Use genetic diagrams to predict the inheritance of a single gene and explain variation

What this dot point is asking

SEAB wants you to use the key genetics terms correctly, to set out a genetic diagram (cross) for a single gene to predict the genotypes, phenotypes and ratios of the offspring, and to explain the difference between continuous and discontinuous variation. Setting out a clear cross is the core skill being tested.

The answer

The key terms

• A gene controls a characteristic; an allele is a version of a gene (for example, the allele for tall or for short).
• Dominant allele: shows its effect even when only one copy is present (written as a capital letter, for example $T$).
• Recessive allele: only shows its effect when two copies are present (written as a small letter, $t$).
• Genotype: the alleles an organism has (for example, $Tt$).
• Phenotype: the characteristic you can see (for example, tall).
• Homozygous: two of the same allele ($TT$ or $tt$). Heterozygous: two different alleles ($Tt$).

Setting out a genetic cross

To predict the offspring of a cross, set out a clear genetic diagram:

1. State the parental phenotypes and genotypes.
2. Show the gametes each parent can make (each gamete carries one allele).
3. Combine the gametes (a grid, called a Punnett square, is clearest) to find the offspring genotypes.
4. State the phenotypes and the ratio.

A worked ratio

Cross a heterozygous tall plant ($Tt$) with a short plant ($tt$), where $T$ (tall) is dominant to $t$ (short):

• $Tt$ makes gametes $T$ and $t$; $tt$ makes gametes $t$ and $t$.
• Combining them gives offspring: $Tt$, $Tt$, $tt$, $tt$.
• Phenotypes: $Tt$ is tall, $tt$ is short.
• Ratio: 1 tall : 1 short.

If instead two heterozygous plants ($Tt \times Tt$) are crossed, the offspring are $TT$, $Tt$, $Tt$, $tt$, giving a 3 tall : 1 short ratio.

Variation

Offspring vary, and variation comes in two kinds:

• Continuous variation: a range of values between two extremes, with many in-between values (for example, height or body mass in humans). Usually controlled by several genes and often affected by the environment.
• Discontinuous variation: a few distinct categories with no in-between values (for example, blood group or tongue-rolling). Usually controlled by one or a few genes and not affected by the environment.

Examples in context

Example 1. Eye colour in a family. The allele for brown eyes is dominant to that for blue. Two brown-eyed parents who are both heterozygous can have a blue-eyed child if the child inherits the recessive allele from each parent, the $3 : 1$ ratio in action.

Example 2. Height and diet. Human height shows continuous variation: it is controlled by many genes, but also affected by the environment, such as childhood diet. This is why height varies smoothly across a population rather than in distinct groups.

Try this

Q1. Define the term dominant allele. [1 mark]

• Cue. An allele that shows its effect in the phenotype even when only one copy is present.

Q2. A cross between two heterozygous plants ($Tt \times Tt$) is made. State the expected phenotype ratio. [2 marks]

• Cue. $3$ dominant (tall) $: 1$ recessive (short).

Q3. State whether blood group is an example of continuous or discontinuous variation, and explain why. [2 marks]

• Cue. Discontinuous, because it falls into a few distinct categories with no in-between values and is controlled by genes alone.