Question

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

Short answer

In scenario (a), where the female is the heterogametic sex, the genotypes and phenotypes of the F1 and F2 generations are as follows: F1: - 50% X+Xrw (normal-sized wings females) - 50% XrwY (normal-sized wings males) F2: - 25% X+Y (normal-sized wings males) - 25% X+Xrw (normal-sized wings females) - 25% XrwY (reduced wings males) - 25% XrwXrw (reduced wings females) In scenario (b), where the male is the heterogametic sex, the genotypes and phenotypes of the F1 and F2 generations are as follows: F1: - 50% X+Xrw (normal-sized wings females) - 50% XrwY (reduced wings males) F2: - 25% X+Y (normal-sized wings males) - 25% X+Xrw (normal-sized wings females) - 25% XrwY (reduced wings males) - 25% XrwXrw (reduced wings females)

Step-by-step solution

Case (a): Female is the heterogametic sex

For this case, we will assume that the female has the genotype \(X_{rw}X_{rw}\) and the male has the genotype \(X_{+}Y\). We will begin by finding the genotypes and phenotypes of the \(F_{1}\) generation.

Calculate \(F_{1}\) genotypes and phenotypes for Case (a)

We can use a Punnett square to determine the genotypes of the offspring. The possible gametes are \(X_{+}\) and \(Y\) from the male and \(X_{rw}\) from the female. Fill in the Punnett square, as shown below: ``` X+ Y Xrw X+Xrw XrwY ``` The genotypes of \(F_{1}\) offspring are: \(50\% X_{+}X_{rw}\) and \(50\% X_{rw}Y\). In terms of phenotypes, all \(F_{1}\) offspring will have normal-sized wings since the X-linked recessive mutation is masked by the dominant allele in heterozygous individuals.

Calculate \(F_{2}\) genotypes and phenotypes for Case (a)

We will now find the genotypes and phenotypes of the \(F_{2}\) generation by self-crossing the \(F_{1}\) offspring. Remember that female offspring have the \(X_{+}X_{rw}\) genotype, so they will produce \(X_{+}\) and \(X_{rw}\) gametes, and male offspring have the \(X_{rw}Y\) genotype, so they will produce \(X_{rw}\) and \(Y\) gametes. The Punnett square for the \(F_{2}\) generation is as follows: ``` X+ Xrw Y X+ X+X+ X+Xrw X+Y Xrw X+Xrw XrwXrw XrwY ``` For the \(F_{2}\) generation, we have the following genotypes and phenotypes: \(25\% X_{+}Y\) normal-sized wings males, \(25\% X_{+}X_{rw}\) normal-sized wings females, \(25\% X_{rw}Y\) reduced wings males, and \(25\% X_{rw}X_{rw}\) reduced wings females. Now let's consider the case where the male is the heterogametic sex.

Case (b): Male is the heterogametic sex

For this case, we will assume that the female has the genotype \(X_{rw}X_{rw}\) and the male has the genotype \(X_{+}Y\). We will start by finding the genotypes and phenotypes of the \(F_{1}\) generation as before.

Calculate \(F_{1}\) genotypes and phenotypes for Case (b)

We can use a Punnett square to determine the genotypes of the offspring. The possible gametes are \(X_{+}\) and \(Y\) from the male and \(X_{rw}\) from the female. Fill in the Punnett square, as shown below: ``` X+ Y Xrw X+Xrw XrwY ``` The genotypes of the \(F_{1}\) offspring are: \(50\% X_{+}X_{rw}\) and \(50\% X_{rw}Y\). In terms of phenotypes, all female \(F_{1}\) offspring will inherit normal-sized wings since the X-linked recessive mutation is masked by the dominant allele in heterozygous individuals. However, all male \(F_{1}\) offspring will inherit reduced wings since they inherit the mutated \(X_{rw}\) from their mother.

Calculate \(F_{2}\) genotypes and phenotypes for Case (b)

We will now find the genotypes and phenotypes of the \(F_{2}\) generation by self-crossing the \(F_{1}\) offspring. Remember that female offspring have the \(X_{+}X_{rw}\) genotype, so they will produce \(X_{+}\) and \(X_{rw}\) gametes, and male offspring have the \(X_{rw}Y\) genotype, so they will produce \(X_{rw}\) and \(Y\) gametes. The Punnett square for the \(F_{2}\) generation is as follows: ``` X+ Xrw Y X+ X+X+ X+Xrw X+Y Xrw X+Xrw XrwXrw XrwY ``` For the \(F_{2}\) generation, we have the following genotypes and phenotypes: \(25\% X_{+}Y\) normal-sized wings males, \(25\% X_{+}X_{rw}\) normal-sized wings females, \(25\% X_{rw}Y\) reduced wings males, and \(25\% X_{rw}X_{rw}\) reduced wings females.

Key concepts

X-linked recessive traits

X-linked recessive traits are genetic traits where the gene responsible is located on the X chromosome. These traits require two copies of the recessive allele for females to express the trait because they have two X chromosomes. In contrast, since males have only one X chromosome, they need only one copy of the recessive allele to express the trait. This pattern of inheritance can lead to more males expressing the trait compared to females because males are hemizygous for the X chromosome.

In genetics notation, we often use symbols like "X_rw" to represent a recessive allele for a particular trait, such as a wing mutation in insects. If an insect inherits an X-linked recessive allele for a trait like reduced wings only from its mother, it will express this phenotype if male, but only carry it if female unless she inherits another X-linked recessive allele from her father.

Heterogametic sex

The term heterogametic sex refers to the sex that has two different types of sex chromosomes. In many organisms, the typical pattern is for males to be the heterogametic sex, having one X and one Y chromosome (XY). On the other hand, females usually have two X chromosomes (XX), making them the homogametic sex. However, in some species, like birds and some fish, the roles are reversed, with males being homogametic (ZZ) and females being heterogametic (ZW).

In our specific exercise, determining the heterogametic sex is crucial since it affects how traits are inherited. If the female is the heterogametic sex (e.g., ZW), her offspring's sex determination and X-linked inheritance patterns change drastically compared to when the male is heterogametic. Understanding which sex is heterogametic guides predictions in inheritance patterns in genetic crosses.

Punnett square

The Punnett square is a handy graphical tool used by geneticists to visualize the possible genotypes of offspring from a particular genetic cross. It helps to predict inheritance patterns by showing how alleles from the parents combine through gametes.

By aligning possible alleles from one parent on the vertical side of the square and those from another parent on the horizontal side, the interior boxes reveal all possible combinations of these alleles in the offspring. This method allows us to see potential genetic outcomes clearly. In the context of X-linked traits, the Punnett square can show the probability of offspring inheriting either dominant or recessive alleles and thus having related phenotypes, such as normal or reduced wing size.

Wing mutation

A wing mutation in genetics refers to a change in a gene that affects the wing structure or development of an organism, often resulting in abnormal wing size or shape. In the context of this problem, the wing mutation is specifically on the X chromosome and is recessive. This means a visible expression of the mutation (like reduced wings) shows up unless masked by a normal dominant allele.

Observing such mutations helps scientists trace genetic links and understand the mechanisms of inheritance. For example, if a female insect with reduced wings has offspring with a male with normal wings, observing the offspring's wings can reveal insights about which sex determines inheritance and environmental versus genetic influences on wing development.

Inheritance patterns

Inheritance patterns describe the predictable patterns seen in the transmission of genes across generations. They follow specific laws discovered by Gregor Mendel, often summarized in simple patterns such as dominant-recessive in single-gene crosses or more complex modes involving multiple genes, incomplete dominance, or linkage.

For X-linked recessive traits like the wing mutation in our insects, the inheritance pattern depends on which sex carries the mutation and whether that sex is heterogametic. If the male is heterogametic (XY), and carries a recessive mutation, his male offspring cannot inherit the trait from him, while his female offspring can become carriers. Conversely, if females are heterogametic (ZW) and carry a recessive mutation, the genetic predictions about phenotypes in offspring differ significantly. Recognizing these patterns helps in predictions about future generations and in understanding genetic diseases.