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Chapter 5: Problem 8

What specific observations (evidence) support the conclusions about sex determination in Drosophila and humans?

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Question: Discuss the specific observations or evidence that support the conclusions about sex determination in Drosophila and humans. Answer: Evidence for sex determination in humans comes from studies on individuals with sex chromosome abnormalities such as Klinefelter syndrome, Turner syndrome, and XYY karyotype, demonstrating that the presence of the Y chromosome is sufficient to determine male sex. In Drosophila, experiments involving chromosomal manipulation, such as Calvin Bridges' work in 1921, provided evidence for the role of the X:A ratio in determining sex. The identification of key genes involved in sex determination, such as the SRY gene in humans and the Sex lethal, transformer, and doublesex genes in Drosophila, also supports their respective mechanisms. Comparative studies between species, like the similarities between the fruit fly gene "doublesex" and the human gene "DMRT1," further demonstrate the involvement of evolutionarily related genes in different sex-determination systems.

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01

Introduction to sex determination in Drosophila and humans

In humans, sex is determined by the presence or absence of the Y chromosome. Male individuals possess one X and one Y chromosome (XY), while females have two X chromosomes (XX). In contrast, Drosophila melanogaster, the fruit fly, relies on the ratio of X chromosomes to autosomes for sex determination. Female fruit flies have a balanced ratio of X chromosomes to autosomes (XX:A = 1), while males have an unbalanced ratio (X:A = 0.5).
02

Evidence for sex determination in humans

The major evidence supporting this conclusion in humans comes from studies on individuals with sex chromosome abnormalities. An individual with Klinefelter syndrome has an additional X chromosome (XXY) and is male, while an individual with Turner syndrome has only one X chromosome (XO) and is female. Additionally, individuals with an XYY karyotype are also male. These observations demonstrate that the presence of the Y chromosome is sufficient to determine male sex.
03

Evidence for sex determination in Drosophila

In Drosophila, evidence for the X:A ratio mechanism was derived from experiments involving chromosomal manipulation. One such experiment conducted by Calvin Bridges in 1921 involved the creation of fruit flies with different X:A ratios. His work demonstrated that X:A ratios of 0.5 produced males, ratios of 1 produced females, and intermediate ratios led to intersex individuals. This provided strong evidence for the role of the X:A ratio in determining sex in Drosophila.
04

Key genes in sex determination

Another important aspect in understanding sex determination is identifying key genes involved. In humans, the SRY gene on the Y chromosome plays a crucial role in determining maleness. Mutations in this gene can result in XY individuals developing as females. In Drosophila, key genes including Sex lethal (Sxl), transformer (tra), and doublesex (dsx) are involved in the sex-determination pathway. The expression of these genes is regulated by the X:A ratio, further supporting the fruit fly's sex determination mechanism.
05

Comparative studies of sex determination

Comparative studies between species help us to understand the evolution of sex-determination mechanisms. Although humans and Drosophila use different strategies for sex determination, research has shown that some elements of the molecular pathways are conserved across species. The discovery that the fruit fly gene 鈥渄oublesex鈥� and the human gene 鈥淒MRT1鈥� have similarities in their DNA-binding domain and function in sex determination demonstrates that evolutionarily related genes can be involved in different sex-determination systems. In conclusion, evidence for sex determination in humans and Drosophila comes from observations of individuals with sex chromosome abnormalities, chromosomal manipulation experiments, identification of key genes involved in the process, and comparative studies. Understanding how these mechanisms operate provides insight into the evolution and development of sexual systems in animals.

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Most popular questions from this chapter

In mice, the Sry gene (see Section 5.2) is located on the Y chromosome very close to one of the pseudoautosomal regions that pairs with the X chromosome during male meiosis. Given this information, propose a model to explain the generation of unusual males who have two X chromosomes (with an Sry-containing piece of the Y chromosome attached to one X chromosome).

Review the Chapter Concepts list on p. \(83 .\) These all center on sex determination or the expression of genes encoded on sex chromosomes. Write a short essay that discusses sex chromosomes as they contrast with autosomes.

Distinguish between the Protenor and Lygaeus modes of sex determination.

Cat breeders are aware that kittens expressing the X-linked calico coat pattern and tortoiseshell pattern (Figure 5.6 ) are almost invariably females. Why?

In chickens, a key gene involved in sex determination has recently been identified. Called \(D M R T 1\), it is located on the \(Z\) chromosome and is absent on the W chromosome. Like SRY in humans, it is male determining. Unlike \(S R Y\) in humans, however, female chickens (ZW) have a single copy while males (ZZ) have two copies of the gene. Nevertheless, it is transcribed only in the developing testis. Working in the laboratory of Andrew sinclair (a co- discoverer of the human \(S R Y\) gene), Craig Smith and colleagues were able to "knock down" expression of \(D M R T 1\) in \(Z Z\) embryos using RNA interference techniques (see Chapter 16 ). In such cases, the developing gonads look more like ovaries than testes [Nature 461: 267 (2009)]. What conclusions can you draw about the role that the DMRT1 gene plays in chickens in contrast to the role the SRY gene plays in humans?
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