Genetics II - B1115 - SP24 - Stud - PDF

Genetics II Chromosome Theory of Inheritance The chromosome theory of inheritance states that genes are located on chromosomes, at a particular locus. The movement of chromosomes during meiosis explains Mendel’s principles of inheritance. List and explain Mendel’s two principles of inheritance. Prin...

Genetics II Chromosome Theory of Inheritance The chromosome theory of inheritance states that genes are located on chromosomes, at a particular locus. The movement of chromosomes during meiosis explains Mendel’s principles of inheritance. List and explain Mendel’s two principles of inheritance. Principle of Segregation The physical separation of alleles during meiosis I is responsible for Mendel’s principle of segregation. Principle of Independent Assortment The genes for different traits assort independently of one another at meiosis I because they are located on different nonhomologous chromosomes. oDifferent nonhomologous chromosomes assort independently of one another. § This phenomenon explains Mendel’s principle of independent assortment. Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Testing the Chromosome Theory Thomas Hunt Morgan adopted fruit flies (Drosophila melanogaster) as a model organism for genetics. Morgan’s first goal was to identify different phenotypes: Wild type is the most common phenotype for each trait. Other phenotypes arise by mutation. Mutants are individuals with traits caused by mutations. Testing the Chromosome Theory This suggests a relationship between sex and inheritance of eye colour in Drosophila. Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Sex Determination In some animals, females are XX, and males are XY. oEach ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome. oIn humans and other mammals, all embryos initially develop immature internal sexual structures of both females and males. § The SRY gene on the Y chromosome encodes a protein that is the trigger for male development. Other animals have different methods of sex determination. Human Sex Chromosomes Most chromosomes come in pairs that match in shape and size. In many animal species there is a special pair of unmatched chromosomes called the sex chromosomes. These chromosomes determine the sex of the individual. The X chromosome consists of Do all of the genes on the sex about 1000 genes and the Y of chromosomes relate to the sex of about 50 genes. the individual? Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Sex-Linked Traits Sex-Linked Traits The general term for genes being on either sex chromosome is sex linkage. oA gene located on the X chromosome is X-linkage. oA gene located on the Y chromosome is Y-linkage. A recessive X-linked trait can be found in: A. B. C. D. females only. males only. Both males and females Neither males nor females A recessive X-linked trait can be found in: A. B. C. D. females only. males only. Both males and females Neither males nor females A Y-linked trait can be found in: A. B. C. D. females only. males only. both males and females. neither males nor females. A Y-linked trait can be found in: A. B. C. D. females only. males only. both males and females. neither males nor females. Review Question A man with haemophilia (a recessive X-linked condition) has a daughter of normal phenotype. She marries a man who is normal for the trait. o What is the probability that a daughter of this mating will be haemophiliac? o What is the probability that the couple will have a haemophiliac son? Review Question In cats, black coat (XB) is codominant with yellow (XY). The coat colour gene is on the X chromosome. Calico cats, which have coats with yellow and black patches, are heterozygous for the coat colour alleles. o Why are most calico cats female? o A calico female, Fluffy, had a litter with one yellow male, two black males, two yellow females, and three calico females. What are the genotype and phenotype of the father? Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Recall the Dihybrid Cross Genetic Linkage When genes are close together on the same chromosome (autosome or sex chromosome), they are linked. o Linkage is the tendency of genes to be inherited together because they are on the same chromosome. Genetic Linkage Genetic Linkage The lack of independent assortment means that the genes show linkage. The male progeny fall into two groups: One group, represented by fewer numbers of progeny, consists of XwyY and XWYY, which are recombinants as they result from a crossover of homologous chromosomes. The other group, nonrecombinants, consist of genotypes found in one of the parents, in this case, the mother. Recombinant and Nonrecombinant Alleles Crossing over in prophase I of meiosis is a physical exchange of chromosome parts. Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Frequency of Recombination The frequency of recombination is a measure of the distance between linked genes. When two genes are on separate chromosomes, the ratio of 1:1:1:1 is expected for the nonrecombinant and recombinant gametes. For two genes found on the same chromosome, they may be located very far from each other and crossing over is likely to occur. Or the genes may be found very close together, making a crossover event less likely. Would you expect to see a 1:1:1:1 ratio when genes are close together on the same chromosome? Frequency of Recombination The frequency of recombination between any two genes ranges from 0% (when crossing over never takes place) to 50% (when the genes are so far apart that crossing over between the genes always takes place). Linked genes have a recombination frequency between 0% and 50%. What is the frequency of recombination for these two genes? Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Genetic Maps The frequency of recombination can be used as a measure of the physical distance between genes. oThese distances are used in the construction of a genetic map, which is a diagram showing the relative position of genes along a chromosome. § The maps are drawn using a scale in which one unit of distance (a map unit) is the distance between genes resulting in 1% recombination. How far apart are w and y (in map units)? Review Questions Determine the sequence of genes along a chromosome based on the following recombination frequencies: W–X, 21 map units; W–Z, 29 map units; X–Y, 23 map units; X–Z, 8 map units; Y–Z, 15 map units. What is the predicted frequency of recombination between X and Z? What process produces recombinants? A. B. C. D. E. chromosome deletion chromosome duplication crossing over nondisjunction X-linkage What process produces recombinants? A. B. C. D. E. chromosome deletion chromosome duplication crossing over nondisjunction X-linkage The frequency of recombination is __________ for genes that are closer together compared to genes that are further apart on the same chromosome. A. larger B. smaller The frequency of recombination is __________ for genes that are closer together compared to genes that are further apart on the same chromosome. A. larger B. smaller Two genes that are 8.2 map units apart will have a recombination frequency of: A. B. C. D. E. 2.05%. 4.1%. 8.2%. 16.4%. It is not possible to determine this. Two genes that are 8.2 map units apart will have a recombination frequency of: A. B. C. D. E. 2.05%. 4.1%. 8.2%. 16.4%. It is not possible to determine this. Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Multiple Alleles When there are more than two alleles of a gene in a population, it is called multiple allelism. o May have dozens of alleles of a single gene. § Example: Humans have three common alleles for ABO blood types: - IA, IB, and i - Each allele codes for a version of an enzyme that adds polysaccharides to the membrane of red blood cells. How many alleles of this gene will an individual have? Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Complete Dominance Alleles of a gene are not always dominant or recessive. Codominance Some alleles display codominance. Neither allele is dominant or recessive to the other. Heterozygotes display the phenotype of both alleles. – Example: ABO blood types § Both IA and IB are dominant to i. § Yet, IAIB heterozygotes produce both polysaccharides, resulting in the AB blood type. Incomplete Dominance Other alleles display incomplete dominance. Heterozygotes have an intermediate phenotype. For example, pure-line plants with red flowers (RR) crossed to pure-line plants with white flowers (rr). Mendel would predict heterozygous offspring with red flowers (Rr). However, Rr offspring have pink flowers. Why does this not support the blending hypothesis? Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Gene-Gene Interactions The expression of many genes depends on the presence or absence of other genes. Two or more genes work together to control a single trait. Genes that modify the phenotypic expression of other genes are said to show epistasis. There are many types of epistasis leading to different modifications of the 9:3:3:1 ratio. Epistasis For example, comb shape in chickens is controlled by two genes (R and P). oThe R allele is expressed differently depending on which allele of P is present. Mendelian Genetics for Two or More Genes Quantitative characters are those that vary in the population along a continuum. ‒Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype. Wheat kernel colour is an example of polygenic inheritance. Environment and Phenotype Another departure from Mendelian genetics arises when the phenotype for a trait depends on environment as well as genotype. An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behaviour. – An organism’s phenotype reflects its overall genotype and unique environmental history. Which of the following is the strongest evidence that a trait might be influenced by polygenic inheritance? A. F1 offspring of parents with different phenotypes have the dominant phenotype. B. F1 offspring of parents with different phenotypes have an intermediate phenotype. C. The trait shows qualitative (discrete) variation. D. The trait shows quantitative variation. Which of the following is the strongest evidence that a trait might be influenced by polygenic inheritance? A. F1 offspring of parents with different phenotypes have the dominant phenotype. B. F1 offspring of parents with different phenotypes have an intermediate phenotype. C. The trait shows qualitative (discrete) variation. D. The trait shows quantitative variation. Mendelian Genetics: Human Traits Humans are not good subjects for genetic research: oGeneration time is too long oParents produce relatively few offspring oBreeding experiments are unacceptable § However, basic Mendelian genetics endures as the foundation of human genetics. Lecture Outline 1. Testing the Chromosome Theory of Inheritance a) Sex Determination b) Sex-linked Traits 2. Genetic Linkage a) Frequency of Recombination b) Genetic Maps 3. Multiple Alleles 4. Complete Dominance, Incomplete Dominance and Codominance 5. Epistasis 6. Pedigrees Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations. Inheritance patterns of particular traits can be traced and described using pedigrees. Pedigrees can also be used to make predictions about future offspring. We can use the multiplication and addition rules to predict the probability of specific phenotypes. Is this disorder caused by a dominant allele or a recessive allele? Behaviour of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele. Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal. Dominantly Inherited Disorders Some human disorders are caused by dominant alleles. oDominant alleles that cause a lethal disease are rare and arise by mutation. Would you expect recessive and dominant disorders to differ in severity and prevalence? If so, why? Why is the pea wrinkled-seed allele a recessive allele? A. Individuals with the allele have lower fitness than that of individuals with the dominant allele. B. It ‘recedes’ in the F2 generation when homozygous parents are crossed. C. The allele is less common than the dominant allele (i.e. the wrinkled allele is a rare mutant). D. The trait associated with the allele is not exhibited in heterozygotes. Why is the pea wrinkled-seed allele a recessive allele? A. Individuals with the allele have lower fitness than that of individuals with the dominant allele. B. It ‘recedes’ in the F2 generation when homozygous parents are crossed. C. The allele is less common than the dominant allele (i.e. the wrinkled allele is a rare mutant). D. The trait associated with the allele is not exhibited in heterozygotes.