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Chapter 4: Problem 1

In this chapter, we focused on many extensions and modifications of Mendellan principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. Answer 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 sexdetermining chromosomes rather than on autosomes? (d) For genes whose expression seems to be tied to the gender of individuals, how do we know whether a gene is X-linked in contrast to exhibiting sex- limited or sex-influenced inheritance? (e) How was extranuclear inheritance discovered?

Short Answer

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Answer: Early geneticists were able to identify non-Mendelian inheritance patterns by observing the phenotypes of offspring resulting from controlled crosses between organisms with different traits and analyzing deviations from expected Mendelian ratios. To differentiate between various forms of inheritance like X-linked traits or extranuclear inheritance, geneticists studied the patterns of inheritance in families or controlled crosses, considering the traits' correlation with sex, and further investigating the genetic mechanisms underlying specific patterns, such as the involvement of mitochondrial DNA in extranuclear inheritance.

Step by step solution

01

(a) Identifying non-Mendelian inheritance patterns

Early geneticists were able to ascertain inheritance patterns that did not fit typical Mendelian ratios by observing the phenotypes of offspring resulting from controlled crosses between organisms with different traits. When the observed ratios of phenotypes deviated from the expected Mendelian ratios, they realized that different forms of inheritance must be at work, such as incomplete dominance, codominance, or multiple-alleles, among others.
02

(b) Discovering gene interactions

Geneticists determined that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs by analyzing the results of controlled crosses. They observed that some traits didn't follow Mendelian inheritance patterns, leading them to investigate the possibility of multiple genes affecting the same characteristic. By analyzing the phenotype ratios of subsequent generations and making careful deductions, geneticists were able to figure out how many gene pairs were involved in the inheritance of a particular characteristic.
03

(c) Genes on sex-determining chromosomes

Geneticists discovered that specific genes are located on sex-determining chromosomes rather than autosomes by studying the patterns of inheritance of sex-linked traits in humans and other organisms. They observed that some traits correlated with the sex of the individual and were inherited in a very specific manner, which could not be explained by their presence on autosomes. Further studies, like those on fruit flies, helped establish that these sex-linked traits were indeed located on sex chromosomes.
04

(d) X-linked vs. sex-limited/influenced inheritance

To know whether a gene is X-linked or exhibits sex-limited or sex-influenced inheritance, geneticists study the patterns of inheritance in families or controlled crosses. X-linked traits will show a specific inheritance pattern, with males being more affected than females, and females often acting as carriers. In contrast, sex-limited or sex-influenced traits have different expression patterns between males and females, but they are autosomal traits and won't show a skewed inheritance pattern based on sex.
05

(e) Discovery of extranuclear inheritance

Extracellular inheritance was discovered when geneticists found that some traits did not seem to fit any known patterns of inheritance, even when considering sex chromosomes. An example of this was the discovery of the "petite" mutation in yeast, which affected the size and growth of yeast colonies. Further studies revealed that the inheritance pattern for this trait was not based on nuclear DNA but rather on the DNA contained within the mitochondria, leading to the understanding of extranuclear inheritance.

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Most popular questions from this chapter

In foxes, two alleles of a single gene, \(P\) and \(p,\) may result in lethality \((P P),\) platinum coat \((P p),\) or silver coat \((p p) .\) What ratio is obtained when platinum foxes are interbred? Is the \(P\) allele behaving dominantly or recessively in causing (a) lethality; platinum coat color?

The specification of the anterior-posterior axis in Drosophila embryos is initially controlled by various gene products that are synthesized and stored in the mature egg following oogenesis. Mutations in these genes result in abnormalitics of the axis during embryogenesis, illustrating maternal effect. How do such mutations vary from those involved in organelle heredity that illustrate extranuclear inheritance? Devise a set of parallel crosses and expected outcomes involving mutant genes that contrast maternal effect and organelle heredity.

What genetic criteria distinguish a case of extranuclear inheritance from (a) a case of Mendelian autosomal inheritance; (b) a case of \(\mathrm{X}\) -linked inheritance?

In humans, the ABO blood type is under the control of autosomal multiple alleles. Red-green color blindness is a recessive X-linked trait. If two parents who are both type A and have normal vision produce a son who is color- blind and type \(0,\) what is the probability that their next child will be a female who has normal vision and is type \(0 ?\)

Labrador retrievers may be black, brown, or golden in color (see the chapter opening photograph on \(\mathrm{p} .53\) ). Although each color may breed true, many different outcomes occur if numerous litters are examined from a variety of matings, where the parents are not necessarily true-breeding. The following results show some of the possibilities. Propose a mode of inheritance that is consistent with these data, and indicate the corresponding genotypes of the parents in each mating. Indicate as well the genotypes of dogs that breed true for each color. (a) black \(\times\) brown \(\longrightarrow\) all black (b) black \(\times\) brown \(\longrightarrow \quad 1 / 2\) black \(1 / 2\) brown (c) black \(\times\) brown \(\longrightarrow \quad 3 / 4\) black \(1 / 4\) golden (d) black \(\quad \times\) golden \(\longrightarrow \quad\) all black (e) black \(\times\) golden \(\longrightarrow \quad 4 / 8\) golden 318 black \(1 / 8\) brown (f) black \(\times\) golden \(\longrightarrow \quad 2 / 4\) golden \(1 / 4\) black \(1 / 4\) brown (8) brown \(\times\) brown \(\longrightarrow \quad 3 / 4\) brown \(1 / 4\) golden (h) black \(\times\) black \(\longrightarrow 9 / 16\) black \(4 / 16\) golden \(3 / 16\) brown
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