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3.6 describe the inheritance of sex-linked genes, and genes that exhibit co-dominance and explain why these do not produce simple Mendelian ratios
describe the inheritance of sex-linked genes, and genes that exhibit co-dominance and explain why these do not produce simple Mendelian ratios
- Mendel was fortunate in his choice of factors as they all showed dominant/recessive characteristics. However, sex-linked genes and genes that are co-dominant do not display the phenotype ratioos predicted by Mendel’s laws.
- An example of sex-linked inheritance is red-green colour blindness in humans. The gene is carried on the X chromosome and there is no corresponding gene on the Y chromosome. Therefore males need only one allele for colour blindness on the X chromosome while females require two. This results in many more males being colour blind than females because the father would have to be colour blind and the mother either colour blind or be a carrier for colour blindness. As you would expect the sex of offspring to be 50% male and 50% female the occurrence of colour blindness is higher in males than would be expected from a simple pair of dominant and recessive genes. Take the cross between a normal female XN XN and a colour-blind male X n Y.
|
XN |
XN |
X n |
XN X n |
XN X n |
Y |
XN Y |
XN Y |
All offspring have normal sight. But if the female is a carrier for colour blindness and crosses with a normal male then 50 % of the males will be colour blind and none of the females.
|
XN |
X n |
XN |
XN XN |
XN X n |
Y |
XN Y |
X n Y |
- Human blood types are another example of co-dominance. Human blood types give different results from Mendelian ratios. When a homozygous male with AA alleles crosses with a homozygous female with BB alleles then all of the offspring will be a different phenotype from the parents (group AB).