Genetics: Dominant and Recessive Traits
Using alleles and Punnett squares to predict inherited traits
Genetics: Dominant and Recessive Traits
Using alleles and Punnett squares to predict inherited traits
Biology - Grade 6-8
- 1
In pea plants, the allele for purple flowers, P, is dominant over the allele for white flowers, p. What flower color would a plant with the genotype Pp have? Explain your answer.
A dominant allele shows its trait when at least one copy is present.
A plant with the genotype Pp would have purple flowers because the dominant allele P masks the recessive allele p. - 2
A pea plant has the genotype pp for flower color, where P means purple flowers and p means white flowers. What is the plant's phenotype?
The plant's phenotype is white flowers because it has two recessive alleles and no dominant allele. - 3
For a trait, B represents a dominant allele for brown eyes and b represents a recessive allele for blue eyes. List the possible genotypes for a person with brown eyes.
The dominant trait can appear with one or two dominant alleles.
The possible genotypes for a person with brown eyes are BB and Bb because both contain at least one dominant B allele. - 4
Use a Punnett square to predict the offspring from a cross between two heterozygous tall pea plants, Tt and Tt. Tall, T, is dominant over short, t. What are the possible genotypes and their ratios?
The possible genotypes are TT, Tt, Tt, and tt. The genotype ratio is 1 TT to 2 Tt to 1 tt. - 5
Using the Tt by Tt cross from the previous problem, what percent of the offspring are expected to be tall?
Count the boxes with at least one capital T.
Seventy-five percent of the offspring are expected to be tall because TT and Tt plants show the dominant tall trait. - 6
A rabbit's black fur allele, F, is dominant over its white fur allele, f. A rabbit with genotype Ff is crossed with a rabbit with genotype ff. Complete the cross and state the expected phenotypes of the offspring.
The expected offspring genotypes are Ff, Ff, ff, and ff. About 50 percent are expected to have black fur and 50 percent are expected to have white fur. - 7
Explain the difference between genotype and phenotype using the example of pea plant height.
Genotype refers to genes, and phenotype refers to the physical trait you can observe.
A genotype is the allele combination, such as TT, Tt, or tt. A phenotype is the visible trait, such as tall or short height. - 8
In a certain plant, smooth seeds, S, are dominant over wrinkled seeds, s. A student observes a plant with smooth seeds. Can the student know for sure whether the plant's genotype is SS or Ss just by looking? Explain.
The student cannot know for sure just by looking because both SS and Ss genotypes produce the smooth seed phenotype. - 9
A dog has the genotype hh for a recessive coat pattern. What must be true about the dog's parents' alleles for this trait?
An offspring with hh must receive one h allele from each parent.
Each parent must have passed one recessive h allele to the dog. Each parent could be Hh or hh for this trait. - 10
A chart shows four offspring from a cross: Aa, Aa, aa, and aa. If A is dominant and a is recessive, what percent of the offspring show the dominant phenotype and what percent show the recessive phenotype?
Fifty percent of the offspring show the dominant phenotype because Aa has the dominant allele. Fifty percent show the recessive phenotype because aa has only recessive alleles. - 11
For a certain flower color trait, red, R, is dominant over white, r. Write the genotype of a plant that is homozygous dominant and the genotype of a plant that is homozygous recessive.
Homozygous means the two alleles are the same.
A homozygous dominant plant has the genotype RR. A homozygous recessive plant has the genotype rr. - 12
A flower with genotype Rr is described as heterozygous. What does heterozygous mean?
Heterozygous means an organism has two different alleles for a gene, such as R and r. - 13
A student says, "A dominant trait is always more common in a population than a recessive trait." Is the student correct? Explain.
Dominance describes how alleles interact, not how often they appear.
The student is not correct. Dominant means the allele can mask another allele, but it does not always mean the trait is more common in a population. - 14
In mice, gray fur, G, is dominant over white fur, g. Two gray mice have some offspring with white fur. What are the most likely genotypes of the two gray parent mice?
The most likely genotypes of the two gray parent mice are Gg and Gg because each parent must carry a recessive g allele to produce a white gg offspring. - 15
A plant breeder crosses a purebred tall pea plant, TT, with a purebred short pea plant, tt. Tall, T, is dominant over short, t. What genotype and phenotype are expected for all first-generation offspring?
Each offspring gets one T allele from the tall parent and one t allele from the short parent.
All first-generation offspring are expected to have the genotype Tt. All are expected to have the tall phenotype because T is dominant over t.