Biol 1408 : Chapter 9 Patterns of Inheritance

Transcription

1 Biol 40 : Chapter 9 Patterns of Inheritance 9.6 Geneticists can use the testcross to determine unknown genotypes A testcross can show whether the unknown genotype includes a recessive allele. A testcross is done by mating an individual of unknown genotype and a homozygous recessive individual. Mendel used testcrosses to verify that he had truebreeding genotypes. The following demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes. 202 Pearson Education, Inc. 9.6 Geneticists can use the testcross to determine unknown genotypes For example, Labrador s have either a black coat color or a chocolate coat color With respect to coat color, we know that black is dominant (B) and chocolate is recessive (b). So, A black lab,the phenotype, is genotypically BB or Bb But a chocolate lab phenotype must be genotypically bb ( thus not cc, since we have to use the same letters) 202 Pearson Education, Inc.

2 9.6 Geneticists can use the testcross to determine unknown genotypes If we want to know if a black lab is of the BB or Bb variety, we can cross it with a homozygous recessive trait ( bb, the brown lab) If the black lab was BB and we cross it with a bb lab, the outcome should be 00 % Bb labs.. Or 00 % black labs But if the black lab was of the Bb variety, it can produce gametes with either B or b traits. Mating with a bb individual would thus produce 50% Bb (black) and 50 % bb (brown) labs 202 Pearson Education, Inc. 9.6 Geneticists can use the testcross to determine unknown genotypes What is the genotype of the black dog? Testcross Genotypes Gametes B? bb Two possibilities for the black dog: BB or Bb B B b Black lab makes either B or B and b gametes Chocolate lab only makes b gametes b Bb b Bb bb Offspring All black black : chocolate 9. CONNECTION: Genetic traits in humans can be tracked through family pedigrees The inheritance of human traits follows Mendel s laws. A pedigree shows the inheritance of a trait in a family through multiple generations, demonstrates dominant or recessive inheritance, and can also be used to deduce genotypes of family members 202 Pearson Education, Inc. 2

3 Human genetics Mendel s law apply to the inheritance of human traits as well. In a simple dominant-recessive inheritance of dominant allele A and recessive allele a, a recessive phenotype always results from a homozygous recessive genotype (aa) but a dominant phenotype can result from either the homozygous dominant genotype (AA) or a heterozygous genotype (Aa). 202 Pearson Education, Inc. 9. CONNECTION: Genetic traits in humans can be tracked through family pedigrees Kristin and Lori are sisters and have a different hairline Kristin Straight hairline Lori Widow s peak Let s assign the allele letter for hairline characteristics H or h. Which of these two traits is dominant and what are the genotypes for this allele in these sisters? HH, Hh or hh? Analyzing a pedigree can help us find the answer. 9. CONNECTION: Genetic traits in humans can be tracked through family pedigrees If two parents show the same characteristic, and one of their children shows the opposite characteristic, then the parents characteristic must be dominant and heterozygous! Assume the opposite. If both parents have the same characteristic and it is recessive? hh x hh the offspring must all be hh So the characteristic of the parents must be dominant. What is one is homozygous dominant and the other heterozygous? HH x Hh the offspring will be HH or Hh Thus all off spring will show the dominant characteristic. Hence, the only way the offspring shows a recessive trait if bot parents are heterozygous for that trait. 3

4 Figure 9.-2 Al Mating Beth KEY Female Male Widow s peak hairline trait Straight hairline trait ST GENERATION Charles Debbie Parents with same trait but with children having opposite trait Children 2ND GENERATION Evelyn Frank Gary Henry Isabel Juliana Straight hairline 3RD GENERATION Widow s peak Kristin Lori Figure 9.-3 KEY Female Male Widow s peak hairline trait H: widow s peak allele Straight hairline trait h: straight allele ST GENERATION Al Beth Charles Debbie Hh Hh hh Hh 2ND GENERATION Evelyn Frank Gary Henry hh hh Hh A parent with a widow s peak who has a child with a straight hairline must be Hh. Isabel Juliana Hh hh Straight hairline 3RD GENERATION Widow s peak Kristin hh Kristin has a straight hairline but her parents do not, so straight hairline must be homozygous recessive (hh). Lori 9. CONNECTION: Genetic traits in humans can be tracked through family pedigrees Kristin has a straight hairline but her parents do not, so straight hairline must be homozygous recessive (hh). Not all genotypes can be determined. Lori could be HH or Hh and there is no way to know (unless she has some children and the pedigree is extended). 4

5 Human genetics Although dominant traits rule over recessive traits, dominant traits are not always the most common trait in nature Wild-type traits is the term used for traits prevailing (common) in nature ( and are thus not necessarily specified by dominant alleles). 202 Pearson Education, Inc. Human genetics Freckles are an example that dominant alleles are not always dominating in nature. While freckles is a governed by a dominant allele, the absence of freckles is more common in our societies Thus the genotype FF and Ff, which produce the phenotype of having freckles, is less common than the genotype ff (not having freckles). 9. Genetic traits and human family pedigrees Example : having freckles Assume two parents that have freckles, and their child has no freckles. Let s also assume No freckles is dominant and designate N for the trait of NOfreckles The genotype of the child can thus be NN or Nn Mom and Dad have freckles (assumed to be recessive). That means, the only way that could be if they both are nn. The next slide shows this in diagram form. 202 Pearson Education, Inc. 5

6 9. Genetic traits and human family pedigrees MOM (freckled) x DAD (freckled) nn x nn Child (Not freckled) genotype = NN or Nn We assumed Not freckled is dominant; that automatically makes having freckles is recessive This diagram shows that the assumption of having freckles being recessive doesn t work. The genotype of the child cannot be explained using the genotypes of the parents. No way Mom and Dad can produce a child with NN or Nn genotype if having freckles is recessive. Thus having freckles is dominant and no freckles is recessive. 202 Pearson Education, Inc. 9. Genetic traits and human family pedigrees Thus freckles is dominant and the child with freckles means, the child is ff What are Mom and Dad in terms of genotype if they have freckles? Mom and Dad must both be Ff, because that is the only way we can have a child with ff MOM (freckled) x DAD (freckled) Possible offspring? FF x FF or FF x Ff or Ff x Ff Child (Not freckled) genotype = ff FF and Ff FF and Ff FF, Ff and ff 202 Pearson Education, Inc. 9. CONNECTION: Genetic traits in humans can be tracked through family pedigrees The use of human pedigree is important to trace certain alleles within families. Demonstration of the presence of dominant or recessive alleles in plants and animals can be done by using test-crosses Not something that can be done with ease on animals with small litters and long pregnancies Not an option for humans to test-cross individuals just for the sake of genetics. 202 Pearson Education, Inc. 6

7 Human Genetic traits Dominant Traits Recessive Traits The following are some examples of human traits with dominant and recessive forms. All of these are minor variations with no major side effects, but do obey Mendel s law of genetics Freckles Widow s peak No freckles Straight hairline Free earlobe Attached earlobe Human Genetic traits Tongue rolling is dominant Hitchkiker s thumb is as well Bent pinky ( dominant) vs straight pinky Dimples in cheek ( dominant) vs no dimples Curly hair ( dominant) vs straight hair Long eyelashes ( dominant) vs short eyelashes Left thumb over right thumb when interlacing fingers is dominant Hair on your middle digits of your fingers is dominant 9. Genetic traits and human family pedigrees Due to the fact that recessive traits only show up when the individual is homozygous for the recessive allele, we can figure out who in the family tree is the carrier. For example, when two people, who have a certain trait, have a child that does not have that trait, one may assume that the trait of the child is recessive. Reason? The only way to have gotten the trait IF both parents have at least one copy of that recessive allele. 202 Pearson Education, Inc. 7

8 9. Genetic traits and human family pedigrees For Example : the characteristic of having attached or free earlobes Which trait is dominant? Let s look at a pedigree! 202 Pearson Education, Inc. 9. Genetic traits and human family pedigrees : Earlobes example First generation (grandparents) Ff Ff ff Ff Second generation (parents, aunts, and uncles) FF or Ff Third generation (two sisters) Female Male Attached Free ff ff Ff Ff ff ff FF or Ff Similar reasoning on this female child, tells us that attached earlobes is recessive (her parents had free earlobes). Thus parents must be Ff. Parents cannot be FF and Ff or both be FF! 9. CONNECTION: Genetic traits in humans can be tracked through family pedigrees First generation (grandparents) Second generation (parents, aunts, and uncles) FF or Ff Third generation (two sisters) Female Male Attached Free Ff Ff ff Ff ff ff Ff Ff ff ff FF or Ff One of the parents had a grandparent and sibling with attached earlobes. Thus the other grandparent must have been Ff as well.

9 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene Inherited human disorders show either. recessive inheritance in which two recessive alleles are needed to show disease, heterozygous parents are the called carriers of the disease-causing allele, and the probability of inheritance increases with inbreeding, mating between close relatives. 2. dominant inheritance in which one dominant allele is needed to show disease and dominant lethal alleles are usually eliminated from the population. 202 Pearson Education, Inc. Figure 9.9A Normal Dd Normal Dd Parents This shows an example of a recessive form of deafness. Normal hearing is thus the dominant allele, deafness due to a mutation in the gene. Only people with both mutated genes become deaf (dd). D Eggs d D DD Normal Dd Normal (carrier) Sperm Offspring d Dd Normal (carrier) dd Deaf 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is recessive and an indiviudal must be homozygous recessive (aa) to get the disorder carried by about in 3 Americans. ( they are heterozygous (Aa ) Dominant human disorders include (so, if an individual has the dominant allele, they will get the disorder ( AA or Aa) achondroplasia, resulting in dwarfism, and Huntington s disease, a degenerative disorder of the nervous system. 202 Pearson Education, Inc. 9

10 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene Dr. Michael C. Ain, a specialist in the repair of bone defects caused by achondroplasia and related disorders. He himself has achondroplasia. So his genotype is either AA or Aa for this trait. The only way he could have gotten this if either both his parents had the disorder or one parent did not and the other did Parents : AA x AA Kids : all AA Parents : AA x Aa Kids : 2 AA, 2 Aa Parents : AA x aa Kids : all Aa Parents : Aa x Aa Kids : AA, 2 Aa, aa Parents : Aa x aa Kids : 2 Aa, 2 aa Parents : aa x aa Kids : all aa Those without the disorder in Blue Table CONNECTION: New technologies can provide insight into one s genetic legacy New technologies offer ways to obtain genetic information before conception, during pregnancy, and after birth. Genetic testing can identify potential parents who are heterozygous carriers for certain diseases. 202 Pearson Education, Inc. 0

11 9.0 CONNECTION: New technologies can provide insight into one s genetic legacy Several technologies can be used for detecting genetic conditions in a fetus. Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits karyotyping and biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease. Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests. 202 Pearson Education, Inc. Figure 9.0A Ultrasound transducer Amniocentesis Amniotic fluid extracted Chorionic Villus Sampling (CVS) Tissue extracted Ultrasound from the transducer chorionic villi Fetus Placenta Uterus Cervix Fetus Placenta Chorionic villi Cervix Centrifugation Uterus Amniotic fluid Fetal cells Cultured cells Several hours Several weeks Biochemical and genetics tests Fetal cells Several hours Several weeks Several hours Karyotyping 9.0 CONNECTION: New technologies can provide insight into one s genetic legacy Blood tests on the mother at 4 20 weeks of pregnancy can help identify fetuses at risk for certain birth defects. Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus. Newborn screening can detect diseases that can be prevented by special care and precautions. 202 Pearson Education, Inc.

12 VARIATIONS ON MENDEL S LAWS 9. Incomplete dominance results in intermediate phenotypes P generation Red RR Gametes R r White rr Mendel s pea crosses always looked like one of the two parental varieties, a situation called complete dominance. For some characters, the appearance of F hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance. F generation F 2 generation Gametes 2 Eggs 2 2 R r R 2 R RR Rr Pink hybrid Rr Sperm 2 rr rr 2 r r 9. Incomplete dominance results in intermediate phenotypes One example of incomplete dominance in humans is hypercholesterolemia, in which genes code for cholesterol (LDL) receptors dangerously high levels of cholesterol occur in the blood and heterozygotes have intermediately high cholesterol levels. HH Homozygous for ability to make LDL receptors LDL receptor LDL Genotypes Hh Heterozygous Phenotypes hh Homozygous for inability to make LDL receptors Cell Normal Mild disease Severe disease 2

13 9.2 Many genes have more than two alleles in the population Although each individual carries, at most, two different alleles for a particular gene, in cases of multiple alleles, more than two possible alleles exist in a population. For example, a certain trait has 3 flavors, A, B and C floating around in a population So, possible combinations for alleles in an individual could be AB or AC or BC ( remember that only two alleles can occur at same time) Think like socks on your feet ( you have many pair but only two socks fit on your feet at any time. Many combinations possible) 9.2 Many genes have more than two alleles in the population Human ABO blood group phenotypes involve three alleles for a single gene. The four human blood groups, A, B, AB, and O, result from combinations of these three alleles. The A and B alleles are both expressed in heterozygous individuals, making both alleles codominant. The alleles code for carbohydrate groups on red blood cells 9.2 Many genes have more than two alleles in the population The alleles are coded as follow Allele I A codes for carbohydrates A on RBC Allele I B codes for carbohydrates B on RBC Allele I O codes for no A or B carbohydrates on RBC ( sometimes referred to as allele i ) So what are the possible combinations? 3

14 Blood Group Carbohydrates Present (Phenotype) Genotypes on Red Blood Cells A B AB I A I A or I A i I B I B or I B i I A I B Carbohydrate A Carbohydrate B Carbohydrate A and Carbohydrate B O ii Neither 9.2 Many genes have more than two alleles in the population Modern Paternal testing uses DNA analysis, but this knowledge of blood groups makes for some quick and easy determinations. For example, a woman claims a man is the father of her baby. She is AB blood type and the baby is blood type A. He claims he is blood type O and thus cannot be the Dad. True or False? 9.2 Many genes have more than two alleles in the population MOM = AB = I A I B DAD = O = ii So possible genotypes (phenotypes) of children I A I B i I A i I B i i I A i I B i Children will have either A or B blood type. 4

15 9.3 A single gene may affect many phenotypic characters Pleiotropy occurs when one gene influences multiple characters. Sickle-cell disease is a human example of pleiotropy. This disease affects the type of hemoglobin produced and the shape of red blood cells and causes anemia and organ damage. Sickle-cell and nonsickle alleles are codominant. Carriers of sickle-cell disease have increased resistance to malaria. An individual homozygous for the sickle-cell allele Produces sickle-cell (abnormal) hemoglobin The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Damage to organs Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis) Other effects Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections 9.4 A single character may be influenced by many genes Many characters result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes on a single phenotypic character. Human skin color is an example of polygenic inheritance. 205 Pearson Education, Inc. 5

16 P generation F generation aabbcc (very light) AABBCC (very dark) 9.4 A single character may be influenced by many genes AaBbCc (medium shade) AaBbCc (medium shade) Sperm F 2 generation Eggs Fraction of population Pearson Education, Inc Skin color 9.5 The environment affects many characters Many characters result from a combination of heredity and the environment. For example, skin color is affected by exposure to sunlight and heart disease and cancer are influenced by genes and the environment. Identical twins show that a person s traits are the results of genetics and the environment Chromosomes determine sex in many species Many animals have a pair of sex chromosomes, designated X and Y, that determine an individual s sex. Among humans and other mammals, individuals with one X chromosome and one Y chromosome are males, and XX individuals are females. In addition, human males and females both have 44 autosomes (nonsex chromosomes). 6

17 9.20 Chromosomes determine sex in many species In mammals (including humans), the Y chromosome has a crucial gene, SRY, for the development of testes, and an absence of the SRY gene directs ovaries to develop. X Y 9.20 Chromosomes determine sex in many species In certain fishes, butterflies, and birds, the sex chromosomes are Z and W, males are ZZ, and females are ZW. Some organisms lack sex chromosomes altogether. In most ants and bees, sex is determined by chromosome number. Females develop from fertilized eggs and thus are diploid. Males develop from unfertilized eggs. Chromosome Males are thus number fatherless and haploid. determines sex 9.20 Chromosomes determine sex in many species In some animals, environmental temperature determines the sex. For some species of reptiles, the temperature at which the eggs are incubated during a specific period of embryonic development determines whether the embryo will develop into a male or female. Global climate change may therefore impact the sex ratio of such species. 7

18 9.2 Sex-linked genes exhibit a unique pattern of inheritance Sex-linked genes are located on either of the sex chromosomes. The X chromosome carries many genes unrelated to sex. Most sex-linked human disorders are due to recessive alleles and seen mostly in males 9.22 CONNECTION: Human sex-linked disorders affect mostly males A male receiving a single X-linked recessive allele from his mother will have the disorder. A female must receive the allele from both parents to be affected. Recessive and sex-linked human disorders include hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting, red-green colorblindness, a malfunction of light-sensitive cells in the eyes, and Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination CONNECTION: Human sex-linked disorders affect mostly males Queen Victoria Albert Alice Louis Alexandra Alexis Czar Nicholas II of Russia Pedigree of Hemophelia in Royal Russian family Female Male Hemophilia Carrier Normal

19 9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution The Y chromosome provides clues about human male evolution because Y chromosomes are passed intact from father to son and mutations in Y chromosomes can reveal data about recent shared ancestry. In 2003, geneticists discovered that about % of males currently living in central Asia have Y chromosomes of striking genetic similarity EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution Further analysis traced their common genetic heritage to a single man living about,000 years ago. In combination with historical records, the data led to the speculation that the Mongolian ruler Genghis Kahn may be responsible for the spread of the telltale chromosome to nearly 6 million men living today 9