Question

Probability and Heredity

Answer 1

Answer and explanation:

1. Punnett suqare A

The crossing of the Guinea Pigs in case A is Bb X Bb, both black, since the dominant allele (B) is black, while the recessive allele (b) is white. The corresponding Punnett square A would be:

Bb X Bb

Alelles      B         b

B             BB       Bb

b             Bb       bb

2. Punnett square B

In this case, the crossing occurs between Bb and bb guinea pigs, phenotypically a (heterozygous) black guinea pig and a (homozygous) white guinea pig.

Bb X bb

Alelles      B         b

b              Bb      bb

b              Bb      bb

3. Punnett Square A shows a cross between two black guinea pigs.

What is the probability that an offspring will be black? White?

Two black guinea pigs, known heterozygotes (Bb), will have a 75% chance that the offspring will be black, and a 25% chance that it will be white.

Offspring based on Punnett square A:

• BB homozygous black guinea pigs: 25%
• Bb heterozygous black guinea pigs: 50%
• bb homozygous white guinea pigs: 25%.

25% BB + 50% Bb = 75% black guinea pigs.

4. What color are the parents shown in Punnett Square B?

Given that the dominant allele is the one that gives the color black and the recessive one the white color, the parents at the Punnett square b crossing are:

• Bb (heterozygous) phenotype: black
• bb (homozygous recessive) phenotype: white.

The dominant alleles are those that, when present, determine the dominant trait -in this case the black color- preventing the recessive trait from manifesting.

5. Which guinea pig parent(s) in Punnett Square B is homozygous?  Which is heterozygous? Explain how you know

According to Punnett square B, the homozygous guinea pig is the white guinea pig, whose genotype is bb, while the heterozygous is the black guinea pig, with the Bb genotype.

To determine when a trait appears homozygous or heterozygous, one must observe the pair of alleles that determine a trait.

• A homozygous genotype has two equal alleles .
• In heterozygous genotype alleles are different.

In Mendelian inheritance, the heterozygous genotype always implies that the phenotype displays the dominant trait, and is considered a hybrid for that trait.

6. What is the probability that an offspring will be black in the cross  shown in Punnett Square B? What is the probability that an offspring  will be white?

According to Punnett square B, where a heterozygous black guinea pig and a homozygous recessive white guinea pig cross, the offspring can be:

• BB homozygous black guinea pigs: None
• Bb heterozygous black guinea pigs: 50%
• bb homozygous white guinea pigs: 50%

The probability that the offspring is black guinea pigs is 50% (heterozygous Bb), while the probability that the offspring is white guinea pigs is 50% (recessive homocytes).