Mendelian Genetics & Inheritance Patterns

Gregor Johann Mendel selected the garden pea (Pisum sativum) for his landmark experiments because the plant is naturally self-pollinating, produces a large number of offspring, has a short generation time, and displays seven pairs of clearly contrasting traits such as tall versus dwarf stem height and round versus wrinkled seed shape. These features allowed him to track inheritance over multiple generations with statistical rigour.

From his monohybrid crosses, Mendel formulated two foundational laws. The Law of Dominance states that when two organisms with contrasting alleles are crossed, the F₁ generation displays only the dominant phenotype. The Law of Segregation (First Law) explains that the two alleles for a trait separate during gamete formation so each gamete carries only one allele. When Tt is crossed with Tt, the F₂ generation yields a 3:1 phenotypic ratio (3 tall : 1 dwarf) and a 1:2:1 genotypic ratio (1 TT : 2 Tt : 1 tt), as illustrated in a standard Punnett square. A test cross, performed by crossing an individual of unknown genotype with a homozygous recessive (tt), reveals whether the parent is homozygous dominant (all dominant offspring) or heterozygous (1:1 ratio).

From dihybrid crosses involving two independent traits, Mendel derived the Law of Independent Assortment (Second Law): alleles of different genes assort independently during gamete formation, yielding a 9:3:3:1 phenotypic ratio in the F₂ generation. This law applies only when the genes reside on different chromosomes; linked genes do not assort independently.

Not all inheritance follows strict Mendelian patterns. In incomplete dominance, the heterozygote shows a blended phenotype, as seen when red and white snapdragons produce pink F₁ flowers with a 1:2:1 F₂ ratio. In co-dominance, both alleles express equally in the heterozygote; the ABO blood group system exemplifies this, where individuals with genotype I^A I^B display both A and B antigens (blood group AB). The ABO system also demonstrates multiple allelism: three alleles (I^A, I^B, i) produce six genotypes and four phenotypes. Pleiotropy occurs when a single gene influences multiple traits, as the HbS allele in sickle cell anemia affects haemoglobin shape, oxygen transport, and organ function simultaneously. Polygenic inheritance is the converse: many genes contribute to a single trait, producing continuous variation as in human skin colour and height.

Sutton and Boveri's Chromosomal Theory of Inheritance established that genes are carried on chromosomes. Thomas Hunt Morgan's experiments on Drosophila demonstrated that genes on the same chromosome are linked and do not assort independently, but crossing over during meiosis creates recombinant offspring. The recombination frequency between two genes serves as a measure of their physical distance, forming the basis of genetic mapping. The key testable concept is distinguishing between co-dominance (both alleles fully expressed, e.g., AB blood group) and incomplete dominance (blended expression, e.g., pink snapdragons), along with the ability to work through Punnett squares for monohybrid and dihybrid crosses.