6.1.2 Patterns of inheritance: monohybrid and dihybrid crosses; the inheritance of codominant and multiple alleles, sex linkage and epistasis; the use of genetic diagrams to predict phenotypic ratios; and the chi-squared test to compare observed and expected results.

What this dot point is asking

OCR wants you to carry out monohybrid and dihybrid crosses, explain the inheritance of codominant and multiple alleles, sex linkage and epistasis, use genetic diagrams to predict phenotypic ratios, and use the chi-squared test to compare observed and expected results.

The answer

Key terms

A gene is a length of DNA coding for a protein; an allele is a version of a gene. Genotype is the alleles present; phenotype is the observable characteristic. Homozygous means two identical alleles, heterozygous two different. A dominant allele is expressed in the heterozygote; a recessive allele only in the homozygote.

Monohybrid and dihybrid crosses

• A monohybrid cross follows one gene. Crossing two heterozygotes (for example $Tt \times Tt$) gives a 3:1 phenotypic ratio of dominant to recessive.
• A dihybrid cross follows two genes on different chromosomes. Crossing two double heterozygotes ($TtRr \times TtRr$) gives a 9:3:3:1 ratio, because the genes assort independently (Mendel's second law).

Use a Punnett square to combine the gametes and read off the genotypes and phenotypes.

Codominance, multiple alleles, sex linkage and epistasis

• Codominance: both alleles are expressed in the heterozygote (for example the AB blood group, where both A and B antigens appear).
• Multiple alleles: a gene with more than two alleles in the population (for example the ABO blood group has $I^A$, $I^B$ and $I^O$).
• Sex linkage: genes on the X chromosome. Males (XY) express a single recessive X-linked allele, so conditions like haemophilia and colour blindness are commoner in males; females can be unaffected carriers.
• Epistasis: one gene affects the expression of another (for example one gene must produce a pigment precursor before a second gene can act), distorting the expected dihybrid ratio.

The chi-squared test

The chi-squared test ($\chi^2$) tests whether observed results differ significantly from those expected by a genetic ratio, or whether any difference is due to chance:

where $O$ is observed and $E$ is expected. State a null hypothesis (no significant difference), find the degrees of freedom (categories minus 1), and compare $\chi^2$ with the critical value at $p = 0.05$:

• if $\chi^2$ is less than the critical value, the difference is not significant (accept the null hypothesis, the data fit the ratio);
• if $\chi^2$ is greater than or equal to the critical value, the difference is significant (reject the null hypothesis).

Examples in context

Example 1. ABO blood groups. The ABO system shows both multiple alleles ($I^A$, $I^B$, $I^O$) and codominance ($I^A$ and $I^B$ are codominant, giving group AB), a single locus illustrating two of the patterns at once.

Example 2. Haemophilia in royal families. Haemophilia, an X-linked recessive condition, passed through European royal families via unaffected female carriers to affected sons, a classic demonstration of sex-linked inheritance.

Try this

Q1. State the expected phenotypic ratio from crossing two organisms heterozygous for two independently assorting genes. [1 mark]

• Cue. 9:3:3:1.

Q2. Explain why X-linked recessive conditions are more common in males than females. [2 marks]

• Cue. Males have only one X chromosome, so a single recessive allele is expressed; females have two X chromosomes and would need two recessive alleles to be affected (otherwise they are carriers).

Q3. Write the formula for the chi-squared statistic. [1 mark]

• Cue. $\chi^2 = \sum \dfrac{(O - E)^2}{E}$.