Question

An allele is an alternative form of a gene that is located at a specific position on a specific chromosome. We usually have two alleles for a trait, one inherited from each parent. The notation iRR, Rr or rr in our example, is called the genotype. When the pair of alleles is identical, they are called homozygous. When the pair is different, they are called heterozygous. Capital letters represent dominant traits; lower case letters represent recessive traits. Dominant traits can mask recessive traits. The colors red and white are called the phenotype.

According to Mendel's law of dominance, if we know the genotypes of the parents with regard to a specific trait, we can use probability to determine the number of offspring displaying each trait.
Use the Punnett square above and the text to help you determine which three statements are true.
A HH is the genotype for the homozygous dominant condition.
B If H = hairy and h = not hairy, following the law of dominance Hh would be a moderately hairy individual.
C In a field of 200 sunflowers, if 52 have orange petals and 148 have yellow, then the genotypes of the parents are YY and yy.
D Given the situation: a heterozygous pink pig (Pp) is crossed with a homozygous white pig (pp). All the offspring will be pink.
E In a cross between two heterozygous pink pigs (Pp), about one-fourth will be white.
F In a population of flies, red eyes are dominant over green. If a homozygous red-eyed fly mates with a homozygous green-eyed fly the probability that they will have red-eyes offspring will be 100%.

Answer 1

The true statements are A, E, and F.

- A: True — HH (two identical dominant alleles) is homozygous dominant.
- E: True — Pp × Pp gives genotypes 1/4 PP : 1/2 Pp : 1/4 pp, so about one-fourth are white (pp).
- F: True — RR × rr yields 100% Rr, and red (dominant) will show in all offspring.

(B, C, and D are false for the reasons: B ignores dominance (Hh shows the dominant hairy phenotype, not "moderately" hairy); C can't produce two different phenotypes if parents were YY and yy; D yields about 50% pink and 50% white, not all pink.)