State Mendel's law of segregation — Biology notes

WHAT is the Law of Segregation?

Key vocabulary you must hold in your head:

Gene = an instruction for a trait. Allele = one version of that gene (e.g. $T$ = tall, $t$ = short).
Diploid (2n) = two copies of each gene. Haploid (n) = one copy (gametes).
Homozygous = two same alleles ( $TT$ or $tt$ ). Heterozygous = two different ( $Tt$ ).

WHY is it true? (First-principles derivation)

We derive the famous 3:1 ratio from segregation — never just quote it.

Step 1 — Each parent is diploid, so a heterozygote is $Tt$ . Why this step? Mendel crossed pure tall ( $TT$ ) × pure short ( $tt$ ). All F₁ are $Tt$ — they look tall because $T$ is dominant, but secretly carry $t$ .

Step 2 — Segregation: the $Tt$ parent makes gametes. The two alleles separate. So gametes are half $T$ , half $t$ : $P(T) = \tfrac{1}{2}, \qquad P(t) = \tfrac{1}{2}$ Why this step? This is the law itself — one allele per gamete, and since there's one $T$ and one $t$ , each is equally likely.

Step 3 — Self-cross F₁ × F₁, combine gametes independently. Each offspring = one gamete from each parent. Multiply probabilities (Punnett square):

♀ \ ♂	$T\ (½)$	$t\ (½)$
$T\ (½)$	$TT\ (¼)$	$Tt\ (¼)$
$t\ (½)$	$Tt\ (¼)$	$tt\ (¼)$

Step 4 — Read off genotype and phenotype ratios. $\text{Genotype: } TT : Tt : tt = 1 : 2 : 1$ Since $T$ is dominant, $TT$ and $Tt$ both look tall: $\text{Phenotype: } \underbrace{(\tfrac14 + \tfrac12)}_{\text{tall}} : \underbrace{\tfrac14}_{\text{short}} = \tfrac34 : \tfrac14 = \boxed{3 : 1}$ Why this step? The 3:1 ratio is not an axiom — it falls out automatically once alleles segregate ½:½ and combine independently.

Figure — State Mendel's law of segregation

HOW does this happen physically? (The mechanism)

Mendel didn't know meiosis — he inferred segregation from numbers. We now know why:

Worked Examples

Common Mistakes (Steel-man + Fix)

Active Recall

Recall Quick self-test (hide the answers!)

What separates during gamete formation? → the two alleles of a gene.
At which meiotic stage? → Anaphase I (homologs to opposite poles).
F₂ genotype ratio? → $1:2:1$ .
F₂ phenotype ratio (full dominance)? → $3:1$ .
How many alleles per gamete? → exactly one.

Recall Feynman: explain to a 12-year-old

You have two coloured marbles in your pocket — one red, one blue. When you give a marble to your friend, you can only hand over one at a time, not both. So each friend gets just one. Later, two friends each bring one marble to make a new pair. That "only one marble per hand" rule is exactly what genes do when making egg and sperm cells: each cell gets one of your two copies, decided by a fair coin flip.

Flashcards

State Mendel's Law of Segregation.

The two alleles of a gene separate during gamete formation so that each gamete carries only one allele; the pair is restored at fertilization.

What is the physical (cellular) basis of segregation?

Separation of homologous chromosomes to opposite poles during anaphase I of meiosis.

In a

Tt \times Tt

cross, what is the F₂ genotype ratio?

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

In a

Tt \times Tt

cross with full dominance, what is the F₂ phenotype ratio?

3 : 1

(dominant : recessive).

How many alleles of a gene does a single gamete carry?

Exactly one.

What gamete proportions does a heterozygote (

Tt

) produce?

One half carry

T

, one half carry

t

(½ : ½).

Why does a recessive trait reappear in F₂ even after disappearing in F₁?

Because alleles stay intact (don't blend); recessive allele is merely hidden in heterozygotes and segregates out again.

What is a test cross and what does it reveal?

Crossing an unknown dominant phenotype with a homozygous recessive (

tt

); offspring ratios reveal whether the unknown is homozygous (all dominant) or heterozygous (1:1).

Does "dominant" mean the allele is more common in a population?

No — dominance affects only the visible phenotype, not allele frequency.

Connections

Meiosis — anaphase I provides the physical mechanism of segregation.
Law of Independent Assortment — Mendel's 2nd law; segregation applied to two genes at once.
Punnett Square — the tool to combine segregated gametes.
Dominance and Recessiveness — why $3:1$ phenotype hides the $1:2:1$ genotype.
Test Cross — using segregation to expose hidden alleles.
Probability in Genetics — multiplication & addition rules behind the ratios.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, Mendel ka Law of Segregation ekdum simple idea hai. Har diploid organism ke paas ek gene ke do alleles hote hain — jaise tall ke liye $T$ aur short ke liye $t$ . Jab body gamete (egg ya pollen) banati hai, to ye dono alleles alag ho jaate hain (segregate), aur har gamete ko sirf ek hi allele milta hai. Bilkul jaise ek pair of socks ko cut karke alag-alag de do. Fertilization ke time phir do gametes milte hain aur pair wapas ban jaata hai.

Iska physical reason meiosis hai. Jo do alleles homologous chromosomes pe baithe hote hain, wo Anaphase I me opposite poles ki taraf khinch jaate hain. Isliye har gamete me ek hi copy jaati hai. Mendel ko meiosis pata nahi tha, usne sirf numbers dekhke ye guess kiya — kya genius tha!

Ab famous 3:1 ratio kaise aata hai? $Tt \times Tt$ cross karo. Har parent ke gametes half $T$ , half $t$ . Punnett square banao to genotype nikalta hai $1\,TT : 2\,Tt : 1\,tt$ . Kyunki $T$ dominant hai, $TT$ aur $Tt$ dono tall dikhte hain, sirf $tt$ short. Isliye phenotype ratio $3:1$ ban jaata hai. Yaad rakho — ye ratio average hai, har bachche ke liye fixed nahi.

Do common galtiyan: (1) "Gamete me dono allele hote hain" — galat, sirf ek. (2) "Dominant allele zyada common ho jaata hai" — galat, dominance sirf dikhne ko affect karta hai, frequency ko nahi. Recessive allele kabhi destroy nahi hota, wo F₂ me wapas aata hai — yahi proof hai ki alleles intact rehte hain, blend nahi hote.

Test yourself — Mendelian Genetics

Connections