The key genetic terms (gene, allele, genotype, phenotype, dominant, recessive, homozygous, heterozygous), monohybrid inheritance and genetic crosses, codominance and sex linkage, and the use of genetic diagrams to predict offspring ratios.

What this dot point is asking

CCEA wants you to use the key genetic terms correctly, explain monohybrid inheritance and complete genetic crosses, explain codominance and sex linkage, and use genetic diagrams (Punnett squares) to predict the genotype and phenotype ratios of offspring. It builds directly on the molecular genetics of the previous dot point, applying the gene concept to how characteristics pass from parents to offspring.

Key genetic terms

These terms are the language of inheritance, so using them precisely is essential. Alleles are written as letters: a capital letter for the dominant allele and the same letter in lower case for the recessive (for example R and r). A homozygous dominant individual is RR, a heterozygous individual is Rr (which shows the dominant phenotype because R masks r), and a homozygous recessive individual is rr (which shows the recessive phenotype). Because every individual carries two alleles of each gene (one from each parent) and passes one to each offspring through the gametes, inheritance can be predicted.

Monohybrid crosses and Punnett squares

The Punnett square is a grid with one parent's gametes across the top and the other's down the side; filling each cell combines the gametes to give the offspring genotypes. From the genotypes you read off the phenotypes using the dominance rule. A 3 to 1 phenotype ratio is the hallmark of a heterozygous cross for a simple dominant and recessive gene. A test cross (crossing an organism showing the dominant phenotype with a homozygous recessive) reveals whether the unknown parent is homozygous (all offspring dominant) or heterozygous (a 1 to 1 ratio of phenotypes).

Codominance and sex linkage

In codominance, neither allele is fully dominant, so both alleles are expressed in the heterozygote, giving a third phenotype. The human ABO blood groups are a classic example: the alleles for A and B are codominant (a person with both shows blood group AB), while the allele for O is recessive. In sex linkage, the gene is carried on a sex chromosome, usually the X chromosome. Because females are XX and males are XY, a male has only one copy of an X-linked gene; so a recessive X-linked condition such as haemophilia or red-green colour blindness is expressed in a male with a single recessive allele, while a female would need two. This is why such conditions are more common in males, and why a carrier mother can pass the condition to her sons.

Examples in context

Example 1. ABO blood groups and transfusions. The codominant A and B alleles and the recessive O allele determine a person's blood group, which must be matched for safe transfusion. Predicting a child's possible blood groups from the parents' genotypes is a direct medical application of codominant inheritance, linking genetics to the health-science focus of the qualification.

Example 2. Genetic counselling for sex-linked disease. A family with a history of haemophilia may seek genetic counselling. Using sex-linkage reasoning and a genetic diagram, a counsellor can estimate the chance that a carrier mother's sons will be affected and her daughters will be carriers, showing how inheritance prediction informs real health decisions.

Try this

Q1. Define the terms genotype and phenotype. [2 marks]

• Cue. Genotype is the alleles an organism has; phenotype is the observable characteristic.

Q2. Two heterozygous parents (Bb) are crossed. State the expected phenotype ratio of the offspring. [2 marks]

• Cue. 3 dominant to 1 recessive (from a 1 BB to 2 Bb to 1 bb genotype ratio).

Q3. Explain why red-green colour blindness is more common in males than females. [2 marks]

• Cue. The gene is on the X chromosome; males have only one X, so one recessive allele is enough, while females need two.