Biology 135A Chromosomal Basis for Inheritance PDF

Biology 135A Chromosomal Basis for Inheritance (Ch. 15) Rajab Introduction Chromosome theory – Thomas Morgan – Linkage – Recombination Mapping genes on chromosomes-Alfred Sturtevant – Recombination frequency Autosomes vs. sex chromosomes Chromosome theory:...

Biology 135A Chromosomal Basis for Inheritance (Ch. 15) Rajab Introduction Chromosome theory – Thomas Morgan – Linkage – Recombination Mapping genes on chromosomes-Alfred Sturtevant – Recombination frequency Autosomes vs. sex chromosomes Chromosome theory: Mendelian anchromosome address on a genes have loci 1000s per chrom. Chromosomes undergo segregation & independent during meiosis assortment. diploid carriotype Chromosomes can be autosomes or sex chromosomes Thomas Morgan: Morgan’s first question: where are genes located? 1st to associate a gene with a specific chromosome – white eye of fruit fly on X chromosome Morgan’s Experiment In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? XEXe or XEy, wont be any homos? this shows that its on the x chromosome XEXe x XEy = XEXE , XExe , XEy , xey white eyes = partially lethal genes so not exactly 25% 4252 flies; 782 white-eyed all male  w+ = dominant Xw+ Xw+ = homo dom? Results Xwy = hemizygous recessive Xw+y= hemi dom Morgan’s symbol system differed from Mendel’s 1. All F1 red → white recessive 2. B/C white eyes expressed only in F2 males, then eye-color gene on the X chromosome [with no corresponding locus on the Y chromosome]  Next Question: Are the genes for body color and wing size in fruit flies located on the same chromosome or different chromosomes? autosomal (not sex) vestigial: doesnt have a function Morgan did a testcross: b+bvg+vg x bbvgvg His expected result w/o linkage (i.e. independent assortment [this is what Mendel would have predicted]): b+bvg+vg / bbvgvg / b+bvgvg / bbvg+vg in a 1:1:1:1 ratio His expected result w/ linkage: b+bvg+vg / bbvgvg in a 1:1 ratio His actual result: Most were as expected with linkage, but got some that were not expected b+bvg+vg / bbvgvg / b+bvgvg / bbvg+vg in a 5:5:1:1 ratio (965:944:206:185) wait what??? (Morgan was first to use that term) How can this be explained? Crossing over This only happened in 17% of the gametes ****** need to know how to do Genetic Recombination: – recombination is the production of offspring with new combinations of traits inherited from two parents – results from: 1. Independent assortment: – in a dihybrid cross, there are parental and recombinant offspring → show example of pea seed color and shape cross 2. Recombination of linked genes by crossing over: – cross over breaks linkages in parental chromosomes to form recombinant chromosomes with new allele combinations 3. Random fertilization Genes are arranged linearly on a chromosome and we can map them → Morgan…again! (Actually…Sturtevant) – Recombination frequency can be used to map the relative positions of loci → genetic mapping = linkage map Alfred Sturtevant’s hypothesis: recombination frequencies reflect distances between genes on a chromosome – the farther apart two genes are, the higher the probability of a cross-over event. – one map unit = 1% recombination frequency = 1 centimorgan Use the following recombination data (same as what Sturtevant had) to map these genes Gene pair Recombination frequency b and vg 17 mu b and cn 9 mu vg and cn 9.5 mu The b−vg recombination frequency is slightly less than the sum of the b−cn and cn−vg frequencies because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes. A second crossover would “cancel out” the first and thus reduce the observed b−vg recombination frequency. Figure 15.8 A partial genetic (linkage) map of a Drosophila chromosome Linkage maps only show the linear sequence of genes along a chromosome, not precise physical locations precise = DNA sequencing  THE TWO GIANTS OF GENETICS Mendel postulated three laws: (1) Alleles segregate (meiosis happens) (2) Dominance (3) Independent assortment Morgan discovered the following: (1) Genes live on chromosomes (2) Chromosomes recombine (cross over) (3) Genes are arranged linearly on chromosomes Autosomes vs. Sex Chromosomes Figure 15.5 Human Sex Chromosomes: – females are XX and males are XY – X and Y behave like homologues during 1,100 genes meiosis but not true homologues – SRY gene on Y determines testes formation w/out SRY, gonads develop as ovaries – males are hemizygous for their sex Y chromosomes – other organisms? 78 genes The transmission of X-linked recessive traits. Sex-Linked Genetic Disorders X-linked disorders in humans: Color-blindness –recessive: 1 in12 men and 1 in 200 women Duchenne muscular dystrophy-recessive Hemophilia-recessive Hypertrichosis-dominant Y-linked disorders: Hairy pinnae (is it dominant or recessive?)  Color blindness test. Can you see the number? Just Kidding!! Haha Go to https://colormax.org/color-blind-test/  Hypertrichosis: werewolf syndrome X-linked dominant  Hairy ears: a phenotype proposed to be Y linked