© 2026 LearnInShort
Two Families of Genetic Disorders
Genetic disorders arise when the hereditary material is altered. Exams split them into two families, and keeping them apart is half the battle.
The first family is chromosomal disorders. A whole chromosome is added or lost, so the total number differs from the normal 46. They arise from nondisjunction, an error in which chromosomes fail to separate during meiosis, so a gamete carries one chromosome too many or too few. Down, Turner and Klinefelter syndromes belong here.
The second family is single-gene disorders, where the chromosome count is normal but one gene carries a fault. They follow Mendelian patterns: autosomal recessive (thalassaemia, sickle-cell anaemia, albinism, phenylketonuria) and X-linked recessive (colour blindness, haemophilia).
Always ask first which family a disorder belongs to. That one decision tells you whether the question is about chromosome numbers or about a cross with an allele.
1. Down syndrome is caused by:
2. A female with only one X chromosome (45,X) has:
3. A carrier mother (XCXc) marries a normal man (XCY). What proportion of their sons is expected to be colour-blind?
4. Which of the following is an autosomal recessive disorder?
5. Klinefelter syndrome is characterised by the chromosome pattern:
6. A colour-blind man (XcY) marries a normal woman who is not a carrier (XCXC). Their children will be:
7. Down syndrome arises mainly because of:
8. X-linked recessive disorders appear more often in males because: