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Chapter 4: Problem 25

In Drosophila, an X-linked recessive mutation, scalloped (sd), causes irregular wing margins. Diagram the \(F_{1}\) and \(\mathrm{F}_{2}\) results if (a) a scalloped female is crossed with a normal male; (b) a scalloped male is crossed with a normal female. Compare these results with those that would be obtained if the scalloped gene were autosomal.

Short Answer

Expert verified
Answer: In scenario (a), when a scalloped female is crossed with a normal male, all female offspring carry the scalloped allele (XX(sd)) while males express the scalloped phenotype (XY(sd)). In scenario (b), when a scalloped male is crossed with a normal female, half of the female offspring carry the scalloped allele (XX(sd+)) and the other half do not (XX(sd)). For both scenarios, the \(F_{2}\) generation shows different inheritance patterns compared to autosomal inheritance. If the scalloped gene were autosomal, the \(F_{1}\) generation would consist of 100% heterozygous offspring, while the \(F_{2}\) generation would result in a 3:1 phenotypic ratio (normal:scalloped), which is different from the observed inheritance pattern in X-linked inheritance.

Step by step solution

01

Understand X-linked inheritance

X-linked inheritance is a mode of inheritance for genes located on the X chromosome. In Drosophila, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Recessive X-linked mutations will only be expressed in males, as they have only one X chromosome, but females need to have two copies of the recessive allele in order to express the trait.
02

Setup the parental genotypes

In scenario (a), the female is scalloped while the male is normal. If we represent the scalloped allele as "sd", this would mean: female = XX(sd) and male = XY(sd+). In scenario (b), the male is scalloped while the female is normal. This would mean: female = XX(sd+) and male = XY(sd).
03

Use Punnett squares to predict \(F_{1}\) generation

For both scenarios, we will use the Punnett square to predict the genotypes of the \(F_{1}\) generation. Scenario (a): | | X(sd+) | Y | |--------|-------|---------| | X(sd) | XX(sd)| XY(sd) | Scenario (b): | | X(sd+) | X(sd+) | |--------|-------|---------| | X(sd) | XX(sd)| XX(sd+) | In scenario (a), all female offspring would carry the scalloped allele (XX(sd)) while males would express the scalloped phenotype (XY(sd)). In scenario (b), half of the female offspring would carry the scalloped allele (XX(sd+)) and the other half would not (XX(sd)).
04

Predict \(F_{2}\) generation

We will now use the same approach to predict the \(F_{2}\) generation. Here, we will cross two heterozygous offspring from scenario (a) and two heterozygous offspring from scenario (b). Scenario (a): XX(sd) x XY(sd) = No change in \(F_{2}\), as all females will still carry the scalloped allele (XX(sd)) and all males will express the scalloped phenotype (XY(sd)). Scenario (b): XX(sd+) x XY(sd) = | | X(sd+) | X(sd+) | |--------|-------|---------| | X(sd) | XX(sd+)| XX(sd+) | | X(sd+) | XY(sd)| XY(sd+) | In the \(F_{2}\) generation, the ratio is 1:1 for normal and scalloped females and 1:1 for normal and scalloped males, as shown in the Punnett square.
05

Compare to autosomal inheritance

If the scalloped gene were autosomal, the inheritance pattern would be different. Assuming the female parent has the genotype sd/sd and the male parent has the genotype sd+/sd+, the \(F_{1}\) generation would consist of 100% heterozygous offspring (sd/sd+). The \(F_{2}\) generation from crossing two heterozygous offspring would result in a 1:2:1 genotypic ratio (sd/sd:sd/sd+:sd+/sd+) and a 3:1 phenotypic ratio (normal:scalloped). This is different from the observed inheritance pattern in X-linked inheritance.

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Most popular questions from this chapter

When summer squash plants (Cucurbita pepo) with discshaped fruits are crossed to ones with long fruits, the \(\mathrm{F}_{1}\) generation all have disc-shaped fruits. When the \(F_{1}\) plants are crossed to each other, the \(\mathrm{F}_{2}\) produce spherical fruits as well as exhibit the two parental strains. The phenotypic ratio is 9: 6: 1 (disc-shaped:spherical:long). (a) Which type of gene interaction is this an example of? (b) Explain the phenotypes observed in terms of the number of gene pairs involved and by designating genotypes for all the fruit shapes in the cross. (Use dashes where required.)

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; (b) platinum coat color?

In rats, the following genotypes of two independently assorting autosomal genes determine coat color: A third gene pair on a separate autosome determines whether or not any color will be produced. The \(C C\) and \(C c\) genotypes allow color according to the expression of the \(A\) and \(B\) alleles. However, the \(c c\) genotype results in albino rats regardless of the \(A\) and \(B\) alleles present. Determine the \(F_{1}\) phenotypic ratio of the following crosses: (a) \(A A b b C C \quad \times \quad\) aaBBcc (b) \(A a B B C C \quad \times \quad A A B b c c\) (c) \(A a B b C c \quad \times \quad\) AaBbcc (d) \(A a B B C c \quad \times \quad A a B B C c\) (e) \(A A B b C c \quad \times \quad A A B b c c\)

In Dexter and Kerry cattle, animals may be polled (hornless) or horned. The Dexter animals have short legs, whereas the Kerry animals have long legs. When many offspring were obtained from matings between polled Kerrys and horned Dexters, half were found to be polled Dexters and half polled Kerrys. When these two types of \(\mathrm{F}_{1}\) cattle were mated to one another, the following \(\mathrm{F}_{2}\) data were obtained: \(3 / 8\) polled Dexters \(3 / 8\) polled Kerrys \(1 / 8\) horned Dexters \(1 / 8\) horned Kerrys A geneticist was puzzled by these data and interviewed farmers who had bred these cattle for decades. She learned thatKerrys were true breeding. Dexters, on the other hand, were not true breeding and never produced as many offspring as Kerrys. Provide a genetic explanation for these observations.

Pigment in mouse fur is only produced when the \(C\) allele is present. Individuals of the \(c c\) genotype are white. If color is present, it may be determined by the \(A, a\) alleles. AA or \(A a\) results in agouti color, while aa results in black coats. (a) What \(F_{1}\) and \(F_{2}\) genotypic and phenotypic ratios are obtained from a cross between \(A A C C\) and aacc mice? (b) In three crosses between agouti females whose genotypes were unknown and males of the aacc genotype, the following phenotypic ratios were obtained:
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