Question

A third grader decided to breed guinea pigs for her school science project. She went to a pet store and bought a male with smooth black fur and a female with rough white fur. She wanted to study the inheritance patterns of these traits. A cross between these two parents resulted in two litters with a total of 15 F1 progeny, all of which were rough and black. Soon, the F1 litters began to produce F2 progeny, and before long the child had 125 F2 guinea pigs. 8 had smooth white coats, 25 had smooth black coats, 23 were rough and white, and 69 were rough and black. Draw the Punnett Square for the cross between the rough black F1s. How are the coat color and texture traits inherited

Answer 1

Answer:

The F₁ generation had all black and rough fur. Thus, black is dominant over white and rough is dominant over smooth.

Let us consider the two genes B and R for fur color and quality of fur respectively.

In gene B, B is the dominant allele which codes for black color and b is a recessive allele which codes for white color (when present in homozygous condition).

Similarly, in gene R, R is a dominant allele which codes for roughness and r being a recessive allele which codes for smooth fur (when present in homozygous condition).

So, the genotype of parents would be rrBB (smooth and black fur) and RRbb(rough and white fur).

The cross would result in the production of offspring all with rough and black fur that is, RrBb.

The selfing of F₁ generation would result in the production of offspring with four types of phenotypes; smooth white coats, smooth black coats, rough and white, and rough and black.

It shows that the alleles for the coat color and texture traits are inherited independently of each other.

The same is explained with the help of Punnett square in the below attachment.

Answer 2

In complete dominance cases heter0zyg0us individuals express the dominant phenotype because the dominant allele masks the expression of the recessive one. Coat color and texture are inherited by complete dominance.

What is a complete dominance?

When the dominant allele completely masks the recessive allele, we talk about complete dominance.

This is the case of individuals that are heter0zyg0us for a particular gene and express the dominant trait.

The dominant allele is hiding the expression of the recessive allele.

Available data:

• male with smooth black fur
• female with rough white fur
• F1) 15 rough and black individuals
• F2) 8 had smooth white coats, 25 had smooth black coats, 23 were rough and white, and 69 were rough and black.

We know that the whole F1 is composed of rough and black individuals. These results suggest that these are the dominant phenotypes, and that both parents are h0m0zyg0tes for both traits.

The results also suggests that the inheritance pattern is complete dominance.

We will assume these traits are coded by single diallelic genes.

• R is the dominant allele and codes for rough
• r is the recessive allele and codes for smooth
• B is the dominant allele and codes for black
• b is the recessive allele and codes for white

1st cross:

Parentals)       rr BB         x        RR bb

Gametes) rB, rB, rB, rB         Rb, Rb, Rb, Rb

F1) Genotype: 100% dihybrid individuals, RrBb

     Phenotype: 100% rough and black individuals.

2nd Cross:

Parentals)    RrBb        x         RrBb

Gametes) RB, Rb, rB, rb    RB, Rb, rB, rb

Punnett square)    RB          Rb           rB           rb

                   RB      RRBB     RRBb      RrBB      RrBb

                   Rb      RRBb      RRbb      RrBb      Rrbb

                   rB       RrBB       RrBb       rrBB       rrBb

                    rb       RrBb       Rrbb       rrBb       rrbb

F2)  Genotype ⇒ among the progeny it is expected to find

• 1/16 RRBB
• 2/16 RRBb
• 1/16 RRbb
• 2/16 RrBB
• 4/16 RrBb
• 2/16 Rrbb
• 1/16 rrBB
• 2/16 rrBb
• 1/16 rrbb

       

      Phenotypes

If the mode of inheritance is by complete dominance, then, the expected phenotypic ratio is 9:3:3:1.

• 9/16 R-B- Rough and black
• 3/16 R-bb rough and white
• 3/16 rrB- Smooth and black
• 1/16 rrbb smooth and white

Let us see the observed phenotypic ratios.

125 individuals in the F2  -------------- 16 possible combinations

8 smooth white ---------------------------- X = 1

25 smooth black -------------------------- X = 3.2 ≅ 3

23 rough and white -----------------------X = 2.9 ≅ 3

69 rough and black ----------------------- X = 8.83 ≅ 9

So, the observed phenotypic ratio is very close to 9:3:3:1 as it was expected to.

We can conclude that the coat color and texture traits inherited by complete dominance.

You will learn more about complete dominance at

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