Chromosomal mechanisms
The sex of most animals is determined at the moment of fertilization by a combination of sex chromosomes. Depending on the type of gametes produced, homogametic (one type of gamete) and heterogametic (two types of gametes) sexes are distinguished.
There are several main types of chromosomal determination:
- XY type (humans, mammals). Females are homogametic (XX), males are heterogametic (XY). In humans, oocytes always carry an X chromosome, while spermatozoa carry either an X or a Y chromosome.
- ZW type (birds, butterflies). An inversion is observed: the female is heterogametic (ZW) and the male is homogametic (ZZ).
- XO type (Orthoptera, spiders). Depends on the number of sex chromosomes. In grasshoppers, females have two chromosomes (XX) and males have one (XO). In moths, the situation is reversed (female is XO).
In addition to the chromosomal mechanism, there is a haplodiploid mechanism (in bees and ants). Diploid individuals develop into females, while haploid individuals develop from unfertilized eggs into males (their spermatogenesis occurs via mitosis). In some reptiles (crocodiles), sex depends on environmental factors, particularly incubation temperature.
Genetic stage: primary determination
In the early stages of embryogenesis, gonads are initially indifferent (genital ridges). Primordial germ cells migrate into them from the yolk sac. Gonadal differentiation begins at week 7 and depends on genetic factors.
Development along the male pathway is triggered by the SRY gene (on the short arm of the Y chromosome). It contains no introns and encodes a transcription factor that activates a cascade of other genes. For example, the autosomal SOX9 gene (chromosome 17) ensures Sertoli cell differentiation, while the WT1 gene (chromosome 11) is essential for intermediate mesoderm formation. The AZF region (Y chromosome) is also critical for normal spermatogenesis; mutations in it lead to oligo- or azoospermia.
Development along the female pathway is triggered if the SRY gene is inactive by week 7. In this case, the DAX1 gene (on the X chromosome) and the FOXL2 gene (on chromosome 3) suppress the expression of male SOX9, directing differentiation toward ovaries.
Hormonal stage: secondary determination
Following gonad formation, the hormonal stage begins. Initially, the embryo has two types of ducts: Wolffian and Müllerian.
In the male body, interstitial Leydig cells synthesize testosterone (which is converted into active dihydrotestosterone by 5-alpha-reductase). It stimulates the transformation of Wolffian ducts into the spermatic ducts and the formation of external genitalia. Sertoli cells secrete anti-Müllerian hormone (AMH, gene on chromosome 19), which causes the regression of Müllerian ducts.
In the female body (weeks 8–10), estrogens are synthesized. Without the influence of androgens and AMH, the Wolffian ducts disappear, while the Müllerian ducts form the uterus, fallopian tubes, and the upper part of the vagina.
Receptor apparatus and pathologies
For hormones to perform their function, target tissues must possess receptors for them. The androgen receptor is encoded by the AR gene located on the long arm of the X chromosome (region Xq11-12).
A mutation in this gene leads to Morris syndrome (testicular feminization). With a 46,XY karyotype and a normal SRY gene, testes form and produce testosterone. However, due to a receptor defect, cells do not respond to the hormone, and phenotypic development proceeds along female lines.
Receptor sensitivity to dihydrotestosterone also plays a role in the development of androgenetic alopecia. In this condition, the frontal and parietal areas of the scalp are overly sensitive to androgens, leading to hair loss. The trait is inherited in an autosomal dominant manner in men and an autosomal recessive manner in women.
Briefly: answers to topical questions
Which organs develop from the Wolffian ducts under the influence of androgens?
During embryogenesis in males, internal reproductive organs and spermatic ducts develop from the Wolffian (mesonephric) ducts. Derivatives of these structures include:
- Vas deferens (ductus deferens) — functions as the primary excretion pathway;
- Seminal vesicles — develop in conjunction with the vas deferens;
- Epididymal duct — forms from the paired mesonephric duct.
In female embryos, these ducts regress and do not develop.
Which enzymes ensure the conversion of testosterone into active metabolites?
The conversion of testosterone into active metabolites is provided by two main enzymes.
- 5-alpha-reductase ($5\alpha$-reductase) — catalyzes the irreversible conversion of the hormone into the active metabolite dihydrotestosterone, which has a high affinity for receptors.
- Aromatase — ensures the process of peripheral conversion of testosterone into estrogen (estradiol).
Thanks to these enzymes, the hormone can exert biological effects not only directly, but also through its derivatives.
A simple association will help remember the chromosome inversion in birds (ZW type): unlike mammals, in birds the "weaker sex" produces different gametes (is heterogametic), while the "stronger sex" produces identical ones (ZZ).
Class and exam questions
What is the balance theory of sex determination?
This is a mechanism characteristic of Drosophila. Sex depends not on the physical presence of the Y chromosome, but on an index: the ratio of the number of X chromosomes to the number of haploid sets of autosomes. An index of 1.0 yields a female, and 0.5 yields a male.
Where do primordial germ cells come from?
Gonocytes (primordial germ cells) form outside the gonads—in the yolk sac of the embryo, after which they migrate into the developing genital ridges.
Why is the phenotype female in Morris syndrome despite the presence of a Y chromosome?
Due to a mutation in the AR gene, target tissues completely lose their sensitivity to male sex hormones. Testosterone is produced by the testes but cannot bind to receptors, so the external organs develop along female lines.
What else to study on this topic
- Organization and functions of the SRY gene at the molecular level
- Genetics of spermatogenesis: the role of the AZF region
- Mechanisms of secondary sex characteristic formation
- Parthenogenesis and ploidy in Hymenoptera insects
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