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Reproduction and genetics

Biology · WAEC and JAMB · SS2 and SS3

The most predictable topic in the paper, because genetics questions follow a fixed structure. Learn to draw a Punnett square cleanly and the marks are routine.

What you need to know

  • Mitosis produces two genetically identical diploid cells for growth and repair.
  • Meiosis produces four genetically different haploid gametes, and introduces variation.
  • Dominant alleles are written with a capital letter, recessive with a small one.
  • Genotype is the genetic make-up; phenotype is the observable characteristic.
  • A monohybrid cross between two heterozygotes gives a 3:1 phenotypic ratio.
  • A test cross with the homozygous recessive reveals whether a dominant organism is homozygous or heterozygous.
  • Sex is determined by XX in females and XY in males; the father determines the sex of the child.
  • Sickle cell is caused by a recessive allele; AS individuals are carriers with resistance to malaria.

Key terms

Allele
One of the alternative forms of a gene occupying the same position on a chromosome.
Homozygous
Having two identical alleles for a trait, such as TT or tt.
Heterozygous
Having two different alleles for a trait, such as Tt.
Carrier
A heterozygous individual who shows the dominant trait but can pass on the recessive allele.

Worked example

Two parents both with genotype AS have a child. What is the probability the child has sickle cell anaemia?

  1. Cross AS x AS. Gametes from each: A and S.
  2. Punnett square gives AA, AS, SA, SS.
  3. Only SS has the disease, which is one outcome in four.

The mistake to avoid

Ratios are expected, not counts. The answer to a monohybrid cross is "3:1", not "three tall and one short" — and the ratio is phenotypic unless genotypic is asked for.

In the exam

Always show the parental genotypes, the gametes, the Punnett square and then the ratio. Each line carries a mark, so a correct answer with no working scores poorly.