What fraction of the progeny of the cross BbTt × BBtt will have black fur and long tails?
Understand the Problem
The question is asking us to determine the genetic outcomes of a cross between two types of mice based on their fur color and tail length. We will use a Punnett square to find the fraction of the progeny that will have black fur and long tails.
Answer
1
Answer for screen readers
The fraction of the progeny that will have black fur and long tails is $1$ or $100%$.
Steps to Solve
- Identify the alleles First, we need to identify the alleles controlling fur color and tail length in the mice. Let's say:
- Black fur (B) is dominant over brown fur (b).
- Long tail (L) is dominant over short tail (l).
- Determine the genotypes of the parents Assuming the two parent mice have the following genotypes:
- Parent 1: homozygous for black fur and homozygous for long tail, which is $BBLL$.
- Parent 2: homozygous for brown fur and homozygous for short tail, which is $bbll$.
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Set up the Punnett Square for the fur color and tail length We will create a Punnett square for both traits (fur color and tail length). The gametes from Parent 1 are $BL$ and the gametes from Parent 2 are $bl$.
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Fill the Punnett Square The Punnett square will look like this:
[ \begin{array}{c|c|c} & BL & BL \ \hline bl & BbLl & BbLl \ bl & BbLl & BbLl \ \end{array} ]
Here, all the offspring have the genotype $BbLl$, which indicates that they all have black fur and long tails.
- Determine the fraction of progeny with black fur and long tails Since all the progeny have the $BbLl$ genotype, the fraction of offspring with black fur and long tails is:
$$ \text{Fraction} = 1 \text{ or } \frac{4}{4} $$
The fraction of the progeny that will have black fur and long tails is $1$ or $100%$.
More Information
In this genetic cross, since one parent is homozygous dominant for both traits and the other is homozygous recessive, all offspring will inherit the dominant traits, resulting in 100% of the progeny displaying black fur and long tails.
Tips
- Confusing dominant and recessive alleles: Always remember that a dominant allele will express its trait over a recessive one.
- Failing to set up the Punnett square accurately: Make sure to carefully fill in all combinations of alleles from the parents.
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