Inheritance and Genetic Crosses
Genetic terms
- Gene โ a section of DNA coding for a polypeptide; allele โ a version of a gene.
- Genotype โ the alleles present; phenotype โ the observable characteristics.
- Homozygous โ two identical alleles; heterozygous โ two different alleles.
- Dominant allele โ expressed in the heterozygote (capital letter); recessive โ only expressed when homozygous (lower case).
- Codominant โ both alleles are expressed together (e.g. blood group AB).
Monohybrid inheritance
Inheritance of a single gene. Use a Punnett square.
Cross two heterozygotes Bb ร Bb:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
- Genotype ratio 1 BB : 2 Bb : 1 bb; phenotype ratio 3 dominant : 1 recessive.
Dihybrid inheritance
Inheritance of two genes at the same time. A cross between two double heterozygotes (AaBb ร AaBb) gives the classic 9 : 3 : 3 : 1 phenotype ratio (when the genes assort independently).
Codominance and multiple alleles
- Codominance: heterozygotes show both phenotypes (e.g. red and white flowers โ roan/speckled, not pink).
- Multiple alleles: a gene with more than two possible alleles in the population (e.g. ABO blood groups: Iแดฌ, Iแดฎ, Iแดผ).
Sex linkage
Genes on the X chromosome are sex-linked. Because males (XY) have only one X, a single recessive allele is expressed โ so X-linked recessive conditions (e.g. red-green colour blindness, haemophilia) are more common in males.
Autosomal linkage and epistasis
- Autosomal linkage: genes on the same chromosome tend to be inherited together (fewer recombinants).
- Epistasis: one gene affects the expression of another, altering the expected ratios.
Chi-squared test
The chi-squared (ฯยฒ) test compares observed results with expected ratios to see if the difference is due to chance. If the calculated ฯยฒ exceeds the critical value (usually at p = 0.05), the difference is significant (something other than chance is happening).
Worked example
Two heterozygous black guinea pigs (Bb) are crossed; black is dominant. What proportion of offspring are white?
- Bb ร Bb โ 1 BB : 2 Bb : 1 bb. Only bb is white โ 1 in 4 (25%). โ
Common mistakes
- Confusing genotype ratio (1:2:1) with phenotype ratio (3:1).
- Treating codominance like incomplete dominance โ codominance shows both alleles, not a blend.
- Forgetting sex-linked recessives affect males more (they have one X).
Exam tips
- Always draw Punnett squares and define your allele symbols first.
- Learn the key ratios: monohybrid 3:1, dihybrid 9:3:3:1.
- Know when and how to apply the chi-squared test and interpret the result vs the critical value.
Key facts to remember
- Define genotype/phenotype, homozygous/heterozygous, dominant/recessive/codominant.
- Monohybrid Bb ร Bb โ 3:1; dihybrid AaBb ร AaBb โ 9:3:3:1; sex-linked recessives commoner in males (XY).
- Chi-squared compares observed vs expected โ significant if ฯยฒ > critical value at p = 0.05.