Solve monohybrid crosses with Punnett squares

3.1.6 · Biology › Mendelian Genetics

Core Concepts (the 20% that explains 80%)

HOW to build a Punnett square — derivation from first principles

Hum Mendel's Law of Segregation par rely karte hain: ek gene ke do alleles gamete formation ke dauran separate ho jaate hain, isliye har gamete ko exactly ek milta hai.

Step-by-step method:

Har parent ka genotype likho.
Har parent ke liye possible gametes list karo (pair ko split karo).
Ek grid banao: ek parent ke gametes columns label karte hain, doosre ke rows.
Har cell ko column allele + row allele combine karke fill karo.
Genotype ratios count karo, phir dominance use karke phenotype ratios mein convert karo.

Figure — Solve monohybrid crosses with Punnett squares

Worked Examples

Worked example Example 1 — Heterozygous × Heterozygous (

$Y y \times Y y$ ) Trait: pea seed color. $Y$ = yellow (dominant), $y$ = green (recessive).

	Y	y
Y	$Y Y$	$Y y$
y	$Y y$	$y y$

Why split into $Y$ and $y$ ? Segregation: har $Y y$ parent half $Y$ gametes aur half $y$ gametes banata hai.
Why fill cells by combining? Har cell = ek fertilization event (ek sperm × ek egg).
Genotype: $1 Y Y : 2 Y y : 1 y y$ .
Phenotype: $3$ yellow $: 1$ green. Why? $Y Y$ aur $Y y$ dono yellow dikhate hain.

Worked example Example 2 — Test cross (

$Y y \times y y$ ) Yeh pata lagane ke liye use hota hai ki koi yellow plant $Y Y$ hai ya $Y y$ .

	Y	y
y	$Y y$	$y y$
y	$Y y$	$y y$

Why use $y y$ as tester? Ek homozygous recessive sirf $y$ contribute karta hai, isliye offspring phenotype directly reveal karti hai unknown parent ke gametes.
Phenotype: $1$ yellow $: 1$ green ( $1 : 1$ ).
Interpretation: koi bhi green offspring dekhna prove karta hai ki yellow parent $Y y$ tha.

Worked example Example 3 — Homozygous dominant × homozygous recessive (

$Y Y \times y y$ , the $F_{1}$ )

	Y	Y
y	$Y y$	$Y y$
y	$Y y$	$Y y$

Why are all offspring $Y y$ ? Ek parent sirf $Y$ de sakta hai, doosra sirf $y$ .
Phenotype: $100%$ yellow. Yeh uniform $F_{1}$ generation hai. Inhi do $F_{1}$ ko cross karne par Example 1 ka $3 : 1$ wapas milta hai.

Ek pure-breeding tall plant ( $T T$ ) ko ek short plant ( $tt$ ) ke saath cross kiya jaata hai. $F_{1}$ ko self-cross kiya jaata hai. Forecast: $F_{2}$ mein kya phenotype ratio hoga? Verify: $F_{1} = T t$ . $T t \times T t \Rightarrow 1 T T : 2 T t : 1 tt \Rightarrow$ 3 tall : 1 short. ✓

Common Mistakes (Steel-manned)

Flashcards

Monohybrid cross kya study karta hai?

Ek single gene/trait ki inheritance generations ke across.

Mendel's Law of Segregation batao.

Ek gene ke do alleles gamete formation ke dauran separate ho jaate hain; har gamete ko exactly ek allele milta hai.

A a \times A a

ka genotype ratio?

1 AA : 2 A a : 1 aa

A a \times A a

ka phenotype ratio (complete dominance)?

3 dominant : 1 recessive.

A a

ka phenotype

AA

jaisa kyun hota hai?

Ek dominant allele kaafi functional product produce karta hai dominant trait dikhane ke liye.

Test cross kya hai aur

aa

kyun use karte hain?

Unknown dominant ko homozygous recessive (

aa

) ke saath cross karna; offspring phenotypes directly unknown ke gametes reveal karti hain.

Test cross

A a \times aa

ka phenotype ratio?

1 dominant : 1 recessive.

AA \times aa

ka result?

Saare offspring

A a

→ 100% dominant phenotype (

F_{1}

uniform).

2×2 Punnett square mein har cell kya probability represent karti hai?

\frac{1}{4}

(equally likely fertilization outcome).

Genotype aur phenotype mein kya fark hai?

Genotype = allele pair (e.g.,

Y y

); phenotype = observable trait (e.g., yellow).

Ek homozygote kitne tarah ke gametes produce karta hai?

Ek.

Ek heterozygote kitne tarah ke gametes produce karta hai?

Do (har ek ~50%).

Recall Feynman: explain to a 12-year-old

Socho ki har parent ke paas do "trait cards" hain lekin ek baby ko sirf EK card de sakta hai. Dad apne do cards shuffle karta hai aur ek deta hai; Mom bhi yahi karti hai. Punnett square ek chhoti si table hai jo har tarika list karti hai jisme Dad ka card Mom ke card se mil sakta hai. Kuch cards "loud" hote hain (dominant) — agar baby ko ek bhi loud card mile, toh wahi dikhta hai. Ek "quiet" (recessive) trait tabhi appear hoti hai jab baby ko do quiet cards milein. Boxes count karo, aur tum predict kar sakte ho ki babies probably kaisi dikhenge!

Connections

Mendel's Law of Segregation — biological basis ki hum alleles kyun split karte hain.
Law of Independent Assortment — ise dihybrid (two-gene) crosses tak extend karta hai.
Probability rules in genetics — square ke peeche product & sum rules.
Incomplete dominance and codominance — jab $3 : 1$ phenotype mein $1 : 2 : 1$ ban jaata hai.
Test cross — monohybrid logic ka practical use.
Genotype vs Phenotype — ratio interpretation ki core distinction.