Distinguish homozygous and heterozygous

3.1.3 · Biology › Mendelian Genetics

HOW to tell them apart (the decision rule)

Gametes se genotype ki derivation (first principles):

Har parent gametes banata hai jo ek allele carry karte hain (yeh hai Law of Segregation — pair alag ho jaata hai).
Fertilization ek gamete ko har parent se fuse karta hai.
Toh offspring genotype = (egg se allele) + (sperm se allele).
Agar dono gametes same allele carry kar rahe the → homozygous. Agar unhone different alleles carry kiye → heterozygous.

Toh zygosity fertilization ke moment pe decide hoti hai ki kaun se do alleles milte hain.

Phenotype consequence (WHY it matters)

Worked Examples

Worked example Example 2 — Monohybrid cross

$T t \times T t$ Step 1: Har parent heterozygous hai → gametes $T$ aur $t$ banata hai. Yeh step kyun? Law of Segregation: pair gametes mein alag ho jaata hai. Step 2: Punnett square:

	$T$	$t$
$T$	$T T$	$T t$
$t$	$T t$	$tt$

Step 3: Genotype ratio $= 1 T T : 2 T t : 1 tt$ . Kyun? Chaar equally likely boxes dete hain ek homozygous-dom, do hetero, ek homozygous-rec. Step 4: Zygosity tally → homozygous : heterozygous = 2 : 2 = 1 : 1. Kyun? $T T$ aur $tt$ dono homozygous hain (2 boxes); dono $T t$ heterozygous hain (2 boxes). Step 5: Phenotype ratio $= 3$ tall $: 1$ short. Kyun? $T T$ aur $T t$ dono tall dikhte hain (3 boxes), sirf $tt$ short hai.

Common Mistakes (Steel-man + Fix)

Flashcards

"Homozygous" ka matlab kya hai?

Ek locus pe dono alleles identical hain (jaise

T T

tt

"Heterozygous" ka matlab kya hai?

Ek locus pe dono alleles different hain (jaise

T t

Genotype aur phenotype mein fark?

Genotype = actual allele pair jo carry kiya jaata hai; phenotype = observable trait.

Kya

tt

homozygous hai ya heterozygous?

Homozygous (homozygous recessive).

Tall plant ka genotype dekh ke kyun nahi bata sakte?

Complete dominance mein

T T

aur

T t

dono tall dikhte hain — dominant, recessive ko mask karta hai.

Agar koi organism recessive trait dikhata hai, toh uska genotype kya hai?

Homozygous recessive (jaise

tt

) — ek akela dominant allele use mask kar deta.

T t \times T t

ka genotype ratio?

1 T T : 2 T t : 1 tt

T t \times T t

offspring mein homozygous-to-heterozygous ratio?

2 : 2 = 1 : 1

Kaun sa cross batata hai ki ek dominant-looking organism homo ya heterozygous hai?

Homozygous recessive (

tt

) ke saath test cross.

Test cross mein kaun sa result prove karta hai ki parent heterozygous tha?

Koi bhi offspring jo recessive trait dikha raha ho, uska appearance.

Heterozygote ka doosra naam?

Hybrid / carrier.

Homozygote ka doosra naam?

True-breeding / pure-breeding.

Recall Feynman: explain to a 12-year-old

Socho har gene ek pair of socks hai jo tumhe mili — ek sock Mom se, ek Dad se. Agar dono socks same colour ki hain, toh woh homozygous hai. Agar socks different colours ki hain, toh woh heterozygous hai. Kuch colours "loud" (dominant) hote hain aur kuch "quiet" (recessive). Agar tum ek loud aur ek quiet sock pahno, log sirf loud wala notice karte hain — toh do alag mixes bahar se same dikh sakte hain! Quiet colour tabhi dikhta hai jab dono socks quiet hon. Isliye ek "short" pea plant ke paas zaroor do quiet (recessive) socks hongi, lekin ek "tall" plant ke paas do loud, ya ek loud + ek quiet ho sakti hai.

Connections

Mendel's Law of Segregation — explain karta hai ki kyun har gamete ek allele carry karta hai.
Dominant and Recessive Alleles — kyun heterozygotes sirf ek trait dikhate hain.
Punnett Square — zygosity ratios count karne ka tool.
Test Cross — unknown zygosity experimentally determine karne ka tarika.
Genotype vs Phenotype — woh layer jo complete dominance mein zygosity chhupa deti hai.
Monohybrid Cross — $1 : 2 : 1$ genotype aur $3 : 1$ phenotype ratios ka source.
Incomplete Dominance and Codominance — special cases jahan heterozygotes alag dikhte hain.

WHAT are we talking about?