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Chapter 4: Problem 1

In this chapter, we focused on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions? (a) How were early geneticists able to ascertain inheritance patterns that did not fit typical Mendelian ratios? (b) How did geneticists determine that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs? How were they able to determine how many gene pairs were involved? (c) How do we know that specific genes are located on the sex-determining chromosomes rather than on autosomes? (d) For genes whose expression seems to be tied to the sex of individuals, how do we know whether a gene is X-linked in contrast to exhibiting sex-limited or sex-influenced inheritance?

Short Answer

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Answer: Early geneticists differentiated between X-linked inheritance and sex-limited or sex-influenced inheritance by carefully examining the patterns of inheritance in offspring. X-linked patterns showed a different pattern of inheritance between males and females, due to the presence of only one X chromosome in males. In contrast, traits with sex-limited or sex-influenced inheritance were influenced by the sex of the offspring but had the same genetic effect on both males and females. Comparing these patterns in carefully designed crosses allowed researchers to determine the nature of the gene's inheritance.

Step by step solution

01

a) Inheritance patterns and Mendelian ratios

Early geneticists were able to ascertain inheritance patterns that did not fit typical Mendelian ratios by carefully examining the results of their breeding experiments. They would track certain traits, observe their patterns in offspring, and compare these observations with the expected Mendelian ratios. When observed ratios did not match the expected ones, they would investigate further by conducting more crosses between different combinations of offspring and/or developing new methodologies.
02

b) Interactions of gene pairs and the determination of the number of genes involved

Early geneticists determined that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs by studying genetic interactions, known as epistasis. They noticed that some traits did not follow the simple Mendelian ratios and started examining patterns involving combinations of more than one gene pair. By carefully examining the results of crosses and analyzing the data, they were able to determine how many gene pairs were involved and develop new inheritance models that accounted for gene interactions.
03

c) Identification of sex-determining chromosomes and gene location

Early geneticists were able to identify specific genes located on the sex-determining chromosomes rather than autosomes by crossing different organisms and examining the inheritance patterns in the offspring. Researchers like Thomas Hunt Morgan observed that certain traits were inherited differently in males and females and hypothesized that they were sex-linked, meaning they were located on the sex chromosomes. Experiments using organisms like fruit flies, which have easily identifiable sex-linked traits such as eye color, confirmed that certain genes were located on the sex-determining chromosomes.
04

d) Determining X-linkage vs. sex-limited or sex-influenced inheritance

Early geneticists were able to discern whether a gene was X-linked or exhibited sex-limited or sex-influenced inheritance by thoroughly examining the patterns of inheritance in the offspring. A gene exhibiting an X-linked pattern would show a different pattern of inheritance between males and females, due to the presence of only one X chromosome in males. On the other hand, traits with sex-limited or sex-influenced inheritance will be influenced by the sex of the offspring, but the effect of the gene will not differ between males and females. Comparing these patterns in carefully designed crosses allowed researchers to determine the nature of the gene's inheritance.

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Most popular questions from this chapter

While vermilion is X-linked in Drosophila and causes the eye color to be bright red, brown is an autosomal recessive mutation that causes the eye to be brown. Flies carrying both mutations lose all pigmentation and are white-eyed. Predict the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of the following crosses: (a) vermilion females \(\times\) brown males (b) brown females \(\times\) vermilion males (c) white females \(\times\) wild-type males

In Drosophila , the \(\mathrm{X}\) -linked recessive mutation vermilion \((v)\) causes bright red eyes, in contrast to the brick-red eyes of wild type. A separate autosomal recessive mutation, suppressor of vermilion \((s u-v),\) causes flies homozygous or hemizygous for \(v\) to have wild-type eyes. In the absence of vermilion alleles, \(s u-v\) has no effect on eye color. Determine the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) phenotypic ratios from a cross between a female with wild-type alleles at the vermilion locus, but who is homozygous for \(s u-v,\) with a vermilion male who has wild-type alleles at the \(s u-v\) locus.

Karl Landsteiner and Philip Levine discovered a glycoprotein expressed on the surface of red blood cells, which exists in two forms, \(M\) and \(N .\) An individual may produce either one or both of them. The alleles \(L^{M}\) and \(L^{N}\) control the expression of the glycoprotein. What type of inheritance does the MN blood group exhibit, and what are the genotypes of the phenotypes observed in the human population?

Another recessive mutation in Drosophila, ebony \((e),\) is on an autosome (chromosome 3) and causes darkening of the body compared with wild-type flies. What phenotypic \(F_{1}\) and \(F_{2}\) male and female ratios will result if a scalloped-winged female with normal body color is crossed with a normal-winged cbony male? Work out this problem by both the Punnett square method and the forked-line method.

What is meant by the term epistasis? Distinguish between epis tasis and dominance. Do not use examples in answering this question.
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