Sex Chromosomes: Why Sons Get Y from Father and X from Mother — and What That Means

Every human being begins with a single cell. In that cell, among the 46 chromosomes, lies the answer to a question that has shaped cultures, determined dynasties, and been catastrophically misunderstood for most of recorded history: what determines whether a person develops as male or female? The answer is chromosomal, precise, and decided at the moment of fertilisation — and it is entirely determined by the father's sperm, not the mother's egg.

The XX/XY system

Females carry two copies of the X chromosome (XX). Males carry one X and one Y chromosome (XY). The X chromosome is large — about 155 million base pairs, carrying approximately 800–900 genes. The Y chromosome is much smaller — about 57 million base pairs, carrying only approximately 70 protein-coding genes, many of them involved in male sex determination and sperm production.

The critical gene on the Y chromosome is SRY (Sex-determining Region Y) — a master switch gene that, when present and active during embryonic development, triggers a cascade that directs the undifferentiated gonads to develop into testes rather than ovaries. Without SRY, development defaults toward the female pathway.

Why the father — not the mother — determines sex

For most of recorded history, women were blamed, punished, and discarded when they "failed to produce sons." Henry VIII annulled two marriages and executed one wife in the pursuit of a male heir. The irony — invisible without genetics — is that the mother has no capacity to influence the sex of her child. Every egg a woman produces carries one X chromosome. That is the only option. The sperm, produced by the father, are the variable: approximately half carry an X chromosome, half carry a Y. At fertilisation it is the sperm that decides.

"The gene is the fundamental unit of heredity — and what we discover in that unit changes everything we thought we knew about identity, fate, and the meaning of what we inherit."

X-linked inheritance: why some traits affect sons more than daughters

The X chromosome carries approximately 800 functional genes, many of them having nothing to do with sex. Conditions caused by recessive alleles on the X chromosome follow a distinctive inheritance pattern: X-linked recessive conditions affect males far more commonly than females, because males have only one X chromosome. If that single X carries the disease allele, there is no second X to provide a working copy.

Historical note: the weight of biological misattribution

The genetic mechanism of sex determination was not understood until the early twentieth century. Thomas Hunt Morgan's work on fruit flies, beginning in 1908, established that sex was chromosomally determined and that genes on the X chromosome followed a distinctive inheritance pattern. The full mechanism of SRY as the master sex-determination switch was not identified until 1990.

Before this understanding, the cultural consequences of biological ignorance were severe. Dynasties were built on the assumption that a woman's body "chose" to produce sons or daughters. The European haemophilia that spread through Queen Victoria's descendants — a condition carried silently by daughters and expressed devastatingly in sons — was attributed to various causes before the genetics became clear. Mukherjee's point in The Gene is that the history of genetics is inseparable from the history of what we thought we knew about inheritance — and what that wrong knowledge was used to justify.

The X and Y chromosomes are not merely sex determinants. They carry functional genes whose expression differences between males and females contribute to differences in disease susceptibility, drug metabolism, immune function, and longevity. The Y chromosome has been slowly losing genes across evolutionary time — from an ancestral autosome carrying hundreds of genes, it has shed all but approximately 70. Whether this erosion will continue is an open question in evolutionary biology.