Genetics: Punnett Squares and Inheritance
Using allele combinations to predict inherited traits
Genetics: Punnett Squares and Inheritance
Using allele combinations to predict inherited traits
Biology - Grade 6-8
- 1
In pea plants, tall (T) is dominant to short (t). One parent is Tt and the other parent is tt. What are the possible genotypes of the offspring, and what fraction of the offspring are expected to be short?
Set up a 2 by 2 Punnett square using T and t from one parent and t and t from the other parent.
The possible genotypes are Tt and tt. Half of the offspring, or 1/2, are expected to be short because only tt shows the recessive short trait. - 2
A trait for purple flowers (P) is dominant over white flowers (p). Cross two heterozygous plants, Pp x Pp. What genotype ratio do you expect?
The expected genotype ratio is 1 PP : 2 Pp : 1 pp. This comes from the four possible combinations in the Punnett square. - 3
Using the cross Bb x Bb, where brown fur (B) is dominant over white fur (b), what phenotype ratio is expected in the offspring?
First find the genotypes BB, Bb, Bb, and bb.
The expected phenotype ratio is 3 brown fur : 1 white fur. Three of the four possible offspring have at least one dominant B allele, and one has bb. - 4
In rabbits, black fur (B) is dominant over brown fur (b). If a homozygous dominant rabbit is crossed with a homozygous recessive rabbit, what will the genotypes and phenotypes of the offspring be?
All of the offspring will have genotype Bb. All of the offspring will have black fur because the dominant B allele masks the recessive b allele. - 5
A plant has genotype Aa. What does this genotype tell you about the plant's alleles, and is the plant homozygous or heterozygous?
Look at whether the two letters are the same or different.
The genotype Aa means the plant has one dominant allele and one recessive allele for the trait. The plant is heterozygous because the two alleles are different. - 6
In guinea pigs, smooth coat (S) is dominant over rough coat (s). Cross Ss with ss. What percentage of the offspring are expected to have rough coats?
Fifty percent of the offspring are expected to have rough coats. The cross produces two Ss offspring and two ss offspring, so 2 out of 4 have the recessive rough phenotype. - 7
Two plants produce some offspring with a recessive trait. What must be true about the parents' alleles for that trait?
A recessive phenotype appears only when an organism has two recessive alleles.
Each parent must carry at least one recessive allele for that trait. An offspring can only show a recessive trait if it inherits a recessive allele from both parents. - 8
In cats, long hair (l) is recessive to short hair (L). A short-haired cat has genotype Ll. Another short-haired cat also has genotype Ll. What fraction of their kittens are expected to have long hair?
One fourth, or 1/4, of the kittens are expected to have long hair. The cross Ll x Ll gives LL, Ll, Ll, and ll, and only ll has long hair. - 9
A Punnett square shows the possible allele combinations from a genetic cross. What information does it help predict, and what can it not tell with certainty?
Think about the difference between probability and a guaranteed result.
A Punnett square helps predict the possible genotypes and phenotypes of offspring and the probability of each outcome. It cannot tell with certainty the exact traits of every individual offspring. - 10
In a flower species, red color (R) is dominant over yellow color (r). If one parent is RR and the other is Rr, what percentage of offspring are expected to have red flowers?
One hundred percent of the offspring are expected to have red flowers. The possible genotypes are RR and Rr, and both include the dominant R allele. - 11
Explain the difference between genotype and phenotype using the example Ee for a trait where E is dominant.
One word describes the genetic makeup, and the other describes the visible trait.
Genotype is the allele combination an organism has, such as Ee. Phenotype is the observable trait that results, and with a dominant E allele present, the organism shows the dominant phenotype. - 12
A student says that if a dominant allele is present, the recessive allele disappears. Is this correct? Explain your answer.
This is not correct. A recessive allele does not disappear when a dominant allele is present. It is still part of the organism's genotype, but its effect is masked in the phenotype by the dominant allele.