Apply models of inheritance and explain the sources of genetic variation in populations

What this dot point is asking

SCSA wants you to use inheritance models to predict offspring ratios and to explain where the variation that those models shuffle actually comes from. Expect Punnett square problems and questions asking you to identify the mode of inheritance from data.

The basics of inheritance

Every individual carries two alleles for each gene, one from each parent. The genotype is the allele combination (for example BB, Bb or bb); the phenotype is the observable trait. A homozygous individual has two identical alleles; a heterozygous individual has two different alleles.

Mendel's key principle is the law of segregation: the two alleles for a gene separate during meiosis so that each gamete carries only one. At fertilisation, alleles combine again at random.

Dominant and recessive inheritance

In simple dominance, a dominant allele masks the effect of a recessive allele in a heterozygote. A monohybrid cross between two heterozygotes (Bb x Bb) gives a 3 to 1 phenotypic ratio in the offspring.

Beyond simple dominance

Not all traits follow a clean dominant or recessive pattern.

• Codominance: both alleles are fully expressed in the heterozygote, as in human AB blood type where both A and B antigens appear.
• Incomplete dominance: the heterozygote shows an intermediate phenotype, as in pink flowers from red and white parents.
• Multiple alleles: a gene has more than two alleles in the population, such as the three alleles of the ABO blood group system.
• Polygenic inheritance: many genes contribute to one continuous trait such as height or skin colour, producing a range of phenotypes rather than distinct categories.

Sex-linked inheritance

Genes carried on the sex chromosomes show sex-linked patterns. Most are X-linked. Because males have only one X (XY), a single recessive allele on the X is expressed, so X-linked recessive conditions such as red-green colour blindness and haemophilia are more common in males. Females (XX) need two copies to show the recessive phenotype but can be unaffected carriers.

Sources of genetic variation

Inheritance models shuffle existing alleles, but variation has deeper sources.

Mutation is the ultimate source of all new alleles. A mutation is a change in the DNA base sequence. Point mutations alter a single base (substitution, insertion or deletion); larger chromosomal mutations affect whole segments or numbers of chromosomes. Only mutations in gametes are passed to offspring. Many mutations are neutral, some harmful, and a few beneficial, providing the raw material for evolution.

Meiosis reshuffles existing alleles through crossing over and independent assortment, producing new combinations in gametes without creating new alleles.

Random fertilisation combines two genetically unique gametes from a vast number of possibilities, multiplying variation again.

Why variation matters

Genetic variation within a population is what allows it to respond to environmental change. If conditions shift, individuals carrying alleles suited to the new environment are more likely to survive and reproduce. A population with little variation is far more vulnerable to disease or environmental pressure, which is why variation is central to the continuity of a species.