Holstein Friesian cattle are a breed of cattle used by dairy farmers in many countries of the world for the high milk yield of their cows. [Figure 4.1] Milk yield in Holstein Friesian cattle is affected by heat stress. Heat stress occurs when homeostatic mechanisms are not enough to keep the body temperature down to normal levels. One of the factors that contributes to heat stress is air temperature.

1. (a)Fig. 4.2 shows: • the mean daily air temperature in Central Europe • the mean monthly milk yield per cow of Holstein Friesian cattle in Central Europe. [Figure 4.2] With reference to Fig. 4.2, describe the trends in air temperature and milk yield from April to August.[2]
2. (b)Many dairy farmers in tropical regions use cattle breeds that are tolerant to heat stress (heat-tolerant cattle). These heat-tolerant cattle: • can tolerate higher air temperatures than Holstein Friesian cattle before heat stress occurs • have milder symptoms of heat stress than Holstein Friesian cattle for the same high air temperatures. Where heat stress does not occur, heat-tolerant cattle produce a lower milk yield than Holstein Friesian cattle under the same conditions. Scientists compared DNA sequences of Holstein Friesian cattle and heat-tolerant cattle for a number of genes known to have an effect on body temperature. Twenty genes were found that had alleles associated only with heat-tolerant cattle. With reference to the information provided, including the data in Fig. 4.2: • state the type (pattern) of phenotypic variation shown by milk yield in cattle • identify factors that cause phenotypic variation in milk yield in cattle. In each case, give a reason for your choice. type (pattern) of phenotypic variation and reason for choice factors that cause phenotypic variation and reason for each choice[3]
3. (c)The scientists found that one of the genes studied, PRLR, has a dominant allele known as SLICK. The SLICK allele was identified in Senepol cattle, a heat-tolerant breed, and is not found in Holstein Friesian cattle. Cattle with the SLICK allele have short hair due to reduced hair growth. Scientists have used selective breeding to introduce the SLICK allele into Holstein Friesian cattle. The milk yields of normal Holstein Friesian cattle and Holstein Friesian cattle with the SLICK allele are shown in Fig. 4.3, during: • March, when the mean daily air temperature is 5°C. • September, when the mean daily air temperature is 14°C. [Figure 4.3] With reference to Fig. 4.3, describe the effect of the SLICK allele on milk yield in Holstein Friesian cattle.[3]
4. (d)The SLICK allele differs from the recessive allele by a single nucleotide deletion. This results in a frameshift mutation and introduces a premature stop codon in the PRLR gene. Scientists can use gene editing to replicate this mutation in Holstein Friesian cattle. This provides a way to introduce the SLICK allele into Holstein Friesian cattle without selective breeding. Compare gene editing and selective breeding for introducing the SLICK allele into Holstein Friesian cattle. Include similarities and differences in your answer.[6]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.