Sex Linkage and Autosomal Linkage

Some genes do not follow the simple patterns of independent assortment described by Mendel. Genes located on the sex chromosomes show distinctive inheritance patterns, and genes on the same autosome tend to be inherited together. Understanding these departures from Mendelian ratios is essential for A-Level Biology.

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Sex Determination in Humans

• Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes.
• Females have two X chromosomes (XX); males have one X and one Y chromosome (XY).
• The SRY gene on the Y chromosome triggers male development by initiating the production of testes-determining factor (TDF).
• During meiosis, a female produces gametes that all contain one X chromosome, while a male produces gametes containing either an X or a Y chromosome.
• The sex of offspring is determined by whether the sperm carries an X or a Y chromosome.

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X-Linked (Sex-Linked) Inheritance

Key Definition: Sex-linked genes are genes located on the X chromosome (or, rarely, the Y chromosome). Because the X chromosome is much larger than the Y chromosome, most sex-linked genes are X-linked and have no corresponding allele on the Y chromosome.

Key Features of X-Linked Inheritance

• Males are hemizygous for X-linked genes — they have only one copy, so any allele on the X chromosome will be expressed, whether dominant or recessive.
• Females can be homozygous dominant, heterozygous (carriers), or homozygous recessive.
• X-linked recessive conditions are more common in males because males need only one copy of the recessive allele to be affected.
• An affected father cannot pass an X-linked condition to his sons (sons inherit the Y chromosome from their father).
• A carrier mother has a 50% chance of passing the recessive allele to each child, regardless of sex.

Writing Sex-Linked Genotypes

Alleles are written as superscripts on the X chromosome symbol:

• X^H = normal allele (e.g., for blood clotting)
• X^h = recessive allele (e.g., for haemophilia)
• The Y chromosome is written simply as Y (no corresponding allele)

Genotype	Sex	Phenotype
X^H X^H	Female	Normal
X^H X^h	Female	Carrier (unaffected)
X^h X^h	Female	Affected (haemophilia)
X^H Y	Male	Normal
X^h Y	Male	Affected (haemophilia)

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Worked Example 1 — Haemophilia

Haemophilia A is an X-linked recessive condition in which the blood fails to clot properly due to a deficiency in clotting factor VIII.

Cross: Carrier female × Normal male Parental genotypes: X^H X^h × X^H Y

Gametes: X^H or X^h (mother); X^H or Y (father)

	X^H	Y
X^H	X^H X^H (normal female)	X^H Y (normal male)
X^h	X^H X^h (carrier female)	X^h Y (affected male)

Offspring:

• 1/4 normal female
• 1/4 carrier female
• 1/4 normal male
• 1/4 haemophiliac male

Key observations:

• 50% of sons are affected.
• No daughters are affected, but 50% of daughters are carriers.
• The condition appears to "skip generations" — an unaffected carrier mother can produce affected sons.

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Worked Example 2 — Red-Green Colour Blindness

Red-green colour blindness is another X-linked recessive condition.

Cross: Colour-blind female × Normal male Parental genotypes: X^b X^b × X^B Y

	X^B	Y
X^b	X^B X^b (carrier female)	X^b Y (colour-blind male)
X^b	X^B X^b (carrier female)	X^b Y (colour-blind male)

Offspring:

• All daughters are carriers (X^B X^b) — phenotypically normal.
• All sons are colour-blind (X^b Y).

Exam Tip: In sex-linked crosses, always write the sex chromosomes explicitly (e.g., X^H X^h, not just Hh). Examiners expect to see the X and Y chromosome notation. Failing to show this loses marks.

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Recognising X-Linked Inheritance in Pedigrees