ZIMSEC O Level · Biology · Form 4
Key terms
- Chromosome: thread of DNA in the nucleus; humans have 23 pairs.
- Gene: a section of DNA that codes for a protein (characteristic).
- Allele: a different form of a gene, e.g. tall (T) or short (t).
- Dominant: shows whenever present. Recessive: shows only when two copies are present.
- Homozygous: two identical alleles (TT or tt). Heterozygous: different alleles (Tt).
- Genotype: the alleles present. Phenotype: the visible feature.
Monohybrid crosses
Worked example: Two heterozygous tall plants (Tt) are crossed. Find the expected offspring.
Gametes: T and t from each parent.
Offspring: TT, Tt, Tt, tt → 3 tall : 1 short.
Sex determination
Females are XX, males XY. Each child has a 50% chance of being a boy.
Variation and mutation
- Continuous variation (height, mass) shows a range; discontinuous variation (blood group) falls into distinct groups.
- A mutation is a change in a gene or chromosome. Radiation and some chemicals increase the rate.
- Natural selection: individuals best suited to their environment survive and pass on their alleles. Artificial selection is used to breed high-yield maize or hardy cattle.
Exam tip: Always show gametes in circles and set out crosses with a Punnett square – method marks are awarded.
Check yourself
- Distinguish between genotype and phenotype.
- A homozygous dominant black bull (BB) is crossed with a red cow (bb). What colour are the calves?
- Give one example of continuous variation in humans.
Show answers
- Genotype is the alleles an organism has; phenotype is its observable characteristic.
- All black (Bb).
- Height or body mass.
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