Calculate the Crossover Value (CoV) and explain the deviation of the observed ratio from the expected.

Parental offspring are those with the same combination of alleles as the original parents. Since the female had alleles in coupling phase (BL/bl), the parental phenotypes are Black body, long wings (BL) and Grey body, short wings (bl).
Recombinant offspring are those with new combinations of alleles due to crossing over. These are Black body, short wings (Bl) and Grey body, long wings (bL).

Number of parental offspring:
Black body, long wings = 420
Grey body, short wings = 410
Total parental offspring = $420 + 410 = 830$

Number of recombinant offspring:
Black body, short wings = 38
Grey body, long wings = 32
Total recombinant offspring = $38 + 32 = 70$

Total offspring = Parental offspring + Recombinant offspring
Total offspring = $830 + 70 = 900$

The Crossover Value (also known as recombination frequency) is calculated as the ratio of recombinant offspring to the total offspring, expressed as a percentage.

1. (b) (ii)
The observed ratio deviates from the expected Mendelian ratio (1:1:1:1 for a dihybrid test cross) because the two genes (body colour and wing length) are linked. Linkage means that the genes are located on the same chromosome and tend to be inherited together. This results in a higher proportion of parental phenotypes (Black body, long wings and Grey body, short wings) and a lower proportion of recombinant phenotypes (Black body, short wings and Grey body, long wings) than would be expected if the genes assorted independently. The occurrence of crossing over during meiosis allows for the formation of recombinant gametes, but at a frequency less than 50%, which is reflected in the calculated crossover value of 7.78%.