Inheritance - Sex Determination (GCSE Biology)
Sex Determination
Sex Determination
As we discussed earlier, normal human body cells contain 23 pairs of chromosomes, so 46 overall. 22 of these control characteristics and are known as autosomes. 1 pair is the sex chromosomes. These chromosomes control gender.
The sex chromosomes differ in males and females.
- Males – The male sex chromosomes are X and Y.
- Females – The female sex chromosomes are X and X.
Sex-Linked Characteristics
Some characteristics are dependent on sex. A sex-linked characteristic is a characteristic in which the gene responsible for the characteristic is located on a sex chromosome (on the X or Y chromosome). This makes the characteristic more common in one sex than in the other.
Colour blindness is an example of a sex-linked disorder. It is caused by a faulty allele carried on the X chromosome. It is a recessive allele so females need 2 copies of the allele to be colour-blind (genotype Xn Xn). If she has one copy, she will be a carrier. However, males only need one copy of the allele (genotype Xn Y). This therefore makes colour blindness more common in males than females.
You must be able to work out the probabilities of sex linked genetic diseases. You can do this again using a Punnett Square.
The biggest difference here is that the top row and left most column are no longer just dependent on the alleles, instead they are dependent on gender. Here a Homozygous Dominant male (XDY) is crossed with a Heterozygous female (XDXd).
Therefore the possible alleles are:
- XDXD – A Homozygous dominant female
- XDY – A Homozygous dominant male
- XDXd – A Heterozygous female
- AdY – A Homozygous recessive man
You can use these value to work out probabilities and ratios in the same way as above.
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