🔬 Sex Determination Mechanisms & Barr Body
Learn XX-XY, ZW-ZZ and genic balance theory of sex determination for CUET Agriculture. Barr body, Bridges theory and environmental factors.
Sex Determination
Sex determination is the process by which the sex of an organism is determined during development. Different organisms use different mechanisms, and sex can be determined at various stages of development:
- Progamic (before fertilization) — sex is determined before fertilization. Example: Honey Bee (Drone formation from unfertilized eggs)
- Syngamic (at fertilization) — sex is determined at the time of fertilization. This is the most common mechanism. Example: Most animals and plants, including humans
- Epigamic (after fertilization) — sex is determined after fertilization, influenced by the environment. Example: Honey Bee (Queen vs Worker — both are genetically female, but nutrition determines their fate)
Types of Chromosomes
Before understanding sex determination, it is important to distinguish between the two types of chromosomes:
- Autosomes — somatic chromosomes that control body (somatic) characters. These are the same in both sexes.
- Allosomes (Heterosomes) — sex chromosomes that are related to sex determination. These differ between males and females.
- Autosomes and Heterosomes were first described by Montgomery
- Sex chromosomes were first discovered by McClung in Grasshopper
- The X-chromosome was discovered by Henking and named it X-body
- Wilson and Stevens proposed the Chromosomal Theory of sex determination
NOTE
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Sex Determination
Sex determination is the process by which the sex of an organism is determined during development. Different organisms use different mechanisms, and sex can be determined at various stages of development:
- Progamic (before fertilization) — sex is determined before fertilization. Example: Honey Bee (Drone formation from unfertilized eggs)
- Syngamic (at fertilization) — sex is determined at the time of fertilization. This is the most common mechanism. Example: Most animals and plants, including humans
- Epigamic (after fertilization) — sex is determined after fertilization, influenced by the environment. Example: Honey Bee (Queen vs Worker — both are genetically female, but nutrition determines their fate)
Types of Chromosomes
Before understanding sex determination, it is important to distinguish between the two types of chromosomes:
- Autosomes — somatic chromosomes that control body (somatic) characters. These are the same in both sexes.
- Allosomes (Heterosomes) — sex chromosomes that are related to sex determination. These differ between males and females.
- Autosomes and Heterosomes were first described by Montgomery
- Sex chromosomes were first discovered by McClung in Grasshopper
- The X-chromosome was discovered by Henking and named it X-body
- Wilson and Stevens proposed the Chromosomal Theory of sex determination
NOTE
In humans, there are 22 pairs of autosomes and 1 pair of sex chromosomes, totaling 23 pairs (46 chromosomes). The sex chromosomes are XX in females and XY in males.
1. Allosomic Sex Determination
(i) XX-XY Type (Lygaeus Type)
First described by Wilson and Stevens in the milkweed bug Lygaeus. This is the most common sex determination system. Two subtypes exist:
(a) XX Female and XY Male
This is the system used by humans and most mammals:
- Female is Homogametic — produces only one type of gamete (A + X). All eggs carry an X chromosome.
- Male is Heterogametic — produces two types of gametes (A + X and A + Y). Sperm can carry either X or Y.
Female (2A + XX) → Gametes: A + X (all same)
Male (2A + XY) → Gametes: A + X or A + Y (two types)
Examples: Humans, Drosophila, most mammals
| Parent | Gametes | |
|---|---|---|
| Female (XX) | X | X |
| Male (XY) | X | Y |
| X (from father) | Y (from father) | |
|---|---|---|
| X (from mother) | XX (Daughter) | XY (Son) |
| X (from mother) | XX (Daughter) | XY (Son) |
Sex ratio: 50% Female : 50% Male
IMPORTANT
In the XX-XY system, the father determines the sex of the offspring. The mother always contributes an X chromosome, but the father can contribute either an X (producing a daughter) or a Y (producing a son).
(b) ZW Female and ZZ Male
This system is the reverse of the XX-XY system:
- In this system, the female is heterogametic (ZW) and produces two types of gametes
- The male is homogametic (ZZ) and produces only one type of gamete
- Found in birds (poultry, peacock), butterflies, some fish, some reptiles
Examples: Poultry, Peacock, Butterflies
TIP
Remember: In the ZW-ZZ system, the mother determines the sex of the offspring (opposite to the XX-XY system in humans). This is often tested in CUET by asking about sex determination in birds.
(ii) XX-XO Type (Protenor Type)
- Female has two X chromosomes (2A + XX) — Homogametic
- Male has only one X chromosome (2A + XO) — Heterogametic
- There is no Y chromosome in males
- Males have one fewer chromosome than females
Examples: Grasshopper, Cockroach
Female (2A + XX) → Gametes: all A + X
Male (2A + XO) → Gametes: A + X or A (without sex chromosome)
NOTE
In the XX-XO system, sex is determined by the number of X chromosomes, not by the presence of a Y chromosome. Males have one X, females have two.
(iii) Haploid-Diploid Mechanism of Sex Determination
Found in Hymenoptera (order of insects) — Honey bees, Wasps, Ants. This is a unique system where sex depends on the number of chromosome sets, not on specific sex chromosomes.
- Diploid (2n) → Female (Queen or Worker)
- Haploid (n) → Male (Drone)
Diploid larva (2n) ─── Fed on Royal Jelly ──→ Queen (Fertile female)
└── Fed on Normal food ──→ Worker (Sterile female)
- Queen bee receives a diploid set of chromosomes (from fertilized egg) → she has 32 chromosomes (2n = 32)
- Drone develops from unfertilized haploid egg (parthenogenesis) → has 16 chromosomes (n = 16)
- Worker bee is also diploid (2n) but is sterile due to nutritional differences (not fed royal jelly during larval development)
| Female (Queen/Worker) | Male (Drone) | |
|---|---|---|
| Chromosome number | 32 (2n) | 16 (n) |
| Development | From fertilized egg | From unfertilized egg (parthenogenesis) |
| Meiosis | Normal | Produces sperm by mitosis-like division |
IMPORTANT
In honey bees, fertilization determines sex: fertilized eggs (2n) become females, unfertilized eggs (n) become males. The difference between Queen and Worker is nutritional (royal jelly), not genetic.
Cytological Basis of Sex Identification
Barr Body Technique (Lyon's Hypothesis)
The Barr body provides a quick cytological method to determine the number of X chromosomes in an individual:
- In human females, out of two X chromosomes, one X is Euchromatin (active, gene-expressing) and the other is Heterochromatin (inactive, condensed)
- The inactive X condenses into a dark-staining body visible during interphase — this is called a Barr Body
- Barr body was discovered by Murray Barr
- The inactivated X chromosome is called Facultative Heterochromatin (meaning it is not always heterochromatic — it was active before inactivation)
IMPORTANT
Barr body count = Number of X chromosomes - 1
This formula allows you to determine the number of X chromosomes from a cell sample.
| Genotype | Sex | Barr Bodies |
|---|---|---|
| Normal female (2A + XX) | Female | 1 Barr body |
| Normal male (2A + XY) | Male | 0 (Absent) |
| Turner syndrome (2A + XO) | Female | 0 (Absent) |
- Barr body can be used to identify sex from cells — useful in forensic science and prenatal diagnosis
- In females, the Barr body can also appear as a drumstick appendage on the nucleus of white blood cells (neutrophils). This is another diagnostic feature used in clinical laboratories.
Lyon's Hypothesis explained
**Mary Lyon** proposed that in female mammals, one of the two X chromosomes is randomly inactivated early in embryonic development. This process is called **X-inactivation** or **lyonization**.Key points:
- Inactivation is random — in some cells the paternal X is inactivated, in others the maternal X
- Once inactivated in a cell, all daughter cells maintain the same X as inactive
- This is why calico cats (tortoiseshell) are almost always female — the random inactivation of X chromosomes carrying different coat colour alleles creates patches of different colours
- The inactive X forms the visible Barr body
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Sex determination types | Progamic (before fertilization), Syngamic (at fertilization, most common), Epigamic (after fertilization, environmental) |
| Autosomes vs Allosomes | Autosomes = somatic chromosomes (same in both sexes) Allosomes (Heterosomes) = sex chromosomes (differ between sexes) |
| Autosomes/Heterosomes described by | Montgomery |
| Sex chromosomes discovered by | McClung in Grasshopper |
| X-chromosome discovered by | Henking (named it X-body) |
| Chromosomal theory of sex determination | Wilson and Stevens |
| Humans: chromosomes | 22 pairs autosomes + 1 pair sex chromosomes = 23 pairs (46 total) |
| XX-XY type (Lygaeus type) | Female = XX (homogametic), Male = XY (heterogametic) Father determines sex Examples: Humans, Drosophila, most mammals |
| ZW-ZZ type | Female = ZW (heterogametic), Male = ZZ (homogametic) Mother determines sex Examples: Birds (poultry, peacock), butterflies |
| XX-XO type (Protenor type) | Female = XX, Male = XO (no Y chromosome) Examples: Grasshopper, Cockroach |
| Haploid-Diploid mechanism | Diploid (2n) = Female; Haploid (n) = Male (Drone) Found in Hymenoptera (honey bees, wasps, ants) |
| Honey bee chromosomes | Queen/Worker = 32 (2n); Drone = 16 (n) |
| Queen vs Worker bee | Both diploid; difference is nutritional — Queen fed royal jelly |
| Drone development | From unfertilized egg (parthenogenesis) |
| Genic Balance Theory | Proposed by C.B. Bridges for Drosophila; sex determined by X/A ratio (not Y chromosome) |
| Sex index (X/A) values | 1.0 = Normal Female; 0.5 = Normal Male; 1.5 = Super Female (Metafemale); <0.5 = Super Male |
| Drosophila Y chromosome | Does NOT determine sex; carries genes for male fertility (spermatogenesis) only |
| Human vs Drosophila sex determination | Humans: Y chromosome essential (SRY/TDF gene) Drosophila: X/A ratio determines sex |
| Barr body | Inactivated X chromosome; discovered by Murray Barr |
| Barr body formula | Barr bodies = X chromosomes − 1 |
| Normal female Barr bodies | 1 Barr body |
| Normal male Barr bodies | 0 (absent) |
| Turner syndrome Barr bodies | 0 (XO genotype) |
| Lyon's Hypothesis | One X chromosome randomly inactivated early in embryonic development; explains calico cats (random X-inactivation creates colour patches) |
| Facultative Heterochromatin | The inactivated X chromosome (was active before inactivation) |
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