Sex determination—the process that decides whether an
organism develops as male or female—has fascinated scientists for centuries.
Before the discovery of sex chromosomes, the genetic factors behind male and
female differences were largely a mystery. The breakthrough discovery of these
chromosomes revolutionized our understanding of how sex is inherited and
controlled at the genetic level.
The Breakthrough: Discovery of Sex
Chromosomes
The story begins in 1910 with the pioneering geneticist
Thomas Hunt Morgan, who studied the fruit fly Drosophila melanogaster—a
tiny insect commonly used in genetic research. Normally, these flies have red
eyes. However, Morgan discovered a male fly with white eyes, a rare mutation.
When this white-eyed male mated with a red-eyed female, the first generation
(F1) offspring all had red eyes. But when these F1 flies bred among themselves,
the second generation (F2) included both red-eyed females and white-eyed males,
with the latter appearing in a predictable ratio.
This observation led Morgan to link eye color inheritance
with specific chromosomes and marked the first evidence of sex-linked traits.
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| Chromosomes in Drosophila |
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| Chromosomes in Man |
Chromosomes and Their Role in Sex
Determination
Chromosomes in Fruit Flies
Prior to Morgan’s discovery, researchers knew that Drosophila
had four pairs of chromosomes (eight in total). Three pairs were
autosomes—chromosomes that are the same in both males and females. The fourth
pair, however, differed between the sexes and was called the sex chromosomes.
- Females:
Have two rod-shaped, identical X chromosomes (XX).
- Males:
Have one rod-shaped X chromosome and one smaller, hooked Y chromosome
(XY).
The unique structure and inheritance patterns of these sex
chromosomes explained the genetic differences between males and females.
Human Chromosomes and Sex Determination
Humans have 23 pairs of chromosomes, with 22 pairs being
autosomes and one pair being sex chromosomes. Like fruit flies:
- Females
carry two X chromosomes (XX).
- Males
carry one X and one Y chromosome (XY).
In human reproduction, every egg cell contains a single X
chromosome. Sperm cells carry either an X or a Y chromosome, and it is the
sperm’s chromosome that determines the sex of the child. If a sperm with an X
chromosome fertilizes the egg, the resulting child will be female (XX). If a
sperm with a Y chromosome fertilizes the egg, the child will be male (XY).
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| Sex determination in human |
The Key Player: The SRY Gene
At the heart of male sex determination lies a gene called SRY—short
for Sex-determining Region Y. Located on the short arm of the Y
chromosome, the SRY gene acts as a genetic switch that triggers the development
of male characteristics.
Without SRY, the embryo typically develops as female. Its
presence initiates the formation of testes and the production of male hormones,
shaping the pathway toward male development.
The discovery of sex chromosomes and the SRY gene has
dramatically advanced our understanding of genetic sex determination. From
Morgan’s fruit flies to modern human genetics, this knowledge provides critical
insights into biology, heredity, and the remarkable complexity of life.
