ACSEE (Form 6) Genetics — practice questions

1. Haemophilia is a failure of the blood to clot normally. 2. The gene responsible is recessive, so it is masked whenever a normal allele is present. 3. It is carried on the X chromosome, making it sex-linked. 4. A male has only one X, so a single recessive allele shows in him; a female needs two to be affected. 5. That is why the condition appears far more often in males, and why writing the alleles without linking them to the X chromosome loses the whole pattern. _Source: NECTA ACSEE 2023 Biology 133/2, Question 4: Genetics_

1. Sickle cell anaemia is controlled by a recessive gene like haemophilia. 2. But that gene sits on an autosome, one of the chromosomes that is not a sex chromosome. 3. So the condition appears with equal frequency in males and females. 4. A genetic cross for it uses ordinary allele letters, with no X or Y attached. 5. Using X and Y chromosomes to represent the sickle cell alleles was the single most common recorded error on this question. _Source: NECTA ACSEE 2023 Biology 133/2, Question 4: Genetics_

1. Write the mother as X^H X^h and the father as X^H Y. 2. The mother passes either X^H or X^h, each with probability 1/2. 3. A son receives the Y from his father, so his single X comes from his mother. 4. So half of the sons receive X^h and, having no second X to mask it, are affected. 5. The probability among sons is therefore 1/2. Across all children it would be 1/4, which is the value often given by mistake. _Source: NECTA ACSEE 2023 Biology 133/2, Question 4: Genetics_