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Honors Biology
Unit 12 – genetics
Name: __________________________
X-Chromosome Inactivation
We know that males have XY sex chromosomes, and they seem to function just fine, so they must be able
to survive with only 1 X chromosome.
Females, however, have XX as sex chromosomes, two of them! So, do we really need two, or do females
have an extra? The answer was discovered in 1961 by Mary Lyon, a British scientist who specialized in
mouse genetics. She hypothesized that we only need one copy of the X chromosome, so therefore in
females, in every cell, one of the two X chromosomes is coiled up into an inactive structure called a Barr
Body, from which genes are never transcribed.
Would males have Barr Bodies? Explain.
THIS explains the cat from the first slide of our notes.
First - In cats – orange coat color is a sex-linked trait. (O) is
dominant over black (o), and is found only on the X
chromosome. Since this is a sex-linked trait, the genotypes are
actually XOXO (orange), or XoXo (black), or XOXo (orange and
black pattern called tortoiseshell) for possible females. XOY is
an orange male, and XoY is a black male.
Why can only females be tortoiseshell?
Can a male EVER be a tortie?
The heterozygote condition actually shows up as a tortie cat because in some cells, the X carrying the O
is turned off (x-inactivation), so the o, or black gene is active, and in some cats the X carrying the little o
is turned off, so the O is active, and we see orange pigment.
But – the cat on the right is a calico – a tortoiseshell with big white spots – how does this happen?
The white-coat gene is a separate gene, not on the sex-chromosome. In some cells of the female above,
the white gene, or white spotting factor, are present and cause these big white spots. If a cat doesn’t
have this “factor”, then it is a regular tortoiseshell cat.
PROBLEM:
1. Cross an orange male with a black female. – What offspring may result?
2.
Cross a black male with a tortoiseshell female – what offspring may result?