Question

An insect species is discovered in which the heterogametic sex is unknown. An X-linked recessive mutation for reduced wing \((r w)\) is discovered. Contrast the \(F_{1}\) and \(F_{2}\) generations from a cross between a female with reduced wings and a male with normal-sized wings when (a) the female is the heterogametic sex. (b) the male is the heterogametic sex.

Short answer

Answer: When the female is the heterogametic sex (a), all the F1 offspring will have normal-sized wings regardless of their sex. In contrast, when the male is the heterogametic sex (b), the F1 generation will have females with normal-sized wings and males with reduced wings. For the F2 generation, when the male is the heterogametic sex, all females will have normal-sized wings, while the males will have a mix of reduced and normal-sized wings.

Step-by-step solution

(a) Female is the heterogametic sex

In this scenario, the female insect would have the sex chromosomes ZW, while the male would have the sex chromosomes ZZ. Since reduced wing (r w) is an X-linked recessive mutation, we'll use "r" to denote the allele linked to the reduced wings while "R" denotes the normal-sized wings allele. Female (ZW): Zr (reduced wings) Male (ZZ): ZRZR (normal-sized wings) F1 generation: Since the female is Zr and the male is ZRZR, the resulting F1 generation's genotypes and phenotypes will be: - Female offspring (ZW): ZrZ (normal-sized wings) - Male offspring (ZZ): ZrZR (normal-sized wings)

(b) Male is the heterogametic sex

In this scenario, the female insect would have the sex chromosomes XX, while the male would have the sex chromosomes XY. As before, we'll use "r" to denote the allele linked to reduced wings and "R" for the normal-sized wings allele. Female (XX): rr (reduced wings) Male (XY): XR Y (normal-sized wings) F1 generation: Since the female is rr, and the male is XR Y, the resulting F1 generation's genotypes and phenotypes will be: - Female offspring (XX): rXR (normal-sized wings) - Male offspring (XY): rY (reduced wings) F2 generation: Since both females and males have different genotypes at this stage, we'll pick one offspring of each sex to mate for the F2 generation: - Female: rXR (normal-sized wings) - Male: rY (reduced wings) The resulting F2 generation's genotypes and phenotypes will be: - Female offspring: (1/2)rXR, (1/2)rR (all females will have normal-sized wings) - Male offspring: (1/2)rY (reduced wings), (1/2)RY (normal-sized wings) In summary, when the female is the heterogametic sex, all F1 offspring have normal-sized wings regardless of sex. When the male is the heterogametic sex, the phenotypes of the F1 generation differ between sexes, and the F2 generation presents the offspring with different wing sizes for both sexes.

Key concepts

Heterogametic Sex

Understanding the concept of heterogametic sex is fundamental in genetics, particularly when discussing sex-linked traits. In species with two sexes, one sex is considered heterogametic and the other homogametic. This categorization is based on the type of sex chromosomes the sexes carry. The heterogametic sex has two distinct sex chromosomes, leading to the production of two types of gametes. In mammals, including humans, males are typically the heterogametic sex, possessing an X and a Y chromosome (XY), producing sperm that can carry either an X or a Y. Females are usually the homogametic sex, with two X chromosomes (XX), and all their eggs carry an X chromosome. However, in some species, like birds and some insects, this is reversed, with females being heterogametic (ZW) and males homogametic (ZZ).

When studying X-linked recessive inheritance, knowing which sex is heterogametic is crucial because it determines how a trait is passed down. This is exemplified in the exercise where different patterns of inheritance occur based on which insect sex is heterogametic. An X-linked recessive trait, such as the reduced wing size in the exercise, will manifest differently in offspring depending on whether the trait is carried by a heterogametic female or a heterogametic male.

F1 and F2 Generations

The terms F1 and F2 generations are integral to genetics and breeding experiments. The F1 generation stands for the 'first filial generation' and consists of offspring resulting from a cross between two parent organisms. The F2 generation, or 'second filial generation', is composed of offspring from crossing individuals of the F1 generation. These terms are often used to describe the outcomes of genetic crosses over successive generations, allowing geneticists to track and predict genetic inheritance patterns.

In the exercise provided, the F1 and F2 generations are used to contrast the potential inheritance of the X-linked recessive trait for reduced wing size in a hypothetical insect species. When the F1 generation is produced from parents with known genotypes, the inheritance of traits in the F1 offspring provides insight into which alleles are dominant or recessive. Breeding between members of this F1 generation then produces the F2 generation, which can exhibit new combinations of these traits and provide a clearer picture of the underlying genetic mechanisms. This successive pattern is essential to understanding genetic inheritance and the predictable ratios of trait distribution.

Sex Chromosomes

Sex chromosomes are a special pair of chromosomes that determine the sex of an organism and carry genes for sex-specific traits. In humans and most animals, these chromosomes are denoted as X and Y. Females typically have two X chromosomes (XX), and males have one X and one Y chromosome (XY). However, as mentioned previously, some species, such as birds and certain insects, follow a different system where the female is ZW and the male is ZZ.

In relation to genetics and inheritance, sex chromosomes are of particular interest because they behave differently than autosomes during meiosis, the process that generates gametes. Traits linked to the X chromosome, for example, can be passed from mother to son or mother to daughter in different patterns from those linked to autosomes. In the exercise scenario where a sex-linked recessive trait like reduced wing size is examined, the inheritance pattern varies considerably depending on whether the trait is located on the X or Z chromosome and which sex is heterogametic. Thus, sex chromosomes play a crucial role in determining the inheritance of sex-linked traits and require special consideration when predicting genetic outcomes.