Mendelian Genetics

Core Review

Foundational Terminology

• Gene: A segment of DNA that codes for a functional product (usually a protein). Each gene resides at a specific locus on a chromosome.
• Allele: An alternative version of a gene. For example, the gene for pea flower color has a purple allele and a white allele.
• Dominant allele: An allele whose phenotype is expressed in a heterozygote (masking the recessive allele). Conventionally represented by an uppercase letter (P).
• Recessive allele: An allele whose phenotype is expressed only when two copies are present (homozygous). Represented by a lowercase letter (p).
• Homozygous: Having two identical alleles for a gene (PP or pp).
• Heterozygous: Having two different alleles for a gene (Pp).
• Genotype: The genetic makeup (the actual alleles: PP, Pp, pp).
• Phenotype: The observable physical or biochemical trait (purple flowers or white flowers).

Mendel's Law of Segregation

Each organism possesses two alleles for each gene (one from each parent). During gamete formation (Anaphase I of meiosis), the two alleles segregate — each gamete receives only ONE allele. At fertilization, the offspring receives one allele from each parent, restoring the pair. This explains the 3:1 phenotypic ratio in the F₂ generation of a monohybrid cross:

• P: PP (purple) × pp (white) → F₁: all Pp (purple)
• F₂: Pp × Pp → ¼ PP, ½ Pp, ¼ pp → 3 purple : 1 white

Mendel's Law of Independent Assortment

Alleles for different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes (or sufficiently far apart on the same chromosome to behave as if unlinked). This arises from the random orientation of bivalents during Metaphase I of meiosis. A dihybrid cross (e.g., YyRr × YyRr, where Y = yellow, y = green, R = round, r = wrinkled) produces the classic 9:3:3:1 phenotypic ratio in the F₂:

• 9/16: yellow, round (dominant for both)
• 3/16: yellow, wrinkled (dominant first, recessive second)
• 3/16: green, round (recessive first, dominant second)
• 1/16: green, wrinkled (recessive for both)

Worked Dihybrid Punnett Square

Cross: YyRr × YyRr

Gametes from each parent: YR, Yr, yR, yr (4 types each, due to independent assortment).

The 4×4 Punnett square yields:

Phenotypic count: 9 yellow-round (at least one Y, at least one R), 3 yellow-wrinkled (at least one Y, rr), 3 green-round (yy, at least one R), 1 green-wrinkled (yyrr).

Probability Rules

Product Rule: The probability of two independent events both occurring is the product of their individual probabilities. Example: probability of a child being homozygous recessive for two unlinked genes (e.g., pp and rr) = ¼ × ¼ = 1/16 if both parents are heterozygous for both traits.

Sum Rule: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities. Example: in a monohybrid cross (Pp × Pp), the probability of a dominant phenotype (PP or Pp) = ¼ + ½ = ¾.

The Test Cross

A test cross determines the unknown genotype of an individual expressing the dominant phenotype. The individual is crossed with a homozygous recessive (pp). If ANY offspring show the recessive phenotype, the unknown parent MUST be heterozygous (Pp). If ALL offspring show the dominant phenotype, the unknown parent is likely homozygous dominant (PP).

Example: A purple-flowered pea plant of unknown genotype is test-crossed with a white-flowered plant (pp).

• If the purple plant is PP → all offspring will be Pp (all purple).
• If the purple plant is Pp → approximately half the offspring will be Pp (purple) and half pp (white).

Common Traps

• "Dominant = more common": False. Dominance describes the relationship between alleles in a heterozygote, NOT population frequency. Polydactyly (extra fingers) is dominant but rare.
• "A 3:1 ratio always means heterozygous parents": A 3:1 ratio in offspring indicates heterozygous × heterozygous. But the DAT may give you ratios and ask you to work BACKWARD to determine parental genotypes.
• "Independent assortment always applies": Linked genes (close together on the same chromosome) do NOT assort independently — their alleles tend to be inherited together unless crossing over separates them.