Q.What are sex-chromosomes and autosomes?
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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. …
The chromosomes of an organism are not all functionally alike — one pair is set aside specifically to decide the individual's sex, while the rest govern the body's other inherited characters. …
Sex chromosomes (XX/XY) decide the sex of an organism; autosomes are all the other chromosomes and control body characters, not sex.
In the human genome there are 23 pairs (46) of chromosomes. These are of two kinds:
Sex chromosomes (allosomes) — The one pair of chromosomes that decides the sex of the individual. In humans these are X and Y. A female is XX and a male is XY. They carry the sex-determining genes (plus some body-character genes).
Autosomes — The remaining 22 pairs of chromosomes that are alike in both males and females. They carry genes controlling somatic (body) characters and play no direct role in determining sex.
…
- CBSE 2026Set V11 markMCQQ.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 …
- CBSE 2025Set ANNUAL1 markMCQQ.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). …
- CBSE 2024Set ANNUAL1 markQ.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 …
- CBSE 2023Set 57/3/11 markMCQQ.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.
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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.
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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.
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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.
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Option (d): "Homozygous sex chromosomes (XX) produce male sex in Drosophila" …
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- CBSE 2023Set ANNUAL1 markMCQQ.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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