Explain polygenic inheritance and codominance with examples related to milk production and coat color. Calculate the milk yield for a Jersey cow with genotype PPQqRr.

The genotype is PPQqRr.

• P contributes 2 dominant alleles (PP).
• Q contributes 1 dominant allele (Q).
• R contributes 1 dominant allele (R). Total number of dominant alleles = $2 + 1 + 1 = 4$.

Each dominant gene adds 30 litres of milk.
Additional milk yield = $4 \text{ dominant alleles} \times 30 \text{ litres/allele} = 120 \text{ litres}$.

The base milk yield for genotype ppqqrr (0 dominant alleles) is 200 litres.
Total milk yield = Base milk yield + Additional milk yield
Total milk yield = $200 \text{ litres} + 120 \text{ litres} = 320 \text{ litres}$.
The milk yield of a Jersey cow with the PPQqRr genotype is $\boxed{\text{320 litres}}$.

2.2.1 The genetic phenomenon is codominance.

2.2.2 Both alleles for coat color (black and white) are expressed equally and simultaneously in the heterozygous individual, resulting in a roan phenotype where both black and white hairs are distinctly visible.

2.3.1 Prepotency is the ability of an individual to transmit its characteristics to its offspring with a high degree of certainty. It is important in genetics for selective breeding, as it allows breeders to consistently pass on desirable traits to future generations, improving livestock or crop varieties.

2.3.2 Atavism is the reappearance of a trait or characteristic in an organism after several generations of absence. It is important in genetics as it indicates the presence of dormant genes that can be expressed under certain conditions, providing insights into evolutionary history and the genetic potential within a species.