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Biology Slide 1 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps 11-5 Linkage and Gene Maps Slide 2 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps It’s easy to see how genes located on the same chromosome assort independently, but what about genes located on the same chromosome? Wouldn’t they generally be inherited together? Slide 3 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Linkage What structures actually assort independently? Slide 4 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Linkage Gene Linkage Thomas Hunt Morgan’s research on fruit flies led him to the principle of linkage. Morgan discovered that many of the more than 50 Drosophila genes he had identified appeared to be “linked” together. They seemed to violate the principle of independent assortment. Example: Fruit flies with reddish-orange eyes and miniature wings Genes for these traits were almost inherited together Slide 5 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Linkage Morgan and his associates grouped the linked genes into four linkage groups. Each linkage group assorted independently but all the genes in one group were inherited together. Each chromosome is actually a group of linked genes. Slide 6 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Linkage Morgan concluded that Mendel’s principle of independent assortment still holds true. Chromosomes assort independently, not individual genes. Slide 7 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps The Chromosomal Basis of Sex • In humans and other mammals there are two varieties of sex chromosomes, designated X and Y. • The Y chromosome is much smaller than the X chromosome. • A person who inherits two X chromosomes develops as female. • A male develops from a zygote containing one X and one Y chromosome. Copyright Pearson Prentice Hall Slide 8 of 18 End Show 11-5 Linkage and Gene Maps The Chromosomal Basis of Sex • In both testes and ovaries, the two sex chromosomes segrgate during meiosis, and each gamete receives one. • Each egg contains one X chromosome. • Half the sperm cells contain a Y chromosome and the other half contain an X chromosome. • Therefore, sex determination is a matter of chance ( a fifty-fifty chance). Slide 9 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Inheritance of Sex-Linked Genes • In addition to their role as carriers of genes that determine sex, the sex chromosomes have many characters unrelated to sex. • A gene located on either sex chromosome is called a sex-linked chromosome. • Fathers pass sex-linked alleles to all of their daughters but to none of their sons. • Mothers pass sex-linked alleles to both sons and daughters. Slide 10 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Inheritance of Sex-Linked Genes • If a sex-linked allele is recessive, a female will express the phenotype only if she is homozygous. • Any male receiving the recessive allele from his mother will express the trait. • More males have sex-linked recessive disorders. Example: Color-blindness Color-blind fathers pass it on to daughters Slide 11 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Slide 12 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Maps Gene Maps Crossing-over during meiosis sometimes separates genes that had been on the same chromosomes onto homologous chromosomes. Crossover events occasionally separate and exchange linked genes and produce new combinations of alleles. Slide 13 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Maps • Alfred Sturtevant, a student of Morgan, reasoned that the farther apart two genes were, the more likely they were to be separated by a crossover in meiosis. • Recombination frequencies can be used to determine the distance between genes. Slide 14 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Maps • Sturtevant created a gene map showing the relative locations of each known gene on one of the Drosophila chromosomes. Slide 15 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Maps • If two genes are close together, the recombination frequency between them should be low, since crossovers are rare. • If they are far apart, recombination rates between them should be high. Slide 16 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Maps Exact location on chromosome 0.0 Chromosome 2 Aristaless (no bristles on antenna) 13.0 Dumpy wing 48.5 Black body 54.5 Purple eye 67.0 Vestigial (small) wing 99.2 Arc (bent wings) 107.0 Speck wing Slide 17 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Gene Maps Exact location on chromosome Chromosome 2 1.3 Star eye 31.0 Dachs (short legs) 51.0 Reduced bristles 55.0 Light eye 75.5 Curved wing 104.5 Brown eye Slide 18 of 18 Copyright Pearson Prentice Hall End Show 11-5 Linkage and Gene Maps Advanced Genetics Video Slide 19 of 18 Copyright Pearson Prentice Hall End Show 11-5 Click to Launch: Continue to: - or - Slide 20 of 18 End Show Copyright Pearson Prentice Hall 11-5 According to Mendel's principle of independent assortment, the factors that assort independently are the a. genes. b. chromosomes. c. chromatids. d. gametes. Slide 21 of 18 End Show Copyright Pearson Prentice Hall 11-5 A chromosome is actually a group of a. independent genes. b. linkage groups. c. crossed-over genes. d. linked genes. Slide 22 of 18 End Show Copyright Pearson Prentice Hall 11-5 Thomas H. Morgan is credited with the discovery of the principle of a. segregation. b. independent assortment. c. gene linkage. d. dominance. Slide 23 of 18 End Show Copyright Pearson Prentice Hall 11-5 Linkage maps can be produced because the farther apart two genes are on a chromosome, a. the less likely they are to assort independently. b. the more likely they are to be linked. c. the more likely they are to be separated by a crossover. d. the less likely they are to be separated by a crossover. Slide 24 of 18 End Show Copyright Pearson Prentice Hall 11-5 If two genes are close together on the same chromosome, they are more likely to a. behave as though they are linked. b. behave independently. c. move to different chromosomes. d. belong to different linkage groups. Slide 25 of 18 End Show Copyright Pearson Prentice Hall END OF SECTION