Monohybrid Crosses, Dominance and Segregation

Genes, alleles and genotypes

A diploid pea plant carries two homologous chromosomes for each chromosome pair. At a gene locus, one allele lies on each homologue. We write T for the allele associated with tallness in Mendel's example and t for its alternative. TT and tt are homozygous genotypes, because both copies match; Tt is heterozygous. Genotype describes the inherited alleles; phenotype describes the observable expression. A tall plant can be TT or Tt, while a dwarf plant is tt under this simple model. These symbols are conventions tied to a particular character, not names for the chromosomes themselves. Calling T 'dominant' describes the heterozygote's phenotype, not a greater probability of inheritance.

Following the F1 generation

Cross a true-breeding tall plant (TT) with a true-breeding dwarf plant (tt). During meiosis, each TT parent gamete receives T and each tt gamete receives t. Fertilisation combines those alleles to produce Tt zygotes. All F1 plants are tall because T is completely dominant to t in this defined character. The dwarf-associated allele has not vanished; it remains present in heterozygotes. This is the critical distinction between expression and transmission. The F1 phenotype cannot alone reveal whether Tt offspring are genetically identical to the tall TT parent; both are tall but have different gamete-producing possibilities.

Segregation produces the F2 pattern

When Tt plants form gametes, the two alleles segregate. Under normal meiosis, half of the gametes bear T and half bear t, in expectation. On self-pollination, each gamete type can unite with either type. The four equally likely combinations are TT, Tt, tT and tt; Tt and tT are the same diploid genotype. Their probabilities are 1/4 TT, 1/2 Tt and 1/4 tt. Thus the genotypic ratio is 1:2:1. With complete dominance, TT and Tt are tall, so the phenotypic ratio is 3 tall:1 dwarf. The 3:1 ratio depends on this specific dominance model, random combination and equal gamete frequencies.

The law of dominance and its limits

The principle of dominance summarises what happens when two different alleles are present and one determines the observed trait under specified conditions. It explains why all of the original F1 peas were tall and why a recessive phenotype reappeared in F2. It is not a universal rule that an allele always masks another. Later examples include incomplete dominance, in which the heterozygote is intermediate, and codominance, in which both allele products are detectable. Dominance is not evidence that a trait is more common, healthier or biologically superior. It is a relationship between alleles for an explicitly defined phenotype.

The law of segregation is more general

Segregation means that, during gamete formation, members of an allele pair separate so that a normal haploid gamete receives one allele at the locus. This occurs because homologous chromosomes separate in meiosis I. Fertilisation restores the diploid pair. Heterozygotes ordinarily generate two allele types, while homozygotes generate one. Segregation still operates in incomplete dominance and codominance, where the F2 phenotype ratio may be 1:2:1 instead of 3:1. The segregation rule concerns transmission; dominance concerns phenotype. Confusing the two makes it impossible to interpret unusual F2 patterns correctly.

A test cross reveals a hidden genotype

A tall plant could be TT or Tt. Cross it with a dwarf tt plant, which can provide only t gametes. If the tall plant is TT, all offspring are Tt and tall. If it is Tt, its T and t gametes yield equal probabilities of tall Tt and dwarf tt offspring. Therefore a 1:1 phenotype ratio supports heterozygosity, while only tall offspring support homozygosity but never prove it conclusively from a small sample. For example, four tall offspring from a Tt × tt cross have probability (1/2)^4 = 1/16. A test cross is an inference tool, not an infallible label.

How to read a Punnett square

Place the gamete types from one parent along columns and those from the other parent along rows. In each cell, combine one allele contribution from each parent. Each cell represents a possible fertilisation outcome; if all gamete combinations are equally likely, cell counts become probabilities. For Tt × Tt, the square contains one TT, two Tt and one tt cell. The square predicts offspring, not gamete frequencies by itself; those frequencies come from meiosis. If gametes are unequal or loci linked, identical-sized boxes need not all have equal probabilities.

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