Distinguish homozygous and heterozygous

HOW to tell them apart (the decision rule)

Derivation of the genotype from gametes (first principles):

Each parent makes gametes that carry one allele each (this is the Law of Segregation — the pair splits).
Fertilization fuses one gamete from each parent.
So the offspring genotype = (allele from egg) + (allele from sperm).
If both gametes happened to carry the same allele → homozygous. If they carried different alleles → heterozygous.

So zygosity is decided at the moment of fertilization by which two alleles meet.

Phenotype consequence (WHY it matters)

Worked Examples

Worked example Example 2 — Monohybrid cross

$Tt \times Tt$ Step 1: Each parent is heterozygous → makes gametes $T$ and $t$ . Why this step? Law of Segregation: the pair separates into gametes. Step 2: Punnett square:

	$T$	$t$
$T$	$TT$	$Tt$
$t$	$Tt$	$tt$

Step 3: Genotype ratio $= 1\,TT : 2\,Tt : 1\,tt$ . Why? The four equally likely boxes give one homozygous-dom, two hetero, one homozygous-rec. Step 4: Zygosity tally → homozygous : heterozygous = 2 : 2 = 1 : 1. Why? $TT$ and $tt$ are both homozygous (2 boxes); the two $Tt$ are heterozygous (2 boxes). Step 5: Phenotype ratio $= 3$ tall $: 1$ short. Why? $TT$ and $Tt$ both look tall (3 boxes), only $tt$ is short.

Common Mistakes (Steel-man + Fix)

Flashcards

What does "homozygous" mean?

Both alleles at a locus are identical (e.g.

TT

tt

What does "heterozygous" mean?

The two alleles at a locus are different (e.g.

Tt

Difference between genotype and phenotype?

Genotype = actual allele pair carried; phenotype = the observable trait.

tt

homozygous or heterozygous?

Homozygous (homozygous recessive).

Why can't you tell a tall plant's genotype by looking?

With complete dominance both

TT

and

Tt

look tall — dominant masks recessive.

If an organism shows a recessive trait, what is its genotype?

Homozygous recessive (e.g.

tt

) — a single dominant allele would mask it.

Genotype ratio of

Tt \times Tt

1\,TT : 2\,Tt : 1\,tt

Homozygous-to-heterozygous ratio in

Tt \times Tt

offspring?

2:2 = 1:1

What cross reveals if a dominant-looking organism is homo or heterozygous?

A test cross with a homozygous recessive (

tt

In a test cross, what result proves the parent was heterozygous?

Appearance of any offspring showing the recessive trait.

Another name for a heterozygote?

Hybrid / carrier.

Another name for a homozygote?

True-breeding / pure-breeding.

Recall Feynman: explain to a 12-year-old

Imagine each gene is a pair of socks you got — one sock from Mom, one from Dad. If both socks are the same colour, that's homozygous. If the socks are different colours, that's heterozygous. Some colours are "loud" (dominant) and others are "quiet" (recessive). If you wear one loud sock and one quiet sock, people only notice the loud one — so two different mixes can look the same from outside! The only way the quiet colour shows is if both socks are quiet. That's why a "short" pea plant must have two quiet (recessive) socks, but a "tall" plant could have two loud, or one loud + one quiet.

Connections

Mendel's Law of Segregation — explains why each gamete carries one allele.
Dominant and Recessive Alleles — why heterozygotes show only one trait.
Punnett Square — tool to count zygosity ratios.
Test Cross — how to experimentally determine unknown zygosity.
Genotype vs Phenotype — the layer that hides zygosity under complete dominance.
Monohybrid Cross — source of the $1:2:1$ genotype and $3:1$ phenotype ratios.
Incomplete Dominance and Codominance — special cases where heterozygotes look distinct.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, har sexually reproduce karne wale organism ke paas har gene ki do copies hoti hain — ek Mummy se, ek Papa se. Yeh do copies hi alleles kehlate hain. Ab sirf ek simple sawaal: yeh dono copies same hain ya different? Agar same hain (jaise $TT$ ya $tt$ ) to bolte hain homozygous (pure ya true-breeding). Agar different hain (jaise $Tt$ ) to bolte hain heterozygous (hybrid ya carrier). Bas itni si baat hai — pura concept isi fork pe khada hai.

Ab important twist: complete dominance mein heterozygous $Tt$ bhi tall dikhta hai, kyunki dominant $T$ , recessive $t$ ko chhupa deta hai. Iska matlab ek tall plant ko dekh kar tum pakka nahi keh sakte ki woh $TT$ hai ya $Tt$ — dono tall lagte hain. Lekin short plant ( $tt$ ) hamesha pakka homozygous recessive hota hai, kyunki agar ek bhi $T$ hota to woh tall ho jaata. Yahi reason hai ki recessive trait "free mein" genotype bata deta hai.

Agar tumhe pata karna ho ki ek tall plant $TT$ hai ya $Tt$ , to test cross karo — usse $tt$ (homozygous recessive) ke saath cross karo. Agar koi bhi short baby nikla, to parent zaroor heterozygous tha. Agar saare tall nikle, to parent $TT$ tha. Yeh trick exam mein bahut puchhi jaati hai.

Ek common galti yaad rakhna: $Tt \times Tt$ cross mein phenotype ratio 3:1 hota hai, par genotype ratio $1:2:1$ hota hai, aur homozygous-to-heterozygous ratio $2:2 = 1:1$ hai. In teeno ko mix mat karna — ratio kaunsa hai pehle yeh confirm karo: genotype ya phenotype.

WHAT are we talking about?

HOW to tell them apart (the decision rule)

Phenotype consequence (WHY it matters)

Worked Examples

Common Mistakes (Steel-man + Fix)

Flashcards

Connections

Concept Map

Hinglish (regional understanding)

Test yourself — Mendelian Genetics

Connections