Genetic Inheritance

Single gene, two alleles. Use Punnett squares.

Ratios:

• Heterozygous × heterozygous → 3:1 (dominant:recessive)
• Heterozygous × homozygous recessive → 1:1 (test cross)

Both alleles expressed equally in heterozygote.

Example: ABO blood groups (multiple alleles)

• $I^A$ and $I^B$ are codominant; $i$ is recessive
• $I^AI^B$ → blood group AB

Two genes on different chromosomes (independently assort).

$\text{AaBb} \times \text{AaBb} \rightarrow 9:3:3:1 \text{ ratio}$

Genes on the X chromosome. Males: XᴬY or XᵃY; Females: XᴬXᴬ, XᴬXᵃ, or XᵃXᵃ.

Males more likely to show recessive X-linked conditions (only need one copy).

Examples: haemophilia, colour blindness.

Genes on the same chromosome → inherited together → don't follow independent assortment.

Crossing over can separate linked genes → recombinant phenotypes (less common).

One gene masks the expression of another gene.

Example: coat colour in mice. If gene E is homozygous recessive (ee), no pigment deposited regardless of other genes → modified ratios (9:3:4 or 12:3:1).

$\chi^2 = \sum \frac{(O - E)^2}{E}$

where O = observed, E = expected.

• Compare calculated $\chi^2$ with critical value at chosen significance level (usually 0.05)
• Degrees of freedom = number of categories − 1
• If $\chi^2$ > critical value → significant difference → reject null hypothesis
• If $\chi^2$ ≤ critical value → no significant difference → accept null hypothesis

• Monohybrid: 3:1 or 1:1; dihybrid: 9:3:3:1 (independent assortment)
• Codominance: both alleles expressed; ABO blood groups
• Sex linkage: X-linked; males more affected
• Linkage: genes on same chromosome; recombinants from crossing over
• Epistasis: one gene masks another; modified ratios
• Chi-squared: test observed vs expected ratios