Genetics is the study of heredity and variation. Gregor Mendel's experiments with garden pea (Pisum sativum) in the 1860s laid the foundation of genetics. He chose pea plants for their contrasting traits, short generation time, easy hybridisation, and large number of offspring.
Mendel's Laws
Law of Dominance: In a cross between pure-breeding parents differing in one trait, the trait that appears in the F1 generation is dominant; the hidden one is recessive.
Law of Segregation (Law of Purity of Gametes): Alleles of a gene separate (segregate) during gamete formation so each gamete carries only one allele of each gene. This explains the 3:1 ratio in F2 of a monohybrid cross.
Law of Independent Assortment: Genes for different traits assort independently into gametes. This explains the 9:3:3:1 ratio in F2 of a dihybrid cross. This law holds only for genes on different chromosomes (non-linked genes).
Monohybrid Cross
Cross TT x tt → F1 all Tt (tall). F1 x F1 → F2: 1TT : 2Tt : 1tt → phenotype ratio 3 tall : 1 dwarf.
In pea, Tall (T) is dominant over dwarf (t). Cross Tt x Tt. Gametes from each: T and t. Punnett square gives TT, Tt, Tt, tt in ratio 1:2:1. Genotype ratio = 1:2:1; phenotype ratio = 3:1.
Testcross (backcross) — cross of F1 (Tt) with homozygous recessive (tt). Offspring: Tt and tt in 1:1 ratio. If testcross gives 1:1, the parent was heterozygous; if all dominant, parent was homozygous dominant.
Dihybrid Cross
Cross RRYY x rryy → F1: RrYy. F1 x F1 → F2 ratio 9:3:3:1 (9 RY_ : 3 Ryy : 3 rrY_ : 1 rryy).
R = round, r = wrinkled; Y = yellow, y = green. F2 ratio: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. This demonstrates independent assortment.
Deviations from Mendelian Ratios
Incomplete dominance: F1 heterozygote shows intermediate phenotype. E.g., Mirabilis jalapa (4 o'clock plant) — Red (RR) x White (rr) → Pink (Rr). F2 ratio: 1 Red : 2 Pink : 1 White (1:2:1).
Codominance: Both alleles fully expressed in heterozygote. E.g., ABO blood group — IA IA = blood group A, IB IB = B, IA IB = AB (both antigens expressed), ii = O.
A man with blood group AB (IA IB) and a woman with blood group O (ii) can have children of blood groups A (IA i) and B (IB i) only. They cannot have an AB or O child.
Epistasis — one gene masks the expression of another. Bombay phenotype (h gene) — individuals with hh genotype cannot express ABO blood group antigens regardless of IA or IB alleles.
Chromosomal Theory and Linkage
Morgan's work on Drosophila showed genes on the same chromosome are linked and do not assort independently. Linked genes give ratios different from Mendelian expectations. Recombination frequency between linked genes = measure of distance between them (1% recombination = 1 centimorgan).
Morgan crossed Drosophila with grey body + normal wing x black body + vestigial wing. In testcross, instead of 1:1:1:1, he got mostly parental types with few recombinants (due to linkage). Crossing over produces recombinants.
Sex-linked inheritance — Colour blindness in humans is X-linked recessive. A carrier mother (XB Xb) x normal father (XB Y) can have: colour-blind sons (Xb Y), normal sons, carrier daughters, normal daughters. Males are more frequently affected because they have only one X chromosome.
Chromosomal Disorders
- Down syndrome (Trisomy 21): 2n = 47, extra chromosome 21; caused by non-disjunction
- Klinefelter syndrome (XXY): male with 47 chromosomes, reduced fertility
- Turner syndrome (XO): female with 45 chromosomes, sterile
- Super Female (XXX): 47 chromosomes
- Super Male / Jacob syndrome (XYY): 47 chromosomes
Common mistakes
- Mendel's law of independent assortment applies only to unlinked genes (on different chromosomes or far apart on the same chromosome).
- In incomplete dominance, the phenotype ratio = genotype ratio (1:2:1), unlike complete dominance (3:1 phenotype).
- Down syndrome is trisomy 21 (not monosomy or other chromosome).
- Blood group AB demonstrates codominance, not incomplete dominance.
Summary
Mendel's laws of dominance, segregation, and independent assortment explain basic inheritance patterns. Deviations include incomplete dominance, codominance, and epistasis. Morgan's work revealed linkage and crossing over. Chromosomal non-disjunction causes aneuploidies like Down, Klinefelter, and Turner syndromes.