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Chapter 4: Problem 7

The trait of medium-sized leaves in iris is determined by the genetic condition \(P P\) '. Plants with large leaves are \(P P\), whereas plants with small leaves are \(P^{\prime} P^{\prime} . A\) cross is made between two plants each with medium-sized leaves. If they produce 80 seedlings, what would be the expected phenotypes, and in what numbers would they be expected? What is the term for this allelic relationship?

Short Answer

Expert verified
Answer: The expected phenotypes and their numbers in the offspring are 20 seedlings with large leaves, 40 seedlings with medium-sized leaves, and 20 seedlings with small leaves. The term for this allelic relationship is "incomplete dominance."

Step by step solution

01

Set up the Punnett Square

Create a 2x2 Punnett Square grid and label each row and column with the respective alleles from each parent plant. In our case, the parent plants are both heterozygous (\(P P'\)), so we can label our square as follows: \begin{tabular}{c c|c c} & & P & P' \\ \hline P & & & \\ P' & & & \\ \end{tabular}
02

Fill the Punnett Square

Fill in the boxes of the square by combining one allele from each parent plant, considering the genetic condition of the offspring. We should get the following completed Punnett Square: \begin{tabular}{c c|c c} & & P & P' \\ \hline P & PP & PP' \\ P' & PP' & P'P' \\ \end{tabular}
03

Interpret the Punnett Square

Based on the Punnett Square, we can see there are three different genetic combinations in the offspring: 1. \(PP\) - Large leaves (25% of the offspring) 2. \(PP'\) - Medium leaves (50% of the offspring) 3. \(P'P'\) - Small leaves (25% of the offspring) The percentages can be found by dividing the number of each genotype by the total number of offspring (4 in this case).
04

Calculate the Expected Number of Offspring

We are given that they produce 80 seedlings. To find the expected number of offspring with each phenotype, multiply the percentage of each phenotype by the total number of seedlings: 1. Large leaves: \(0.25 * 80 = 20\) seedlings 2. Medium leaves: \(0.50 * 80 = 40\) seedlings 3. Small leaves: \(0.25 * 80 = 20\) seedlings
05

Determine the Term for the Allelic Relationship

The term for this type of allelic relationship is "incomplete dominance." In incomplete dominance, neither of the alleles is completely dominant over the other, resulting in a heterozygous individual showing a blend of the traits associated with each allele. In our example, the medium-sized leaves are the result of the blended traits from the large (\(P P\)) and small (\(P' P'\)) leaf alleles.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Genetic Traits
Genetic traits are characteristics or features that are passed down from one generation to the next through genes. These traits can include physical attributes like height, leaf size, or hair color.
In the world of genetics, understanding how these traits are inherited is key to predicting how offspring will look or behave. When discussing plants, like irises, genetic traits are often explained using alleles, which are different versions of a gene. For example, in this exercise, leaf size is determined by the alleles represented as \(P\) for large leaves and \(P'\) for small leaves.
  • Genes come in pairs, and each parent contributes one allele for each trait.
  • Depending on which alleles are passed down, offspring can inherit different traits.
Predicting these traits often involves using tools like Punnett Squares, which help visualize how alleles combine from the parents to the offspring.
Incomplete Dominance
Incomplete dominance is a fascinating concept in genetics where the traits of the offspring are not simply "dominant" or "recessive" but a mix.
In an incomplete dominance scenario, like with the irises in our problem, the heterozygous combination of alleles produces a third, blended trait. In our example:
  • The \(PP\) genotype leads to large leaves.
  • The \(P'P'\) genotype gives small leaves.
  • However, when the alleles \(P\) and \(P'\) come together (\(PP'\)), they result in medium-sized leaves.
Having a spectrum of trait expressions due to incomplete dominance shows how complex and interesting genetics can be.
It also highlights that not all genetic traits follow the simple "either/or" patterns often discussed, instead, they can express more subtle "in-between" forms.
Heterozygous
The term heterozygous refers to having two different alleles for a specific gene. This is an important concept in genetics because it influences how traits are inherited.
In the given exercise with iris plants, each parent was heterozygous, denoted as \(PP'\).
  • "Hetero" means different, so heterozygous organisms have one allele of each type.
  • This can lead to unique characteristics, especially in cases of incomplete dominance.
When a plant inherits alleles \(P\) and \(P'\) from its parents, it expresses a trait that is not seen in either parent if they were both homozygous for another trait.
This not only makes for a diverse set of traits within a species but also allows for interesting variations in offspring, as seen with the medium-sized leaves produced in this example.

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Most popular questions from this chapter

In a cross in Drosophila involving the X-linked recessive eye mutation white and the autosomally linked recessive eye mutation sepia (resulting in a dark eye), predict the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of crossing true-breeding parents of the following phenotypes: (a) white females \(\times\) sepia males (b) sepia females \(\times\) white males Note that white is epistatic to the expression of sepia.

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In humans, the \(A B O\) blood type is under the control of autosomal multiple alleles. Color blindness is a recessive X-linked trait. If two parents who are both type \(A\) and have normal vision produce a son who is color-blind and is type \(\mathrm{O},\) what is the probability that their next child will be a female who has normal vision and is type \(\mathrm{O} ?\)
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