Question

You are studying an enzyme in mice whose activity can be easily measured by taking blood samples and doing an assay that measures the product of the enzyme. You are not sure on what chromosome the gene for this enzyme is located. The following enzyme activities are measured in one set of crosses:

	Enzyme Activity
Fathers	4 mg/ml
Mothers	306 mg/ml
Female Offspring of the Crosses	151 mg/ml

All differences are statistically significant. Male offspring of the cross have not been tested.  

From this data propose (and briefly explain) the following, making clear from your answers any dominance relationships.

a. logical genotypes for all three of the type of individuals if the gene is on an autosome  

b. logical genotypes for all three of the type of individuals if the gene is sex-linked  

c. IF the gene is sex-linked, would the male offspring (not shown in the table) have enzyme activities like their fathers, their mothers or their sisters?

Answer 1

a. Assuming the gene is present on autosomes.

• The father has 4 mg/ml, and the mother has 306 mg/ml activity.
• So, let us say the father has recessive genes, and the mother has dominant ones.
• Hence father genotype is 'aa' and mother genotype is 'AA'.
• Their daughter has 151 mg/ml activity, indicating heterozygous nature (mix of both father and mother). Hence her genotype may be 'Aa'.

b. If genes are on sex chromosomes

• Father genotype will be XaY and the mother genotype will be XAXA.
• So her daughter genotype is XAXa (daughter receives an X chromosome from both father and mother).

c. If sex-linked, like the above case,

• Then the son genotype will be (Note that mother gives her X chromosome to son and father gives Y chromosome) = XAY .
• Since the son has one dominant allele XA, his enzyme activity will be the same as his mother.