Describe chromosomal theory of sex determination.

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Q. Describe chromosomal theory of sex determination.

Telangana TsbieTelangana Board of Intermediate Education 2025Subjective· 8mImportance★★★★★

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Concept understanding — Chromosomal Sex Determination Mechanisms

Chromosomal Sex Determination Mechanisms

Long before anyone knew what a chromosome actually did, a German biologist named

Henking (1891) was studying spermatogenesis in a firebug (an insect) under the

microscope. He noticed that during meiosis, a particular nuclear structure behaved

oddly: it moved to one pole of the cell without a visible partner to pair with, so

about half the sperm received it and half didn't. Unable to say what it was, he simply

labelled it the "X body". It was only later work — showing this body was a real

chromosome, and that its presence or absence in a sperm directly decided the offspring's

sex — that let the "X body" be recognised for what it truly was: the first

identification of a sex chromosome. Henking's own observation is why we still call

it the X chromosome today, and it is the historical starting point of the whole

chromosomal theory of sex determination.

The Main Systems

Once chromosomes were established as the basis of sex determination, several distinct

systems were found across different organism groups:

XX–XO type (e.g. grasshopper, some insects): females have two X chromosomes (XX), males have only one (XO — the "O" means no partner chromosome at all, not a second sex chromosome). A male produces two kinds of sperm — half with the X, half without — so sperm determine the offspring's sex.
XX–XY type (e.g. Drosophila, humans): females are XX (homogametic — produce only one kind of egg), males are XY (heterogametic — produce two kinds of sperm, X- bearing and Y-bearing). Again, it is the male's gamete that decides the offspring's sex.
ZZ–ZW type (e.g. birds, some fish and reptiles): this system is the reverse of XY in one important way — here it is the female who is heterogametic (ZW) and the male who is homogametic (ZZ). So in birds, it is the mother's egg (Z-bearing or W-bearing) that determines the sex of the offspring, not the father's sperm. …

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The chromosomal theory of sex determination, established by Henking, McClung, Sutton and Boveri, showed that sex is determined by specific sex chromosomes contributed by the parents, not simply by environmental or other factors.

Background: In many organisms, chromosomes exist as homologous pairs (autosomes) that are the same in both sexes, but one pair (the sex chromosomes) differs between males and females and determines the sex of the offspring. This was demonstrated by early cytological studies (Henking's identification of the "X-body", and McClung's, Sutton's and Boveri's chromosome studies), which correlated the segregation and inheritance pattern of Mendelian factors with the behaviour of chromosomes during meiosis - forming the basis of the chromosomal theory of inheritance and sex determination.

XX-XY type (seen in humans, Drosophila, and most mammals):

Humans have 23 pairs of chromosomes: 22 pairs of autosomes (same in both sexes) and 1 pair of sex chromosomes.
Females have two X chromosomes (XX) - they are the homogametic sex, producing only X-bearing eggs.
Males have one X and one Y chromosome (XY) - they are the heterogametic sex, producing two types of sperm: 50% carrying an X chromosome and 50% carrying a Y chromosome.
At fertilisation, if an X-bearing sperm fertilises the egg, the zygote is XX (develops as female); if a Y-bearing sperm fertilises the egg, the zygote is XY (develops as male).
Since the sex chromosome contributed by the sperm (father) determines the offspring's sex, the male is said to determine the sex of the child in this system.

Other chromosomal sex-determination systems (for completeness):

…

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set V11 markMCQ
$Q. Identify the correct match.$
(a) XO - Male grasshopper
(b) XX - Male Drosophila
(c) ZW - Male bird
(d) ZZ - Female bird
›Reveal solutionSolution

The grasshopper male is XO; the other matches are reversed.

In grasshoppers, sex determination is of the XO type: the male has a single X (XO) while the female is XX. The other options are wrong: in Drosophila XX is female (male is XY); in birds the fem …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. In which of the following organism XO type of Sex determination is found?$
(a) Human
(b) Grasshopper
(c) Drosophila
(d) Honey Bee
›Reveal solutionSolution
In the XO type of sex determination (seen in grasshoppers and many other insects), males have only a single X chromosome and no second sex chromosome, while females have two X chromosomes.

Different organisms use different chromosomal mechanisms of sex determination:

XO type — males are XO (one X, no Y; total chromosome number is one less than in females), females are XX. This is characteristic of grasshoppers and several other insects. Males produce two types of sperm — one carrying the X chromosome and one carrying no sex chromosome — so the sperm determines the sex of the offspring.

XY type — seen in humans (males XY, females XX; option a is therefore incorrect for XO). …
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CBSE 2024Set ANNUAL1 mark
$Q. Write type of sex determination in Grasshopper.$
›Reveal solutionSolution

Grasshoppers show the XO type of sex determination, in which males lack a second sex chromosome altogether.

In grasshoppers, females have two X chromosomes (XX), while males have only a single X chromosome and no corresponding second sex chromosome (XO). During meiosis, the male produces two types of sperm — half carrying the X chromosome and half carrying no sex chromosome. Fusion with the female's X-bearing egg then determines the offspring's sex: an X-b …

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CBSE 2023Set 57/3/11 markMCQ
$Q. Choose the set of sex determining chromosomes that indicates the correct sex of the respective organism.$
(a) Homozygous sex chromosomes (ZZ) determine female sex in birds
(b) XO type of sex chromosomes determine male sex in grasshoppers
(c) XXY condition in humans, as found in Turner Syndrome, determines male sex
(d) Homozygous sex chromosomes (XX) produce male sex in Drosophila
›Reveal solutionSolution
The key idea is that sex determination systems vary across organisms — the XO system in grasshoppers makes males XO (heterogametic) and females XX (homogametic). Among the given options, only option (b) is correct.

The question tests your understanding of the different sex determination mechanisms found in nature. Many students memorise "XX = female, XY = male" for humans and assume it works everywhere — but that's a dangerous shortcut. Different organisms have evolved completely different systems.

Let's examine each option carefully.

Option (a): "Homozygous sex chromosomes (ZZ) determine female sex in birds"

Birds use the ZZ-ZW system, which is the reverse of the mammalian XY system. In birds, the female is the heterogametic sex (ZW) and the male is homogametic (ZZ). So ZZ gives a male, not a female. This option is wrong.

Option (b): "XO type of sex chromosomes determine male sex in grasshoppers"

Grasshoppers use the XO system (also called XX-XO). Females have two X chromosomes (XX), while males have only one X (XO — the O means "absent"). So the male is XO, and the female is XX. This matches the statement exactly. This option is correct.

Option (c): "XXY condition in humans, as found in Turner Syndrome, determines male sex"

There are two errors here. First, Turner Syndrome in humans is XO (monosomy X), not XXY. Second, XXY in humans is Klinefelter Syndrome, which produces a male phenotype (though with some differences). But the option says "as found in Turner Syndrome" — that's factually wrong. The condition named is incorrect, so the option is wrong.

Option (d): "Homozygous sex chromosomes (XX) produce male sex in Drosophila" …
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CBSE 2023Set ANNUAL1 markMCQ
$Q. The sex chromosome in male grasshopper is:$
(a) XX
(b) XY
(c) XO
(d) ZZ
›Reveal solutionSolution
Grasshoppers use the XO sex-determination system; males are XO (one X, no Y) and females are XX.

In many insects, including the grasshopper, sex is determined by the number of X chromosomes rather than by a distinct Y chromosome.

Females have two X chromosomes (XX) and produce only one type of egg (X-bearing).

Males have only one X chromosome and no Y — written as XO. During meiosis, males produce two types of sperm: 50% carry the X chromosome and 50% carry no sex chromosome at all (O). …
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Q.Describe chromosomal theory of sex determination.

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