Sex Linkage & Determination: Genetics Study Notes

Sex Linkage & Determination

Learning Outcomes

• Understand chromosomal sex-determining systems.

• Describe inheritance of sex-linked disorders such as hemophilia and colour blindness.

• Explain dosage compensation and X inactivation.

Chromosomes

Chromosome Number and Arrangement

Species differ in their chromosomal number and arrangement, which influences genetic inheritance and sex determination.

• Chromosome numbers vary widely among species (e.g., humans: 46, fruit fly: 8, chicken: 78).

• Challenges exist in developing a unified theory for chromosomal sex determination.

• Multiple mechanisms for sex determination have been proposed.

Species	Chromosome Number (2n)
Human	46
Dog	78
Fruit fly	8
Chicken	78
Pea	14

Sex Determination Mechanisms

Overview

Sex determination involves the mixing of genomes by fertilization and fusion of gametes, with mechanisms varying across organisms.

• Genotypic sex determination: Determined by genetic composition (chromosomes).

• Environmental sex determination: Influenced by environmental factors (e.g., temperature in reptiles).

• Chromosomal sex determination: Based on specific sex chromosomes (e.g., XY, XO, ZW systems).

Chromosomal Sex Determination Systems

Major Systems

• XY System: Found in humans and many animals.

• XO System: Found in insects like crickets and grasshoppers.

• ZW System: Found in birds, snakes, and some insects.

• Haplodiploidy System: Found in bees and ants.

XY Sex Determination System

In the XY system, males are heterogametic (XY) and females are homogametic (XX).

• Males: XY (heterogametic sex)

• Females: XX (homogametic sex)

SRY Gene

The SRY gene on the Y chromosome triggers development of male reproductive organs.

• SRY causes undifferentiated gonads to form testes.

• Testes produce testosterone (develops Wolffian ducts) and anti-Mullerian hormone (degrades Mullerian ducts).

XO Sex Determination System

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

• Males: XO (one X chromosome)

• Females: XX (two X chromosomes)

• Common in crickets, grasshoppers, and some insects.

ZW Sex Determination System

In the ZW system, females are heterogametic (ZW) and males are homogametic (ZZ).

• Females: ZW (heterogametic sex)

• Males: ZZ (homogametic sex)

• Found in birds, snakes, and some insects.

Haplodiploidy Sex Determination System

Sex is determined by the number of chromosome sets.

• Haploid set: Male (from unfertilized eggs)

• Diploid set: Female (from fertilized eggs)

• Examples: Bees and ants.

Sex-Linked Inheritance

Discovery Timeline

Key discoveries in sex chromosome research:

• Karl Henking (1891): Identified 'X body' in insect sperm.

• Thomas Hunt Morgan (1910): Discovered sex-linked inheritance in Drosophila.

Morgan's Discovery

In 1910, Morgan found a male Drosophila with white eyes, leading to the study of sex-linked inheritance.

• White eye color is a recessive trait on the X chromosome.

• Sex-linked traits exhibit criss-cross inheritance (gene transmission from mother to son or father to daughter).

Sex-Linked Inheritance Patterns

• Dominant inheritance: Trait appears in individuals with one dominant allele on the X chromosome.

• Recessive inheritance: Trait appears only if both X chromosomes carry the recessive allele (females) or the single X chromosome in males.

Genotype	Phenotype
XDXd (female)	Dominant trait expressed
XdY (male)	Recessive trait expressed
XDY (male)	Dominant trait expressed
XdXd (female)	Recessive trait expressed

Genotypes and Expression

• Males: Hemizygous for X chromosome (one copy of each gene).

• Females: Homozygous or heterozygous (two copies of each gene).

Example Questions

• Genotype of male/female with sex-linked recessive trait not expressed: Male: XBY; Female: XBXb (carrier).

• Genotype of male/female with sex-linked dominant trait not expressed: Male: XbY; Female: XbXb.

Sex-Linked Disorders

Haemophilia

Haemophilia is a disorder where blood does not clot properly due to lack of clotting factors.

• Normal: Clotting factors form a stable plug to stop bleeding.

• Haemophilia: Deficiency in clotting factors leads to prolonged bleeding.

Severity	Clotting Factor Level	Symptoms
Severe	<1%	Frequent spontaneous bleeding
Moderate	1-5%	Bleeding after minor injury
Mild	6-30%	Bleeding after surgery or major injury

Colour Blindness

Colour blindness is a deficiency in color perception, not actual blindness.

• Most common form is red-green color blindness, inherited as an X-linked recessive trait.

• Normal vision vs. color blind vision can be tested using Ishihara plates.

Dosage Compensation

Genetic Dosage Problem

Females have two X chromosomes, males have one. Without compensation, females would produce twice as much X-linked gene product.

Mechanism

• Dosage compensation equalizes protein production from X-linked genes in both sexes.

• In female mammals, one X chromosome is inactivated (X inactivation).

X-Inactivation

Barr Body

Murray Barr discovered condensed, darkly staining bodies (Barr bodies) in female cells, representing inactivated X chromosomes.

• Barr body = inactivated X chromosome.

Lyon Hypothesis

• X inactivation occurs randomly in somatic cells early in embryonic development.

• All progeny cells have the same X chromosome inactivated.

X-Inactivation in Cats

Female tortoiseshell (calico) cats show random distribution of orange and black patches due to X inactivation.

• "Black-orange" coloration gene is X-linked.

• In heterozygous females, patches form where either the orange or black allele is active.

• In males (hemizygous), only one color is expressed.

Chromosomal Abnormalities and Barr Bodies

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

Sex-Linked Punnett Square Example

Normal Male × Carrier Female (Colour Blindness)

• Let B = normal vision, b = colour blind.

• Cross: XBY × XBXb

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

Key Steps

1. Write genotypes of parents.

2. Cross them using Punnett square.

3. Observe resulting phenotypes.

Summary

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

• Dosage compensation and X inactivation ensure balanced gene expression between sexes.

• Sex-linked disorders such as haemophilia and colour blindness follow predictable inheritance patterns.

Additional info: Academic context and explanations have been expanded for clarity and completeness.