Question

In Drosophila subobscura, the presence of a recessive gene called grandchildless (gs) causes the offspring of homozygous females, but not those of homozygous males, to be sterile. Can you offer an explanation as to why females and not males are affected by the mutant gene?

Short answer

Short Answer: The recessive "grandchildless" gene affects the offspring of homozygous females but not males in Drosophila subobscura due to differences in their reproductive systems. The gene may negatively impact egg development in females, leading to sterility in their offspring, while leaving sperm development or function in males unaffected.

Step-by-step solution

Understanding recessive genes and their inheritance

The grandchildless gene is recessive, which means that it is only expressed when an individual has two copies of the gene (one from each parent). If a heterozygous individual (carrier) has both a dominant and a recessive allele, the dominant allele will be expressed, and the individual will not be affected by the grandchildless gene. When two carriers mate, there is a 25% chance that their offspring will inherit two recessive alleles and be affected by the gene.

Examining Drosophila reproductive biology

In Drosophila, as in many other organisms, male and female reproductive systems are different. Males produce sperm, while females produce eggs. During reproduction, the male's sperm fertilizes the female's eggs. The differences between male and female reproductive systems may contribute to the observed effect of the grandchildless gene only affecting the offspring of homozygous females.

Postulating why the grandchildless gene affects females but not males

One possible explanation for the grandchildless gene affecting the offspring of homozygous females and not homozygous males could be related to the gene's effect on the egg production or development in homozygous females. Since females contribute not only their genetic material but also the cytoplasm and other organelles to their offspring, it is plausible that the expression of the grandchildless gene leads to abnormalities in the egg's cytoplasm, which in turn impacts the development of the offspring and leads to sterility. In contrast, males contribute only genetic material in the form of sperm, and if the grandchildless gene has no effect on sperm development or function, the offspring of homozygous males would not be affected.

Observing the generational effect

In conclusion, the offspring of homozygous females carrying the recessive grandchildless gene are sterile, whereas the offspring of homozygous males are not. This can be attributed to the different roles of male and female reproductive systems, where the gene could have a detrimental effect on the egg's development in homozygous females. On the other hand, the lack of effect in offspring of homozygous males suggests that the mutant gene does not impact sperm development or function.

Key concepts

Recessive Gene

Understanding recessive genes is crucial to grasp how genetic traits are passed down through generations. A recessive gene, such as the grandchildless gene in Drosophila, remains hidden unless an individual inherits two copies—one from each parent. In genetic terms, these individuals are known as homozygous recessive. They exhibit the trait associated with this gene because there is no dominant allele present to mask it.
If an individual carries one recessive and one dominant allele, they are termed heterozygous. The dominant trait will be expressed, effectively concealing the recessive gene. When two heterozygous individuals reproduce, there is a 25% chance that their offspring will inherit two recessive alleles, thereby expressing the recessive trait, like sterility in the case of the grandchildless gene.

• The recessive gene appears only if two copies are present.
• Heterozygous individuals carry, but do not exhibit, the recessive trait.
• Offspring of two carriers have a 25% chance of being affected by a recessive gene.

Drosophila

Drosophila, commonly known as fruit flies, are a pivotal model organism in genetics due to their simple reproductive systems and short life cycles. They offer valuable insights into genetic inheritance and mutations. The fruit fly's relatively small genome and large number of offspring make it ideal for observing inheritance patterns and gene functions.
Researchers often turn to Drosophila for studying how specific genes can affect development and reproduction due to their clear genetic markers and observable phenotypic changes. For example, the grandchildless gene in Drosophila serves as an excellent subject for examining gene-induced sterility across generations.

• Drosophila are instrumental in genetics research.
• They have a short life cycle, aiding rapid study of gene inheritance.
• Mutations in Drosophila can provide insights into human genetics.

Female Sterility

Female sterility in genetics refers to the inability of an organism to reproduce due to genetic factors. In the case of Drosophila, the grandchildless gene responsible for female sterility is expressed in homozygous recessive females. To understand why this occurs, it's essential to consider the unique roles in reproduction each sex plays.
Female Drosophila contribute not only genetic material but also cytoplasmic elements from the egg to their offspring. These components are vital for early development. When the grandchildless gene is expressed, it likely causes abnormalities in the egg's cytoplasm or its development, resulting in sterility.
The eggs from homozygous females cannot support proper development due to this genetic interference. Meanwhile, males only pass on their genetic material through sperm, with no effect on sterility if the gene is homozygous in them.

• Female sterility often results from genetic abnormalities affecting egg development.
• The grandchildless gene interrupts critical reproductive processes in female Drosophila.
• Males carry the gene but do not exhibit sterility as they lack egg-related responsibilities.