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Lecture 12

BIOL 207 (10/09/13) Lecture 12: Sex Chromosomes (end) | Lecture 13: Meiotic Recombination

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Biology (Biological Sciences)
Mike Harrington

BIOL 207 (October 9, 2013) Lecture 12: Sex Chromosomes (con’t) | Lecture 13: Meiotic Recombination X-Linked Genes  Females are either heterozygous or homozygous o Heterozygous: same gene different alleles  Males are hemizygous (“hemi-” = half) o The only have 1 X-chromosome out of the 2 sex chromosomes Drosophila White Gene  The white eye gene in Drosophila is x-linked and recessive. o Heterozygous females will have red eyes  Therefore, they can only have white eyes if they have the genotype (w /w ) o Males with 1 recessive mutant allele will end up with white eyes even though they do not have to copies of it. o Example: A white eyed female (w /w ) is crossed with a red-eyed male (w /Y). What will the next generation look like?  Red eyed females and white eyed males will be the two exhibited phenotypes of the next generation  Here’s the punnet square to show how this comes about: Cat Coat Colour  There is an x-linked red gene o O = orange o o = black  Female cats that are heterozygous for this gene will exhibit orange and black spotted coats o Homozygous for ‘O’ will give them orange coats o Homozygous for ‘o’ will give then black coats  For males, though, it is one or the other because they only have one X chromosome o If X – chromosome has ‘O’ allele, cat will be orange and vice versa if the allele is ‘o’  Why do female cats exhibit co-dominance in the two alleles? o They have 2 X-chromosomes and each on has a different allele for coat colour. o In any one chromosome pair, on will have an allele for orange coat colour and the other one will have the allele for the black coat colour. o However, one of the two chromosomes will be inactive for that gene  You see orange fur for the cells that have an active orange fur allele  You see black fur for the cells that have an active black fur allele Mutations in X-linked Genes in Humans  Human males can also be hemizygous  If an x-linked gene were to be mu
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