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Chapter 4: Problem 39

Suppose that a recessive gene that produces a short tail in mice is located in the pseudoautosomal region. A short-tailed male mouse is mated with a female mouse that is homozygous for a normal tail. The \(\mathrm{F}_{1}\) mice from this cross are intercrossed to produce the \(\mathrm{F}_{2}\). Give the phenotypes, as well as their proportions, of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) mice?

Short Answer

Expert verified
In the F1 generation, all mice have normal tails. In the F2 generation, 75% have normal tails, and 25% have short tails.

Step by step solution

01

Understanding Parental Genotypes

In this problem, the gene for tail length is located in the pseudoautosomal region. The male mouse has a short tail, indicating he has two recessive alleles (for simplicity, assume genotype as tt). The female mouse is homozygous for a normal tail, meaning she has two dominant alleles, genotype TT.
02

Determining F1 Genotypes and Phenotypes

When the short-tailed male (tt) is crossed with the normal-tailed female (TT), all offspring (F1 generation) will inherit one 'T' allele from the mother and one 't' allele from the father, resulting in a genotype of Tt for all F1 mice. Since 'T' (normal tail) is dominant over 't' (short tail), all F1 mice will have normal tails.
03

Intercrossing F1 Generation to Obtain F2

The F1 mice with genotype Tt are intercrossed. This means Tt (from one F1 mouse) is crossed with Tt (from another F1 mouse). We use a Punnett square to determine the probabilities: - TT: Normal tail - Tt: Normal tail - Tt: Normal tail - tt: Short tail
04

Calculating F2 Phenotypic Proportions

From the Punnett square, we observe the following genotypic ratio: 1 TT : 2 Tt : 1 tt. In terms of phenotypes, 3 out of 4 have a normal tail (1 TT + 2 Tt = 3 normal) and 1 out of 4 has a short tail (1 tt). Thus, the phenotypic ratio is 3 normal tails to 1 short tail.

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Key Concepts

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

Punnett Square
The Punnett Square is a handy tool used in genetics to predict the genotypes and phenotypes of offspring. It鈥檚 a simple grid that helps visualize how alleles from each parent can combine in offspring. In our exercise, we use a Punnett Square to understand the likelihood of different tail lengths in mice's babies.
  • The tool consists of a grid where each box inside represents a possible genotype of an offspring.
  • The alleles from one parent are placed on the top row and those from the other parent on the side column.
  • By filling in the squares, we can see all possible combinations of parental alleles and determine the offspring's possible genotypes and phenotypes.
Using our example, crossing F1 mice with Tt genotypes results in a 3:1 phenotypic ratio of normal tails to short tails.
Pseudoautosomal Region
The pseudoautosomal region is a critical area of study in genetic inheritance. It is a segment of the X and Y chromosomes where crossing over occurs during meiosis. In our example, the gene for tail length in mice is located in this region, which is why it behaves differently than most X- or Y-linked traits.
  • This region allows the exchange of genetic material between the two sex chromosomes.
  • Genes located here, like the one for short tails, often exhibit autosomal patterns of inheritance.
  • As seen with the short tail gene, both males and females can inherit these traits, albeit with different frequencies and outcomes.
This is why studying traits in the pseudoautosomal region requires a different approach than typical sex-linked traits.
Recessive Gene
Recessive genes are dominant鈥檚 quieter counterparts in genetics. They can be overshadowed by dominant genes but can still show their effects when paired together. In our problem, the recessive gene 't' produces a short tail in mice.
  • For a recessive trait to be visible, both alleles in the genotype must be recessive (tt in our mice scenario).
  • Recessive traits often skip generations, appearing only when two carriers pass on the allele.
  • Recessive features can provide insights into the genetic health and diversity of a population.
Understanding recessive genes helps explain why some traits seem to "disappear" in certain generations only to reappear later.
Phenotypic Ratio
Phenotypic ratio refers to the relative number of offspring displaying certain traits. It is determined after filling out a Punnett square and analyzing the resulting genotypes. In our example, the phenotypic ratio describes the proportion of normal tail mice to short tail mice in the F2 generation.
  • We derive it from the observable characteristics (phenotypes) of the offspring.
  • It鈥檚 crucial for predicting the likelihood of offspring displaying specific traits, aiding breeders and geneticists in their work.
  • In our mouse scenario, the ratio is 3:1, meaning three mice with normal tails for every one with a short tail.
Phenotypic ratios provide a practical way to connect genotypic possibilities to real-world outcomes, highlighting the pattern of genetic inheritance in populations.

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Most popular questions from this chapter

A geneticist discovers a male mouse with greatly enlarged testes in his laboratory colony. He suspects that this trait results from a new mutation that is either Y linked or autosomal dominant. How could he determine whether the trait is autosomal dominant or Y linked?

A color-blind woman and a man with normal vision have three sons and six daughters. All the sons are color blind. Five of the daughters have normal vision, but one of them is color blind. The color-blind daughter is 16 years old, is short for her age, and has not undergone puberty. Explain how this girl inherited her color blindness.

Miniature wings in Drosophila are due to an X-linked gene \(\left(\mathrm{X}^{m}\right)\) that is recessive to an allele for long wings \(\left(\mathrm{X}^{+}\right) .\) Sepia eyes are produced by an autosomal gene \((s)\) that is recessive to an allele for red eyes \(\left(s^{\top}\right)\) a. A female fly that has min iature wings and sepia eyes is crossed with a male that has normal wings and is homozygous for red eyes. The \(\mathrm{F}_{1}\) flies are intercrossed to produce the \(\mathrm{F}_{2}\). Give the phenotypes, as well as their expected proportions, of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) flies. b. A female fly that is homozygous for normal wings and has sepia eyes is crossed with a male that has miniature wings and is homozygous for red eyes. The \(F_{1}\) flies are intercrossed to produce the \(\mathrm{F}_{2}\). Give the phenotypes, as well as their expected proportions, of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) flies.

Give the typical sex-chromosome complement found in the cells of people with Turner syndrome, with Klinefelter syndrome, and with androgen-insensitivity syndrome. What is the sex-chromosome complement of triplo-X females?

Identical twins (also called monozygotic twins) are derived from a single egg fertilized by a single sperm, creating a zygote that later divides into two (see Chapter 6 ). Because identical twins originate from a single zygote, they are genetically identical. Caroline Loat and her colleagues examined nine measures of social, behavioral, and cognitive ability in 1000 pairs of identical male twins and 1000 pairs of identical female twins (C. S. Loat, et al. 2004. Twin Research \(7: 54-61\) ). They found that, for three of the measures (prosocial behavior, peer problems, and verbal ability), the two male twins of a pair tended to be more alike in their scores than were two female twins of a pair. Propose a possible explanation for this observation. What might this observation indicate about the location of genes that influence prosocial behavior, peer problems, and verbal ability?
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