What is a monohybrid cross?
A monohybrid cross studies the inheritance of a single characteristic controlled by one gene that has two contrasting alleles — one dominant and one recessive. Gregor Mendel worked with pea plants, crossing plants that differed in one trait such as stem height. By convention the dominant allele is written as a capital letter (T for tall) and the recessive allele as the matching small letter (t for short).
The F1 and F2 generations
When a pure-breeding tall plant (homozygous dominant, TT) is crossed with a pure-breeding short plant (homozygous recessive, tt), all offspring in the first filial generation (F1) are heterozygous (Tt) and tall. The recessive trait seems to disappear because the dominant allele masks it. When two F1 plants (Tt × Tt) are crossed, the short trait reappears.
Example
Tt × Tt using a Punnett square gives gametes T and t from each parent. The F2 combinations are TT, Tt, Tt, tt. The genotype ratio is 1 TT : 2 Tt : 1 tt, and the phenotype ratio is 3 tall : 1 short, the classic 3:1 ratio.
Test cross and the Law of Segregation
Because TT and Tt plants both look tall, a test cross with a homozygous recessive plant (tt) is used to find an unknown genotype. If any short offspring appear, the unknown parent must be heterozygous (Tt). Mendel's Law of Segregation states that the two alleles of a gene separate during the formation of gametes, so each gamete carries only one allele.
Key idea
Monohybrid cross of two heterozygotes → genotype ratio 1:2:1, phenotype ratio 3:1. A test cross of a heterozygote with a recessive gives a 1:1 ratio.
Remember
Capital letter = dominant allele; small letter = recessive allele. Homozygous means two identical alleles; heterozygous means two different alleles.