the genes he could have. Remember that the Y chromosome will not have a gene for the trait. A male who has the C gene on his X chromosome will be able to see red and green. If he has the c gene on his X chromosome he will be colorblind. There are two possible gene combinations for the male. One will allow him to see red and green. What would happen if a woman with Cc genes and a man with a c gene on his X chromosome had children? Do a Punnett square to find the chance one of their daughters will have colorblindness. Genetic Disorders Each day nearly 600 babies are born in the U.S. with some type of disorder. Some of the disorders are inherited. Let s look at a few of them. Errors in chromosome number Some people are born with more or fewer than 46 chromosomes. This happens when the sperm or egg cell does not have 23 chromosomes. During meiosis, the sister chromatids are supposed to pull apart from each other. Sometimes the sister chromatids stick together and instead of being separated into two separate cells they both end up in one of the gametes. That cell now has an extra chromosome. If these cells were eggs, one would have fewer chromosomes, or 22. If this egg joined a sperm, the child would only have 45 chromosomes. If an egg with an extra chromosome, or 24, joined with a sperm the child would have 47 chromosomes. In either case the child would not have the correct number of chromosomes. If an organism does not have the correct number of chromosomes there is something wrong with the organism.
15 15 Not pulling apart can happen to almost any pair Unusual Sex Chromosome of chromosomes. When it happens to the sec patterns chromosomes, certain traits show up. The table to the female Male right shows sex chromosomes patterns that sometimes XO YO show up in people. The O stands for a missing sex XXX XXY chromosome. Notice the boy and the girl who have only one sex chromosome. The sex chromosome for one parent is missing. What are some problems that result from having one of these chromosome patterns? A YO male will die before it is born. An XO female will have a condition called Turner s syndrome. An XXY male will have something called Klinefelters syndrome. People with Turner s or Klinefelters cannot make sex cells but they can live. Down syndrome is another genetic disorder caused by an autosome not separating during meiosis. This causes a person with Down syndrome to have an extra chromosome number 21. For this reason, Down syndrome is also called Trisomy 21. A person with Down syndrome learns more slowly than most children his age. Children with Down syndrome often have heart problems. Genetic Disorders and Sex Chromosomes You have studied traits controlled by genes on the sex chromosome. The ability to see red and green was one of them. There are also serious genetic disorders controlled by genes on the X chromosome. A certain recessive gene on the X chromosome can cause a rare disorder called hemophilia. Remember that hemophilia is a disorder in which a person s blood does not clot. Bleeding from a cut or bruise may take hours to stop. Hemophilia almost always shows up in male children. How would it be possible for a female child to inherit hemophilia? Genetic Disorders and Autosomes Genetic disorders can also be caused by genes found on the autosomes. Dyslexia is a genetic disorder that is also called word blindness. It is cause by a dominant gene. People with dyslexia see and write some letters of the alphabet or parts of words backwards. They have trouble learning to read. Twenty years ago, a child born with PKU would spend its life in a hospital. PKU is a genetic disorder in which some chemicals in the body do not breakdown as they should. These chemicals can harm the brains cells. Today babies are tested for PKU soon after birth. A child born with PKU can be given a special diet, starting within the first few weeks of life. This diet has only small amounts of the chemicals that will not break down properly. The child can grow normally with this diet. Genetic counseling It runs in the family. Have you heard this comment before? It means that many family members have a certain trait. The trait could be found in parents, grandparents, brothers, sisters, or other relatives. Family members can have the same hair color, eye color, or shape of nose. Genetic disorders run in families, too. How can a couple find out whether a disorder in their family will show up in their children? They can seek genetic
16 16 counseling. Genetic counseling is the use of genetics to predict and explain traits in children. A genetic counselor can tell whether a problem is caused by genes. A genetic counselor asks a lot of questions about many members of a family. Then the counselor makes some conclusions. A genetic counselor can help answer the following questions: 1. How did their baby get the disorder? 2. If the baby is healthy, does it have a problem gene? 3. If the trait dominant or recessive? 4. What will happen to the baby s health, as it gets older? 5. What are the chances that future children will have the trait? The baby of a young couple died when it was two years old. The doctor told the couple that their baby had cystic fibrosis. This is a genetic disorder in which the lungs and pancreas don t work the way they should. There are very serious breathing problems. Many children with this disorder die at an early age. The baby s parents went to a genetic counselor, who told them that the disorder was caused by having two recessive genes, ff. Both parents appeared normal but they were each heterozygous for the dominant and recessive genes, Ff. The counselor showed the parents a drawing of a family pedigree like the one below. A pedigree is a diagram that can show how a certain trait is passed along in a family. A pedigree may be used to trace a family trait and to predict whether future generations will have the trait. The couple learned that there was a one in four chance of any of their children being born with the disorder and that tests such as amniocentesis can tell whether a fetus has the disorder. Talking to the counselor gave them an idea of what to expect if they decided to have more children. Genetics Part 4: DNA Life s Code Many different kinds of codes are used for many different reasons. Braille is an example of a code. It is used by a visually impaired person to help him or her read. Another code is one that is found in each of your cells. It is a code that determines all of your traits. It directs your body to be you. What you see in the picture to the right is a model of a chemical molecule. How this molecule acts as a code to determine your body s traits is what this chapter is all about. The DNA Molecule Within each of your cells is a chemical that controls life. It is what we call genes. Now you will find out how this chemical makes up the genes, how it determines all of your traits, and how it passes an organism s instructions from generation to generation. You will see why this chemical has been called the code of life. DNA Structure
17 17 The picture above is a model of a DNA molecule. The letters DNA stand for deoxyribonucleic acid. We will use only the letters DNA each time we mention this molecule. DNA is a molecule that makes up genes and determines the traits of all living things. Humans, birds, mushrooms, plants, protozoans, and bacteria all have DNA. All living things contain DNA in their cells. Scientists often use models to explain things that are very complex. We shall use a model of DNA to explain what it looks like and how it does its job. The figure to the left shows a ladder. It is made up of two upright sidepieces and many rungs. The right side of the picture shows a small part of the DNA model. Note that it looks something like the ladder. An actual DNA molecule is much longer than the model. Scientists have estimated that a single DNA molecule in a human cell may contain about 100 million rungs. There are six features of the DNA molecule. 1. DNA has two main sides. These sides are like the upright parts of a ladder. 2. The sides are made of two different molecules that alternate along each side. One is called ribose and is a sugar. The other molecule is a phosphate. 3. There are parts that connect the two sides together. These parts look like the rungs of a ladder. 4. Nitrogen bases form then rungs of a DNA molecule. 5. There are four different bases in DNA. They are adenine (A), thymine (T), cytosine (C), and guanine (G). The abbreviation for each base is given in parentheses after its name. 6. The four bases join each other in a special way. Adenine always bonds to thymine, and cytosine always bonds to guanine. You should know one more thing about the shape of DNA. Imagine holding each end of a ladder and twisting it. You would end up with a structure that looks like a double spiral as seen in the pictures on the previous page. DNA and Chromosomes DNA is in every cell in your body. It is also in every cell of all other living things. Where in these cells is it found? You probably already know DNA makes up parts of the chromosomes found in the nucleus. Look carefully at the figure below. It shows how DNA is related to chromosomes. Keep in mind that this drawing is many times larger than the real thing. Now think about this: Where in the cell can we find genes? Genes are parts of chromosomes. We can define a gene in three ways. First, it can be defined as a short piece of DNA. Second, it is a certain number of bases (rungs on the ladder) on the DNA molecule. In addition, a gene can be defined as
18 a small section of a chromosome that determines traits. All three definitions are correct. 18 How DNA works How does DNA direct a plant to make chlorophyll? How does DNA direct the formation of a sickle-shaped or round red blood cell? How does DNA direct all traits in all living things. Let s compare DNA and how it works to a computer. A computer reads electrical messages stored within its memory bank. These messages are stored in code form. DNA is the code cellular information is stored. The nitrogen bases in the DNA molecule spell out the message in code form The order of the nitrogen bases A, T, C, and G, is the coded message. Diagram 1 shows the order of bases somewhere on a section of DNA. Let s suppose this order directs the formation of round red blood cells. Now, let s suppose we look at the same place on the DNA of another person. We see the message in Diagram 2. This order of bases directs the formation of sickle cells. G A G T G A G G C T T C Diagram 1 C T C A C T C C G A A G G A G T G A G G C T A C Diagram 2 C T C A C T C C G A T G In this example, the orders of the bases in DNA are slightly different. This difference causes the traits to be very different. One person has normal round red blood cells. The other person has sickle shaped cells that do not carry as much oxygen as normal round cells. This makes the person very tired most of the time. Because the sickle cells do not carry as much oxygen the person is anemic and the condition is called sickle cell anemia. New orders of nitrogen bases can make new messages. Each message gives the cell instructions for a different trait. Think about our alphabet for a moment. The letters of the alphabet are always the same. We put the letters into different orders to form different words. The letters w, o, and l form the word owl. They also can form the word low. Each order codes for a different meaning. How many different letters are in the DNA alphabet? Time to make the proteins DNA directs the making of proteins in cells. Remember from your study of cells that ribosomes are the cell parts where proteins are made. You also probably remember that ribosomes are not found in the nucleus but attached to the ER or floating in the cytoplasm. If the recipe to make a protein is kept in the nucleus and it is built outside the nucleus, how does the information get from the nucleus to the cytoplasm?
19 19 The nucleus has to have a helper molecule called RNA to control protein manufacture in the ribosomes. The abbreviation RNA stands for ribonucleic acid. RNA is different from DNA in three ways. 1. RNA is made of a single stand instead of a double strand. If you took a ladder and cut all the rungs down the middle that would be what the RNA molecule would look like. 2. The sugar that alternates with the phosphates to make up the sidepiece is not deoxyribose as in DNA but just plain ribose sugar. 3. All the nitrogen bases are the same as in DNA except RNA does not have thymine (T) instead it has uracil. DNA has A, T, C, G; RNA has A, U, C, G. There are three types of RNA. The type of RNA that makes up the ribosomes is called ribosomal RNA or rrna for short. RNA that carries the message to make a protein from the nucleus to the ribosome is called messenger RNA or mrna for short. Somehow the information has to be transferred from the DNA in the nucleus to the mrna molecule. This process is called transcription. During transcription, the DNA molecule unwinds and splits down the middle between the bases. You can see a picture of this to the left. Once the DNA molecule opens up, an RNA molecule is made using one side of the DNA as a pattern. When this happens if the DNA has a G the RNA would have a C, and vice versa. The difference comes when the DNA has an A the RNA would not have a T since there is no T in RNA. Instead it would have a U. You can see an example of an RNA strand in diagram 3. DNA strand G A G T G A G G C T T C Diagram 3 RNA strand C U C A C U C C G A A G Notice where there is a T on the DNA stand the RNA base is still A. When the message gets to the ribosome, the ribosome acts as a worktable for making the protein. When RNA arrives at the ribosomes, the message it carries must be decoded before it can be used to direct the formation of the protein. This process is called translation. In translation, the mrna strand is read by the ribosome. Each three mrna bases code for an amino acid. The ribosome then sends another type of RNA out to get the correct amino acid. This type of RNA transfers the amino acid to the protein. It is called transfer RNA or trna. trna has three special nitrogen bases which are the opposite of the bases on the mrna. For example, if the mrna reads UUC then the bases on the trna read AAG. In this way, only the amino acid the mrna codes for can added to the protein. Given the bases order of a piece of DNA, you can figure out what the order of the amino acids in the protein is codes for.
20 All you need to accomplish this is the chart below and the knowledge of how to change DNA into RNA. 20 How the genetic message changes You have learned that newly formed cells have the same genetic message as the cell from which they came. Sometimes they do not. What happens when the message changes? Can scientists change the message of living things? Mutations Sometimes errors happen when chromosomes are copied. The bases A, T, C, and G may join incorrectly. Joining incorrectly results in a change called a mutation. A mutation is any change in copying the DNA message. What happens if you hit the wrong key on the computer keyboard? Doesn t the computer get the wrong message? Cells are like the computer. A wrong base in the DNA gives the cell the wrong message. The result is that the wrong type of protein is made. This change may cause a different trait to appear. What causes mutations to occur? Many mutations are simply the results of copying mistakes. An error may take place in the pairing of bases when DNA is copied during cell reproduction. Other mutation may be caused by something from outside the cell. Certain chemicals and some forms of radiation can cause mutations. Radiation is energy that is given off by atoms. The sun releases a large amount of radiation. X rays, ultraviolet light, and visible light are examples of kinds of radiation from the sun. Very powerful radiation, such as X rays and ultraviolet light can cause mutations.
21 21 Kind of mutations There are several types of mutations. They are named differently depending on what happens to the nitrogen base sequence. For example, if you have the strand of DNA that has the base order, CGTACG it will code for a protein containing the amino acids alanine and cysteine. When a deletion mutation happens, one or more of the nitrogen bases is cut out. Instead of having: CGTACG, after a deletion of the first G, you would have CTACG which codes for only one amino acid, aspartate. In a duplication mutation, one or more of the bases are duplicated. So, using our strand of DNA, CGTACG, after a duplication mutation of the first G, the strand would read CGGTACG. This mutated strand now codes for alanine and methionine. In an inversion mutation a section of the DNA is snipped out and put back in backwards. Using our strand of DNA (CGTACG), an inversion mutation of the first two bases would give the code GCTACG which codes for arginine and cysteine. An insertion mutation occurs when one or more bases are added to the strand of DNA. Again using the DNA, CGTACG, by inserting a T at the beginning a new strand is made with the order TCGTACG. This new piece of DNA codes for the amino acids serine and methionine. As you can see, none of the mutation examples result in the correct order of amino acids in the protein that is suppose to be made from that strand. This may be a minor difference that doesn t really affect the organism or can cause its death. Cloning Do you know a set of identical twins? If you do, they are clones. That s because identical twins are two children that form from the splitting of one fertilized egg. They have exactly the same genes. You know that two clones have the same genes. They are exact copies of each other. Since the identical twins have the same genes they must have the same DNA. Identical twins result for the fertilization of a single egg by a single sperm. This is normal. What happens after the fertilization is what is not normal. The single zygote splits in two but each of the cells forms a whole organism on its own. Fraternal twins result from the fertilization of two different eggs by two different sperm. These twins are not clones since it is unlikely they got exactly the same genes from both parents. Fraternal twins do not have to be the same sex. They are like any two other siblings except they share the same uterus at the same time. Can humans clone animals? The answer is yes. When an organism is cloned the nucleus is destroyed from an egg. A body cell is donated from the organism to be cloned and its nucleus is removed. The nucleus is then inserted into the egg that lacks the nucleus. The egg is acts as a zygote and begins to divide. The zygote then is implanted in the uterus of a pregnant organism and it develops into a new organism. Splicing Genes Have you ever spliced a wire or broken film? If you have, then you know that the word splice means to insert, or join together. Today, scientists can splice genes. They join genes from one kind of living thing with the genes of a different kind of living
22 22 thing. This splicing works because the genetic code for all living things is the same. They all have the same four bases, A, T, C, and G. All living things use the same genetic code to translate the DNA language into the protein language. What is different is the order in which the bases appear. In gene splicing, a bacterium receives a section of DNA from another organism, such as a human. The new DNA combination that is formed in the bacterium is recombinant DNA. Recombinant DNA is the DNA that is formed when DNA from one organism is put into the DNA of another organism. Gene splicing produces bacteria that can make certain chemicals. For example, scientists splice human genes that control the making of insulin into bacteria. The bacteria then make insulin in large amounts. Diabetics benefit because they can t make their own insulin. Recombinant DNA has produced plants that are not harmed by chemical sprays. Some sprays kill weeds but also kill crop plants. By splicing spray-resistant genes into crop plants, the crop plant is unharmed by sprays. Splicing of human genes into bacteria also has helped make human growth hormone. This hormone is given to children when their pituitary gland does not make enough of it. The real future of gene splicing may be in the ability to cure certain genetic diseases.
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