Question

In a monohybrid cross between heterozygous individuals, three out of four offspring will display the (blank)
phenotype. In Mendel’s experiments, this monohybrid cross always resulted in a(n) (blank)
ratio of dominant to recessive phenotypes. In a dihybrid cross between individuals that are heterozygous for each trait, the four possible offspring phenotypes will be observed in the ratio of (blank)
.

Answer 1

The answers would be:

First blank: 3:1

Second blank: 9:3:3:1

Let's try and see how a Punnet Square would look like for each scenario:

MONOHYBRID

Rr x Rr          R - Dominant                  r- recessive

      R      r     

R   RR    Rr                       Genotype:                                Phenotype                  

r    Rr     rr                       RR (1)                                        Dominant Trait

                                       Rr   (2)                                       Dominant Trait

                                       rr    (1)                                        Recessive Trait

As you can see, you have 3 results out of 4 that will come out as domimant phenotypes, and 1 out of 4 as Recessive Phenotype. We can therefore conclude that the ratio of dominant to recessive is 3:1.

DIHYBRID

RrYy x RrYy

            RY           Ry           rY        ry

RY      RRYY      RRYy     RrYY     RrYy

Ry      RRYy       RRyy     RrYy      Rryy

rY       RrYY       RrYy      rrYY      rrYy

ry       RrYy        Rryy      rrYy       rryy

Breaking it down to genotype and phenotype:

Genotype                  Phenotype

RRYY (1)              Dominant:Dominant

RRYy  (2)             Dominant:Dominant

RrYY  (2)             Dominant:Dominant

RrYy   (4)             Dominant: Dominant

RRyy  (1)              Dominant:Recessive

Rryy   (2)             Dominant:Recessive

rrYY   (1)              Recessive: Dominant

rrYy   (2)              Recessive: Dominant

rryy   (1)               Recessive:Recessive

Let's look at the four possible phenotype expressions based on our breakdown and count how many combinations we see:

Dominant:Dominant             9  

Dominant:Recessive            3

Recessive:Dominant            3

Recessive:Recessive           1

So the ratio would be:

9 : 3 : 3 : 1