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Chapter 7: Problem 24

In German cockroaches, bulging eyes \((b u)\) are recessive to normal eyes \(\left(b u^{+}\right)\), and curved wings \((c v)\) are recessive to straight wings \(\left(c v^{+}\right) .\) These two traits are encoded by autosomal genes that are linked. A cockroach has genotype \(b u^{+} b u c v^{+} c v\), and the genes are in repulsion. Which of the following sets of genes will be found in the most common gametes produced by this cockroach? a. \(b u^{+} c v^{+}\) b. \(b u c v\) c. \(b u^{+} b u\) d. \(c v^{+} c v\) e. \(b u c v^{+}\) Explain your answer.

Short Answer

Expert verified
The most common gametes are \( bu^{+} cv \) and \( bu \, cv^{+} \), matching option e.

Step by step solution

01

Understand the Concept of Genes in Repulsion

Genes in repulsion mean that the dominant alleles for different traits are present on different homologous chromosomes. In this case, the cockroach's genotype is organized as \( bu^{+} cv \) and \( bu \, cv^{+} \), meaning that the dominant allele \( bu^{+} \) is paired with the recessive allele \( cv \), and the dominant allele \( cv^{+} \) is paired with the recessive allele \( bu \).
02

Identify Common Gamete Types

Since the genes are linked and arranged in repulsion, the most common gametes produced will maintain this original arrangement. The two most probable gamete types will be those that reflect the parental arrangement: \( bu^{+} cv \) and \( bu \, cv^{+} \). These arrangements result from the lower likelihood of crossing over events between linked genes.
03

Match with Answer Choices

Compare the most common gamete types \( bu^{+} cv \) and \( bu \, cv^{+} \) with the provided options. \( bu \, cv^{+} \) matches option e, which correctly represents one of the most common gamete types from this cockroach.

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Key Concepts

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

Autosomal Genes
Genetic linkage is a fascinating concept, especially when dealing with autosomal genes. Autosomal genes are located on non-sex chromosomes, known as autosomes. These genes come in pairs, as organisms inherit one set from each parent. In the context of the German cockroach, the genes responsible for eye shape and wing contour are both autosomal. Unlike sex chromosomes, autosomes appear as matched pairs. This similarity implies genes they carry are inherited similarly, regardless of the sex of the offspring. It鈥檚 essential to understand that the concept of linkage especially applies to these autosomal genes when they are close together on the same chromosome.
Recessive Traits
Recessive traits are expressions that only manifest when an organism possesses two recessive alleles. For the German cockroach, both bulging eyes \( (bu) \) and curved wings \( (cv) \) represent recessive traits. Unlike dominant traits where a single dominant allele can determine the trait鈥檚 expression, recessive traits require a pair to be visible. When discussing these traits, heterozygous organisms, like our cockroach example, carry one recessive and one dominant allele for each trait. This means the dominant traits (normal eyes and straight wings) are exhibited instead of the recessive ones, unless two recessive alleles are present.
Gamete Formation
Gamete formation is a critical process ensuring genetic diversity. In organisms, cells undergo meiosis to produce gametes. For a cockroach with the genotype \( bu^{+} bu \; cv^{+} cv \), gametes are formed by copying the paired genes and randomly separating them into different cells.

Process of Gamete Formation

  • The original cellular configuration splits, with each gamete receiving one allele from each pair.
  • When genes are linked and in repulsion, like our cockroach example, the original arrangement is more likely retained.
  • Consequently, the most common gametes reflect the initial parental gene order, leading to higher occurrence of the expected pairs \( bu^{+} cv \) and \( bu \, cv^{+} \).
Chromosomal Arrangement
Understanding chromosomal arrangement provides insight into how genes are inherited. When considering linkage, autosomal genes often cluster closely on the same chromosome. In this setup, the genes' proximity influences the likelihood of being inherited together.

Genes in Repulsion

  • The cockroach genotype features repulsion, meaning each chromosome has a dominant and recessive allele for different traits.
  • This arrangement results in the dominant eye shape gene \( bu^{+} \) being located on the same chromosome as the recessive wing shape gene \( cv \), and vice versa.
  • The consequence of this arrangement is a decreased chance of crossing over between these linked genes, reinforcing their transmission in the existing pattern to subsequent generations.

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Most popular questions from this chapter

In \(D .\) melanogaster, ebony body \((e)\) and rough eyes \((r o)\) are encoded by autosomal recessive genes found on chromosome 3 ; they are separated by \(20 \mathrm{~m} . \mathrm{u} .\) The gene that encodes forked bristles \((f)\) is \(\mathrm{X}-\) linked recessive and assorts independently of \(e\) and \(r o .\) Give the phenotypes of progeny and their expected proportions when a female of each of the following genotypes is test-crossed with a male. a. \(\begin{array}{cc}e^{+} & r o^{+} \\\\\hline e & r o\end{array}\frac{f^{+}}{f}\) b. \(\begin{array}{cc}e^{+} & r o\\\\\hline e & r o^{+}\end{array}\frac{f^{+}}{f}\)

What effect does crossing over have on linkage?

In cucumbers, heart-shaped leaves ( \(h l\) ) are recessive to normal leaves \((H l)\), and having numerous fruit spines \((n s)\) is recessive to having few fruit spines \((N s) .\) The genes for leaf shape and for number of spines are located on the same chromosome; findings from mapping experiments indicate that they are 32.6 m.u. apart. A cucumber plant having heart-shaped leaves and numerous spines is crossed with a plant that is homozygous for normal leaves and few spines. The \(\mathrm{F}_{1}\) are crossed with plants that have heartshaped leaves and numerous spines. What phenotypes and phenotypic proportions are expected in the progeny of this cross?

In a testcross for two genes, what types of gametes are produced with (a) complete linkage, (b) independent assortment, and (c) incomplete linkage?

What is a lod score, and how is it calculated?
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