An allele is a version of a gene
Diploid organisms usually carry two alleles of a gene, one on each homologous chromosome. A homozygote carries identical alleles; a heterozygote carries different alleles. Genotype describes the allele combination, while phenotype is the observable outcome produced by genotype in an environment.
Dominant does not mean common, stronger or better. It means that one copy produces the defined phenotype in a heterozygote. A recessive phenotype appears when no dominant allele is present under the conditions being considered.
Segregation is chromosome behaviour
During meiosis I, homologous chromosomes separate. Because alleles of one gene occupy corresponding loci on a homologous pair, the two alleles segregate into different gametes. A heterozygote Aa therefore produces A and a gametes in approximately equal proportions when segregation is normal.
Fertilisation randomly combines gametes. A Punnett square does not cause a ratio; it displays the probabilities created by meiosis and random fertilisation.
Identify alleles carried by each parent.
Separate alleles into valid gametes.
Combine one gamete from each parent.
Count genotype or phenotype outcomes.
Monohybrid ratios depend on the question
For Aa × Aa with complete dominance, genotypes occur in a 1 AA : 2 Aa : 1 aa ratio, while phenotypes occur in a 3 dominant : 1 recessive ratio. The two ratios answer different questions and should never be interchanged.
A test cross pairs an individual showing a dominant phenotype with a homozygous recessive partner. If recessive offspring appear, the unknown parent must carry the recessive allele.
| Cross | Genotype outcome | Phenotype outcome |
|---|---|---|
| AA × aa | All Aa | All dominant |
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 dominant : 1 recessive |
| Aa × aa | 1 Aa : 1 aa | 1 dominant : 1 recessive |
Dihybrid crosses are two probability problems
If two genes assort independently, a double heterozygote AaBb forms AB, Ab, aB and ab gametes in equal proportions. The classic AaBb × AaBb phenotypic ratio is 9:3:3:1 only when both genes show complete dominance, assort independently and offspring classes have equal survival.
The product rule is often faster than a sixteen-box square. For example, the probability of aa and B_ can be calculated as P(aa) × P(B_), provided the genes assort independently.
When simple Mendelian ratios change
Incomplete dominance produces an intermediate heterozygous phenotype, so the phenotypic ratio of a heterozygote cross may match the 1:2:1 genotype ratio. In codominance, both alleles are detectably expressed, as in the AB blood group.
Linked genes occupy the same chromosome and may not assort independently. Crossing over can create recombinant gametes, but parental combinations are often more frequent when loci are close. Sex-linked genes also require attention to which sex carries one or two copies of the relevant chromosome.
Heterozygote resembles one homozygote.
Heterozygote is intermediate.
Both allele products are expressed.
Genes may not assort independently.
Worked dihybrid cross: AaBb × Aabb
Assume independent assortment and complete dominance. At A, Aa × Aa gives dominant phenotype probability ¾. At B, Bb × bb gives dominant phenotype probability ½. Therefore P(A_B_) = ¾ × ½ = ⅜. Similarly A_bb is ⅜, aaB_ is ⅛, and aabb is ⅛. The ratio is 3:3:1:1, not 9:3:3:1.
For 80 offspring, expected counts are 30, 30, 10 and 10. These are long-run averages rather than mandatory counts for every family. Linked loci can violate the independent gamete frequencies used here, while incomplete dominance can change the relationship between genotype and visible phenotype. Establish parental genotypes and assumptions before choosing a familiar ratio.
The ideas to carry forward
- Segregation reflects separation of homologous chromosomes.
- Genotype and phenotype ratios answer different questions.
- Dihybrid probabilities can be multiplied only when events are independent.
- Dominance pattern, linkage and sex linkage can change expected ratios.
Answer first. Then reveal the marking logic.
01Why does Aa × Aa produce a 1:2:1 genotype ratio but a 3:1 phenotype ratio under complete dominance?2 marks · show the biological link
Answer: AA and Aa are different genotypes but share the dominant phenotype, so their probabilities combine into the phenotypic class.
02What evidence in offspring suggests that two genes are linked?2 marks · show the biological link
Answer: Parental allele combinations occur more often than recombinant combinations, deviating from independent-assortment expectations.
03What is P(Aabb)?2 marks · show the biological link
Answer: P(Aa) × P(bb) = ½ × ½ = ¼.
04Why not 9:3:3:1?2 marks · show the biological link
Answer: One parent is bb, so the B-locus cross is not Bb × Bb.
05Must exactly ten of eighty be aabb?2 marks · show the biological link
Answer: No. Random samples vary around their expected counts.
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