Sex Linkage and Determination: Mechanisms, Inheritance, and Dosage Compensation

Sex Linkage & Determination

Introduction

This section explores the genetic and chromosomal mechanisms underlying sex determination, the inheritance of sex-linked traits, and the biological processes that ensure dosage compensation between sexes. Key examples include human disorders such as hemophilia and color blindness, as well as the phenomenon of X-inactivation.

Chromosomes and Sex Determination Mechanisms

Chromosomes

• Species differ in chromosomal number and arrangement, which influences their mechanisms of sex determination.

• Early cytogenetic studies revealed the presence of distinct sex chromosomes in many species.

Species	Chromosome Number (2n)	Sex Chromosome System
Human	46	XX/XY
Dog	78	XX/XY
Chicken	78	ZW/ZZ
Fruit fly	8	XX/XY
Grasshopper	24	XX/XO

Sex Determination Mechanisms

• Sex determination involves the 'mixing' of genomes by fertilization and fusion of gametes.

• Mechanisms include environmental, genetic, and chromosomal systems.

• Different species use different triggers for male or female development.

Chromosomal Sex Determination Systems

XY System

• Males are the heterogametic sex (XY), producing two types of gametes (X or Y).

• Females are the homogametic sex (XX), producing only X-bearing gametes.

• Common in mammals and fruit flies.

SRY Gene

• The SRY (Sex-determining Region Y) gene on the Y chromosome initiates testis development in mammals.

• Testes produce testosterone (promotes Wolffian duct development) and anti-Müllerian hormone (degrades Müllerian ducts).

XO System

• Sex is determined by the presence or absence of the X chromosome.

• Males: XO (one X chromosome), Females: XX (two X chromosomes).

• Found in grasshoppers, crickets, and some insects.

ZW System

• Females are the heterogametic sex (ZW), males are homogametic (ZZ).

• Common in birds, snakes, and some insects.

Haplodiploidy System

• Sex is determined by ploidy: haploid (unfertilized) eggs develop into males, diploid (fertilized) eggs develop into females.

• Seen in bees and ants.

Sex-Linked Inheritance

Discovery of X and Y Chromosomes

• Key discoveries by scientists such as Karl Henking, Thomas Hunt Morgan, and others established the role of sex chromosomes in inheritance.

• Thomas Hunt Morgan's work with Drosophila melanogaster (fruit flies) demonstrated sex-linked inheritance patterns.

Morgan's Discovery

• In 1910, Morgan identified a white-eyed male fruit fly, leading to the discovery that eye color was linked to the X chromosome.

• This established the concept of sex-linked (X-linked) inheritance.

Patterns of Sex-Linked Inheritance

• Sex-linked traits are produced by genes located on the X chromosome.

• Males are hemizygous for X-linked genes (one copy), while females can be homozygous or heterozygous (two copies).

• Sex-linked traits often show criss-cross inheritance: passed from mother to son or father to daughter.

Dominant vs. Recessive Inheritance

Inheritance Type	Genotype (Female)	Genotype (Male)	Phenotype
Dominant	XDXd or XDXD	XDY	Affected
Recessive	XdXd	XdY	Affected
Carrier (Recessive)	XDXd	—	Not affected (carrier)

Example: Sex-Linked Punnett Square

Cross between a normal male (XBY) and a carrier female (XBXb):

	XB	Xb
XB	XBXB (unaffected daughter)	XBXb (carrier daughter)
Y	XBY (unaffected son)	XbY (affected son)

Sex-Linked Disorders

Haemophilia

• Haemophilia is an X-linked recessive disorder affecting blood clotting.

• Individuals with haemophilia lack sufficient clotting factors, leading to prolonged bleeding.

Severity	Clotting Factor Level	Prevalence
Severe	<1%	60%
Mild	6–30%	25%
Moderate	1–5%	15%

Colour Blindness

• Colour blindness is an X-linked recessive trait resulting in the inability to distinguish certain colors.

• More common in males due to hemizygosity for the X chromosome.

Dosage Compensation and X-Inactivation

Dosage Compensation

• Ensures equal expression of X-linked genes in males (XY) and females (XX).

• Prevents females from producing double the amount of X-linked gene products compared to males.

X-Inactivation

• One X chromosome in each somatic cell of female mammals is randomly inactivated early in embryonic development (Lyon hypothesis).

• The inactivated X chromosome forms a condensed structure called a Barr body.

• All progeny cells from the original cell maintain the same X chromosome inactivated.

X-Inactivation in Cats

• Female tortoiseshell (calico) cats display random patches of orange and black fur due to X-inactivation.

• In heterozygous females, cells with the active orange allele produce orange fur, while those with the active black allele produce black fur.

• Males (hemizygous) show only one color (black or orange).

Barr Bodies and Chromosomal Abnormalities

Phenotype	Chromosome Composition	Number of X Chromosomes	Number of Barr Bodies
Normal female	XX	2	1
Normal male	XY	1	0
Turner syndrome (female)	XO	1	0
Triple X syndrome (female)	XXX	3	2
Klinefelter syndrome (male)	XXY	2	1

Summary

• Sex determination and sex-linked inheritance are governed by chromosomal mechanisms and gene expression regulation.

• Dosage compensation, primarily through X-inactivation, ensures balanced gene expression between sexes.

• Understanding these mechanisms is crucial for interpreting patterns of inheritance and the manifestation of genetic disorders.