Middle School Life Science : Use Punnett Squares
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Example Questions
Example Question #1 : Punnett Squares
The allele for blue eyes is "t" while the allele for brown eyes is "T". What percentage of offspring produced by two parents with blue eyes would also have blue eyes?
50%
0%
100%
25%
100%
The answer is 100%. Since both parents had to have homozygous recessive alleles for blue eyes in order to express them, they both must have "tt" for a genotype. The following punnet square shows the results.
Example Question #1 : Punnett Squares
The allele for longer necks in giraffes is dominant to the allele for shorter necks.
What are the phenotypes of the cross between two homozygous parents, one with a long neck and one with a short neck?
50% long necks and 50% short necks
75% long necks and 25% short necks
100% long necks
100% short necks
100% long necks
The answer is 100% long necks. A homozygous dominant crossed with a homozygous recessive parent is shown below.
Example Question #1 : Punnett Squares
Brown fur is dominant to white fur in a species of rabbit and is represented with the alleles "B" and "b".
What are the resulting percentages of genotypes of a cross between a homozygous dominant parent and a heterozygous parent?
100% BB
50% BB, 25% Bb, 25% bb
50% BB and 50% Bb
75% BB and 25% Bb
50% BB and 50% Bb
The answer is 50% BB and 50% Bb, as shown in the punnet square below. The phrase "homozygous dominant" means the genotype of one parent is BB while "heterozygous" is a genotype of Bb.
Example Question #1 : Use Punnett Squares
The parents genotypes from this cross are ______.
homozygous dominant and heterozygous
homozygous recessive and heterozygous
homozygous dominant and homozygous recessive
two heterozygous
homozygous recessive and heterozygous
The answer is "homozygous recessive and heterozygous" because homozygous means two of the same allele, while heterozygous means two different alleles. Dominant alleles are represented by capital letters and recessive alleles are represented by lowercase letters.
Example Question #1 : Use Punnett Squares
The allele for black feet in a species of duck is dominant to the allele for brown feet. What are the phenotypes of the offspring from the cross shown in the punnet square above?
75% B and 25% b
25% BB, 25% bb, and 50% Bb
75% black feet and 25% brown feet
25% black feet and 75% brown feet
75% black feet and 25% brown feet
The answer is "75% black feet and 25% brown feet" because phenotypes are the physical expression of an allele pair and the dominant allele for black feet will overpower the allele for brown feet.
Example Question #5 : Punnett Squares
In butterflies, the gene for black wings (B) is dominant to the gene for blue wings (b). For which of the following parental genotypes would 100% of offspring be expected to be blue?
BB x bb
Bb x Bb
BB x BB
bb x bb
bb x bb
The answer is "bb x bb" because in all the other scenarios, the black gene would be dominant over the blue gene in at least one offspring.
Example Question #6 : Punnett Squares
In poodles, the allele for black fur (B) is dominant to the gene for brown fur (b). Two poodles are crossed. One poodle is homozygous for black fur, and the other is heterozygous. What percentage of the offspring are expected to have black fur?
75%
100%
0%
50%
25%
100%
The answer is 100%. All offspring are expected to have black fur because all offspring will have at least one dominant allele for black fur which will overpower any allele for brown fur.
Example Question #7 : Punnett Squares
Polycystic Kidney Disease (PKD) is a disease that can cause kidney failure. This disease is usually caused by a dominant allele. What is the chance that a child will have PKD if the father is unaffected and the mother is heterozygous for PKD?
75%
0%
50%
25%
100%
50%
The answer is "50%."
Example Question #8 : Punnett Squares
A dog gives birth to 5 puppies. What percentage of chromosomes does each puppy share with its mother?
50%
75%
100%
25%
50%
The answer is 50% because each puppy shares 50% of chromosomes with its mother and 50% with its father.
Example Question #1 : Punnett Squares
In a population of rats at a pet store, the allele for black fur is dominant (B) and the allele for brown fur is recessive (b). A rat with the genotype BB is crossed with a rat with the genotype Bb. Which describes the phenotype of the parent rats?
Both are black.
One is black and one is black with brown spots.
Both are brown.
One is black and one is brown.
Both are black.
The answer is "Both are black." because each rat has a dominant allele for black fur.
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