Use the test cross to determine genotype

3.1.8 · Biology › Mendelian Genetics

WHAT is a test cross?

Unknown parent ya toh:

$T T$ (homozygous dominant) hai, ya
$T t$ (heterozygous) hai

Tester hamesha $tt$ hota hai.

WHY use the homozygous recessive as the tester?

Agar hum $T T$ ko tester ki jagah use karein, toh har offspring tall hoga, unknown parent ki parwah kiye bina — result uninformative hoga. Recessive tester information ko maximise karta hai.

HOW to derive the offspring ratios from first principles

Hum Law of Segregation use karte hain: har parent har gamete mein ek gene ke exactly ek allele pass karta hai, aur dono alleles equally alag hote hain (heterozygote ke liye 50/50).

Case A — Unknown homozygous dominant $T T$ hai:

$T T$ se gametes: saare $T$ . $tt$ se gametes: saare $t$ .

$T T \times tt \Rightarrow all T t$

Har offspring $T t$ hai → saare dominant phenotype dikhate hain. Phenotype ratio = 100% dominant (saare tall).

Case B — Unknown heterozygous $T t$ hai:

$T t$ se gametes: $\frac{1}{2} T$ , $\frac{1}{2} t$ . $tt$ se gametes: saare $t$ .

	$T$ ( $\frac{1}{2}$ )	$t$ ( $\frac{1}{2}$ )
$t$	$T t$	$tt$

$T t \times tt \Rightarrow \frac{1}{2} T t : \frac{1}{2} tt$

Phenotype ratio = 1 dominant : 1 recessive (50% tall, 50% short).

Figure — Use the test cross to determine genotype

Common Mistakes (Steel-manned)

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Active Recall Flashcards

What is a test cross?

Ek unknown genotype wale individual (dominant phenotype) aur ek homozygous recessive individual ke beech cross, unknown genotype determine karne ke liye.

Why is the tester always homozygous recessive?

Yeh sirf recessive alleles donate karta hai, isliye offspring phenotypes poori tarah unknown parent se decide hote hain — information maximise hoti hai.

Test cross result: all offspring dominant. Genotype of unknown parent?

Most likely homozygous dominant (TT) — lekin large numbers ke saath hi certain hota hai.

Test cross result: ~1 dominant : 1 recessive. Genotype of unknown parent?

Heterozygous (Tt).

What genetic law explains why a Tt parent gives 50% T and 50% t gametes?

The Law of Segregation.

Why is a homozygous dominant (TT) a useless tester?

TT aur Tt dono unknown parents ke saath yeh saare-dominant offspring deta hai, isliye dono ko distinguish nahi kar sakta.

If a Tt parent is test-crossed, what is the chance all 4 offspring are dominant by luck?

(1/2)^4 = 1/16 ≈ 6%, isliye chote samples mislead kar sakte hain.

Difference between test cross and back cross?

Test cross specifically homozygous recessive ke saath hota hai; back cross kisi bhi parental genotype ke saath hota hai.

Connections

Law of Segregation — woh principle jis par test cross rely karta hai
Homozygous vs Heterozygous — woh do possibilities jo hum distinguish karte hain
Punnett Square — offspring ratios predict karne ka tool
Dominant and Recessive Alleles — kyun recessive alleles chhup sakte hain
Dihybrid Test Cross — isse do genes tak extend karna (1:1:1:1 expect karein)
Genotype vs Phenotype — woh core gap jo test cross bridge karta hai