Principles of Inheritance and Variation

Mendel chose the garden pea (Pisum sativum) because of several advantages: 1. Availability of contrasting characters: • Pea has 7 pairs of contrasting traits (tall/dwarf, round/wrinkled seed, etc.) that are easily observable and distinguishable 2. Bisexual flowers: • Pea flowers are bisexual (have both stamens and pistil) and are naturally self-fertilising • Allowed easy controlled cross-pollination by hand 3. Cleistogamous flowers: • Flowers naturally self-pollinate before opening → ensures pure lines • Easy to maintain pure breeding lines 4. Easy artificial cross-pollination: • Large flowers are easy to manipulate — emasculation (removal of stamens) and cross-pollination done easily 5. Short life cycle: • Annual plant with short generation time → results available in one season 6. Large number of offspring: • Produces many seeds per cross → statistically reliable ratios 7. Inexpensive and easy to grow: • Readily available, cheap, easy to cultivate in garden 8. Distinct and stable traits: • The 7 characters Mendel chose show clear-cut differences without intermediates • Traits remained stable across generations 9. Both true-breeding varieties and hybrids available: • True-breeding lines for each trait already existed for use in experiments

(a) Dominance vs Recessiveness: Dominance: • The allele that expresses its effect and masks the effect of the other allele • Expressed in both homozygous (AA) and heterozygous (Aa) conditions • Represented by capital letter (e.g., T for tallness) • Example: T (tall) is dominant over t (dwarf) Recessiveness: • The allele whose effect is masked in the presence of a dominant allele • Expressed only in homozygous condition (tt) • Represented by lowercase letter • Example: t (dwarf) is recessive; expressed only in tt (b) Homozygous vs Heterozygous: Homozygous: • Both alleles for a trait are identical (same) • E.g., TT (homozygous dominant) or tt (homozygous recessive) • Breeds true — produces only one type of gamete • Also called pure-breeding Heterozygous: • The two alleles for a trait are different • E.g., Tt (heterozygous) • Does not breed true — produces two types of gametes (T and t) • Also called hybrid (c) Monohybrid vs Dihybrid: Monohybrid: • Cross involving one pair of contrasting characters (one gene locus) • Example: Tt × Tt (tall × tall) • F2 phenotypic ratio: 3:1 • F2 genotypic ratio: 1:2:1 Dihybrid: • Cross involving two pairs of contrasting characters (two gene loci) • Example: TtRr × TtRr • F2 phenotypic ratio: 9:3:3:1 (when genes on separate chromosomes) • Demonstrates the Law of Independent Assortment

Formula: Number of gamete types = 2ⁿ, where n = number of heterozygous loci Given: heterozygous for 4 loci (all 4 genes are heterozygous) Number of gamete types = 2⁴ = 16 different types of gametes Explanation: • For each heterozygous locus (Aa), the organism produces 2 types of gametes: A or a • For 4 independent heterozygous loci: 2 × 2 × 2 × 2 = 16 combinations Example: AaBbCcDd organism produces gametes: ABCD, ABCd, ABcD, ABcd, AbCD, AbCd, AbcD, Abcd, aBCD, aBCd, aBcD, aBcd, abCD, abCd, abcD, abcd = 16 types This assumes the 4 loci are on separate chromosomes (independent assortment).

Law of Dominance states: When two alternative forms of a character (alleles) are present together in a hybrid, only one (dominant) is expressed; the other (recessive) is suppressed in the F1 generation. Monohybrid Cross Experiment: Parents (P): TT (tall) × tt (dwarf) • Pure tall plant (TT) × pure dwarf plant (tt) F1 generation: All plants are Tt (tall) • The T allele is dominant — it is expressed • The t allele is recessive — it is suppressed (hidden) • All F1 hybrids are tall → Law of Dominance demonstrated F2 generation (F1 × F1: Tt × Tt): • Gametes from each parent: T or t Punnett square: T t T | TT | Tt | t | Tt | tt | F2 genotypes: 1 TT : 2 Tt : 1 tt F2 phenotypes: 3 Tall : 1 Dwarf (3:1 ratio) Observations proving the Law: 1. F1 plants are ALL tall → dominant trait (T) completely masks recessive trait (t) 2. Recessive trait (dwarf) reappears in F2 → it was not lost, only suppressed 3. In F2: 3 tall : 1 dwarf — confirming dominant (T) is expressed in TT and Tt, recessive (t) only in tt Conclusion: Tallness (T) is dominant over dwarfness (t). In a heterozygote (Tt), only the dominant allele determines the phenotype.

Test Cross: A cross between an organism with dominant phenotype but unknown genotype (could be homozygous dominant AA or heterozygous Aa) and a homozygous recessive organism (aa). Purpose: To determine whether the dominant phenotype organism is homozygous (AA) or heterozygous (Aa). Design: Case 1 — If unknown organism is homozygous dominant (AA): AA × aa → All offspring: Aa (all dominant phenotype) Ratio: 100% dominant : 0% recessive → All offspring show dominant phenotype → Parent is AA Case 2 — If unknown organism is heterozygous (Aa): Aa × aa → Offspring: Aa (dominant) and aa (recessive) Ratio: 1 dominant : 1 recessive (1:1) → Half offspring show recessive phenotype → Parent is Aa Example: Tall pea plant (T?) × dwarf pea plant (tt): • If all offspring tall → parent is TT • If offspring are 1 tall : 1 dwarf → parent is Tt The homozygous recessive parent (aa/tt) is called the test parent because: • It contributes only recessive alleles • So the phenotype of offspring directly reveals the gametes (and genotype) of the unknown parent Conclusion: A 1:1 ratio in the offspring indicates heterozygosity; a 1:0 ratio indicates homozygosity.

Cross: Homozygous female × Heterozygous male Let dominant allele = A, recessive allele = a Homozygous female can be either: Case 1: Homozygous dominant female (AA) × Heterozygous male (Aa) Female gametes: all A Male gametes: A or a Punnett square: A (female) A (female) A (♂) | AA | AA | a (♂) | Aa | Aa | F1 genotypes: 1 AA : 1 Aa (= 2 AA : 2 Aa) F1 phenotypes: ALL show dominant phenotype (100%) Ratio: All dominant : 0 recessive Case 2: Homozygous recessive female (aa) × Heterozygous male (Aa) Female gametes: all a Male gametes: A or a Punnett square: a (female) a (female) A (♂) | Aa | Aa | a (♂) | aa | aa | F1 genotypes: 1 Aa : 1 aa F1 phenotypes: 1 dominant (Aa) : 1 recessive (aa) Ratio: 1:1 Conclusion: When a homozygous dominant female is crossed with a heterozygous male, all F1 offspring show the dominant phenotype. When a homozygous recessive female is crossed with a heterozygous male, offspring show 1:1 ratio (like a test cross).

Autosomes and sex chromosomes are similar to each other in the following conditions: 1. In females (XX condition): • Females carry two X chromosomes (XX) • The two X chromosomes behave like autosomes in terms of being a matched (homologous) pair • Like autosomes, both X chromosomes can pair, cross over, and segregate normally during meiosis 2. In homogametic sex (ZZ condition in birds): • In birds, males are ZZ (homogametic) — both sex chromosomes are identical • ZZ males are similar to autosomes in that they have a matched pair 3. In organisms where sex chromosomes are not differentiated: • In some organisms, the sex chromosomes look morphologically identical to autosomes • Example: early stages of sex chromosome evolution 4. Structural similarity: • The sex chromosomes (particularly both X chromosomes in females) contain the pseudoautosomal regions (PARs) that behave like autosomes — recombination occurs here during meiosis • These PAR regions on X and Y (or X and X) are functionally like autosomes In males (XY): X and Y chromosomes are NOT similar — they differ in size and gene content. Y is smaller and has fewer genes. They can only pair at pseudoautosomal regions. Conclusion: Autosomes and sex chromosomes are most similar in females (XX) where the two sex chromosomes are homologous pairs, similar to autosomes.

(a) Codominance: • Both alleles are equally expressed in the heterozygote — neither is dominant or recessive • Both alleles contribute equally to the phenotype • The F1 hybrid shows BOTH parental traits simultaneously Example: ABO Blood Group system (multiple alleles: Iᴬ, Iᴮ, i) • Iᴬ and Iᴮ are codominant alleles • Person with genotype IᴬIᴮ has blood group AB • Both A antigens (from Iᴬ) and B antigens (from Iᴮ) are expressed on red blood cells • Neither allele masks the other — both are fully expressed Another example: Roan coat in cattle • Red (Cᴿ) × White (Cᵂ) → Roan (CᴿCᵂ): both red and white hairs present (b) Incomplete Dominance: • Neither allele is completely dominant — heterozygote shows an intermediate phenotype • The F1 hybrid shows a blending of the two parental traits • Also called partial dominance or blending inheritance (though not true blending) Example: Snapdragon (Antirrhinum) / Four O'Clock plant (Mirabilis jalapa): Cross: Red (RR) × White (rr) F1: Rr → Pink flowers (intermediate) F2: RR (Red) : Rr (Pink) : rr (White) = 1:2:1 Note: In incomplete dominance, phenotypic and genotypic ratios in F2 are BOTH 1:2:1 (Unlike Mendelian dominance where phenotypic ratio is 3:1 and genotypic is 1:2:1) Difference: • Codominance: both phenotypes expressed simultaneously (e.g., AB blood) • Incomplete dominance: intermediate (blended) phenotype (e.g., pink from red × white)

Multiple Alleles: • When a gene has more than two allelic forms (more than two variants) in a population, it is called multiple alleles • A single individual carries only two alleles (one on each homologous chromosome), but in the population, more than two variants exist • Example: ABO blood group gene has three alleles: Iᴬ, Iᴮ, and i ABO Blood Group System: Controlled by a single gene (I gene) with 3 alleles: Iᴬ, Iᴮ, and i Allele properties: • Iᴬ: codes for enzyme that adds A antigen to RBC surface • Iᴮ: codes for enzyme that adds B antigen to RBC surface • i: codes for non-functional enzyme (no antigen added) • Iᴬ and Iᴮ are both dominant over i, but codominant to each other Blood groups and genotypes: | Blood Group | Antigens on RBC | Antibodies in Plasma | Genotypes | |---|---|---|---| | A | A | Anti-B | IᴬIᴬ or Iᴬi | | B | B | Anti-A | IᴮIᴮ or Iᴮi | | AB | A and B | None | IᴬIᴮ | | O | None | Anti-A and Anti-B | ii | Example of a cross: Parents: Blood group A (IᴬIᴮ) × Blood group O (ii) Wait — IᴬIᴮ is blood group AB. Correct example: Parents: Blood group A (Iᴬi) × Blood group B (Iᴮi) Offspring: IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B), ii (O) — all four blood groups possible! Importance: Critical for blood transfusion compatibility.