12.7 Sex Linkage | Inheritance | Biology 11

Sex linkage refers to the inheritance of genes that are located on the sex chromosomes ( $X$ or $Y$ ). Because males ( $X Y$ ) and females ( $X X$ ) carry different complements of sex chromosomes, the expression of sex-linked alleles differs between the two sexes.

Types of Sex Linkage

1. X-Linked Inheritance

Genes located on the non-homologous region of the X chromosome are called X-linked genes. Because males have only one X chromosome, they are said to be hemizygous for X-linked genes.

Genotype	Sex	Phenotype (recessive trait)
$X^{A} X^{A}$	Female	Normal (homozygous dominant)
$X^{A} X^{a}$	Female	Normal (carrier)
$X^{a} X^{a}$	Female	Affected
$X^{A} Y$	Male	Normal
$X^{a} Y$	Male	Affected

Key point: A single recessive allele on the X chromosome is sufficient to express the trait in males. This is why X-linked recessive disorders are far more common in males than in females.

2. Y-Linked (Holandric) Inheritance

Genes located exclusively on the Y chromosome are called holandric genes (from Greek holos = whole, andros = male).

Present only in males
Passed directly from father to all sons
Daughters never inherit Y-linked traits
Example: the SRY gene (Sex-determining Region Y), which triggers the development of testes and male characteristics

Criss-Cross Inheritance

Criss-cross inheritance is the characteristic transmission pattern of X-linked recessive traits:

$Affected father X^{a} Carrier daughter X^{a} Affected grandson$

The trait appears to skip a generation and alternates between sexes:

An affected father passes $X^{a}$ to all daughters (who become carriers)
Carrier daughters pass $X^{a}$ to 50% of their sons (who are affected)

This pattern is the hallmark of X-linked recessive inheritance in pedigree analysis.

Important X-Linked Recessive Disorders in Humans

Disorder	Gene / Defect	Affected Population
Haemophilia A	Deficiency of clotting Factor VIII	Males predominantly
Haemophilia B	Deficiency of clotting Factor IX	Males predominantly
Red-Green Colour Blindness	Defective cone photoreceptor pigments	~8% of males, ~0.5% of females
Duchenne Muscular Dystrophy (DMD)	Absent dystrophin protein → progressive muscle wasting	Males predominantly

Worked Example: Colour Blindness Cross

Cross: Colour-blind man ( $X^{c} Y$ ) × Homozygous normal woman ( $X^{C} X^{C}$ )

$X^{c} Y \times X^{C} X^{C}$

	$X^{C}$	$X^{C}$
$X^{c}$	$X^{C} X^{c}$ (carrier daughter)	$X^{C} X^{c}$ (carrier daughter)
$Y$	$X^{C} Y$ (normal son)	$X^{C} Y$ (normal son)

Results:

All daughters: carriers ( $X^{C} X^{c}$ ) — phenotypically normal
All sons: normal vision ( $X^{C} Y$ )
0% of sons are colour-blind

Cross: Carrier woman ( $X^{C} X^{c}$ ) × Normal man ( $X^{C} Y$ )

	$X^{C}$	$Y$
$X^{C}$	$X^{C} X^{C}$ (normal daughter)	$X^{C} Y$ (normal son)
$X^{c}$	$X^{C} X^{c}$ (carrier daughter)	$X^{c} Y$ (colour-blind son)

Results:

50% of sons are colour-blind
50% of daughters are carriers

Summary Table

Feature	X-Linked Recessive	Y-Linked (Holandric)
Chromosome	X	Y
Affected sex	Mostly males	Only males
Transmission	Criss-cross (father → carrier daughter → grandson)	Father → all sons
Carrier possible?	Yes (females)	No
Example	Haemophilia, Colour blindness	SRY gene

Genotype

Sex

Phenotype (recessive trait)

X^{A} X^{A}

Female

Normal (homozygous dominant)

X^{A} X^{a}

Female

Normal (carrier)

X^{a} X^{a}

Female

Affected

X^{A} Y

Male

Normal

X^{a} Y

Male

Affected

Disorder

Gene / Defect

Affected Population

Haemophilia A

Deficiency of clotting Factor VIII

Males predominantly

Haemophilia B

Deficiency of clotting Factor IX

Males predominantly

Red-Green Colour Blindness

Defective cone photoreceptor pigments

~8% of males, ~0.5% of females

Duchenne Muscular Dystrophy (DMD)

Absent dystrophin protein → progressive muscle wasting

Males predominantly

X^{C}

X^{C}

X^{c}

X^{C} X^{c}

(carrier daughter)

X^{C} X^{c}

(carrier daughter)

Y

X^{C} Y

(normal son)

X^{C} Y

(normal son)

X^{C}

Y

X^{C}

X^{C} X^{C}

(normal daughter)

X^{C} Y

(normal son)

X^{c}

X^{C} X^{c}

(carrier daughter)

X^{c} Y

(colour-blind son)

Feature

X-Linked Recessive

Y-Linked (Holandric)

Chromosome

Affected sex

Mostly males

Only males

Transmission

Criss-cross (father → carrier daughter → grandson)

Father → all sons

Carrier possible?

Yes (females)

Example

Haemophilia, Colour blindness

SRY gene