(a) (i) What is the difference between the genotype and phenotype of an individual? (ii) State two factors each which determine each of the terms in (a)(i)....

Answer Details

(a)(i) Difference between genotype and phenotype

The genotype is the genetic constitution of an individual, that is, the actual set of alleles (genes) it carries for a trait, for example BB, Bb or bb. The phenotype is the observable physical or physiological expression of that trait, for example black coat colour or white coat colour, which results from the genotype interacting with the environment.

(a)(ii) Two factors that determine each term

Genotype is determined by:

• the genotypes (alleles) of the parents from which the individual inherits its genes;
• mutation, that is, changes in the genes or chromosomes.

Phenotype is determined by:

• the genotype of the individual;
• the environment (for example diet, temperature and general health).

(b) Breeding experiment: BB (black) × bb (white)

Let B = allele for black coat (dominant) and b = allele for white coat (recessive).

Parental cross (P): a homozygous black dog (BB) crossed with a homozygous white dog (bb). Each parent contributes one gamete type, so every offspring receives one B and one b allele. All the first filial generation (F1) are therefore Bb (black).

F1 × F1 cross: Bb × Bb. The genetic diagram (Punnett square) below shows the F2 offspring.

From the genetic diagram the F2 genotypes are 1 BB : 2 Bb : 1 bb, giving a phenotypic ratio of 3 black : 1 white.

Total number of F2 puppies = 240.

(b)(i) Number of white puppies

White puppies (bb) form \(\tfrac{1}{4}\) of the offspring:

\[ \text{White} = \frac{1}{4} \times 240 = 60 \text{ white puppies} \]

(b)(ii) Number of black puppies

Black puppies (BB and Bb) form \(\tfrac{3}{4}\) of the offspring:

\[ \text{Black} = \frac{3}{4} \times 240 = 180 \text{ black puppies} \]

Answers: 60 white puppies and 180 black puppies in the second filial generation.