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Ch. 10 - Eukaryotic Chromosome Abnormalities and Molecular Organization
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 10 - Eukaryotic Chromosome Abnormalities and Molecular OrganizationProblem 12c
Chapter 10, Problem 12c

A pair of homologous chromosomes in Drosophila has the following content (single letters represent genes):
Chromosome 1RNMDHBGKWU
Chromosome 2RNMDHBDHBGKWU
What term best describes the unusual structure that forms during pairing of these chromosomes?

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Identify the gene sequences on both homologous chromosomes: Chromosome 1 has the sequence RNMDHBGKWU, and Chromosome 2 has the sequence RNMDHBDHBGKWU.
Compare the sequences of the two chromosomes to identify any differences. Notice that Chromosome 2 has an extra segment 'DHBG' inserted between 'D' and 'K'.
Understand that during meiosis, homologous chromosomes pair up and align gene by gene. However, the extra segment in Chromosome 2 will cause misalignment.
Recognize that this misalignment leads to the formation of a loop structure in the chromosome with the extra segment (Chromosome 2) to accommodate the additional genetic material.
Conclude that the term describing this unusual structure is a 'deletion loop' or 'insertion loop,' depending on the perspective, as it forms due to the presence of an extra segment in one homologous chromosome.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes in a diploid organism that have the same structure and gene sequence but may contain different alleles. In Drosophila, like in other organisms, these chromosomes pair during meiosis, allowing for genetic recombination and ensuring that each gamete receives one chromosome from each pair.
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Chromosome Structure

Chromosomal Pairing

Chromosomal pairing occurs during meiosis when homologous chromosomes align closely along their lengths. This process is crucial for the exchange of genetic material through crossing over, which increases genetic diversity. The unusual structure that forms during this pairing can lead to unique configurations, especially when there are differences in gene content or structure.
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Chromosome Structure

Synapsis

Synapsis is the process during prophase I of meiosis where homologous chromosomes become closely associated, forming a structure known as a synaptonemal complex. This complex facilitates the exchange of genetic material between chromatids, which is essential for proper segregation of chromosomes and genetic variation in offspring.
Related Practice
Textbook Question

An animal heterozygous for a reciprocal balanced translocation has the following chromosomes:

      MN • OPQRST

      MN • OPQRjkl

      cdef • ghijkl

      cdef • ghiST

Identify the gametes produced by alternate segregation. Which, if any, of these gametes are viable?

Textbook Question

A pair of homologous chromosomes in Drosophila has the following content (single letters represent genes):

Chromosome 1RNMDHBGKWU

Chromosome 2RNMDHBDHBGKWU

Diagram the pairing of these homologous chromosomes in prophase I.

Textbook Question

An animal heterozygous for a reciprocal balanced translocation has the following chromosomes:

      MN • OPQRST

      MN • OPQRjkl

      cdef • ghijkl

      cdef • ghiST

Diagram the pairing of these chromosomes in prophase I.

Textbook Question

A pair of homologous chromosomes in Drosophila has the following content (single letters represent genes):

Chromosome 1RNMDHBGKWU

Chromosome 2RNMDHBDHBGKWU

What term best describes this situation?

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Textbook Question

The most common reason a physician might recommend that a woman have maternal serum screening and a karyotype analysis is concern that her fetus may have Down syndrome. Go to the OMIM website at www.ncbi.nlm.nih.gov/omim and look up Down syndrome (OMIM 190685).

Look at the 'Mapping' and 'Molecular Genetics' sections and describe what is meant by the Down syndrome critical region (DSCR).

Textbook Question

A pair of homologous chromosomes in Drosophila has the following content (single letters represent genes):

Chromosome 1RNMDHBGKWU

Chromosome 2RNMDHBDHBGKWU

How does the pairing diagrammed in part (b) differ from the pairing of chromosomes in an inversion heterozygote?