Genetics
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.