Sex Determination
பாலின நிர்ணயம்
Stop blaming the mother for the sex of the child. It is a biological impossibility. The father’s sperm is the sole determinant of the zygote's chromosomal fate.
The idea
Human sex determination is chromosomal. Females are homogametic (XX), producing only X-bearing ova. Males are heterogametic (XY), producing 50% X-bearing and 50% Y-bearing sperm. Fertilization of an ovum by an X-sperm results in a female (XX), while a Y-sperm results in a male (XY).
Why it matters
It is a genetic mechanism to maintain the sex ratio in the population through chromosomal inheritance.
NEET lens
NEET focuses on the 'Heterogametic' nature of the human male. Do not confuse this with bird sex determination (ZW-ZZ), which is the reverse. Questions often test the probability of male vs female offspring; it is always 50:50. Memorize that the Y-chromosome carries the SRY gene, which is the master switch for male development.
Clinical significance
Aneuploidy involving sex chromosomes leads to clinical syndromes like Turner syndrome (XO) or Klinefelter syndrome (XXY). These occur due to non-disjunction during meiosis, not the standard fertilization process. Understanding normal sex determination is the baseline for diagnosing these genetic disorders.
Introduction
Sex determination in humans is chromosomal — XX = female, XY = male — with the SRY gene on the Y chromosome driving testicular development. The mother always contributes X; the father contributes X (→ daughter) or Y (→ son), so sex is determined by the sperm.
Core concept
Human sex chromosomes: Females: 44 autosomes + XX (46,XX). Males: 44 autosomes + XY (46,XY). SRY gene (Sex-determining Region Y): located on short arm of Y chromosome. SRY → TDF (Testis-Determining Factor). Undifferentiated gonad: both sexes have bipotential gonad. SRY present → Sertoli cells differentiate → produce AMH + testosterone → male development. No SRY → default female development (ovaries develop). Female as 'default': in absence of SRY/TDF, gonad becomes ovary. Hormonal cascade: Testosterone → Wolffian duct → epididymis, vas deferens, seminal vesicle. AMH → Müllerian duct regression. DHT (dihydrotestosterone, from testosterone via 5α-reductase) → external genitalia masculinization.
Go deeper
De la Chapelle syndrome (XX male): rare individuals with 46,XX karyotype who are phenotypically male. Cause: SRY gene translocated from Y to X (during paternal meiosis) → SRY on X chromosome → testicular development despite no Y. Proves SRY is sufficient for male sex determination.
From Class 10
Whether an embryo develops as male or female is determined by chromosomes — specifically, whether it receives an X or Y chromosome from the father. The Y chromosome carries the gene for maleness.