Explain codominance (e.g., ABO blood groups)

3.2.2 · Biology › Extensions of Mendelian Genetics

Codominance KYUN hoti hai? (pehle mechanism)

Compare karo:

Complete dominance → recessive allele typically ek non-functional / no product banata hai; ek working copy kaafi hai, isliye sirf ek phenotype dikhta hai.
Incomplete dominance → ek functional allele aadha product banata hai, full phenotype ke liye kaafi nahi → ek intermediate appear hota hai (dosage effect).
Codominance → do alag functional products, dono detectable → dono phenotypes dikhte hain.

ABO blood group system

Genotype → Phenotype table

Genotype	RBC par Antigens	Blood Group	Dominance shown
$I^{A} I^{A}$	A	A	—
$I^{A} i$	A	A	$I^{A}$ dominant over $i$
$I^{B} I^{B}$	B	B	—
$I^{B} i$	B	B	$I^{B}$ dominant over $i$
$I^{A} I^{B}$	A aur B	AB	codominance
$ii$	kuch nahi (sirf H)	O	recessive

Figure — Explain codominance (e.g., ABO blood groups)

Cross KAISE karte hain (derive karo, memorise mat karo)

Worked example Cross 1:

$I^{A} i \times I^{B} i$ — kya in parents ka O child ho sakta hai? Yeh setup kyun? Har parent heterozygous hai, ek chhupa hua $i$ carry kar raha hai. Hume gametes track karni hogi.

Parent 1 gametes: $I^{A}, i$ . Parent 2 gametes: $I^{B}, i$ . Punnett square:

	$I^{B}$	$i$
$I^{A}$	$I^{A} I^{B}$ (AB)	$I^{A} i$ (A)
$i$	$I^{B} i$ (B)	$ii$ (O)

Yeh result kyun? Dono parents recessive $i$ carry karte hain; jab dono $i$ gametes milte hain → $ii$ → group O. Phenotypic ratio: $1 AB : 1 A : 1 B : 1 O$ . Punchline: Do parents (A aur B) charon blood groups produce kar sakte hain, jismein AB (codominance) aur O (recessive) bhi shamil hain.

Worked example Cross 2:

$I^{A} I^{B} \times ii$ — ek AB parent aur ek O parent Interesting kyun hai? Log AB child expect karte hain, lekin yeh impossible hai! AB parent gametes: $I^{A}, I^{B}$ . O parent gametes: $i, i$ .

	$i$	$i$
$I^{A}$	$I^{A} i$ (A)	$I^{A} i$ (A)
$I^{B}$	$I^{B} i$ (B)	$I^{B} i$ (B)

Ratio: $1 A : 1 B$ . Na AB, na O. Yeh step kyun? AB parent kabhi $i$ gamete nahi banata, aur O parent sirf $i$ banata hai — toh ek child ko hamesha ek antigen-allele + ek $i$ milta hai, exactly ek antigen express hota hai.

Common mistakes (steel-manned)

Active recall

Codominance kya hai?

Ek pattern jismein dono alleles heterozygote mein fully aur simultaneously express hote hain, dono distinct phenotypes dikhte hain (blend nahi).

Codominance vs incomplete dominance?

Codominance = dono phenotypes alag-alag fully expressed; incomplete dominance = ek intermediate/blended phenotype.

Blood group AB ka genotype?

I^{A} I^{B}

— dono antigens A aur B bante hain (codominance).

AB blood group codominant kyun hai, blend kyun nahi?

RBC par A aur B dono antigens fully aur alag-alag bante hain; kuch bhi intermediate nahi banta.

ABO mein kaun sa allele recessive hai aur uska phenotype kya hai?

i

recessive hai;

ii

mein koi antigen nahi banta → blood group O.

Blood group A ke liye possible genotypes?

I^{A} I^{A}

aur

I^{A} i

Kya AB (parent) × O (parent) mein AB ya O child ho sakta hai?

Nahi — offspring sirf A ya B hote hain (1:1), kyunki AB A/B deta hai aur O sirf i deta hai.

"Multiple alleles" ka matlab asal mein kya hai?

Ek gene ke population mein do se zyada alleles hain, lekin har diploid individual phir bhi sirf do carry karta hai.

n alleles ke saath kitne genotypes possible hain?

n (n + 1) /2

; ABO ke liye (n=3) yeh 6 deta hai.

I^{A}

aur

I^{B}

enzymes kaun sa molecule modify karte hain?

RBCs par H antigen (

I^{A}

N-acetylgalactosamine add karta hai,

I^{B}

galactose add karta hai).

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho tumhare red blood cells chhote tables hain, aur har table par ek flag lag sakta hai. Tumhare genes instructions hain ki kaun sa flag lagao. "A" instruction flag A lagata hai. "B" instruction flag B lagata hai. Agar tumhe mom se A instruction mili aur dad se B instruction, toh tumhare cells koi weird half-A-half-B flag nahi banate — woh bas dono flags, A aur B, ek saath lagate hain. Yahi group AB hai, aur yeh "ek saath dono" rule codominance hai. "O" instruction ek tuta hua pen hai jo koi flag nahi draw karta — toh yeh tab hi dikhta hai jab dono tumhari instructions tuti hui hon.

Connections

Incomplete Dominance — contrast: blend vs both-shown
Multiple Alleles — ABO model system hai
Complete Dominance and the Punnett Square
Mendel's Law of Segregation — hidden alleles wapas kyun aate hain
Antigens and Immune Recognition — blood transfusion se clinical link
Sickle Cell Trait — molecular level par HbA aur HbS ki codominance

Explain codominance (e.g., ABO blood groups)

Codominance KYA hai?

Codominance KYUN hoti hai? (pehle mechanism)

ABO blood group system

Genotype → Phenotype table

Cross KAISE karte hain (derive karo, memorise mat karo)

Common mistakes (steel-manned)

Active recall

Connections

Concept Map