Dominant Alleles: A dominant allele is one that is always expressed in the phenotype, even if only one copy is present. In genetic notation, these are represented by uppercase letters (e.g., ).
Recessive Alleles: A recessive allele is only expressed in the phenotype when the organism is homozygous for that allele (i.e., it has two copies). It is masked by the presence of a dominant allele and is represented by lowercase letters (e.g., ).
Law of Segregation: During the formation of gametes (eggs and sperm), the two alleles for a trait separate so that each gamete carries only one allele. This ensures that offspring receive 50% of their genetic material from each parent.
Step 1: Identify Parental Genotypes: Determine the alleles of both parents. For example, if crossing a homozygous dominant individual with a homozygous recessive one, the genotypes are and .
Step 2: Set up the Grid: Place the alleles of one parent along the top and the alleles of the other parent down the left side of a grid.
Step 3: Fill the Squares: Combine the alleles from the top and side into each corresponding box to determine the possible genotypes of the offspring.
Step 4: Analyze Ratios: Calculate the probability of each genotype and phenotype. In a cross between two heterozygotes (), the expected phenotypic ratio is (dominant to recessive).
| Feature | Homozygous | Heterozygous |
|---|---|---|
| Allele Composition | Two identical alleles (e.g., or ) | Two different alleles (e.g., ) |
| Breeding Outcome | Produces identical offspring if self-pollinated | Produces offspring with varying traits |
| Phenotype | Expresses the trait of the specific allele | Expresses the dominant trait only |
Letter Selection: Always choose letters where the uppercase and lowercase versions look distinct (e.g., and are better than and ) to avoid confusion during rapid calculations.
Probability Calculations: Express results as fractions (e.g., ), percentages (25%), or ratios (). Ensure you read the question carefully to see which format is required.
Working Backwards: If an offspring shows a recessive trait, you immediately know its genotype is homozygous recessive (). This allows you to deduce that both parents must have carried at least one recessive allele.
Sanity Check: In a monohybrid cross of two heterozygotes, the recessive phenotype should appear in approximately 25% of the offspring. If your Punnett square shows 0% or 50%, re-check your parental genotypes.
Dominant does not mean 'Common': A common misconception is that dominant alleles are more frequent in a population. Dominance only describes how alleles interact to produce a phenotype, not their prevalence.
Mixing Genotype and Phenotype Ratios: Students often provide the genotype ratio (e.g., ) when the question asks for the phenotype ratio (e.g., ). Always double-check which 'type' is requested.
Incorrect Allele Separation: Ensure that each parent's pair of alleles is split correctly. A parent with genotype must contribute either or to a gamete, never both.
