Question

In Drosophila, an X-linked recessive mutation, scalloped (sd) causes irregular wing margins. Diagram the \(F_{1}\) and \(F_{2}\) 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

Answer: In Cross A (scalloped female x normal male), the F1 generation phenotypes are all normal females and all scalloped males. In Cross B (scalloped male x normal female), the F1 generation phenotypes are all normal females and all normal males. If the scalloped gene were autosomal, both males and females in the F1 generation would be heterozygous normal in both Cross A and Cross B, displaying normal phenotypes due to the dominance of the normal allele.

Step-by-step solution

Cross A: Scalloped Female x Normal Male

First, we will analyze the cross between a scalloped female and a normal male. Since the mutation is X-linked recessive, let's use "X" to represent the X chromosome and "sd" to denote the scalloped mutation. For this cross, the female will have the genotype X(sd)X(sd) (homozygous for the scalloped mutation), and the male will have the genotype X(normal)Y (normal X chromosome and Y chromosome). Let's perform the cross and determine the genotypes and phenotypes of the \(F_{1}\) generation: Female gametes: X(sd) and X(sd) Male gametes: X(normal) and Y Possible combinations of gametes: 1. X(sd) X(normal) 2. X(sd) Y \(F_{1}\) genotypes: 1. All females will have the genotype X(sd) X(normal) 2. All males will have the genotype X(sd)Y \(F_{1}\) phenotypes: 1. All females will be normal, as they carry one normal allele which is dominant over the recessive scalloped allele. 2. All males will be scalloped since they carry the recessive scalloped allele on their single X chromosome.

Cross B: Scalloped Male x Normal Female

Now, we will analyze the cross between a scalloped male and a normal female. The male will have the genotype X(sd)Y (carrying the scalloped mutation on the X chromosome), and the female will have the genotype X(normal)X(normal) (normal X chromosomes). Let's perform the cross and examine the genotypes and phenotypes of the \(F_{1}\) generation: Female gametes: X(normal) and X(normal) Male gametes: X(sd) and Y Possible combinations of gametes: 1. X(normal) X(sd) 2. X(normal) Y \(F_{1}\) genotypes: 1. All females will have the genotype X(normal) X(sd) 2. All males will have the genotype X(normal)Y \(F_{1}\) phenotypes: 1. All females will be normal since they carry one normal allele that is dominant over the recessive scalloped allele. 2. All males will be normal because they carry a normal X chromosome without the scalloped mutation.

Autosomal Gene Comparison

If the scalloped gene were autosomal, the inheritance would be different, as autosomal genes are present in both males and females and not linked to the sex chromosomes. In both Cross A and Cross B, the \(F_{1}\) generation would be composed of heterozygous normal individuals carrying one dominant normal allele and one recessive scalloped allele. Both males and females would express the normal phenotype since the normal allele is dominant. The \(F_{2}\) generation resulting from an \(F_{1}\times F_{1}\) cross would display a typical Mendelian 3:1 ratio (3 normal to 1 scalloped) in both males and females.

Key concepts

Drosophila Genetics

Drosophila melanogaster, commonly known as the fruit fly, is a prime organism used in genetic research due to its simple chromosome structure, short life cycle, and ease of care. Its genetic makeup consists of four pairs of chromosomes: three autosomal pairs and one pair of sex chromosomes, similar to humans. In Drosophila genetics, mutations can provide visible phenotypic changes, like the scalloped wing mutation. Understanding these mutations and how they are inherited through generations is a fundamental aspect of genetic studies.

For instance, the X-linked recessive mutation with scalloped wings in fruit flies is an excellent example of how genetic traits are inherited differently depending on their location (X chromosome or autosomal chromosomes). By examining the results of specific crosses, we can observe how recessive and dominant alleles are expressed and how this impacts the fruit flies' phenotype — a powerful demonstration of the principles of heredity.

Mendelian Genetics

Mendelian genetics, named after Gregor Mendel, forms the foundation of understanding heredity and genetic variation. Mendel's laws — the Law of Segregation and the Law of Independent Assortment — describe how alleles, the forms of a gene, separate and recombine to form offspring genotypes. These principles explain the inheritance patterns of traits, predicting ratios of phenotypes in offspring like those seen in Drosophila crosses.

In the scenario where the scalloped wing trait is X-linked, Mendel’s principles still apply but are modified slightly due to its linkage to the sex chromosome. In traditional Mendelian crosses involving autosomal genes, we expect certain phenotype ratios in the offspring, but X-linkage disrupts these ratios due to the distinct inheritance patterns of sex chromosomes.

Sex-Linked Traits

Sex-linked traits are associated with genes located on sex chromosomes. In humans and Drosophila, these are the X and Y chromosomes. X-linked traits can show different patterns of inheritance in males and females because males have only one X chromosome (XY), whereas females have two (XX).

An X-linked recessive trait like the scalloped wing in fruit flies will always be expressed in males when present because they lack a second X chromosome that could potentially carry a dominant allele. Females, on the other hand, must inherit two copies of the recessive allele to exhibit the trait. This unique pattern of inheritance leads to differences in the distribution of X-linked traits between sexes, providing a clear example of how sex chromosomes play a critical role in genetic outcomes.

Autosomal Genes

Autosomal genes are those located on non-sex chromosomes (autosomes). They exhibit the classic Mendelian inheritance patterns and are inherited equally by males and females. Autosomal dominant traits require only one allele to be expressed, while autosomal recessive traits require two copies of the recessive allele.

Comparing autosomal inheritance to X-linked inheritance, as discussed in our Drosophila example, we can see marked differences. If the scalloped wing trait were autosomal, it would not show sex-biased expression patterns, and we would expect to see similar inheritance patterns in both male and female fruit flies. This illustrates how the chromosomal location of a gene has crucial implications for the inheritance and expression of a genetic trait.