Lecture 7: Inheritance and Chromosomes PDF

Go to Discussion Section today/tomorrow and bring completed homework (NOT IN RED PEN OR PENCIL) Average score on quiz was 85% 15% beach 22% forest 63%...

Go to Discussion Section today/tomorrow and bring completed homework (NOT IN RED PEN OR PENCIL) Average score on quiz was 85% 15% beach 22% forest 63% city  Learning objectives (Lecture 7) Chromosomes and meiosis as the mechanism for Mendel’s segregation and independent assortment Understanding sex chromosomes and sex-linkage (Exceptions to Mendelian inheritance) Be able to describe gene linkage and recombinants 2 The Chromosome Theory of Inheritance The chromosome theory of inheritance Arose out of Sutton and Boveri’s observations of meiosis (1902-1903) States that genes are located on chromosomes, at a particular locus The physical separation of alleles during meiosis I is responsible for Mendel’s Principle of Segregation Shown on the next slide 3  Rr genotype Dominant allele Recessive allele for seed shape for seed shape Mendel developed the principle of segregation Chromosomes replicate The two members of each gene pair must segregate Meiosis I They separate into Alleles segregate different gamete cells during the Meiosis II Gametes formation of eggs and sperm in the parents Pairs of alleles are separated during meiosis I in the formation of gametes. 4 Meiosis Explains Mendel’s Principles The genes for different traits assort independently of one another at meiosis I because They are located on different nonhomologous chromosomes Shown on the next slide This phenomenon explains Mendel’s Principle of Independent Assortment His two principles form that backbone of his Theory of Inheritance, which still stands today, although with some extensions/modifications. 5  RrYy genotype Replicated chromosomes prior to meiosis Alleles for seed shape Alleles for seed color Meiosis I Meiosis I Meiosis II Meiosis II Gametes 1/4 RY 1/4 ry 1/4 Ry 1/4 rY PRINCIPLE OF INDEPENDENT ASSORTMENT: The genes for seed shape and seed color assort independently, because (1) they are located on different chromosomes and (2) these chromosomes have two equally likely ways of lining up before they are segregated. 6  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 7 Testing the Chromosome Theory Let’s examine one such mutant, resulting in flies that were white-eyed We’ll learn about one of the modifications of Mendel’s Principles that allowed them to be more general 8 The White-Eyed Mutant Red eyes are the wild-type eye color in fruit flies; while white eyes are a mutation Morgan mated a wild-type female with a mutant male All of the F1 progeny had red eyes Morgan did the reciprocal cross F1 females had red eyes But F1 males had white eyes This suggests a relationship between sex and inheritance of eye color in Drosophila 9 Sex Linkage and the Chromosome Theory X and Y chromosomes are called sex chromosomes They determine the sex of the offspring Females have two X chromosomes Males have an X and a Y A gene being on the X chromosome is X-linked A gene being on the Y chromosome is Y-linked The general term for genes being on either sex chromosome is sex linkage 10 Sex Linkage and the Chromosome Theory Sex chromosomes Pair during meiosis I Segregate during meiosis II Gametes have either an X or a Y chromosome Females produce all X gametes Males produce half X gametes and half Y gametes 11  X chromosome Y chromosome Meiosis I Meiosis II Gametes 50% of sperm contain 50% of sperm contain X chromosome Y chromosome 12 Sex Linkage and the Chromosome Theory Morgan proposed that the gene for white eye color in fruit flies is located on the X chromosome So, he was proposing that the gene for this phenotype exhibited X-linked inheritance He also proposed that red-eye was dominant to white-eye (Genes on non-sex chromosomes are said to be autosomal and show autosomal inheritance) 13  In females, the phenotype is determined by the pair of alleles they have, with Red eye dominant to White (a) One half of reciprocal cross (b) Other half of reciprocal cross Male Male Male gametes Male gametes Female Female gametes gametes Female Female Females Males Females Males In males, the phenotype is determined by the single allele 14 Imagine that the female is heterozygous on the X for eye color… What is the ratio and phenotype(s) of males XWXw produced from this cross? 15 Sex Linkage and the Chromosome Theory So, Morgan was able to show that the allele causing the white-eyed phenotype was recessive and X-linked by Forming the hypothesis Making predictions for the results of specific crosses And confirming by performing the crosses We now know of many other sex-linked genes 16 Exceptions to simple Mendelian Inheritance 1) Sex linkage 2) Linkage 3) Codominance 4) Incomplete dominance 5) Pleiotropy (single gene affects many traits) 6) Gene – gene interaction (epistasis) 7) Gene – environment interaction (G x E) 8) Polygenic inheritance of quantitative traits 17 Linkage: What Happens When Genes Are Located on the Same Chromosome? Mendel’s Principle of Independent Assortment applied to his pea traits We now know that this was because the genes affecting these traits were all on different chromosomes We’ve seen that during meiosis, chromosomes behave like Mendel’s “genetic particles” What happens with genes on the same chromosome? We call this “linkage” 18 Linkage: What Happens When Genes Are Located on the Same Chromosome? Linkage is the tendency of genes to be inherited together because they’re on the same chromosome Linked genes Are predicted to always be transmitted together during gamete formation Should violate the principle of independent assortment (it’s dependent assortment) There are varying degrees of linkage 19 Genes on Chromosome II of the fruit fly 20 Test for Independent Assortment In a di-hybrid cross, are the F2 ratios 9:3:3:1? IF Yes; infer that the genes are unlinked; they reside on different chromosomes. IF No; infer the genes are linked; they reside on the same chromosome. Linkage tells you how close together the genes are on the same chromosome and the degree of linkage is used to map genes. 21  Two linked genes (one for eye color, one for body color) on the X chromosome Gray body Yellow body White eyes Red eyes Meiosis I Meiosis II Female gametes Two, rather than four, types of gametes 22 The Role of Crossing Over Do genes on the same chromosome always stay linked? Morgan performed an experiment He mated two flies That were heterozygous for two sex-linked traits A few male offspring were recombinant Had combinations of alleles not found in the parents Linked alleles sometimes, but not always, stay together 23  The Role of Crossing Over Two traits: Red-eyed (XW ) White-eyed (Xw) Gray body (XY) Yellow body (Xy) Cross: XwYXWy x XwYY Examined male offspring X from female 24 The Role of Crossing Over Expectations: Mendel’s Principal of Independent Assortment? The female from previous slide ? Four male phenotypes – all possible two-trait combinations seen in equal numbers Complete linkage? ? Two male phenotypes seen in equal numbers Same as the two X-chromosomes in the female White-Gray Red-Yellow 25 The Role of Crossing Over Results: neither expectation supported Substantial majority of flies supported completely dependent inheritance 1.4% of flies showed new combinations of alleles (not found in Mother) We call these “recombinant” Crossing-over occurred between the two genes 26  Will genes undergo independent assortment if they are on the sam e chrom osome? Linked genes w ill alw ays stay linked, violating the principle of independent assortment. Linked genes w ill adhere to the principle of independent assortment. Put it all together… Red-eyed White-eyed Why examine only male gray-bodied gray-bodied fem ale m ale XwY XWy XwY Y offspring? Recessive phenotypes Because these tw o genes are X-linked, m ale offspring w ill have only one copy of each gene, from their m other; the tw o possible m ale offspring genotypes are XwY Y and XWyY revealed Four m ale genotypes are possible (XwYY : XWyY : X WyY : XWYY) and w ill occur w ith equal frequency. Male offspring Phenotype Genotype Num ber Four m ale genotypes w ere observed (rather than tw o), but not the equal Recom binant frequencies genotypes predicted by independent assortm ent Neither hypothesis is fully supported. Independent assortment does not apply to linked genes —linked genes segregate together except w hen crossing over and genetic 27 recom bination have occurred. The Role of Crossing Over Morgan proposed that Gametes with new, recombinant genotypes… …Were generated when crossing over occurred during prophase of meiosis I Linked genes are inherited together unless crossing over occurs When crossing over takes place, genetic recombination can occur 28  Crossing over during meiosis I Meiosis II gametes Female Recombinant chromosomes Maternal chromosomes 29 The Role of Crossing Over Genes are more likely to cross over when they are far apart from each other Percentage of recombinant offspring can be used to estimate the relative distance between genes Frequency of crossing over can be used to create a genetic map A diagram showing the relative positions of genes along a particular chromosome 30  Gene 1 Gene 2 Gene 2 Crossing Crossing over is rare over occurs between frequently genes that between are close genes that together are far apart Gene 3 31