Eye colour in fruits flies is sex-linked. Red eye colour R is dominant to white eye colour. A heterozygous red-eyed female fly was crossed with a white eyed male.

Red eye color (dominant) is $R$, white eye color (recessive) is $r$. Since it's sex-linked, the alleles are on the X chromosome. Females are XX, males are XY.

• Heterozygous red-eyed female: She has two X chromosomes, one carrying the dominant red allele and one carrying the recessive white allele. Her genotype is $X^R X^r$.
• White-eyed male: He has one X chromosome and one Y chromosome. Since he has white eyes (recessive trait) and only one X chromosome, his genotype is $X^r Y$.

The parental genotypes are:
Female: $\boxed{X^R X^r}$
Male: $\boxed{X^r Y}$

2. (a) (ii)

The female produces gametes $X^R$ and $X^r$.
The male produces gametes $X^r$ and $Y$.

From the Punnett square, the possible genotypes are:

• $X^R X^r$
• $X^r X^r$
• $X^R Y$
• $X^r Y$

Each genotype appears once out of four possibilities.
The genotypic ratio is $\boxed{1 X^R X^r : 1 X^r X^r : 1 X^R Y : 1 X^r Y}$.

2. (b)
The actual phenotype ratio might differ from the expected ratio due to random fertilization of gametes, especially when the sample size (number of offspring) is small. Genetic crosses predict probabilities, and observed results may vary from these probabilities by chance.

2. (c)
Two disorders due to non-disjunction are:

• Down Syndrome (Trisomy 21)
• Klinefelter Syndrome (XXY)