Describe sex-linked inheritance — Biology notes

WHAT is sex-linkage?

WHY does sex matter here? In humans (and Drosophila):

Females are XX → they have two copies of every X-linked gene.
Males are XY → they have only one copy of each X-linked gene.

WHY males show recessive traits more often

If allele frequency of the recessive allele is $q$ (say $q=0.08$ for red-green colour-blindness):

$P(\text{affected male}) = q = 0.08, \qquad P(\text{affected female}) = q^2 = 0.0064$

So ~12× more affected males than females. HOW we know: a male's single X comes straight from his mother; if that one allele is recessive-defective, he is affected.

Deriving the inheritance pattern from first principles

We only need two facts:

A son gets his X from mother, Y from father.
A daughter gets one X from each parent.

HOW to build any cross: write the genotype with the allele on the chromosome ( $X^A$ , $X^a$ , $Y$ ), then combine gametes in a Punnett square.

Worked Example 1 — Carrier mother × normal father

Trait: X-linked recessive (e.g. colour-blindness). Mother $X^AX^a$ (carrier), Father $X^AY$ .

	$X^A$ (mum)	$X^a$ (mum)
$X^A$ (dad)	$X^AX^A$ normal ♀	$X^AX^a$ carrier ♀
$Y$ (dad)	$X^AY$ normal ♂	$X^aY$ affected ♂

Daughters: ½ normal, ½ carriers — none affected. Why this step? Dad gives every daughter a healthy $X^A$ , masking the recessive.
Sons: ½ normal, ½ affected. Why this step? Dad gives sons only $Y$ , so a son's phenotype is decided entirely by mum's X.

➡ Classic clue: trait "skips" from carrier mother to half her sons.

Worked Example 2 — Affected father × normal homozygous mother

Father $X^aY$ (affected), Mother $X^AX^A$ .

	$X^A$	$X^A$
$X^a$ (dad)	$X^AX^a$ carrier ♀	$X^AX^a$ carrier ♀
$Y$ (dad)	$X^AY$ normal ♂	$X^AY$ normal ♂

All daughters are carriers (got dad's $X^a$ ). All sons are normal (got dad's $Y$ , not his X).
Why this matters: an affected father never passes an X-linked trait to his sons — a key diagnostic of X-linkage. This is criss-cross inheritance: dad's trait shows up in his grandsons via his daughters.

Worked Example 3 — Reciprocal crosses are NOT equivalent

This is the signature test for sex-linkage. Compare:

(a) White-eyed female Drosophila $X^wX^w$ × red-eyed male $X^{W}Y$ → all sons white, all daughters red.
(b) Red-eyed female $X^WX^W$ × white-eyed male $X^wY$ → all offspring red.

Why this step? For an autosomal gene, reversing which parent is which gives identical offspring ratios. For an X-linked gene the results differ because the X travels differently through fathers vs mothers. Morgan used exactly this to prove genes sit on chromosomes (1910).

Figure — Describe sex-linked inheritance

Other patterns to know

Common mistakes

Forecast-then-Verify

Recall Feynman: explain to a 12-year-old

Everyone has two "instruction books" for most body parts — one from mum, one from dad — so if one has a typo, the other usually covers for it. But boys have a special pair where one book (the Y) is tiny and mostly blank. So for the genes only written in the X book, a boy has just one copy. If that single copy has a typo, nothing covers for it, and he gets the trait. Girls have two full X books, so they almost always have a backup. That's why things like colour-blindness show up in boys way more often.

Active-recall flashcards

#flashcards/biology

What does "hemizygous" mean?

Having only one copy of a gene because it lies on the X with no matching allele on the Y (true of males for X-linked genes).

Why are X-linked recessive traits more common in males?

Males have one X, so a single recessive allele (

X^aY

) is expressed; females need two (

X^aX^a

A son inherits his single X chromosome from which parent?

His mother.

Can an affected father pass an X-linked trait to his sons?

No — he gives sons his Y; sons get their X from the mother only.

What is criss-cross inheritance?

A father's X-linked trait passes to his daughters (carriers) and then appears in his grandsons.

For X-linked recessive frequency

q

, give affected-male and affected-female frequencies.

Males

= q

; Females

= q^2

Why are reciprocal crosses the diagnostic test for sex-linkage?

For autosomal genes the results are identical; for X-linked genes swapping the carrier parent's sex changes the offspring ratios.

Carrier mother × normal father — describe the sons and daughters.

Sons: ½ affected, ½ normal. Daughters: ½ carriers, ½ normal, none affected.

What is a Y-linked (holandric) gene's transmission pattern?

Father → every son → every grandson; never to females.

Affected father with an X-linked DOMINANT trait — which children are affected?

All daughters affected, no sons (sons get his Y).

Are males hetero- or homozygous for X-linked genes?

Neither — they are hemizygous (only one allele).

Who first used reciprocal Drosophila eye-colour crosses to prove genes are on chromosomes?

Thomas Hunt Morgan (1910).

Connections

Mendelian Genetics — Law of Segregation — sex-linkage is an extension, not a violation, of segregation.
Sex Determination — XY and XO systems — the chromosomal basis that makes hemizygosity possible.
Pedigree Analysis — using these rules to read family trees.
Hardy-Weinberg Equilibrium — source of the $q$ vs $q^2$ population frequencies.
Linkage and Crossing Over — Morgan's Drosophila work that established the chromosome theory.
Colour Blindness and Haemophilia — the two textbook human X-linked recessives.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, sex-linked inheritance ka matlab hai ki kuch genes humare sex chromosomes (X ya Y) par hote hain, autosomes par nahi. Females XX hoti hain, yaani har X-linked gene ki do copies, lekin males XY hote hain — sirf ek copy. Is "ek copy" wali baat ko hemizygous kehte hain. Yahi pura khel ka asli reason hai.

Ab socho colour-blindness jaisa X-linked recessive trait. Ladki ko trait dikhne ke liye dono X kharaab chahiye ( $X^aX^a$ ), par ladke ko sirf ek kharaab X kaafi hai ( $X^aY$ ), kyunki uska Y to khaali hai, koi backup nahi. Isliye boys mein ye traits kahin zyada common hote hain. Frequency $q$ ho to males mein $q$ , females mein $q^2$ — matlab boys ka chance kaafi zyada.

Do golden rules yaad rakho: Beta apni single X Maa se leta hai, aur Beti hamesha Papa ki X leti hai. Isliye affected papa apne beton ko trait kabhi nahi de sakta (unhe sirf Y milta hai), par sabhi betiyan carrier ban jaati hain — ise criss-cross inheritance kehte hain. Aur ek important test: reciprocal crosses. Autosomal gene ke liye maa-baap ka role ulta karne se result same aata hai, par X-linked mein result alag aata hai — Morgan ne isi se 1910 mein prove kiya ki genes chromosomes par hote hain.

Exam tip: pehle genotype chromosome ke saath likho ( $X^A$ , $X^a$ , $Y$ ), phir Punnett square banao, aur beta/beti ko alag-alag dekho. Bas yahi technique har sex-linked question solve kar degi.

Test yourself — Extensions of Mendelian Genetics

Connections