Question

B. Albinism Most humans (and vertebrates, in general) are genetically "programmed" to produce a brown skin pigment known as melanin. Albinism, the inability to produce melanin, is inherited in humans as an autosomal recessive trait. An individual with genotype AA or Aa will have normally pigmented skin, whereas an individual with genotype aa will produce no melanin. The albino's skin is very pale white. The white color occurs because one or more steps in the biosynthetic pathway for melanin do not occur because of an inactive enzyme. The inactive enzyme blocks the completion of the pathway. The biosynthetic pathway that leads to the production of melanin is illustrated below. Selected enzyme names are surrounded by a rectangle. An arrow leading away from a substrate points to the product the enzyme produces. 1. After looking at the figure, Phenylalanine explain the reason many forms of albinism are caused by the Phenylalanine hydroxylase presence of an inactive tyrosinase Tyrosine molecule. Tyrosinase DOPA Explain why, when the normal allele that produces tyrosinase is Tyrosinase present, melanin can be produced even when the abnormal allele is Dopaquinone present. Tyrosinase +Cysteine Quinones 5-5-cysteinyldopa and 3. Predict the frequencies of all 2-s-cysteinyldopa genotypes and phenotypes expected to result from the cross of an albino male with a Eumelanin Pheomelanin heterozygous female. Use the (black pigment) Punnett Square method with the (red pigment) correct allele symbols shown above. 2. 4. Do the same for a mating of two heterozygotes. Use the Punnett Square method with the correct allele symbols shown above. 5. What do you know about the genotypes of the parents of an albino child?

Answer 1

1. Tyrosinase catalyzes three steps in the pigment biosynthesis.
All genes exhibit an equal probability of accumulating mutations.
Since tyrosinase is involved at three steps, mutations in tyrosinase are more likely to result in the mutant phenotype

2. Albinism is an autosomal recessive disorder.
The presence of a single functional allele is sufficient to cause the WT phenotype.

3. A = WT dominant allele
a = Mutant recessive allele
Parental cross: Aa X aa
Progeny: Aa aa
Phenotypic ratio = 1:1
Genotypic ratio = 1:1

4. Parental cross: Aa X Aa
Progeny: AA Aa Aa aa
Phenotypic ratio = 3:1
Genotypic ratio = 1:2:1

5. Parents of an albino child have to possess at least one mutant allele.
They can be,
1. Aa and Aa (Both are carriers)
2. aa and aa (Both are affected)
3. Aa and aa