The female parent has Blood group O, so her genotype is ii. She passes an i allele to all her offspring.

Here are the answers to your questions:

1.5.1

• a) W: The female parent has Blood group O, so her genotype is $ii$. She passes an $i$ allele to all her offspring. One child has Blood group A, meaning its genotype is $I^A i$. This child received $I^A$ from W. Another child has Blood group B, meaning its genotype is $I^B i$. This child received $I^B$ from W. Therefore, W must possess both $I^A$ and $I^B$ alleles. The genotype of W is $\boxed{I^A I^B}$.

• b) X: The male parent has Blood group A. Since his mother is Blood group O ($ii$) and his father is W ($I^A I^B$), his genotype must be $I^A i$. This male parent and X have a child with Blood group O, whose genotype is $ii$. For the child to be $ii$, it must have received an $i$ allele from the father ($I^A i$) and an $i$ allele from the mother (X). Therefore, X must carry at least one $i$ allele. The possible genotypes for X that include an $i$ allele are $\boxed{I^A i, I^B i, ii}$.

1.5.2

Haemophilia is an X-linked recessive disorder. Males have one X and one Y chromosome. If their single X chromosome carries the recessive allele for haemophilia, they will express the disorder because there is no dominant allele on the Y chromosome to mask it. Females have two X chromosomes, so they typically need two copies of the recessive allele to be affected; if they have one recessive and one dominant allele, they are usually just carriers.