Gen. Biology Notes ( C-9)
Objectives:
1.
Distinguish between monohybrid and dihybrid problem types.
2.
Articulate how possible combinations of alleles from both parents are potentially passed on to
offspring.
3.
Decide likely hood of achieving a specific genotype or phenotype from a given di-hybrid cross.
Di-hybrid Crosses
Up until this point, we have worked problems that only deal with one gene (trait) and two alleles. It is
possible to track outcomes for multiple traits with two alleles. Whenever two traits are
examined simultaneously, we consider the case to be a di-hybrid cross. Three traits would be a
tri-hybrid cross. Consider the following example key:
F= flat nose
f=long nose
R= red eyes
r= green eyes
The two traits we examine simultaneously are nose shape and eye color. Note that these two traits
have just two alleles each. The genotypes of parents then would contain FOUR alleles and not
two as in the monohybrid crosses. A heterozygous male for nose shape and homozygous,
recessive for eye color would have the genotype of Ffrr. If we crossed this male with a female
that was heterozygous for both traits the cross would look like this. FfrrxFfRr.
Because we have four alleles for each genotype the punnet square grows from 4 to 16 boxes. What
would the square size look like if you were performing a trihybrid cross?
It is very important to remember that each Parent only gets to donate one chromosome with one allele
on it for each trait to their respective sperm or egg cell. So, the male can have four possible
contributions of alleles to sperm cells and they are as follows: Fr, Fr, fr, and fr. Likewise, the
female can have four possible contributions of these two chromosomes to an egg cell and they
are as follows: FR,Fr,fR and fr. We would work this problem just like the monohybrid from this
point forword. See below for example:
Fr
FR
Fr
FFRr
fr
fr
FFRr
Fr
fR
fr
If you finish the square you can see that each potential offspring has four alleles in their genotype just
like their parents. From here we write the genotypes and phenotypes in proportional form
which is different from the monohybrids where we used a ratio form.
So we would simple count how many out of 16 had the genotype FFRr and then convert to percentage
to report the chance of a child having that genotype. The same goes for the phenotypes. How
many kids out of 16 are flat nosed and have red eyes and so on and so forth?