A freshwater fish species, Oryzias latipes, has individuals with four body colour patterns, as shown in Table 5.1. phenotype red white red with black spots white with black spots Table 5.1 body colour pattern Two unlinked genes determine the body colour patterns shown in Table 5.1. One gene controls whether the body colour is red or white: • dominant allele R = red • recessive allele r = white. The other gene controls whether black spots are present or not present: • dominant allele B = with black spots • recessive allele b = without black spots. A fish that is homozygous recessive at both loci is white. Genetic crosses were carried out to investigate the inheritance of the four different body colour patterns. Males that were red with black spots, and homozygous at both loci, were crossed with females that were white. The F1 offspring were all red with black spots. These F1 offspring were then crossed to produce the F2 generation.

1. (a)Table 5.2 shows the observed numbers obtained of each of the four different phenotypes for the F2 generation. phenotype red with black spots white with black spots red white Table 5.2 observed 279 95 96 30 expected 281.25 93.75 93.75 31.25 O-E 1.25 2.25 (O-E)2 1.5625 5.0625 x² = (O-E)2 E 0.017 0.054 Table 5.2 compares the observed numbers with the numbers that would be expected in the F2 generation for a normal dihybrid ratio. Calculate x2 for the F2 generation by completing Table 5.2. The formula for x² is: x² = Σ (Ο-Ε)2 E[3]
2. (b)The critical value at p = 0.05 and 3 degrees of freedom is 7.815. Comment on whether the null hypothesis should be accepted or rejected.[2]
3. (c)To produce the F2 generation, red males with black spots, XrYRBb, were crossed with red females with black spots, XRXrBb. Complete the Punnett square in Fig. 5.1 to show the genotypes and phenotypes of the F2 generation. : Use the symbols XR, Xr and YR for the alleles of the gene that controls body colour. Use the symbols B and b for the alleles of the gene that controls whether black spots are present or not. Some of Fig. 5.1 has been completed for you. female gametes male gametes XRB X'B XRb Xrb XRXBB XRXBb XXBb XB female female female red + black spots XRYRBB red + black spots white + black spots XrYRBB XRYRBb XrYRBb YRB male male male male red + black spots red + black spots red + black spots red + black spots XRXBb XXBb XRXrbb Xrb female female female red + black spots white + black spots red + no spots XrYRBb XrYRbb YRb male male red + black spots red + no spots Fig. 5.1[4]
4. (d)Explain why there are no white males or males that are white with black spots in the F2 generation.[2]
5. (e)In another cross, red males with the genotype X'YRbb were mated with white females with the genotype X'X'bb. All the male offspring were expected to be red and all the female offspring were expected to be white. The observed results showed that the offspring included two red females out of 253 and one white male out of 198. Suggest an explanation for this unexpected result.[2]

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