Apply Mendelian inheritance to predict genotype and phenotype ratios and explain sources of variation.

What this dot point is asking

Key genetics vocabulary

Understanding inheritance starts with precise terms.

• Gene: a length of DNA coding for a characteristic.
• Allele: one version of a gene (for example, a tall allele or a short allele).
• Genotype: the alleles an organism carries (for example, Tt).
• Phenotype: the observable characteristic (for example, tall).
• Homozygous: two identical alleles (TT or tt).
• Heterozygous: two different alleles (Tt).
• Dominant: an allele expressed in the phenotype even when heterozygous (shown with a capital).
• Recessive: an allele only expressed when homozygous (shown with a lowercase).

Monohybrid crosses

A monohybrid cross follows a single gene. A Punnett square predicts offspring ratios. Crossing two heterozygous tall plants (Tt x Tt) gives offspring in the ratio 1 TT : 2 Tt : 1 tt, a genotype ratio of 1:2:1. Because T is dominant, the phenotype ratio is 3 tall : 1 short.

A test cross crosses an organism showing the dominant phenotype with a homozygous recessive individual to find out whether the first organism is homozygous or heterozygous. If any recessive offspring appear, the unknown parent must have carried a recessive allele.

Dihybrid crosses

A dihybrid cross follows two genes at once. Crossing two individuals heterozygous for both genes (for example RrYy x RrYy in peas, where R is round and Y is yellow) gives the classic 9:3:3:1 phenotype ratio: 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. This ratio only holds when the two genes assort independently, which requires them to be on different chromosomes (or far apart on the same chromosome).

Variations on simple dominance

Not all inheritance is simple dominant and recessive.

• Incomplete dominance: heterozygotes show an intermediate phenotype (for example, red x white flowers giving pink).
• Codominance: both alleles are fully expressed in the heterozygote (for example, AB blood type, where both A and B antigens appear).
• Multiple alleles: a gene with more than two possible alleles in the population, such as the ABO blood group with alleles A, B, and O.

Sex linkage

In humans, females are XX and males are XY. Genes on the X chromosome that have no matching allele on the smaller Y chromosome are sex-linked. Because males have only one X, a single recessive allele on it is expressed. This is why X-linked recessive conditions such as red-green colour blindness and haemophilia are more common in males.

Pedigrees

A pedigree is a family tree that tracks a trait across generations. Squares represent males, circles represent females, and shaded symbols are affected individuals. By analysing who is affected, you can deduce whether a trait is dominant or recessive and whether it is autosomal or sex-linked. For example, if two unaffected parents have an affected child, the trait must be recessive, because the parents both carried a hidden allele.

Sources of variation

Genetic variation within a species comes from three main sources:

• Meiosis: crossing over and independent assortment shuffle alleles into new combinations in gametes.
• Random fertilisation: any sperm can fertilise any egg, multiplying the possible combinations.
• Mutation: changes to the DNA sequence create entirely new alleles, the ultimate source of all variation.

Environmental factors can also affect the phenotype (for example, diet affecting height), but only genetic changes are inherited.