Heredity and Common Genetic Diseases
This domain extends Mendelian genetics into sex determination and human genetic disease. The exam asks you to state Mendel's laws, work out a cross with probabilities, explain why a son inherits colour blindness from a carrier mother, and distinguish sex-linked from autosomal disorders.
Mendelian rules
- Law of dominance: in a heterozygote the dominant allele is expressed.
- Law of segregation: the two alleles of a gene separate into different gametes.
- Law of independent assortment: alleles of different genes assort independently during gamete formation.
- Genotype is the allele pair (TT, Tt, tt); phenotype is the visible trait. A test cross with the recessive parent reveals whether a tall plant is TT or Tt.
Sex determination in humans
- Female 44 + XX, male 44 + XY. The ovum always carries X; the sperm carries X or Y, so the father determines the sex of the child.
Common genetic diseases
- Colour blindness: X-linked recessive; the gene is on the X chromosome with no counterpart on Y.
- Haemophilia: X-linked recessive, defective clotting factor, so bleeding does not stop easily.
- Thalassaemia: autosomal recessive, faulty haemoglobin chain, anaemia needing transfusion.
- Down syndrome: trisomy 21, an extra chromosome 21, associated with increased maternal age.
Worked example
A colour-blind father (XcY) marries a carrier mother (XCXc). Mother's gametes XC and Xc; father's Xc and Y. Children: XCXc carrier daughter, XcXc colour-blind daughter, XCY normal son, XcY colour-blind son. Each child therefore has a 1 in 4 chance of being colour-blind; half of the sons and half of the daughters are affected.
Traps that cost marks
- A carrier mother looks normal; the recessive allele is hidden.
- Colour blindness is commoner in males because they have only one X.
- Down syndrome is an autosomal trisomy, not a sex-chromosome disorder.