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Download Ch 15: Sex Determination & Sex Linkage
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Ch 15: Sex Determination & Sex Linkage • Human somatic cells contain 23 pairs of chromosomes – 22 pairs of autosomes (same in males & females) – 1 pair of sex chromosomes (XX or XY) • Females have 2 matching sex chromosomes • Males have 1 X and 1 Y • Sex chromosomes carry genes that affect other traits • a gene on one of the sex chromosomes is called: SEX-LINKED • When a gene is located on the X chromosome, females receive 2 copies of the gene, and males receive only 1 copy – Example: Color-blindness (c) is recessive to normal vision (C), and it is located on the X chromosome; hemophilia LE 15-4b P Generation Ova (eggs) Sperm F1 Generation Ova (eggs) F2 Generation Sperm EXAMPLE PROBLEM: • A female heterozygous for normal vision: (we say she has normal vision, but is a carrier of the colorblindness allele) XC Xc • A male who is colorblind: Xc Y What is the probability that: a) they will have a son who is colorblind? b) they will have a daughter who is colorblind? c) their first son will be colorblind? d) their first daughter will be carrier? XC Xc a) 1:4 (25%) c X XC Xc Xc Xc b) 1:4 (25%) Y XC Y d) 1:2 (50%) Xc Y c) 1:2 (50%) EXAMPLE PROBLEM: • Hemophilia is a hereditary disease in which the blood clotting process if defective. Classic hemophilia results from an abnormal or missing clotting factor VIII; it is inherited as an Xlinked recessive disorder (h). • If a man without hemophilia and a woman who is a carrier of the hemophilia allele have children, what is the probability that… H X Y x H X h X what is the probability that: a) they will have a daughter with hemophilia? b) they will have a son with hemophilia? c) their first son will have hemophilia? d) their first daughter will be a carrier? XH XH Y Xh XH XH XH Xh XH Y Xh Y a) 0:4 (0%) b) 1:4 (25%) c) 1:2 (50%) d) 1:2 (50%) Pedigree Charts The Big Question… • It may be easy to see that genes located on different chromosomes assort independently but what about genes located on the same chromosome? Thomas Morgan’s Research • Morgan identified more than 50 genes on Drosophila’s 4 chromosomes. • He discovered that many seemed to be “linked” together – They are almost always inherited together & only rarely become separated • Grouped genes into 4 linkage groups Morgan’s Conclusion • Each chromosome is actually a group of linked genes • BUT Mendel’s principle of independent assortment still holds true • It is the chromosome that assorts independently!! – Mendel missed this because 6 of the 7 traits he studied were on different chromosomes. So… • If 2 genes are found on the same chromosome are they linked forever? – NO!! • Crossing-Over during Meiosis can separate genes – Increases genetic diversity Gene Maps • Alfred Sturtevant was a graduate student working in Morgan’s lab part-time in 1911 • He hypothesized that the farther apart 2 genes are on a chromosome the more likely they are to be separated by crossingover • The rate of at which linked genes are separated can be used to produce a “map” of distances between genes Alfred Sturtevant 1891-1970 17% LE 15-6a Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) Replication of chromosomes Replication of chromosomes Meiosis I: Crossing over between b and vg loci produces new allele combinations. Meiosis I and II: No new allele combinations are produced. Meiosis II: Separation of chromatids produces recombinant gametes with the new allele combinations. Ova Gametes Recombinant chromosomes Sperm LE 15-6b Sperm Ova Gametes Ova Testcross offspring Sperm 965 Wild type (gray-normal) 944 Blackvestigial Parental-type offspring 206 Grayvestigial 185 Blacknormal Recombinant offspring Recombination 391 recombinants 100 = 17% = frequency 2,300 total offspring Gene Maps • This map shows the relative locations of each known gene on a chromosome – We have used this technique to help construct maps of the Human Genome LE 15-7 Recombination frequencies 9% 9.5% 17% b cn vg Chromosome Recombination frequency between b and cn: 9% Recombination frequency between vg and cn: 9.5% Recombination frequency between b and vg: 17% IB Linkage Symbolism • Instead of writing TtBb for a two-gene cross (indicating the genes are on different chromosomes), you will see the following TB notation indicating the genes are linked: – Imagine each line is a chromosome and the 2 lines are the homologous pair – What would be an example of a recombinant? – What process led to this recombinant? tb LE 15-11 Barr bodies: during embryological development, 1 of the 2 “X” sex-chromosomes in each cell randomly condenses and is inactivated Two cell populations in adult cat: Active X Early embryo: Orange fur X chromosomes Cell division Inactive X and X chromosome Inactive X inactivation Black fur Allele for orange fur Allele for black fur Active X -1 gene for fur color is located on the “X” chromosome -orange and black are alleles -which embryological cell in the female inactivates, determines the coat color LE 15-12 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 n+1 n–1 n–1 n+1 n–1 n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis I n If a condition is sex linked… A. B. C. D. It is inherited on the “X” sex chromosome only It is inherited on the “Y” sex chromosome only It is inherited on either the “X” or the “Y” sex chromosome and you need to inherit 2 copies It is inherited on either the “X” or the “Y” sex chromosome How is a gene map produced? A. B. C. D. Using known gene positions and then hypothesizing the locations of nearby genes Using electron microscopes to see the locations of specific genes that have been radioactively stained Using recombination frequencies which show that genes with low percents of recombination are closer together Using mutation frequencies which show that genes with low percents of mutation are closer together If two genes are linked, what does this mean? A. B. C. D. They are part of the same haploid gamete They can only be inherited together They are located on the same chromosome They are located on either the “X” or “Y” chromosome In a testcross of T b what tB would be an example of a phenotypic recombinant? (T= tall; B= black) A. Tall-white B. C. D. Tall-black Short-white All of the above are recombinants If Genes A and B recombine at a 12% frequency and Genes A and C recombine at a 7% frequency, which Genes are closest together? A. B. C. D. Genes A and B Genes B and C Genes A and C You can’t tell from the information provided.