Mendelian Inheritance

By the end of this lesson, you will be able to apply Mendel's laws to predict offspring ratios using Punnett squares, distinguish dominant from recessive alleles, and solve monohybrid and dihybrid cross problems.

Mendel's Contributions

Gregor Mendel (1860s) used pea plants to discover the laws of inheritance — before genes or DNA were known. His genius was in counting offspring ratios across thousands of plants, revealing predictable mathematical patterns.

Key Vocabulary

Gene: a segment of DNA that codes for a trait.

Allele: one of two or more versions of a gene at a specific locus.

Homozygous: two identical alleles (AA or aa).

Heterozygous: two different alleles (Aa).

Dominant: allele expressed in heterozygous condition (uppercase, A).

Recessive: allele masked in heterozygous condition (lowercase, a).

Genotype: the alleles an organism carries (AA, Aa, aa).

Phenotype: the observable trait produced by the genotype.

Mendel's Laws

Law of Segregation: the two alleles for each gene separate during gamete formation; each gamete carries one allele. The alleles recombine randomly at fertilization.

Law of Independent Assortment: alleles for different genes are inherited independently of each other (if the genes are on different chromosomes). This gives the 9:3:3:1 ratio in dihybrid crosses.

Monohybrid Cross

Parents: Aa × Aa

Punnett square:

	A	a
A	AA	Aa
a	Aa	aa

Genotype ratio: 1 AA : 2 Aa : 1 aa

Phenotype ratio: 3 dominant : 1 recessive (if A is dominant over a)

Dihybrid Cross

Parents: AaBb × AaBb (A=seed color, B=seed shape)

Expected offspring ratio: 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb (9:3:3:1)

This ratio holds when genes are on different chromosomes (independent assortment).

Example 1 — Monohybrid Cross

In pea plants, tall (T) is dominant over short (t). Cross Tt × tt.