Topic 4

Topic 4

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Genetics is the branch of biology that studies how traits pass from parents to offspring through genes and alleles. A gene is a DNA sequence at a specific locus on a chromosome; an allele is an alternative version of that gene.

Key formulas to memorize:

• Monohybrid F₂ ratio: 3:1 (dominant : recessive) under complete dominance
• Dihybrid F₂ ratio: 9:3:3:1 (independent assortment, genes on separate chromosomes)
• Test cross ratio: 1:1 — heterozygous × homozygous recessive
• Hardy-Weinberg equilibrium: p² + 2pq + q² = 1 (p = dominant allele frequency, q = recessive)

Exam high-yield pointers:

1. Mendel’s First Law (Segregation): each gamete receives one allele per gene. This explains the 3:1 ratio in monohybrid crosses.
2. Dominant alleles mask recessive ones in heterozygotes; the recessive phenotype only appears in homozygotes.
3. Codominance (e.g., AB blood group) produces a distinct heterozygote phenotype, NOT a blending — this is a frequent trap in FMGE MCQs.
4. Multiple alleles (ABO system: I^A, I^B, i) — three alleles, four blood groups. Know which alleles are dominant over i.

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🟡 Standard — Regular Study (2d–2mo)

Core Definitions

• Genotype — the genetic makeup of an individual (e.g., I^A I^B for blood group AB).
• Phenotype — the observable trait (blood group A, B, AB, or O).
• Homozygous — two identical alleles (AA or aa); Heterozygous — two different alleles (Aa).
• Pure line — homozygous for all loci; breeds true upon selfing.

Mendel’s Laws

Law of Segregation (First Law): During gamete formation, the two alleles for a character separate so each gamete receives only one allele. Hence a heterozygote (Aa) produces gametes carrying either A or a in equal 1:1 ratio. This underpins the 3:1 phenotypic ratio in an F₂ monohybrid cross.

Law of Independent Assortment (Second Law): Alleles of different genes segregate independently during gamete formation — valid when genes are on separate chromosomes. The dihybrid cross produces the 9:3:3:1 phenotypic ratio in F₂.

Dominance Relationships

Type	Definition	FMGE Example
Complete dominance	Dominant allele fully masks recessive	Tall (T) vs short (t) pea plant
Incomplete dominance	Heterozygote shows intermediate phenotype	Snapdragon flower: red × white → pink F₁
Codominance	Both alleles expressed equally	AB blood group (I^A I^B)
Overdominance	Heterozygote superior to both homozygotes	Sickle-cell trait

Test Cross and Back Cross

A test cross (unknown genotype × homozygous recessive) reveals whether the unknown is homozygous dominant or heterozygous. A 1:1 phenotypic ratio in progeny confirms heterozygosity.

A back cross (F₁ × either parent) can restore homozygous lines if the recessive parent is used.

Practice Pattern

FMGE typically asks one genetics question from monohybrid ratios, blood group inheritance, or Mendelian law application. The 3:1 ratio is the most common entry point.

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🔴 Extended — Deep Study (3mo+)

Deviations from Classic Mendelian Ratios

Epistasis — one gene modifies the expression of another. In recessive epistasis (e.g., 9:3:4 ratio in coat color), the homozygous recessive at the epistatic locus masks both alleles of the hypostatic gene. In dominant epistasis, the dominant allele at one locus masks the other gene (12:3:1 ratio).

Pleiotropy — a single gene influences multiple phenotypic traits. Phenylketonuria (PKU) demonstrates this: one mutated PAH allele causes intellectual disability, fair skin, and eczema simultaneously. This is testable in FMGE under genetic disorders.

Polygenic inheritance — multiple genes contribute to one trait, producing continuous variation. Human skin color spans a spectrum; inheritance involves at least three genes with additive effects (e.g., AABBDD = darkest, aabbdd = lightest). The ratio approximates a bell curve.

Multiple alleles — more than two allele forms exist in a population. The ABO blood system uses three alleles: I^A, I^B, and i. I^A and I^B are codominant; both are dominant over i. Six genotypes produce four phenotypes. A person with genotype I^A I^B has blood group AB — this is NOT incomplete dominance (no blending occurs).

Linkage and Chromosome Mapping

Genes on the same chromosome are linked and tend to inherit together, deviating from independent assortment. Crossing over during meiosis produces recombinant gametes. Genetic distance in map units = (Number of recombinants × 100) / Total progeny.

Hardy-Weinberg Equilibrium

In a large, randomly mating population with no selection, migration, or mutation: p² + 2pq + q² = 1, where p = dominant allele frequency, q = recessive allele frequency. If observed genotype frequencies deviate significantly, evolutionary forces are acting.

Common Mistakes in FMGE

1. Confusing codominance with incomplete dominance — AB blood group is codominant, not intermediate.
2. Assuming 3:1 applies when genes are linked — linked genes produce parental-type ratios close to 1:1 in a dihybrid test cross.
3. Misidentifying the F₁ generation — in incomplete dominance, F₁ is phenotypically intermediate, not showing either parent’s full trait.

Practice Prompts

1. A woman with blood group A (genotype I^A i) marries a man with blood group B (genotype I^B i). List all possible genotypes and phenotypes of their children with ratios.
2. In pea plants, tall (T) is dominant over short (t). A test cross yields 212 tall and 188 short progeny. What is the genotype of the tall parent, and why?

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