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This article is for academic and educational purposes only, intended for Pharm.D students studying anatomy and physiology. It does not promote or discuss explicit content but focuses on medical and pharmaceutical knowledge.
Introduction to Sex Determination:
There are two different types of sexes that participate in sexual reproduction. It is natural to wonder from which parent the baby inherits its sex. The process of sex determination involves genetic and environmental factors that decide whether the offspring will be male or female.
In some species, environmental signals such as temperature influence the sex of the baby. For instance, in certain reptiles, the incubation temperature determines whether the hatchling will be male or female. In other animals like snails, sex can change as they are not genetically fixed. However, in human beings, sex is genetically determined — meaning, the genes inherited from the parents decide the child’s sex.
Determining the Sex in Humans:
To understand how sex determination occurs in humans, it is essential to know the following:
- Humans have 23 pairs of chromosomes. Out of these, 22 pairs are autosomes, and one pair is the sex chromosome, which plays an active role in sex determination.
- Both males and females carry two sex chromosomes.
- Males have one X and one Y chromosome (XY), both of which are active.
- Females have two X chromosomes (XX), in which one X chromosome is active.
- The XY sex-determination system is found in humans, mammals, some insects, and a few plant species.
- All children inherit an X chromosome from their mother, regardless of their sex. Thus, the sex of the child depends on the type of chromosome inherited from the father.
- If the sperm contributes a Y chromosome, the child will be a boy (XY); if it contributes an X chromosome, the child will be a girl (XX).
- A man’s semen contains approximately 50% X-bearing and 50% Y-bearing sperm cells, which is why it is possible for men to have multiple children of the same gender.
Chromosomal Basis of Sex Determination:
When viewed under a microscope, the 46 human chromosomes in a normal somatic cell can be identified by their size, shape, and staining pattern, forming 23 pairs. An entire set of chromosomes arranged in decreasing order of size and centromere position is called a karyotype.
Of the 23 pairs, 22 are autosomes, which look identical in both males and females. The 23rd pair represents the sex chromosomes — these differ between males and females. In females, the pair consists of two X chromosomes, while in males, there is one X and one much smaller Y chromosome.
The Y chromosome contains only about 231 genes, which is less than 10% of the number found on the largest autosome (chromosome 1). During meiosis, spermatocytes divide to produce two types of sperm — half carrying an X chromosome and half carrying a Y chromosome. Oocytes, on the other hand, contain only X chromosomes.
If an X-bearing sperm fertilizes the ovum, the resulting zygote will develop into a female (XX). If a Y-bearing sperm fertilizes the ovum, the zygote will develop into a male (XY). Therefore, it is the father’s chromosome that determines the sex of the child.
Role of the SRY Gene in Sex Determination:
Both male and female embryos develop identically until around the 7th week after fertilization. At this point, one or more genes initiate a cascade of events that lead to male development. The key gene responsible is the SRY (Sex-determining Region of the Y chromosome).
The SRY gene acts as a molecular switch that activates male differentiation. If this gene is present and functional, the fetus develops testes and becomes male. In the absence of SRY, the fetus develops ovaries and becomes female.
Genetic Evidence and Case Studies:
Several human case studies have confirmed the critical role of the SRY gene:
- Phenotypic females with an XY genotype were found to have mutated or nonfunctional SRY genes, preventing normal male development.
- Phenotypic males with an XX genotype were discovered to have a small fragment of the Y chromosome (including the SRY gene) inserted into one of their X chromosomes, allowing them to develop as males.
Thus, the presence or absence of a functional SRY gene is the ultimate genetic factor that determines whether the embryo develops as male or female.
Detailed Notes:
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