B14: Inheritance

Genotype is the allele combination an organism carries, while phenotype is an observable characteristic produced by genotype and environment. A dominant allele is expressed in a heterozygote, a recessive allele requires two copies for expression, and codominant alleles are both expressed in the heterozygote.

For a genetic cross, define allele symbols, write parental genotypes, show possible gametes, combine them in a Punnett square or equivalent diagram, and state genotype and phenotype probabilities. Use one letter for alleles of the same gene and avoid changing symbols midway.

In the ABO blood group system, $I^{A}$ and $I^{B}$ are codominant and $I^{O}$ is recessive, giving four phenotypes from three alleles. Human sex determination uses $XX$ and $XY$ chromosomes: ova carry $X$, while sperm carry $X$ or $Y$, so the fertilising sperm determines the chromosome combination.

A mutation can change a gene sequence, as in sickle cell anaemia, or chromosome number, as in the 47 chromosomes associated with Down syndrome. Mutations arise spontaneously, while ionising radiation and some chemicals can increase mutation rate. A mutation is not automatically beneficial or inherited by offspring.

Continuous variation shows a range and is often influenced by many genes and environment, while discontinuous variation forms distinct categories. Mutation, meiosis, random fertilisation, and environmental conditions contribute to variation. Distinguish inherited variation from an acquired change that is not passed through alleles.

Natural selection requires heritable variation, a selection pressure, differential survival or reproduction, and inheritance of advantageous alleles. Over generations, those alleles become more common. Individuals do not evolve because they need a feature; population allele frequencies change.