Genetics: Sex Determination in Humans

 

Genetics: Sex Determination in Humans

Sex determination is the biological process by which the genetic sex of an individual is established. In humans, sex is determined by the XX–XY chromosomal mechanism, where females possess two X chromosomes (XX) and males possess one X and one Y chromosome (XY).

Introduction

Most organisms reproduce sexually. During sexual reproduction, offspring inherit chromosomes from both parents.

Humans possess:

• 46 chromosomes (2n)
• 23 pairs of chromosomes
• 22 pairs = Autosomes
• 1 pair = Sex chromosomes

The sex chromosomes determine whether the individual develops as a male or female.

Human Chromosomes

Chromosome Type	Number
Autosomes	44 (22 pairs)
Sex chromosomes	2
Total	46

Female: 44 + XX

Male: 44 + XY

Sex Chromosomes

X Chromosome

The X chromosome is comparatively large.

Characteristics:

• Contains more than one thousand genes.
• Controls many body functions besides sex determination.
• Carries genes for:
• Blood clotting
• Vision
• Muscle development
• Brain development
• Numerous metabolic functions

Y Chromosome

The Y chromosome is much smaller.

Characteristics:

• Contains relatively few genes.
• Responsible mainly for male sex determination.
• Carries the SRY (Sex-determining Region Y) gene.
• Essential for development of testes.

The Y chromosome does not contain many genes unrelated to male reproductive development.

Human Sex Determination Mechanism

Humans show: XX–XY type of sex determination

Female = XX

Male = XY

This system is called male heterogamety because males produce two different kinds of gametes.

Homogametic and Heterogametic Individuals

Homogametic

Produces only one type of gamete regarding sex chromosomes.

Example: Female (XX). Produces only: 22 + X ova

Heterogametic

Produces two types of gametes.

Example: Male (XY), Produces:

• 22 + X sperm
• 22 + Y sperm

Thus, Human male is heterogametic. Human female is homogametic.

Formation of Gametes

Female

Oogenesis produces ova. Each ovum contains:

• 22 autosomes
• X chromosome

Therefore, every egg is identical regarding sex chromosome. Egg = 22 + X

Male

During spermatogenesis, Meiosis separates X and Y chromosomes. Therefore, Two types of sperm are produced.

Approximately:

• 50% carry X chromosome
• 50% carry Y chromosome

Types:

22 + X

22 + Y

Hence, the father contributes either an X or a Y chromosome.

Chromosomal Basis of Sex Determination

At fertilization, Mother always contributes: X chromosome, Father contributes either:

X chromosome or Y chromosome

Possible combinations:

Egg	Sperm	Zygote	Sex
X	X	XX	Female
X	Y	XY	Male

Therefore, Probability:

Female = 50%

Male = 50%

This occurs purely by chance.

Meiosis and Formation of Sex-specific Gametes

During meiosis in males, homologous sex chromosomes segregate so that half the sperm receive an X chromosome and half receive a Y chromosome. Females, having two X chromosomes, produce ova that all carry an X chromosome.

Why is Father Responsible for Sex of Child?

This is among the most important concepts.

Mother: Always contributes X chromosome.

Father contributes: Either X or Y.

Therefore, Father determines the sex of the child.

If father contributes:

X

↓

XX

↓

Girl

If father contributes:

Y

↓

XY

↓

Boy

Hence, Scientifically, the mother cannot determine the sex of the child.

Molecular Basis of Sex Determination

Modern genetics has shown that the decisive factor is not merely the Y chromosome but a specific gene present on it.

SRY Gene

Full form: Sex-determining Region of Y chromosome

Location: Short arm of Y chromosome

Function: Initiates development of testes in the embryo.

Role of SRY Gene

SRY activates genes responsible for:

• Testis formation
• Testosterone production
• Male reproductive tract development
• Male secondary sexual characteristics

If SRY is absent,

The embryo follows the female developmental pathway.

Thus,

Presence of SRY

↓

Testes develop

↓

Male hormones secreted

↓

Male phenotype

Development of Gonads

Initially, Embryonic gonads are undifferentiated. They can develop into either:

• Testes
• Ovaries

If Y chromosome with SRY is present:

↓

Testes develop.

If Y chromosome absent:

↓

Ovaries develop.

Role of Testosterone

Testes secrete:

• Testosterone
• Anti-Müllerian hormone (AMH)

These hormones:

• Develop male reproductive organs
• Prevent development of female reproductive ducts
• Produce male secondary sexual characters

Role of X Chromosome

The X chromosome:

• Carries genes essential for survival.
• Is required in both sexes.
• Controls several non-sexual traits.

Examples:

• Colour vision
• Blood clotting
• Muscular function

This explains why X-linked disorders are more common in males.

Why Y Chromosome is Small

Compared with X chromosome, Y chromosome:

• Has fewer genes
• Contains mainly genes related to male fertility and sex determination
• Is inherited from father to son

Probability of Birth of Boy or Girl

Each fertilization event is independent.

Possible outcomes: XX or XY

Each probability: 50% Therefore, Expected ratio:1 : 1

Actual families may differ because each pregnancy is an independent event.

Common Misconceptions

Myth

Mother decides the baby's sex.

Fact

Father determines whether the offspring receives X or Y chromosome.

Myth

Eating certain foods produces boys.

Fact

No scientific evidence. Only chromosomal combination determines sex.

Myth

Certain positions during intercourse determine sex.

Fact

Completely false. Sex depends only on fertilizing sperm.

Comparison with Other Organisms

Organism	Female	Male	Sex determined by
Humans	XX	XY	Male
Birds	ZW	ZZ	Female
Honey bee	Diploid	Haploid	Fertilization
Grasshopper	XX	XO	Male

Important Terminology

Autosomes

Chromosomes other than sex chromosomes.

Sex Chromosomes

Chromosomes involved in sex determination.

---

Homogametic

Produces one type of gamete.

Example: Female (XX)

Heterogametic

Produces two types of gametes.

Example: Male (XY)

Gamete

Haploid reproductive cell.

Fertilization

Fusion of male and female gametes.

Zygote

Diploid cell formed after fertilization.

SRY Gene

Master regulator initiating male development.

Flow Chart

Female

↓

XX

↓

Only X eggs

•  

Male

↓

XY

↓

50% X sperm

50% Y sperm

↓

Fertilization

↓

XX → Female

XY → Male

↓

Sex ratio = 1 : 1

High Yield Facts

• Human sex determination is XX–XY type.
• Female is homogametic.
• Male is heterogametic.
• Mother contributes only X chromosome.
• Father contributes X or Y chromosome.
• SRY gene is present on Y chromosome.
• Presence of Y chromosome initiates male development.
• Expected sex ratio = 1 : 1.
• Sex of child is determined by the sperm that fertilizes the ovum. (Sathee)

Frequently Asked Facts

Question	Answer
Human female genotype	XX
Human male genotype	XY
Homogametic sex	Female
Heterogametic sex	Male
Sex determining chromosome	Y chromosome
Gene responsible	SRY
Sex determining parent	Father
Egg chromosome	X
Sperm chromosomes	X or Y
Expected sex ratio	1 : 1

 

Summary

• Humans exhibit male heterogamety (XX–XY system).
• Females (XX) produce only X-bearing ova.
• Males (XY) produce equal numbers of X-bearing and Y-bearing sperm.
• Fertilization by an X-bearing sperm results in XX (female).
• Fertilization by a Y-bearing sperm results in XY (male).
• The SRY gene on the Y chromosome initiates testis formation and male differentiation.
• The expected sex ratio at fertilization is 1:1.
• The father's sperm genetically determines the sex of the child.