In this video we are going to look at what a dihybrid cross is.
A dihybrid cross is when we are looking at two trait traits.
For example if we have a plant and look at two traits such flower colour and stem length.
The flower colour trait has two possible alleles, a dominant big R that codes for red flowers,
and a recessive little r that codes for white flowers.
The stem length trait also has two possible alleles, a dominant big T that codes for tall
stems, and a recessive little t that codes for short stems.
Genotype refers to the genetic make-up, in other words the specific genes an individual
has.
There are 9 genotypes when looking at these two traits together; RRTT, RRTt, RRtt, RrTT,
RrTt, Rrtt, rrTT, rrTt, rrtt.
Even though there are 9 possible genotypes, there are only four phenotypes.
Phenotypes refer to the physical expression of the genes in an individual.
For the traits of flower colour and stem length, you can have plants with red flowers and tall
stems, plants with red flowers and short stems, plants with white flowers that are tall and
plants with white flowers that are short.
For a dihybrid cross we want to cross two individuals based on these two traits.
For example we could do a dihybrid true breeding cross of two homozygous parents plants (this
is known as the P or parental generation).
We would be crossing one that is homozygous dominant for both traits (RRTT) so a plant
that has red flowers that is tall, with one that is homozygous recessive for both traits
(rrcc), so a plant that has white flowers that is also short.
This is when a punnet square is utilised.
For a dihybrid cross, the punnett square is going to be four by 4 and have 16 squares.
Each combination of alleles from one of the parents plants is put along the top of the
square.
So for the first parent the first big R with the first big T go over the first square,
then this first big R with the second big T go over the second, then the second big
R with the first big T go over the third square and the second big R with the second big T
go over the fourth.
Then for the second parent plant the same thing, with each combination going along the
side of the punnett square.
Then to fill in the punnett square you take the alleles from the corresponding top and
side, keeping the r alleles first and capitals first.
This gives you your potential offspring from this cross.
In this cross all the potential offspring have the same genotypes, they are all heterozygous
for both traits, (RrTt) and all have the same phenotype of having red flowers and tall stems.
The offspring of this first cross would be considered the F1 generation.
We could then cross two individuals from this F1 generation in another dihybrid cross where
we would be crossing two plants that are heterozygous for both traits (RrTt x RrTt)
Again we have a punnet square with 8 squares.
Each combination of alleles from one of the parents plants are put along the top of the
square.
The same happens with the second parent along the side of the punnett square.
Then for each square you take the alleles from the corresponding top and side, keeping
the r alleles and capitals first.
The offspring from this second cross would be considered the F2 generation.
This time we get 9 different genotypes in the offspring; 1/16 are RRTT, 2/16 are RRTt,
1/16 are RRtt, 2/16 are RrTT, 4/16 are RrTt, 2/16 are Rrtt, 1/16 are rrTT, 2/16 are rrTt
and 1/16 are rrtt.
From these we get four different phenotypes with a ratio of 9:3:3:1.
9 have red flowers and tall stems, 3 have red flowers with short stems, 3 have white
flower with tall stems and 1 has white flowers with short stems.
Thank you for watching, if you found this video helpful be sure to subscribe and hit
that like button.