Genetics
A
dihybrid cross involves two
characteristics that are
controlled by two separate
genes with two or more
alleles. The genes are located on different
chromosomes and the alleles are therefore not necessarily
inherited together. When the same alleles for two or more genes are inherited together (such as the gene for high production of
ear wax and the one for strong-smelling
sweat), this is known as
Autosomal Linkage and is
calculated using the
Mendellian ratios for a
monohybrid cross.
The process for a dihybrid cross is essentially the same as for the
monohybrid cross, except that two
separate crosses are occuring at the same moment, i.e. a cross occurs with more than one
characteristic. The typical example brings us back to
Gregor Mendel and his
pea plants (see explanation on
monohybrid cross), except that we are investigating
height (
T/
t) and
also seed
shape (
R/
r).
Once again, the letter
T represents the
dominant allele (tall) and the letter
t represents the
recessive allele (dwarf). Similarly, the letter
R represents the
dominant allele of another gene (round seed) and the letter
r represents the
recessive allele (
wrinkled seed). A
homozygous dominant specimen would therefore have a
genotype of
TTRR, whereas a homozygous recessive
plant would be
ttrr. Being homozygous, the
gametes of each
parent would be the same (
TR or
tr, depending on the specimen) and therefore the cross would be
written as follows:
tr tr
TR TtRr TtRr
TR TtRr TtRr
The resulting
offspring are all
heterozygous in genotype (
TtRr) but, since the dominant allele
expresses the tall and round-seeded
phenotype, all appear
identical to the homozygous dominant parent (
TTRR).
If these heterozygous offspring were
interbred, the result would be
somewhat different. Firstly, the gametes will not be identical, since there are four different
combinations: the true dominant (
TR), the true recessive (
tr) and the two heterozygous combinations (
Tr and
Rt). This cross is calculated like so:
TR Tr tR tr
TR TTRR TTRr TtRR TtRr
Tr TTRr TTrr TtRr Ttrr
tR TtRR TtRr ttRR ttRr
tr TtRr Ttrr ttRr ttrr
The resulting
offspring are quite a
mixture of alleles, but there are clearly two homozygous
specimens (
TTRR and
ttrr) and four identical to the parents (
TtRr). The
remaining offspring are of various combinations, but the
phenotype ratio is 9:3:3:1. There are nine of the dominant combination (tall with round seeds), one true recessive (dwarf with wrinkled seeds) and three of each of the other combinations (tall with wrinkled seeds, dwarf with round seeds).
As with the
monohybrid cross, when the heterozygous
form (
TtRr) is
crossed with either homozygous form (
TTRR or
ttrr), the ratio is 1:1 (or rather, since there are four combinations and phenotypes, the ratio is 1:1:1:1). These are the same combinations as mentioned in the above
paragraph, but in equal
amounts.
Source: Notes from A level Biology