A monohybrid cross involves a single characteristic that is controlled by one gene with two or more alleles. The most commonly used example for monohybrid crosses is Mendel's experiments on pea plants (Pisum sativum) involving tall and dwarf specimens.

The letter T represents the dominant allele (tall) and the letter t represents the recessive allele (dwarf). A homozygous dominant specimen would have a genotype of TT, whereas a homozygous recessive plant would be tt. Being homozygous, the gametes of each parent would be the same (T or t, depending on the specimen) and therefore the cross would be written as follows:

       t       t

T    Tt     Tt

T    Tt     Tt

The resulting offspring are all heterozygous in genotype (Tt) but, since the dominant allele expresses the tall phenotype, all appear identical to the homozygous dominant parent (TT).

If these heterozygous offspring were interbred, the result would be somewhat different:

       T       t

T    TT     Tt

t    Tt       tt

The resulting offspring include two heterozygous specimens and two homozygous (dominant and recessive), although the phenotype ratio is 3:1 in terms of tall:dwarf.

In this way, it is simple to calculate that TT X Tt would result in two dominant homozygous (TT) and two heterozygous (Tt), and that Tt X tt would result in two recessive homozygous (tt) and two heterozygous (Tt).

Source: Notes from A level Biology

See also dihybrid crosses, although I may not get around to it for a while.

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