Question

Describe the principles of genetics proposed by Mendel.

Answer 1

Gregor Mendel (mid-1800s) discovered the basic rules of inheritance by crossing true‑breeding pea plants and analyzing large numbers of offspring. From his experiments he proposed several key principles that form the foundation of classical genetics:

1. Unit factors in pairs
- Heritable “factors” (now called genes) exist as discrete units and occur in pairs in organisms (one factor inherited from each parent).

2. Dominance and recessiveness (Law of Dominance)
- When two different alleles are present in a pair, one allele may mask the expression of the other. The expressed allele is dominant; the masked one is recessive. Example: tall (T) is dominant to short (t) in peas.

3. Segregation (Law of Segregation)
- The two alleles for a trait separate (segregate) during gamete formation so each gamete carries only one allele. At fertilization, offspring receive one allele from each parent, re‑forming the pair.
- Consequence: a monohybrid F2 generation from two heterozygotes (Tt × Tt) shows a phenotypic ratio of ~3:1 (dominant:recessive) and a genotypic ratio of 1 TT : 2 Tt : 1 tt.

4. Independent assortment (Law of Independent Assortment)
- Alleles of different genes segregate into gametes independently of one another (so the inheritance of one trait generally does not affect the inheritance of another), assuming the genes are on different chromosomes or far apart on the same chromosome.
- Consequence: a dihybrid cross between two heterozygotes (AaBb × AaBb) yields an F2 phenotypic ratio of ~9:3:3:1.

Additional concepts and modern refinements
- Mendel’s “factors” are genes and the alternative forms are alleles; genotypes determine potential traits and phenotypes are their expression.
- Mendel’s laws describe particulate (not blending) inheritance.
- There are exceptions/modifications: linkage (genes close together on a chromosome), incomplete dominance, codominance, multiple alleles, polygenic traits, pleiotropy and environmental effects — all of which extend or modify Mendel’s simple rules.

Mendel’s experimental design (true‑breeding lines, controlled crosses, reciprocal crosses, large sample sizes, quantitative counting) and his clear formulation of these principles made his work the cornerstone of classical genetics.