Question

The ability of the human body to break down the red color in beets is controlled by an autosomal dominant allele. The inability is recessive, detected by red coloration of the urine (we will call this phenotype 'secretor).
1. If a nonsecretor woman with a secretor father marries a nonsecretor man who in a previous marriage had a secretor daughter, what is the probability that their first child will be:__________.
a. a secretor girl
b. a nonsecretor girl
c. a secretor boy
2. What is the probability that their first two children will be nonsecretors of either sex?

Answer 1

Answer:

Explanation:

Let the ability to break down the red color in beets be represented by the allele B. The inability would be represented by the allele b.

A nonsecretor's genotype would be BB or Bb while a secretor's genotype would be bb.

A nonsecretor woman with a secretor father would be a carrier with genotype Bb. A nonsecretor man who in a previous marriage had a secretor daughter would also be a carrier with genotype Bb. If the two marries:

            Bb     x     Bb

           BB    2Bb    bb

1.

(a) probability of their first child will be a secretor girl = probability of having a girl and being a secretor.

Probability of having a girl = 1/2

Probability of being a secretor = 1/4

probability of their first child will be a secretor girl = 1/2 x 1/4 = 1/8

(b) Probability of their first child being a nonsecretor girl = probability of having a girl and being a nonsecretor.

Probability of having a girl = 1/2

Probability of being a nonsecretor = 3/4

Probability of their first child being a nonsecretor girl = probability of having a girl and being a nonsecretor = 1/2 x 3/4 = 3/8

2. Probability that their first two children will be nonsecretors of either sex = probability of their first being a nonsecretor and of either sex and probability of their second being a nonsecretor and of either sex.

  = 3/4 x 3/4 = 9/16