Question

Multiple Choice

1. You count 1000 seeds from a monohybrid cross (i.e., single-locus heterozygote crossed with single-locus heterozygote). How many seeds do you expect to display the dominant phenotype? a. 1000 b. 750 c. 500 d. 250 2. Which of the following is among the purposes of a genetic dissection analysis? a. To determine how two alleles at a locus interact with one another. b. To determine the order of intermediaries in a genetic pathway. c. To determine whether a mutation is a gain-of-function or a loss-of-function mutation. d. To determine which allele is the wild type.

Answer 1

Answer 1:

Let the dominant allele be A and recessive allele be "a". Parents are heterozygous at both loci, thus genotype of both mates will be Aa. The gametes produced by each parent will be A, a. The cross can be shown as:

	A	a
A	AA	Aa
a	Aa	aa

Probability of homozygous dominant, AA = 1/4 or 0.25

Thus, in a population of 1000, number of homozygous dominant,

=0.25×1000

=250.

Correct choice should be:

(d) 250

Answer 3:

Homozygous dominant have flower size of 6cm, whereas homozygous recessive has flower size of 2cm. The heterozygous has a size of 5cm. This can be possible if heterozygous allele is not completely dominant over recessive allele. Thus, correct choice should be:

(b) Incomplete dominance.

Answer 4:

Parents are AaLlYy × AaLlYy.

We need to find the probability of offspring with dominant phenotype. This can be calculated using product rule.

Genotype for dominant genotype will be A_L_Y_.

Probability of A_ :

A_ means AA or Aa. Upon crossing the alleles at first locus of each parent, Aa×Aa , we will get:

	A	a
A	AA	Aa
a	Aa	aa

p(A_) = 3/4.

Similarly, p(L_) and p(Y_) will be 3/4.

Hence,

p(A_L_Y_)

=3/4 × 3/4 × 3/4

= 27/64

Correct choice should be:

(c) 27/64