A genetic disorder caused by an autosomal recessive allele i

A genetic disorder caused by an autosomal, recessive allele is characterized by falling asleep in genetics lectures when the lectures begin at 2pm. It is also characterized by waking up as the lecture ends. The disease is known to be 80% penetrant. Complete expressivity occurs in only 20% of people that exhibit the disorder. Incomplete expression occurs only when the person falls asleep, but fails to wake up as the lecture ends. a. Must the percentages of penetrance and expressivity sum to 100 Explain b. What percentage of students who are homozygous for the recessive allele will both fall asleep at 2pm and still be sleeping when the lecture ends? Show your work.

Solution

Answer:

a. Penetrance is defined as the percentage of individuals with a given genotype who exhibit the phenotype associated with that genotype.

Expressivity measures the extent to which a given genotype is expressed at the phenotypic level.

They measure respectively the percentage of cases in which the gene is expressed and the level of expression. So, it is not necessary for these to sum up to 100%.

b. The disease shows 80% penetrance, i.e, 80% of the students who has the homozygous recessive genptype expresses the phenotype.

Let the total number of students with homozygous recessive allele be x.

So, 0.8x shows the phenotype (disease).

20% of the individuals show complete expressivity. So, the percentage of individuals showing incomplete expressivity (falls asleep at 2pm and continues sleeping when the lecture ends) is 80%.

80% of 0.8x = 0.64 x

Therefore, 64% of the students who are homozygous for the recessive allele will show incomplete penetrance.