Ch. 3 - Cell Division and Chromosome Heredity

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook

Sanders 3rd Edition

Ch. 3 - Cell Division and Chromosome Heredity

Problem 28

Chapter 3, Problem 28

In Drosophila, the X-linked echinus eye phenotype disrupts formation of facets and is recessive to wild-type eye. Autosomal recessive traits vestigial wing and ebony body assort independently of one another. Examine the progeny from the three crosses shown below, and identify the genotype of parents in each cross.

Verified step by step guidance

Step 1: Understand the traits and their inheritance patterns. The echinus eye phenotype is X-linked recessive, meaning males have only one X chromosome and will express the phenotype if they inherit the recessive allele. The vestigial wing and ebony body traits are autosomal recessive and assort independently, so their inheritance follows Mendel's laws of independent assortment.

Step 2: Analyze each cross separately by examining the phenotypes of the progeny. For the X-linked echinus trait, determine the sex and eye phenotype ratios to infer the parental genotypes, considering that males inherit their X chromosome from their mother and females inherit one X from each parent.

Step 3: For the autosomal recessive traits (vestigial wing and ebony body), use the phenotypic ratios of the offspring to deduce whether the parents are homozygous dominant, heterozygous carriers, or homozygous recessive for each trait. Remember that independent assortment means you can treat each trait separately and then combine the genotypes.

Step 4: Use Punnett squares to model each cross. For the X-linked trait, set up a Punnett square with the mother's X chromosomes and the father's X and Y chromosomes. For the autosomal traits, set up separate Punnett squares for each trait and then combine the results to predict the expected phenotypic ratios.

Step 5: Compare the expected phenotypic ratios from your Punnett squares with the observed progeny phenotypes in each cross. Adjust the assumed parental genotypes accordingly until the predicted ratios match the observed data, thereby identifying the most likely genotypes of the parents in each cross.

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Key Concepts

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

X-linked Inheritance

X-linked inheritance refers to genes located on the X chromosome. In Drosophila, males have one X and one Y chromosome, so recessive X-linked traits like echinus eye phenotype appear in males if they inherit the mutant allele. Females require two copies of the recessive allele to express the trait, making the inheritance pattern distinct from autosomal traits.

Autosomal Recessive Traits and Independent Assortment

Autosomal recessive traits, such as vestigial wings and ebony body in Drosophila, require two copies of the mutant allele for expression. Independent assortment means these genes are on different chromosomes or far apart on the same chromosome, so alleles segregate independently during gamete formation, producing predictable phenotypic ratios in progeny.

Genotype Determination from Crosses

Determining parental genotypes involves analyzing progeny phenotypes and their ratios from genetic crosses. By comparing observed offspring traits to expected Mendelian ratios for X-linked and autosomal traits, one can infer the genotype of each parent, including heterozygosity or homozygosity for specific alleles.

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Textbook Question

Lesch–Nyhan syndrome (OMIM 300322) is a rare X-linked recessive disorder that produces severe mental retardation, spastic cerebral palsy, and self-mutilation.

What is the probability that the first son of a man whose brother has Lesch–Nyhan syndrome will be affected?

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Textbook Question

In humans, SRY is located near a pseudoautosomal region (PAR) of the Y chromosome, a region of homology between the X and Y chromosomes that allows them to synapse during meiosis in males and is a region of crossover between the chromosomes. The diagram below shows SRY in relation to the pseudoautosomal region.

About 1 in every 25,000 newborn infants is born with sex reversal; the infant is either an apparent male but with two X chromosomes or an apparent female but with an X and a Y chromosome. Explain the origin of sex reversal in human males and females involving the SRY gene. (Hint: See Experimental Insight 3.1 for a clue about the mutational mechanism.)

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Textbook Question

In an 1889 book titled Natural Inheritance (Macmillan, New York), Francis Galton, who investigated the inheritance of measurable (quantitative) traits, formulated a law of 'ancestral inheritance.' The law stated that individuals inherit approximately one-half of their genetic traits from each parent, about one-quarter of the traits from each grandparent, one-eighth from each great grandparent, and so on. In light of the chromosome theory of heredity, argue either in favor of Galton's law or against it.

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Textbook Question

A wild-type Drosophila male and female are crossed, producing 324 female progeny and 161 male progeny. All their progeny are wild type.

Propose a genetic hypothesis to explain these data.

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Textbook Question

A wild-type Drosophila male and female are crossed, producing 324 female progeny and 161 male progeny. All their progeny are wild type.

Design an experiment that will test your hypothesis, using the wild-type progeny identified above. Describe the results you expect if your hypothesis is true.

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Textbook Question

Drosophila has a diploid chromosome number of 2n = 8, which includes one pair of sex chromosomes (XX in females and XY in males) and three pairs of autosomes. Consider a Drosophila male that has a copy of the A₁ allele on its X chromosome (the Y chromosome is the homolog) and is heterozygous for alleles B₁ and B₂, C₁ and C₂, and D₁ and D₂ of genes that are each on a different autosomal pair. In the diagrams requested below, indicate the alleles carried on each chromosome and sister chromatid. Assume that no crossover occurs between homologous chromosomes.

What is the genotype of cells produced by mitotic division in this male?

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