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Chapter 3: Problem 21

In sheep, lustrous fleece results from an allele \((L)\) that is dominant over an allele ( \(l\) ) for normal fleece. A ewe (adult female) with lustrous fleece is mated with a ram (adult male) with normal fleece. The ewe then gives birth to a single lamb with normal fleece. From this single offspring, is it possible to determine the genotypes of the two parents? If so, what are their genotypes? If not, why not? PICTURE CANT COPY

Short Answer

Expert verified
Yes, the genotypes are ewe \( Ll \) and ram \( ll \).

Step by step solution

01

Identify Genotypes

A lustrous fleece results from the dominant allele \( L \), while a normal fleece results from the recessive allele \( l \). To exhibit the dominant trait, a sheep can be either \( LL \) (homozygous dominant) or \( Ll \) (heterozygous). A sheep with a normal fleece must be \( ll \) (homozygous recessive). The ewe with lustrous fleece can be \( LL \) or \( Ll \), and the ram with normal fleece must be \( ll \).
02

Analyze the Offspring

The lamb has a normal fleece, which means its genotype is \( ll \). This indicates it inherited one \( l \) allele from each parent. Since the ram's genotype is \( ll \), it has contributed one \( l \) allele. Therefore, the ewe must also be able to pass on an \( l \) allele, confirming that the ewe's genotype must be \( Ll \).
03

Determine Parent Genotypes

Based on the information that the offspring has a normal fleece and the ram's genotype is \( ll \), the only way for the lamb to inherit a recessive phenotype is if the ewe was heterozygous \( Ll \). Therefore, the genotypes of the ewe and ram are \( Ll \) and \( ll \) respectively.

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Key Concepts

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

Dominant and Recessive Alleles
In genetics, traits such as lustrous or normal fleece in sheep are controlled by specific alleles. An allele is a variant form of a gene and can be either dominant or recessive. Knowing the type of alleles present helps us predict the appearance of the trait in an organism.
In our case:
  • The dominant allele is represented by \( L \), which causes lustrous fleece. A single copy of this allele can express the lustrous trait.
  • The recessive allele is represented by \( l \), which results in normal fleece. This trait is expressed only when two \( l \) alleles are present.
If a sheep has at least one \( L \) allele, it will have lustrous fleece. However, a sheep will have normal fleece only if it possesses two recessive \( l \) alleles. Recognizing whether an allele is dominant or recessive is essential for predicting genetic outcomes in offspring.
Genotype Determination
To determine an individual's genotype (the combination of alleles they possess), we observe their physical traits and apply genetic principles. The example of the sheep helps illustrate these principles.
When the ewe has lustrous fleece, it can be either heterozygous (\( Ll \)) or homozygous dominant (\( LL \)). Here is how we figure out the actual genotype:
  • The ram, displaying normal fleece, must be homozygous recessive (\( ll \)).
  • Seeing that their offspring (the lamb) has normal fleece, the only possible genotype is (\( ll \)).
  • This (\( ll \)) genotype of the offspring indicates it inherited one \( l \) allele from each parent.
This concludes the ewe's genotype must be (\( Ll \)). By examination, understanding the physical traits gives us the insight to determine the underlying genotypes.
Inherited Traits
When traits are passed from parents to offspring, understanding the inheritance patterns helps predict possible outcomes. Each offspring receives an allele from each parent. For dominant traits, receiving one dominant allele is enough to display the trait, whereas recessive traits require two recessive alleles to manifest.
Consider our scenario:
  • The ewe passes a lustrous trait allele (\( L \)). However, since the lamb displays normal fleece, the ewe must have also carried the recessive allele (\( l \)).
  • The ram, having only normal fleece, always passes a recessive allele (\( l \)).
The interaction of these inherited alleles determines the trait the offspring exhibits. Understanding inherited traits allows us to map out potential genetic combinations and their implications in future generations.

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Most popular questions from this chapter

In cucumbers, dull fruit \((D)\) is dominant over glossy fruit \((d)\), orange fruit \((R)\) is dominant over cream fruit \((r),\) and bitter cotyledons \((B)\) are dominant over non-bitter cotyledons \((b) .\) The three characters are encoded by genes located on different pairs of chromosomes. A plant homozygous for dull, orange fruit and bitter cotyledons is crossed with a plant that has glossy, cream fruit and non-bitter cotyledons. The \(\mathrm{F}_{1}\) are intercrossed to produce the \(\mathrm{F}_{2}\) a. Give the phenotypes and their expected proportions in the \(\mathrm{F}_{2}\) b. An \(\mathrm{F}_{1}\) plant is crossed with a plant that has glossy, cream fruit and nonbitter cotyledons. Give the phenotypes and expected proportions among the progeny of this cross.

Two distinct phenotypes are found in the salamander Plethodon cinereus: a red form and a black form. Some biologists have speculated that the red phenotype is due to an allele that is dominant over an allele for black. Unfortunately, these salamanders will not mate in captivity, so the hypothesis that red is dominant over black has never been tested. One day, a genetics student is hiking through the forest and finds 30 female salamanders, some red and some black, laying eggs. The student places each female with her eggs (about 20 to 30 eggs per female) in a separate plastic bag and takes them back to the lab. There, the student successfully raises the eggs until they hatch. After the eggs have hatched, the student records the phenotypes of the juvenile salamanders, along with the phenotypes of their mothers. Thus, the student has the phenotypes for 30 females and their progeny, but no information is available about the phenotypes of the fathers.Explain how the student can determine whether red is dominant over black with this information on the phenotypes of the females and their offspring. PICTURE CANT COPY

Alleles \(A\) and \(a\) are located on a pair of metacentric chromosomes. Alleles \(B\) and \(b\) are located on a pair of acrocentric chromosomes. \(A\) cross is made between individuals having the following genotypes: \(A a\) \(B b \times a a b b\) a. Draw the chromosomes as they would appear in each type of gamete produced by these individuals. b. For each type of progeny resulting from this cross, draw the chromosomes as they would appear in a cell at \(\mathrm{G}_{1}, \mathrm{G}_{2},\) and metaphase of mitosis.

In the California poppy, an allele for yellow flowers \((C)\) is dominant over an allele for white flowers \((c) .\) At an independently assorting locus, an allele for entire petals \((F)\) is dominant over an allele for fringed petals (f). A plant that is homozygous for yellow and entire petals is crossed with a plant that has white and fringed petals. A resulting \(\mathrm{F}_{1}\) plant is then crossed with a plant that has white and fringed petals, and the following progeny are produced: 54 yellow and entire; 58 yellow and fringed; 53 white and entire; and 10 white and fringed. a. Use a chi-square test to compare the observed numbers of progeny having each phenotype with those expected for the cross. b. What conclusion can you draw from the results of the chi-square test? c. Suggest an explanation for the results.

Suppose that you are raising Mongolian gerbils. You notice that some of your gerbils have white spots, whereas others have solid coats. What type of crosses could you carry out to determine whether white spots are due to a recessive or a dominant allele?
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