Lecture_11_Morgan (6 per page)

Lecture_11_Morgan (6 per page) - Morgan, genes and...

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1 Morgan, genes and chromosomal inheritance - Morgan first traced a gene to a specific chromosome - Genes are linked on a chromosome - Geneticists can determine relative locations and distances of genes on chromosomes based on analyses of recombinant frequencies - Errors and exceptions in chromosomal inheritance Readings: Chapter 15 Topics not covered: X-inactivation in female mammals; Inheritance of organelle genes. Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome Morgan made used, Drosophila melanogaster , a fruit fly species that eats fruit. Fruit flies are prolific breeders and have a generation time of two weeks. Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males). Morgan discovered a single male fly with white eyes instead of the usual red. The normal character phenotype is the wild type . Alternative traits are mutant phenotypes . Fig. 15.3 The white-eyed mutation is on the X chromosome,a sex-linked gene . Females (XX) may have two red-eyed alleles or may be heterozygous and have red eyes. Males (XY) have only a single allele and will be red eyed if they have a red-eyed allele or white-eyed if they have a white- eyed allele. Fig. 15.4 When Morgan crossed white-eyed male with a red- eyed female, all the F 1 offspring had red eyes, The red allele appeared dominant to the white allele. Crosses between the F 1 offspring produced the classic 3:1 phenotypic ratio in the F 2 offspring. Surprisingly, the white-eyed trait appeared only in males. All the females and half the males had red eyes. Morgan concluded that a fly’s eye color was linked to its sex. Sex-linked and autosomal traits The white-eyed Drosophila trait is sex-linked . This trait controlled by a gene on a sex chromosome . The other chromosomes are autosomes . Genes on these are said to be autosomal . Genes on the same chromosome are “linked.” Geneticists can determine relative locations and distances of genes on chromosomes based on analyses of recombinant frequencies of genes Two mechanisms for genetic recombination: 1. Independent assortment of chromosomes at meiosis I. 2. Crossover between homologous chromosomes at prophase of meiosis I.
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2 Independent assortment of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome. Y y R r y y r r YR yr yr Y yR r yyrr YELLOW -ROUND seed (heterozygous) green-wrinkled seed (hom ozygous) Testcross 1 : 1 O f s p r i n g (parental) ( parental) Genotype Phenotype Gamete genotype YELLOW - ROUND green- wrinkled Gamete
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This note was uploaded on 04/29/2008 for the course BIO 113 taught by Professor Swenson during the Spring '08 term at Ohio State.

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Lecture_11_Morgan (6 per page) - Morgan, genes and...

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