Sex Determination
BiologyPrinciples of Inheritance and VariationFor NEET aspirants
Sex determination is the mechanism that decides whether a zygote develops into a male or a female. This page traces the chromosomal basis of sex from Henking's X body, then covers the XO type in grasshoppers, the XY type in humans and Drosophila, the ZW type in birds and the haplodiploid system of the honey bee, as in the NCERT Class 12 chapter Principles of Inheritance and Variation. NEET often asks which sex is heterogametic, chromosome numbers in honey bees and how the sperm decides the sex of a child in human sex determination.
On this page1The X body2XO type3XY type4Male and female heterogamety5Sex in humans6Honey bee7Exam essentials8Quick revision9Solved examples10Practice
Key Points at a Glance
- ★ Must learn Henking (1891) traced a nuclear structure, the X body, in 50 per cent of the sperms of some insects; it was later shown to be the X chromosome.
- Sex chromosomes decide sex; all the other chromosomes are autosomes.
- ★ Must learn XO type (grasshopper): female XX, male XO; the male has one chromosome less.
- XY type (humans, Drosophila): female XX, male XY; both sexes have the same number of chromosomes.
- ★ Must learn XO and XY are male heterogamety; ZW in birds is female heterogamety (female ZW, male ZZ).
- ★ Must learn Humans: 23 pairs = 22 pairs of autosomes + 1 pair of sex chromosomes; the sperm decides the sex of the child.
- Every pregnancy carries a 50 per cent chance of a male and a 50 per cent chance of a female child.
- ★ Must learn Honey bee (haplodiploid): fertilised egg gives a diploid female (32); unfertilised egg gives a haploid male, the drone (16).
- Drones form sperms by mitosis; they have no father and no sons, but have a grandfather and can have grandsons.
1. The Chromosomal Basis of Sex: Henking's X Body
- The mechanism of sex determination long puzzled geneticists.
- The first clues to its genetic or chromosomal basis came from experiments on insects.
- Cytological observations in many insects led to the idea of a genetic or chromosomal basis of sex determination.
- ★ Exam imp Henking (1891) traced a specific nuclear structure all through spermatogenesis in a few insects.
- He observed that 50 per cent of the sperms received this structure after spermatogenesis, while the other 50 per cent did not.
- He named it the X body, but could not explain its significance.
- Later work showed that Henking's X body was in fact a chromosome, and it was named the X chromosome.
Key idea
Henking's X body, seen in only half the sperms, turned out to be the X chromosome.
2. The XO Type
- In a large number of insects, sex determination is of the XO type.
- All eggs carry an additional X chromosome besides the other chromosomes (autosomes).
- Some sperms carry the X chromosome, while others do not.
- ★ Exam imp An egg fertilised by a sperm with an X chromosome becomes a female; one fertilised by a sperm without an X chromosome becomes a male.
- So the male has one chromosome fewer than the female: the two sexes have unequal chromosome numbers.
- Because the X chromosome is involved in determining sex, it was called the sex chromosome, and the rest of the chromosomes were named autosomes.
- ★ Exam imp Grasshopper: an example of the XO type. The male has only one X chromosome besides the autosomes; the female has a pair of X chromosomes.
★ Very important Sex chromosomes are the chromosomes involved in determining sex and differing between the sexes; autosomes are all the other chromosomes, common to both sexes. In the XO type, the male has one chromosome fewer than the female.
Memory Trick
O means zero, not a chromosome. In XO, the ‘O’ shows that the second sex chromosome is absent: the male has a single X and one chromosome fewer than the XX female.
Quick Recall: tap to check
Name an insect with the XO type of sex determination.
The grasshopper.
Which chromosomes did the rest of the set come to be called?
Autosomes; the X was called the sex chromosome.
Do XO males and XX females have the same number of chromosomes?
No. The XO male has one chromosome fewer.
Key idea
In XO insects such as the grasshopper, the presence or absence of an X in the sperm decides the sex.
3. The XY Type
- Studies of many other species followed, to understand the mechanism of sex determination.
- ★ Exam imp In a number of other insects and in mammals, including man, the XY type is seen: males and females have the same number of chromosomes.
- Males have an X chromosome, but its partner is distinctly smaller; it is called the Y chromosome.
- Females have a pair of X chromosomes.
- Both sexes carry the same number of autosomes. So males have autosomes plus XY, and females have autosomes plus XX.
- In human beings and in Drosophila, males have one X and one Y chromosome; females have a pair of X chromosomes besides the autosomes.
XO type
- Female XX, male XO
- Male has one chromosome fewer
- Example: grasshopper
XY type
- Female XX, male XY
- Same chromosome number in both sexes
- Examples: humans, Drosophila
Key idea
XY males and XX females have equal chromosome numbers; the small Y marks the male.
4. Male and Female Heterogamety
- In both the XO and the XY types, males produce two different types of gametes: with or without an X chromosome (XO type), or with an X or a Y chromosome (XY type).
- ★ Exam imp Such a mechanism, in which the male produces two kinds of gametes, is called male heterogamety.
- In some other organisms, such as birds, a different mechanism is seen.
- Here the total number of chromosomes is the same in males and females.
- ★ Exam imp But the female produces two different types of gametes in terms of the sex chromosomes: female heterogamety.
- To mark the difference, the two different sex chromosomes of a female bird are called the Z and W chromosomes.
- Females have one Z and one W chromosome; males have a pair of Z chromosomes besides the autosomes.
- In chicken, the sex chromosomes are ZZ in the male and ZW in the female.
| Type | Female | Male | Heterogametic sex | Example |
|---|---|---|---|---|
| XO | XX | XO | Male | Grasshopper |
| XY | XX | XY | Male | Humans, Drosophila |
| ZW | ZW | ZZ | Female | Birds (chicken) |
★ Very important Heterogametic sex: the sex that forms two kinds of gametes for the sex chromosomes; homogametic sex: the sex that forms one kind. In XO and XY systems the male is heterogametic; in birds the female is heterogametic.
- So in birds, the egg, not the sperm, decides the sex of the chick: every sperm carries Z, while an egg carries either Z (giving a male, ZZ) or W (giving a female, ZW).
Memory Trick
Birds are the reverse. Mammals: father is XY, so the father decides. Birds: mother is ZW, so the mother decides. Only the female bird carries W.
NEET Focus
Students often mark the male bird as heterogametic. In birds the female (ZW) is heterogametic and the male (ZZ) homogametic. In both the XO and XY types the male is heterogametic, but only in the XO type do the sexes differ in chromosome number.
Key idea
The sex that forms two kinds of gametes for the sex chromosomes (the XO or XY male, the ZW female) is heterogametic, and it decides the sex of the offspring.
5. Sex Determination in Humans
- In humans, the sex-determining mechanism is of the XY type.
- ★ Exam imp Of the 23 pairs of chromosomes, 22 pairs are exactly the same in males and females: these are the autosomes.
- A pair of X chromosomes is present in the female; an X and a Y chromosome together determine male characteristics.
- During spermatogenesis, males produce two types of gametes: 50 per cent of the sperms carry an X chromosome and 50 per cent carry a Y chromosome, besides the autosomes.
- Females produce only one type of ovum, with an X chromosome.
- The ovum has an equal probability of being fertilised by a sperm carrying X or one carrying Y.
- If the ovum is fertilised by an X-carrying sperm, the zygote develops into a female (XX).
- If the ovum is fertilised by a Y-carrying sperm, the zygote develops into a male (XY).
★ Very important The genetic make-up of the sperm determines the sex of the child. In each pregnancy there is always a 50 per cent probability of a male and a 50 per cent probability of a female child.
- In our society, women are unfortunately blamed for giving birth to female children, and have been ostracised and ill-treated because of this false notion.
- Genetics shows that the mother contributes an X chromosome in every case; it is the father's sperm that decides the sex.
Memory Trick
Mother gives X; father decides. Every ovum carries X, so the sperm's X or Y settles the sex: X from father gives a daughter, Y from father gives a son.
Quick Recall: tap to check
How many autosomes does a human somatic cell have?
44 (22 pairs), plus one pair of sex chromosomes.
What percentage of human sperms carry a Y chromosome?
50 per cent.
Is the sperm or the egg responsible for the sex of a chick?
The egg, because the female bird is ZW and forms Z-bearing and W-bearing eggs.
Key idea
In humans the ovum always gives X; the X or Y of the sperm decides the sex, with equal chances.
6. Sex Determination in the Honey Bee
- ★ Exam imp In the honey bee, sex depends on the number of sets of chromosomes an individual receives.
- An offspring formed by the union of a sperm and an egg develops as a female (queen or worker).
- An unfertilised egg develops as a male (drone) by parthenogenesis.
- So males have half the number of chromosomes of females.
- ★ Exam imp Females are diploid, with 32 chromosomes; males are haploid, with 16 chromosomes.
- This is called the haplodiploid sex-determination system.
6.1 Special features of the haplodiploid system
- Males produce sperms by mitosis, since they are already haploid.
- Males have no father, because they develop from unfertilised eggs.
- So males cannot have sons: their sperms only fertilise eggs, which become females.
- But a male has a grandfather (his mother's father) and can have grandsons (through his daughters).
Memory Trick
Drone: no father, no son; but a grandfather and grandsons. Unfertilised egg, so no father; his sperms make only daughters, so no sons; his mother had a father, and his daughters can lay unfertilised eggs.
Tips and Tricks
For honey-bee numbers, halve or double 32. Female 32, egg 16, drone 16, sperm 16. Sperms come from mitosis (16 to 16), eggs from meiosis (32 to 16).
Key idea
In honey bees, fertilisation decides sex: diploid females (32) from fertilised eggs, haploid drones (16) from unfertilised eggs.
7. Exam Essentials
Pairs to Match
| Organism or term | Matches with |
|---|---|
| Henking (1891) | X body in 50 per cent of insect sperms |
| Grasshopper | XO type; male XO |
| Humans | XY type; male heterogamety |
| Drosophila | XY type; male XY |
| Birds (chicken) | ZW type; female heterogamety |
| Female bird | ZW |
| Male bird | ZZ |
| Honey bee | Haplodiploid sex determination |
| Drone | Haploid male, 16 chromosomes |
| Queen or worker | Diploid female, 32 chromosomes |
| Parthenogenesis | Development of an unfertilised egg into a drone |
| Sperm formation in drones | Mitosis |
| Human autosomes | 22 pairs |
Exceptions
- In the XO type, the male has one chromosome fewer than the female, unlike the XY and ZW types.
- In birds the female is heterogametic, unlike humans and Drosophila.
- Honey bee sex depends on the number of chromosome sets, not on sex chromosomes.
- Drones form sperms by mitosis, not meiosis.
- A drone has no father and no sons.
- In humans the mother does not decide the sex of the child; the sperm does.
Numbers to Remember
- 1891: Henking's X body; seen in 50 per cent of sperms.
- Humans: 23 pairs = 22 pairs of autosomes + 1 pair of sex chromosomes.
- Human sperms: 50 per cent X-bearing, 50 per cent Y-bearing.
- Chance of a male or a female child in each pregnancy: 50 per cent each.
- Honey bee: female 32 (diploid), male 16 (haploid).
8. Quick Revision
- Clues to chromosomal sex determination came from insects.
- Henking (1891): X body in 50 per cent of sperms; later identified as the X chromosome.
- Sex chromosomes decide sex; the rest are autosomes.
- XO type (many insects, grasshopper): female XX, male XO; male has one chromosome fewer.
- XY type (many insects, mammals, humans, Drosophila): female XX, male XY; equal chromosome numbers.
- Y is distinctly smaller than X.
- XO and XY: male heterogamety (male forms two kinds of gametes).
- Birds: female heterogamety; female ZW, male ZZ; equal chromosome numbers.
- In birds, the egg decides the sex of the chick.
- Humans: 22 pairs of autosomes + XX (female) or XY (male).
- Sperms are 50% X and 50% Y; ova all carry X.
- The sperm decides the sex; each pregnancy has a 50% chance of either sex.
- Blaming women for female children is a false notion.
- Honey bee: fertilised egg gives female (32); unfertilised egg gives drone (16) by parthenogenesis.
- Drones: sperms by mitosis; no father, no sons; a grandfather and grandsons.
9. Solved Examples
Solved Example 1
Match List I with List II.
List I: A. Grasshopper, B. Human, C. Chicken, D. Honey bee
List II: I. Haplodiploid, II. Female heterogamety (ZW), III. XO type, IV. XY type
Choose the correct answer:
(A) A-III, B-IV, C-II, D-I
(B) A-IV, B-III, C-II, D-I
(C) A-III, B-II, C-IV, D-I
(D) A-I, B-IV, C-II, D-III
List I: A. Grasshopper, B. Human, C. Chicken, D. Honey bee
List II: I. Haplodiploid, II. Female heterogamety (ZW), III. XO type, IV. XY type
Choose the correct answer:
(A) A-III, B-IV, C-II, D-I
(B) A-IV, B-III, C-II, D-I
(C) A-III, B-II, C-IV, D-I
(D) A-I, B-IV, C-II, D-III
Solution:
Answer: (A). Grasshopper is XO (III), humans XY (IV), chicken ZW (II) and the honey bee haplodiploid (I).
Solved Example 2
Read the statements.
A. In the XO type, males have one chromosome fewer than females.
B. In birds, the male is heterogametic.
C. Human females produce only one type of ovum.
D. The honey bee drone develops from an unfertilised egg.
E. Drones produce sperms by meiosis.
Choose the correct answer:
(A) A, C and D only
(B) A, B and C only
(C) B, D and E only
(D) A, C, D and E only
A. In the XO type, males have one chromosome fewer than females.
B. In birds, the male is heterogametic.
C. Human females produce only one type of ovum.
D. The honey bee drone develops from an unfertilised egg.
E. Drones produce sperms by meiosis.
Choose the correct answer:
(A) A, C and D only
(B) A, B and C only
(C) B, D and E only
(D) A, C, D and E only
Solution:
Answer: (A). B is wrong: in birds the female (ZW) is heterogametic. E is wrong: drones are haploid and form sperms by mitosis.
Solved Example 3
Arrange the events leading to the birth of a son in humans in the correct order.
A. A Y-bearing sperm fertilises the ovum
B. The male forms X-bearing and Y-bearing sperms in equal numbers
C. The zygote has the sex chromosomes XY
D. The zygote develops into a male child
Choose the correct answer:
(A) B, A, C, D
(B) A, B, C, D
(C) B, C, A, D
(D) A, C, B, D
A. A Y-bearing sperm fertilises the ovum
B. The male forms X-bearing and Y-bearing sperms in equal numbers
C. The zygote has the sex chromosomes XY
D. The zygote develops into a male child
Choose the correct answer:
(A) B, A, C, D
(B) A, B, C, D
(C) B, C, A, D
(D) A, C, B, D
Solution:
Answer: (A). Spermatogenesis gives X and Y sperms (B); a Y sperm fertilises the X-bearing ovum (A); the zygote is XY (C) and develops into a male (D).
Solved Example 4
A female honey bee has 32 chromosomes. How many chromosomes are there in a drone and in its sperm, respectively?
(A) 16 and 8
(B) 32 and 16
(C) 16 and 16
(D) 32 and 32
(A) 16 and 8
(B) 32 and 16
(C) 16 and 16
(D) 32 and 32
Solution:
Answer: (C). The drone is haploid (16) and forms sperms by mitosis, so its sperms also have 16 chromosomes.
Solved Example 5
Which of the following statements is NOT correct?
(A) Henking named the X body
(B) Both sexes have the same number of autosomes in the XY type
(C) In humans, the mother decides the sex of the child
(D) In birds, males have a pair of Z chromosomes
(A) Henking named the X body
(B) Both sexes have the same number of autosomes in the XY type
(C) In humans, the mother decides the sex of the child
(D) In birds, males have a pair of Z chromosomes
Solution:
Answer: (C). The ovum always carries X; the sperm's X or Y decides the sex.
Solved Example 6
Statement I: In the XY type, males and females have the same number of chromosomes.
Statement II: In the XO type, all eggs carry an additional X chromosome.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect
Statement II: In the XO type, all eggs carry an additional X chromosome.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect
Solution:
Answer: (A). Both are correct. In the XO type, the sperm with or without an X decides whether the egg becomes female or male.
10. Practice Questions
Practice Questions
- Match List I with List II.
List I: A. Male heterogamety, B. Female heterogamety, C. Parthenogenesis, D. X body
List II: I. Henking, II. Birds, III. Drone, IV. Humans
(A) A-IV, B-II, C-III, D-I (B) A-II, B-IV, C-III, D-I (C) A-IV, B-II, C-I, D-III (D) A-III, B-II, C-IV, D-IAnswer: (A). Humans show male heterogamety, birds female heterogamety, drones arise by parthenogenesis, and Henking named the X body. - Read the statements about honey bees.
A. Workers are diploid females.
B. Drones have 32 chromosomes.
C. A drone can have grandsons.
D. A drone has a father.
Choose the correct answer: (A) A and C only (B) A, B and C only (C) B and D only (D) A, C and D onlyAnswer: (A). Drones are haploid (16), and they develop from unfertilised eggs, so they have no father. - Which of the following has the XO type of sex determination? (A) Human (B) Drosophila (C) Grasshopper (D) ChickenAnswer: (C). The male grasshopper has only one X chromosome besides the autosomes.
- Which statement about birds is NOT correct? (A) The female is ZW (B) The male is ZZ (C) Males and females have the same number of chromosomes (D) The sperm decides the sex of the chickAnswer: (D). The female forms Z and W eggs, so the egg decides the sex.
- How is the sex-determination mechanism different in birds? Is the sperm or the egg responsible for the sex of the chicks?Answer: Birds show female heterogamety: the female is ZW and the male ZZ. All sperms carry Z, while eggs carry Z or W, so the egg decides the sex of the chick.
- Do the male and female have the same number of chromosomes in the XO type?Answer: No. The male (XO) has one chromosome fewer than the female (XX).
- How is sex determined in human beings?Answer: Humans have the XY type. Females (XX) form one type of ovum, with X. Males (XY) form two types of sperms in equal numbers, with X or with Y. An ovum fertilised by an X sperm gives a female (XX); by a Y sperm, a male (XY). The sperm decides the sex, and each pregnancy has a 50 per cent chance of either sex.
Common Mistakes to Avoid
Watch out
- Saying the human mother decides the sex of the child. Correct: the sperm (X or Y) decides; every ovum carries X.
- Marking the male bird as heterogametic. Correct: the female bird (ZW) is heterogametic.
- Saying both sexes have equal chromosome numbers in the XO type. Correct: the XO male has one chromosome fewer.
- Giving drones 32 chromosomes. Correct: drones are haploid with 16; females are diploid with 32.
- Saying drones form sperms by meiosis. Correct: drones are haploid and form sperms by mitosis.
- Saying a drone has a father. Correct: a drone develops from an unfertilised egg, so it has no father, but it has a grandfather.
- Placing the grasshopper in the XY type. Correct: the grasshopper is the standard example of the XO type.
- Writing 23 pairs of autosomes in humans. Correct: 22 pairs of autosomes and 1 pair of sex chromosomes.
Frequently Asked Questions
What was Henking's X body?
In 1891 Henking traced a nuclear structure through spermatogenesis in some insects and found it in only 50 per cent of the sperms. He named it the X body but could not explain it. Later studies showed it was a chromosome, which was named the X chromosome.
What is the difference between the XO and XY types of sex determination?
In both, females are XX. In the XO type, as in grasshoppers, males have only one X and so one chromosome fewer than females. In the XY type, as in humans and Drosophila, males have an X and a smaller Y, so both sexes have the same number of chromosomes.
What is male heterogamety and female heterogamety?
In male heterogamety, the male forms two kinds of gametes for the sex chromosomes, as in XO and XY systems. In female heterogamety, the female forms two kinds of gametes, as in birds, where the female is ZW and the male ZZ.
Who decides the sex of a child in humans?
The father's sperm decides it. Every ovum carries an X chromosome, while half the sperms carry X and half carry Y. An X-bearing sperm gives a daughter (XX) and a Y-bearing sperm a son (XY). So blaming women for daughters is a false notion.
Why is the chance of a boy or a girl always 50 per cent in humans?
Males produce X-bearing and Y-bearing sperms in equal numbers, and an ovum has an equal chance of being fertilised by either. So in every pregnancy there is a 50 per cent probability of a male child and a 50 per cent probability of a female child.
How is sex determined in birds?
Birds show female heterogamety. The female has one Z and one W chromosome, while the male has two Z chromosomes, and both have the same total number of chromosomes. Because the female forms Z and W eggs, the egg decides whether the chick is male or female.
What is haplodiploid sex determination in honey bees?
Sex depends on the number of chromosome sets. A fertilised egg develops into a diploid female (queen or worker) with 32 chromosomes. An unfertilised egg develops by parthenogenesis into a haploid male, the drone, with 16 chromosomes.
Why does a drone have no father but a grandfather?
A drone develops from an unfertilised egg, so it has a mother but no father. Its mother, the queen, came from a fertilised egg, so she had a father, who is the drone's grandfather. A drone's sperms form only daughters, so it has no sons but can have grandsons.
Previous year questions on Sex Determination
7 questions from past papers, each with a step-by-step solution.
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