Describe sex-linked inheritance — Hinglish notes

3.2.7 · Biology › Extensions of Mendelian Genetics

Sex-linkage KYA hai?

WHY sex yahan matter karta hai? Humans (aur Drosophila) mein:

Females XX hoti hain → unke paas har X-linked gene ki do copies hoti hain.
Males XY hote hain → unke paas har X-linked gene ki sirf ek copy hoti hai.

Males recessive traits zyada kyun dikhate hain

Agar recessive allele ki frequency $q$ hai (maano $q = 0.08$ red-green colour-blindness ke liye):

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

Toh ~12 guna zyada affected males hain females se. HOW hum jaante hain: ek male ka akela X seedha uski mother se aata hai; agar woh ek allele recessive-defective hai, toh woh affected hai.

First principles se inheritance pattern derive karna

Hume sirf do facts chahiye:

Ek bete ko uska X mother se, Y father se milta hai.
Ek beti ko ek X har parent se milta hai.

HOW koi bhi cross banate hain: genotype likho allele ko chromosome par rakhke ( $X^{A}$ , $X^{a}$ , $Y$ ), phir gametes ko Punnett square mein combine karo.

Worked Example 1 — Carrier mother × normal father

Trait: X-linked recessive (jaise colour-blindness). Mother $X^{A} X^{a}$ (carrier), Father $X^{A} Y$ .

	$X^{A}$ (mum)	$X^{a}$ (mum)
$X^{A}$ (dad)	$X^{A} X^{A}$ normal ♀	$X^{A} X^{a}$ carrier ♀
$Y$ (dad)	$X^{A} Y$ normal ♂	$X^{a} Y$ affected ♂

Daughters: ½ normal, ½ carriers — koi affected nahi. Yeh step kyun? Dad har beti ko ek healthy $X^{A}$ deta hai, jo recessive ko mask kar deta hai.
Sons: ½ normal, ½ affected. Yeh step kyun? Dad sons ko sirf $Y$ deta hai, isliye bete ka phenotype poori tarah mum ke X se decide hota hai.

➡ Classic clue: trait "skip" karta hai carrier mother se uske aadhe beton tak.

Worked Example 2 — Affected father × normal homozygous mother

Father $X^{a} Y$ (affected), Mother $X^{A} X^{A}$ .

	$X^{A}$	$X^{A}$
$X^{a}$ (dad)	$X^{A} X^{a}$ carrier ♀	$X^{A} X^{a}$ carrier ♀
$Y$ (dad)	$X^{A} Y$ normal ♂	$X^{A} Y$ normal ♂

Saari betiyaan carriers hain (dad ka $X^{a}$ mila). Saare bete normal hain (dad ka $Y$ mila, uska X nahi).
Yeh kyun matter karta hai: ek affected father apni koi X-linked trait apne beton ko kabhi nahi deta — yeh X-linkage ki ek key diagnostic baat hai. Yeh criss-cross inheritance hai: dad ki trait uski betiyon ke zariye uske natiyon mein dikhti hai.

Worked Example 3 — Reciprocal crosses EQUIVALENT nahi hote

Yeh sex-linkage ka signature test hai. Compare karo:

(a) White-eyed female Drosophila $X^{w} X^{w}$ × red-eyed male $X^{W} Y$ → saare bete white, saari betiyaan red.
(b) Red-eyed female $X^{W} X^{W}$ × white-eyed male $X^{w} Y$ → saari offspring red.

Yeh step kyun? Ek autosomal gene ke liye, kaun sa parent kaun sa hai yeh ulta karne se identical offspring ratios milte hain. Ek X-linked gene ke liye results alag hote hain kyunki X fathers vs mothers ke through alag travel karta hai. Morgan ne exactly yahi use karke prove kiya ki genes chromosomes par hote hain (1910).

Figure — Describe sex-linked inheritance

Doosre patterns jo jaanne chahiye

Common galtiyan

Forecast-then-Verify

Recall Feynman: ek 12-saal ke bacche ko explain karo

Har kisi ke paas body ke zyaatar parts ke liye do "instruction books" hoti hain — ek mum se, ek dad se — isliye agar ek mein koi typo ho, toh doosri usually cover kar leti hai. Lekin ladkon ke paas ek special pair hoti hai jahan ek book (Y) chhoti aur mostly blank hoti hai. Toh jo genes sirf X book mein likhe hain, ek ladke ke paas sirf ek copy hoti hai. Agar woh akali copy mein typo ho, toh kuch cover nahi karta, aur use woh trait mil jaati hai. Ladkiyon ke paas do full X books hoti hain, isliye unke paas almost hamesha backup hota hai. Isliye colour-blindness jaisi cheezein ladkon mein bahut zyada aati hain.

Active-recall flashcards

#flashcards/biology

"Hemizygous" ka matlab kya hai?

Ek gene ki sirf ek copy hona kyunki woh X par hai aur Y par koi matching allele nahi (males ke liye X-linked genes ke case mein sach).

X-linked recessive traits males mein zyada common kyun hain?

Males ke paas ek X hota hai, isliye ek akela recessive allele (

X^{a} Y

) express hota hai; females ko do chahiye (

X^{a} X^{a}

Ek beta apna akela X chromosome kis parent se inherit karta hai?

Apni mother se.

Kya ek affected father apni X-linked trait apne beton ko de sakta hai?

Nahi — woh beton ko apna Y deta hai; bete apna X sirf mother se lete hain.

Criss-cross inheritance kya hai?

Ek father ki X-linked trait uski betiyon (carriers) tak jaati hai aur phir uske natiyon mein dikhti hai.

X-linked recessive frequency

q

ke liye, affected-male aur affected-female frequencies batao.

Males

= q

; Females

= q^{2}

Reciprocal crosses sex-linkage ka diagnostic test kyun hain?

Autosomal genes ke liye results identical hote hain; X-linked genes ke liye carrier parent ka sex badalne se offspring ratios badal jaate hain.

Carrier mother × normal father — beton aur betiyon ko describe karo.

Bete: ½ affected, ½ normal. Betiyaan: ½ carriers, ½ normal, koi affected nahi.

Y-linked (holandric) gene ka transmission pattern kya hai?

Father → har beta → har pota; kabhi females tak nahi.

X-linked DOMINANT trait wala affected father — kaun se bacche affected hain?

Saari betiyaan affected, koi beta nahi (beton ko uska Y milta hai).

Kya males X-linked genes ke liye hetero- ya homozygous hote hain?

Dono nahi — woh hemizygous hote hain (sirf ek allele).

Kisne reciprocal Drosophila eye-colour crosses use karke prove kiya ki genes chromosomes par hain?

Thomas Hunt Morgan (1910).

Connections

Mendelian Genetics — Law of Segregation — sex-linkage ek extension hai, segregation ka violation nahi.
Sex Determination — XY and XO systems — chromosomal basis jo hemizygosity ko possible banata hai.
Pedigree Analysis — family trees padhne ke liye in rules ka use.
Hardy-Weinberg Equilibrium — $q$ vs $q^{2}$ population frequencies ka source.
Linkage and Crossing Over — Morgan ka Drosophila work jisne chromosome theory establish ki.
Colour Blindness and Haemophilia — do textbook human X-linked recessives.