BIOL 207 (October 9, 2013)
Lecture 12: Sex Chromosomes (con’t) |
Lecture 13: Meiotic Recombination
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
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
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