Chapter 14 Mendel and the Gene Idea*

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1 Chapter 14 Mendel and the Gene Idea* *Lecture notes are to be used as a study guide only and do not represent the comprehensive information you will need to know for the exams. Drawing from the Deck of Genes Genetic principles are involved in the transmission of traits from parents to offspring in humans and other organisms. The gene idea is parents pass on discrete heritable units genes that retain their separate identities in offspring. An organism s collection of genes is more like a deck of cards than a pail of paint. Like playing cards, genes can be shuffled and passed along, generation after generation. Genetics as we it know today was started by Gregor Mendel (page 267). Mendel developed his theory of inheritance using pea plants (Figure 14.1) several decades before chromosomes were initially discovered. Concept 14.1 : Mendel used the scientific approach to identify two laws of inheritance Mendel used a garden pea plant to carry out his experiments. Mendel s Experimental, Quantitative Approach Mendel s academic background prepared him to carry out his experiments. Mendel used the garden at his abbey to grow peas. He selected different varieties of peas to carry out his studies. Peas have a short generation time and produce a lot of offspring. A heritable feature that varies among individuals is called a character. Each variant for a character is called a trait. Mendel was able to control mating between plants. The reproductive organs of a pea plant are in the flowers the male portion is called the stamen, and makes pollen that contains the sperm; the female portion is called the carpel, and makes the eggs. Pea plants can self-fertilize. Mendel cross-fertilized his plants in order to control the traits he was tracking (Figure 14.2). Each developing zygote then developed in a plant embryo in a seed (pea). Mendel only tracked those characters that occurred in two distinct alternate forms. Mendel started his experiments with true-breeding parents parents that showed the same traits over many generations. When Mendel crossed two true-breeding plants of different traits, for example, purple flower and white flower, this process of mating, or crossing, is called hybridization. The true breeding plants are referred to as the P generation (parental generation), and their hybrid offspring is called the F 1 generation (first filial generation, filial is Latin for son ); using the F 1 generation for mating produces the F 2 generation (second 1

2 filial). Because Mendel studied his crosses up to the F 2 generation, he was able to deduce two (2) fundamental principles of heredity: 1) the law of segregation, and 2) the law of independent assortment. The Law of Segregation Mendel allowed his F 1 generation to self-pollinate and planted their seeds (from Figure 14.2), the white flower reappeared in the F 2 generation. The data with the F 2 fit into a 3:1 ratio (Figure 14.3). His results showed that the purple is the dominant trait and white is the recessive trait. Mendel observed the same 3:1 pattern in other characters (Table 14.1). This was evidence that the heritable factor had not been diluted or destroyed. From this data Mendel deduced the law of segregation. Mendel s Model Mendel developed a model to explain his results. There are four (4) concepts that comprise this model. First alternate versions of genes account for variations in inherited characters. Genes exist in two (2) versions. The alternate versions of genes is called alleles (Figure 14.4). Each gene is a sequence of nucleotides at a specific position, or locus, on a chromosome. This means the nucleotide sequence varies slightly for each allele / gene. Second for each character, an organism inherits two copies of a gene, one from each parent. These are also called alleles of the gene. Recall that in somatic cells, an organism inherits a copy of each gene from each parent. A diploid organism has two sets of chromosomes, one set inherited from each parent. Third if the two alleles at a locus differ, then one, the dominant allele, determines the organism s appearance; the other, the recessive allele, has no noticeable effect on the organism s appearance. Fourth the law of segregation, states that the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes. Thus, an egg or sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete. This refers to the distribution of chromosomes to different gametes in meiosis. A handy diagrammatic device for predicting the outcome of the parental mating is using a Punnett square (Figure 14.5). A capital letter is used to symbolize the dominant allele, for example, F, G, T, H, Q, C; and a lower case letter is used to symbolize the recessive allele, for example, f, g, t, h, q, c. 2

3 Useful Genetic Vocabulary: Homozygous: an organism that has an identical allele pair for a character, for example, FF, dd. Heterozygous: an organism that has a non-identical allele pair for a character, for example, Ff, Dd; this is not true-breeding. An organism s traits do not always reveal its genetic composition. It is important to distinguish between an organism s appearance and genetic make-up (Figure 14.6). Phenotype: the organism s appearance, the traits you can see with your own eyes. Genotype: the genetic composition of the organism, the nucleotide sequence of the allele (gene). The Testcross Cross an unknown genotype which could be Pp or PP with a homozygous recessive, pp, to determine the unknown allele pair (Figure 14.7). A testcross is breeding an organism of unknown genotype with a recessive homozygote. The Law of Independent Assortment Monohybrid / monohybrid cross follows one particular character. Mendel s second law of inheritance was determined by following two characters at the same time, or dihybrids. The cross to determine their outcome is called a dihybrid cross (Figure 14.8). Alleles are sorting independently of each other during meiosis (gamete formation). Mendel s dihybrid experiments are the basis for the law of independent assortment, which states that each pair of alleles segregates independently of each other pair of alleles during gamete formation. This applies to genes that are on different chromosomes. Concept 14.2 : Probability laws govern Mendelian inheritance Probability math is used to determine genetic outcomes on a scale from 0 to 1. An event that will not occur has 0 probability. An event that will occur has a probability of 1. A useful example is a coin toss. Each coin toss is an independent event. The Multiplication and Addition Rules Applied to Monohybrid Crosses Multiplication rule states that to determine a probability of an outcome, multiply the probability of one event by the probability of the other event (Figure 14.9). 3

4 The addition rule states that the probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities. Concept 14.3 : Inheritance patterns are often more complex than predicted by simple Mendelian genetics Not all heritable traits are determined by just two alleles of the same gene. The basic principles of segregation and independent assortment apply even to more complex patterns of inheritance. Extending Mendelian Genetics for a Single Gene Not all alleles are completely dominant or recessive. Degrees of Dominance Alleles can show different degrees of dominance and recessiveness in relation to each other. Complete dominance is when the expression of the dominant allele completely masks the expression of the recessive allele in the heterozygote. The phenotype of the heterozygote looks exactly like the homozygous dominant. For some genes neither allele is completely dominant, and the F 1 hybrids have a phenotype somewhere in between the two parents. This is called incomplete dominance (Figure 14.10). The heterozygote in incomplete dominance expresses a bit of the traits of both parents. Another variation on dominance relationships between alleles is codominance, where both alleles are expressed equally in the heterozygous. The Relationship Between Dominance and Phenotype The allele that is called dominant derives its designation from being able to see it in the phenotype. Frequency of Dominant Alleles Some dominant alleles do not occur very frequently in the population. Multiple Alleles The ABO blood group of humans is a good example of multiple alleles being expressed at the same time (Figure 14.11). Pleiotropy A single gene can affect a number of characteristics in an organism, like sickle cell anemia. 4

5 Extending Mendelian Genetics for Two or More Genes Two or more genes are involved in determining a particular phenotype. Epistasis Epistasis is the phenotypic expression of a gene at one locus alters that of a gene at a second locus. Follow the example written for Figure Polygenic Inheritance Characters of an organism that vary in the population. These characters do not follow the simple Mendelian either-or basis. Characters that vary are called quantitative characters, and usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypic character. This is the opposite of pleiotropy. An example is skin pigmentation (Figure 14.13). Nature and Nurture: The Environmental Impact on Phenotype When the phenotype for a character depends not only on genotype, but also the environment. A genotype generally is not associated with a rigidly defined phenotype, but rather with a range of phenotypic possibilities due to environmental influences. This is called the norm of reaction for a genotype (Figure 14.14). Characters that fall into this category are called multifactorial, meaning that many factors, both genetic and environmental, collectively influence phenotype. Concept 14.4 : Many human traits follow Mendelian patterns of inheritance Basic Mendelism endures as the foundation of human genetics. Pedigree Analysis Analyze the results of matings that have already occurred for people. This is done by collecting family history for a particular trait, and assembling that history on a chart called a pedigree. This chart describes the traits of parents and children across a few generations (Figure 14.15). 5

6 Recessively Inherited Disorders Genetic disorders that are known to be inherited as simple recessive traits. The Behavior of Recessive Alleles Only shows in the homozygous recessive allele pair, example, aa. Those who are heterozygote for the trait, Aa, they may not express the disorder, but rather they are carriers (Figure 14.16). Cystic Fibrosis Cystic fibrosis is most common in people of European descent. About one of every 25 are carriers of the cystic fibrosis allele. The normal gene for this allele produces a chloride ion membrane transport protein. For those with a mutation in this gene, they are unable to transport the chloride ion. As a result, people with cystic fibrosis tend to collect mucus around their internal organs, which makes it difficult for them to breathe and digest food. Sickle-Cell Disease Sickle cell disease (Figure 14.17) is most common among people of African descent. It affects one out of every 400 African Americans. Sickle cell disease is caused by a single mutation in the gene that codes for the hemoglobin protein in red blood cells. Those who are homozygous for the trait have sickle cell disease. Those who are heterozygous for the trait are carriers. It is thought that since this trait is common among African people who live near the equator, where malaria is endemic, this is a trait that reduces the incidence of acquiring malaria in the heterozygote individual. Dominantly Inherited Disorders Some human disorders are due to dominant alleles. One condition is achondroplasia, or dwarfism. Those who are dwarves (Figure 14.18) are heterozygous for the allele pair. Therefore, normal height is recessive. A lethal dominant allele in an individual can cause death of people before they can mature and reproduce. Huntington s Disease: A Late-Onset Lethal Disease Huntington s Disease is the result of a lethal dominant allele that is not expressed until later in adulthood. It is a degenerative disease of the nervous system. It has no obvious phenotype until the person is years old. Multifactorial Disorders Many people are susceptible to diseases that have a multifactorail basis genotype and environment. For example, heart disease, diabetes, cancer, and certain mental illnesses. Our lifestyle can have a tremendous effect on certain phenotypes. 6

7 Genetic Testing and Counseling Genetic counselors can provide genetic information to people who want to conceive, or may be pregnant, about their children. Counseling Based on Mendelian Genetics and Probability Rules Use the rule of multiplication to determine the chance that parents who are carriers could pass the disease along to their offspring. Tests for Identifying Carriers Determine if the parents are heterozygous carriers of the recessive allele. Tests can be done (Figure 14.19) that can determine if the parents are homozygous dominant, or carry the recessive allele. Fetal Testing Amniocentesis can determine if a developing fetus has a disease (Figure 14.19a). Amniotic fluid is removed and tested. Another test is chorionic villus sampling (CVS) (Figure 14.19b), where the cells of the chorionic villi of the placenta from the fetus is tested. These cells are rapidly growing, so they can be used immediately for testing. Ultrasound can be used to determine any anatomical abnormalities. Newborn Screening Some genetic disorders are determined at birth by biochemical testing. 7