Genetics, inheritance and variation
What this domain tests
DNA, genes and chromosomes; monohybrid inheritance and Punnett squares; sex determination and sex-linked conditions; mutation; and continuous versus discontinuous variation. Genetic cross questions carry easy marks that are lost through careless ratio writing.
Core rules
- A gene is a length of DNA coding for a protein; alleles are alternative forms of a gene. Homozygous means two identical alleles, heterozygous two different ones.
- A dominant allele is expressed even when only one copy is present; a recessive allele is expressed only when both copies are recessive. Genotype is the allele combination; phenotype is the visible characteristic.
- Humans have 46 chromosomes in 23 pairs. Sex chromosomes are XX in females and XY in males, so the father's sperm determines the sex of the child.
- In a monohybrid cross of two heterozygotes (Bb x Bb) the genotypic ratio is 1 BB : 2 Bb : 1 bb and the phenotypic ratio is 3 dominant : 1 recessive.
- A test cross with a homozygous recessive partner reveals whether a dominant-looking individual is homozygous or heterozygous.
- Sex-linked genes sit on the X chromosome. Males with XY show X-linked recessive conditions such as haemophilia and red-green colour blindness after inheriting one copy; females need two.
- Variation is genetic (mutation, crossing over, independent assortment, random fertilisation) or environmental, and is continuous (height, mass) or discontinuous (blood group, tongue rolling).
Worked example
Two brown-eyed parents have a blue-eyed child. Explain using a cross. Brown must be dominant (B) and blue recessive (b). A blue-eyed child is bb, so each parent contributed a b allele; both parents are heterozygous Bb. The cross Bb x Bb gives BB, Bb, Bb and bb. Three quarters of offspring are expected to be brown-eyed and one quarter blue-eyed, a 3:1 ratio. The one-in-four chance applies independently to each child.
Traps that cost marks
Giving the genotypic ratio when the phenotypic ratio was asked for. Saying a dominant allele is the more common allele in a population. Forgetting that a haemophiliac son inherits the allele from his mother. Writing that the mother determines the baby's sex.