Quick Reference
Dominant ≠ common
Dominance describes expression in a heterozygote, not population frequency.
Genotype ≠ phenotype
The same phenotype can arise from more than one genotype in a simple dominant/recessive model.
Probability is multiplicative
For independent events, the probability of both events occurring is the product of their probabilities.
Sex-linked means chromosome context
The inheritance pattern depends on where the gene is located and which sex chromosomes carry it.
1. The Vocabulary You Must Not Mix Up
| Term | Meaning | Common distractor |
|---|---|---|
| Allele | A variant of a gene at a locus. | Not the same as a chromosome. |
| Genotype | Allelic/genetic composition. | Not the observable trait itself. |
| Phenotype | Observable characteristic or measurable outcome. | Not necessarily a one-to-one map to a genotype. |
| Homozygous | Two identical alleles at a locus. | Do not confuse with “dominant.” |
| Heterozygous | Two different alleles at a locus. | A heterozygote can show a dominant phenotype in a simple complete-dominance model. |
2. Monohybrid Crosses
Consider a simple complete-dominance model where A is dominant and a is recessive. Crossing two heterozygotes gives AA, Aa, Aa and aa in the four cells of a 2×2 Punnett square. That produces an expected genotype ratio of 1:2:1 and, under complete dominance, a phenotype ratio of 3:1.
3. Test Crosses
A test cross is used to infer the genotype of an individual showing a dominant phenotype by crossing it with a homozygous recessive individual. The offspring pattern can provide evidence about whether the unknown parent is homozygous or heterozygous under the model being tested.
4. Dihybrid Crosses & Independent Assortment
For two independently assorting genes with complete dominance and equal gamete probabilities, the classical F₂ phenotype ratio is 9:3:3:1. The ratio is not a magical property of all two-gene crosses; it depends on assumptions, including independent assortment and the specified dominance model.
5. Sex-Linked Inheritance
For an X-linked recessive trait, males typically have one X chromosome and therefore one copy of X-linked genes, while females typically have two X chromosomes. That changes the probability structure of inheritance. The question must identify the sex chromosomes and the parental genotypes before the cross is built.
Do not automatically apply the 3:1 autosomal phenotype ratio to sex-linked questions. Build the cross with the actual gametes.
6. Non-Mendelian Patterns
Intermediate phenotype
The heterozygote phenotype differs from either homozygote and may appear intermediate.
Both expressed
Both alleles contribute to the heterozygote phenotype.
Population-level allele set
A gene can have more than two allelic forms in a population even though one individual is diploid.
Genes on chromosomes
Genes physically close together may not assort independently because recombination probability depends on distance.
7. The Four Genetics Traps
- Calling a dominant allele “better” or “more common.” Neither follows from dominance alone.
- Forgetting that the two parents can contribute the same or different gametes depending on genotype.
- Applying a memorized ratio without checking the assumptions that produced it.
- Ignoring sex chromosomes in an X-linked problem.
Frequently Asked Questions
PM&DC — Uniform Curriculum MDCAT-2025 (official PDF) · OpenStax — Biology 2e: Characteristics and Traits
Scope: scientific explanations are written for student use and simplified where appropriate. Exact exam wording, syllabus scope and current administrative rules should be checked against PM&DC documents.