Lesson 1 of 4 · Form 4 Biology
Genetics and inheritance
20 min
Key terms
- Gene: a segment of DNA controlling a characteristic. Allele: an alternative form of a gene.
- Dominant allele shows in the heterozygote (T); recessive shows only when homozygous (tt).
- Genotype: genetic make-up (Tt). Phenotype: observable feature (tall).
- Homozygous: TT or tt. Heterozygous: Tt.
Monohybrid cross (Mendel)
Cross two heterozygous tall plants: Tt × Tt
- Gametes: T, t × T, t
- Offspring: TT, Tt, Tt, tt
- Phenotype ratio 3 tall : 1 short; genotype ratio 1 : 2 : 1.
Test cross
Cross an organism showing the dominant trait with a homozygous recessive. If any offspring show the recessive trait, the parent was heterozygous.
Sex determination
Females XX, males XY. The father determines the sex of the child. Chance of a boy = ½.
Sex-linked traits
Carried on the X chromosome, e.g. haemophilia and colour blindness; more common in males.
Codominance
Both alleles show, e.g. roan cattle (red and white hairs), the AB blood group.
Mutations
Sudden changes in genes or chromosomes, e.g. sickle cell (gene mutation), Down syndrome (an extra chromosome 21).
Key points to remember
- Tt × Tt gives 3 : 1
- Test cross uses a homozygous recessive
- Males are XY
- Haemophilia is sex-linked
Test yourself
