Sex Determination by Dr.M.Jothimuniyandi

Sex Determination - Dr.M.Jothimuniyandi -Genetic Sex Determination -Environmental Sex Determination -Genic Sex Determination -Hormonal Regulation in Sex Differentiation-Disorders of Sex Development (DSD) in Humans-Evolution of Sex Determination Mechanisms

1. Sex Determination Dr.M.Jothimuniyandi Assistant Professor

2. Sex determination Sex determination is the biological process by which an organism develops into either a male or a female. In vertebrates, the mechanism of sex determination can vary across species, and it typically involves genetic and environmental factors.

3. Genetic Sex Determination (GSD) This is the most common form of sex determination in vertebrates, where the combination of sex chromosomes determines an individual's sex. a) XX/XY System Found in mammals , including humans, as well as some reptiles and amphibians. Males have an XY chromosome pair, while females have XX. The Y chromosome carries the SRY (Sex-determining Region Y) gene, which initiates the development of male characteristics by triggering the formation of testes. The testes then produce testosterone, leading to male differentiation. In the absence of the Y chromosome (i.e., XX), the default development path is female, as the ovaries form, and estrogen guides female development.

4. Genetic Sex Determination (GSD) b) ZW/ZZ System Found in birds, some reptiles, amphibians, and insects . In this system, females are ZW (heterogametic), and males are ZZ (homogametic). The exact trigger for sex determination in the ZW system is less well understood than in the XX/XY system, but it is thought to involve the DMRT1 gene located on the Z chromosome, which plays a critical role in male development.

5. Genetic Sex Determination (GSD) c) XX/XO System Seen in some insects , such as grasshoppers. Females have two X chromosomes (XX), while males have only one X chromosome (XO). The absence of a second X chromosome leads to male development.

6. Environmental Sex Determination (ESD) In some vertebrates, environmental factors, such as temperature or social interactions, can determine the sex of an individual. a) Temperature-dependent Sex Determination (TSD) Found in many reptiles , such as turtles, crocodiles, and some lizards. The sex of the offspring is determined by the temperature at which eggs are incubated during a critical period of development. In turtles , lower temperatures typically produce males, while higher temperatures produce females. In crocodiles , intermediate temperatures often produce males, while both lower and higher extremes can produce females. The exact mechanism involves temperature-sensitive regulation of key genes, like aromatase , which converts testosterone into estrogen , influencing gonadal development.

7. Environmental Sex Determination (ESD) b) Social Sex Determination Found in some species of fish , such as clownfish . These species are often sequential hermaphrodites , meaning they can change sex during their lifetime based on social structure. For example, in clownfish, if the dominant female dies, the largest male in the group will change sex to become the new female. This change is controlled by complex interactions between hormones and environmental cues.

8. Genic Sex Determination (GSD) In some species, sex is determined by specific genes rather than entire sex chromosomes. Found in some reptiles, amphibians , and fish . Specific genes, such as dmrt1 in reptiles, are responsible for determining the sex of the organism without the need for distinct sex chromosomes. These genes regulate the development of the gonads into either testes or ovaries, much like the SRY gene in mammals.

9. Hormonal Regulation in Sex Differentiation Regardless of the primary sex determination mechanism (chromosomal, environmental, or genetic), hormones play a crucial role in the differentiation of sexual characteristics. Testosterone and its derivatives (like dihydrotestosterone ) promote the development of male secondary sexual characteristics, such as the development of male reproductive organs, increased muscle mass, and hair growth. Estrogens promote the development of female secondary sexual characteristics, including breast development, the menstrual cycle, and fat distribution. Aromatase , an enzyme that converts testosterone to estrogen , is a key player in many species, influencing whether an embryo develops male or female traits.

10. Disorders of Sex Development (DSD) in Humans In humans, genetic anomalies can result in mismatches between chromosomal, gonadal, and phenotypic sex, leading to conditions classified as Disorders of Sex Development . Androgen Insensitivity Syndrome (AIS) : Individuals with XY chromosomes are genetically male but have a defective androgen receptor, making their bodies insensitive to male hormones. As a result, they develop female characteristics. Congenital Adrenal Hyperplasia (CAH) : Genetic disorder in XX individuals that leads to excessive androgen production, causing masculinization of external genitalia.

11. Evolution of Sex Determination Mechanisms Sex determination mechanisms are thought to have evolved multiple times independently across different species. This evolutionary plasticity allows organisms to adapt to varying environmental conditions and ecological pressures. For instance, temperature-dependent sex determination in reptiles may have evolved to maximize reproductive success under specific environmental conditions, while chromosomal sex determination provides a more stable mechanism in mammals.

12. Sex Determination Source: https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQ3EzplNX3I1ZYUsIWUQLRxaaEZx1p2UytalJPSg4xjjc_ixA7tzjQEQHH_P0XglERQLB8&usqp=CAU

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