# Mendelian Genetics — Part A (NEET Biology Class 12): Laws of Inheritance, Blood Groups & Ratios

Source: https://tracknation.in/story/mendelian-genetics-part-a-neet-biology-class-12-laws-of-inheritance-blood-groups-ratios (TrackNation — free UPSC notes with PYQs)

> NEET Class 12 Principles of Inheritance Part A, mined from NCERT: Mendel's pea experiments and his laws of Dominance, Segregation and Independent Assortment; monohybrid (3:1, 1:2:1) and dihybrid (9:3:3:1) crosses, the test cross; incomplete dominance (Antirrhinum), codominance and ABO blood groups, multiple alleles, polygenic inheritance and pleiotropy (PKU). With 12 MCQs, 12 NEET PYQs and an Active Recall prompt. Part B covers chromosomal theory, linkage, sex determination, mutation and genetic

## Mendel & the Garden Pea

🎯 **NEET priority: High-yield.** *Genetics is the single biggest scorer in NEET Biology — this chapter alone brings 8–10 questions a year. Every ratio (3:1, 1:2:1, 9:3:3:1) and every ABO genotype is fair game. Master it cold.*

Why does a tall pea crossed with a dwarf one give *only* tall offspring — and then
dwarfs reappear a generation later? **Gregor Mendel** answered this with seven
years of patient pea-breeding, and his rules still anchor every genetics question NEET asks.

### The man and the method

- Mendel worked on the **garden pea** (*Pisum sativum*) for
**seven years (1856–1863)** and was the first to apply **statistics and
mathematics** to biology — his large sample sizes gave his data real credibility.

- He chose **14 true-breeding lines** — i.e. **seven pairs of
contrasting traits**. A **true-breeding** line is one that, after continuous
self-pollination, shows stable trait expression over many generations.

### The seven characters (memorise the table)

| **Character**

 | **Dominant trait**

 | **Recessive trait**

 |
| Stem height

 | Tall

 | Dwarf

 |
| Flower colour

 | Violet

 | White

 |
| Flower position

 | Axial

 | Terminal

 |
| Pod shape

 | Inflated

 | Constricted

 |
| Pod colour

 | Green

 | Yellow

 |
| Seed shape

 | Round

 | Wrinkled

 |
| Seed colour

 | Yellow

 | Green

 |

**Exam trap:** NEET asks the exact pairs — note **pod colour** is
green (dominant)/yellow (recessive), the *opposite* of **seed colour**
yellow (dominant)/green (recessive). Mix these up and you lose the mark.

## The Monohybrid Cross: Dominance, Segregation & the Test Cross

A **monohybrid cross** follows a single gene. Mendel crossed true-breeding
**tall (TT)** × **dwarf (tt)** peas:

- **F1** = all **Tall (Tt)** — the dwarf trait vanishes.

- Self-pollinate F1 → **F2** = **3 Tall : 1 Dwarf**
(phenotypic **3:1**), but genotypically **1 TT : 2 Tt : 1 tt = 1:2:1**.
The dwarf trait reappears, proving alleles don't blend.

- The **Punnett square** (developed by **Reginald C. Punnett**)
maps these outcomes by probability.

### Mendel's first two laws

- **Law of Dominance:** characters are controlled by discrete
**factors (genes)** that occur in **pairs**; in a dissimilar pair, one
factor (**dominant**) masks the other (**recessive**). This explains
why F1 shows one trait and F2 gives the **3:1** ratio.

- **Law of Segregation:** the two alleles of a pair **separate
(segregate)** during gamete formation, so each gamete carries **only one**
allele — there is **no blending**. This is also called the **law of purity of
gametes**.

### The test cross

A dominant phenotype (e.g. a tall plant) can be **TT** or **Tt** —
you can't tell by looking. A **test cross** crosses it with the
**homozygous recessive (tt)**: if any recessive offspring appear, the unknown was
heterozygous. It is the standard way to **determine an unknown genotype**.

## When Dominance Isn't Complete: Incomplete Dominance, Codominance & ABO

Mendel's clean dominant/recessive picture has important exceptions — all heavily tested.

### Incomplete dominance — a blended phenotype

Sometimes the F1 is **intermediate** between the parents. In the
**snapdragon / dog flower (*Antirrhinum*)**, true-breeding
**red (RR)** × **white (rr)** gives a **pink (Rr)** F1.
On selfing, F2 is **1 Red : 2 Pink : 1 White** — the **genotype ratio
(1:2:1) is unchanged**, but the phenotype ratio is no longer 3:1 because R is not fully
dominant over r.

### Codominance & the ABO blood groups

In **codominance** the F1 resembles **both** parents — both alleles
express fully. The classic example is **human ABO blood grouping**, controlled by
gene **I** with **three alleles: IA, IB and i**.
IA and IB each make a different surface sugar; **i makes none**.
When IA and IB are together (group **AB**), **both
sugars appear** — that is codominance. There are **6 genotypes and 4 phenotypes**.

| **Genotype**

 | **Blood group**

 | **Note**

 |
| IAIA or IAi

 | A

 | IA dominant over i

 |
| IBIB or IBi

 | B

 | IB dominant over i

 |
| IAIB

 | AB

 | codominance — both expressed

 |
| ii

 | O

 | no sugar produced

 |

**Multiple alleles:** ABO is also the textbook case — **three alleles**
govern one gene, yet any **individual carries only two**; multiple alleles are seen
only at the **population** level.

## The Dihybrid Cross & Independent Assortment

A **dihybrid cross** follows **two genes** at once. Mendel crossed
round-yellow (**RRYY**) × wrinkled-green (**rryy**) seeds.

- **F1** = all **round, yellow (RrYy)**.

- F1 produces **four gamete types** — **RY, Ry, rY, ry** —
each 25%.

- Selfing F1 → **F2** in the famous **9:3:3:1** ratio
(9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green) across 16 squares.

**Law of Independent Assortment:** when two pairs of traits are combined, the
**segregation of one pair is independent of the other**. That independence is exactly
why 9:3:3:1 = (3 round : 1 wrinkled) × (3 yellow : 1 green).

## Beyond Mendel: Polygenic Inheritance & Pleiotropy

Two more patterns extend Mendel — both NEET favourites.

### Polygenic inheritance — traits on a gradient

- Many traits aren't "either/or" but vary across a **continuous range**
(human **height**, **skin colour**). These are controlled by
**three or more genes** and are called **polygenic traits**.

- The effect of each allele is **additive**, and the
**environment** also contributes. For skin colour (genes A, B, C), the most dominant
alleles (**AABBCC**) give the darkest skin and all-recessive (**aabbcc**)
the lightest; intermediate counts give intermediate shades.

### Pleiotropy — one gene, many effects

A **pleiotropic gene** produces **multiple phenotypic effects**,
usually by acting on a metabolic pathway. The example is **phenylketonuria (PKU)**:
a single mutation in the gene for the enzyme **phenylalanine hydroxylase** causes
**both mental retardation and reduced hair/skin pigmentation**.

### Why this matters for NEET

- Ratio recall is automatic marks: **3:1** (monohybrid), **1:2:1**
(genotype / incomplete dominance), **9:3:3:1** (dihybrid).

- ABO is asked every which way — genotype↔blood group, codominance vs multiple alleles,
"which genotype gives O" (only **ii**).

- Classic traps: incomplete dominance changes *phenotype* ratio but not
*genotype* ratio; pleiotropy = one gene→many traits, NOT many genes→one trait (that's
polygenic).

## Test Yourself: MCQs, PYQs & Active Recall

Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.

## Practice Questions

**Q1.** In a monohybrid cross, the F2 genotypic ratio is:

- (a) 1:1

- (b) 3:1

- (c) 9:3:3:1

- (d) 1:2:1

**Answer: (d)** — F2 genotypes are 1 TT : 2 Tt : 1 tt (1:2:1); the phenotypic ratio is 3:1.

**Q2.** Mendel studied how many pairs of contrasting traits in pea?

- (a) 5

- (b) 14

- (c) 10

- (d) 7

**Answer: (d)** — Seven pairs (from 14 true-breeding lines): stem height, flower colour, flower position, pod shape, pod colour, seed shape, seed colour.

**Q3.** The Law of Segregation is also known as the law of:

- (a) Linkage

- (b) Purity of gametes

- (c) Dominance

- (d) Independent assortment

**Answer: (b)** — A gamete carries only one allele of a pair (no blending), so it is 'pure' — the law of purity of gametes.

**Q4.** A test cross is performed by crossing the individual with a:

- (a) F1 hybrid

- (b) Homozygous recessive

- (c) Homozygous dominant

- (d) Heterozygote

**Answer: (b)** — Crossing the unknown with the homozygous recessive reveals its genotype from the offspring.

**Q5.** Incomplete dominance in Antirrhinum gives an F2 phenotypic ratio of:

- (a) 9:3:3:1

- (b) 2:1

- (c) 3:1

- (d) 1:2:1

**Answer: (d)** — RR red, Rr pink, rr white → F2 is 1 red : 2 pink : 1 white (1:2:1); phenotype ratio equals the genotype ratio here.

**Q6.** Blood group AB is an example of:

- (a) Pleiotropy

- (b) Incomplete dominance

- (c) Epistasis

- (d) Codominance

**Answer: (d)** — In I^A I^B, both I^A and I^B express their own surface sugar — codominance (the F1 resembles both parents).

**Q7.** How many genotypes and phenotypes exist for human ABO blood groups?

- (a) 6 genotypes, 4 phenotypes

- (b) 6 genotypes, 6 phenotypes

- (c) 4 genotypes, 6 phenotypes

- (d) 3 genotypes, 3 phenotypes

**Answer: (a)** — Three alleles (I^A, I^B, i) give 6 genotypes and 4 phenotypes (A, B, AB, O).

**Q8.** The genotype of a person with blood group O is:

- (a) I^B i

- (b) ii

- (c) I^A I^B

- (d) I^A i

**Answer: (b)** — Only ii gives group O — allele i produces no surface sugar.

**Q9.** The dihybrid F2 phenotypic ratio in Mendel's cross was:

- (a) 1:1:1:1

- (b) 3:1

- (c) 1:2:1

- (d) 9:3:3:1

**Answer: (d)** — 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green = 9:3:3:1, derived from (3:1)(3:1).

**Q10.** Human skin colour and height are examples of:

- (a) Polygenic inheritance

- (b) Pleiotropy

- (c) Codominance

- (d) Incomplete dominance

**Answer: (a)** — Both are continuous traits controlled by three or more genes with additive allele effects — polygenic inheritance.

**Q11.** Phenylketonuria, where one gene affects pigmentation and mental development, illustrates:

- (a) Pleiotropy

- (b) Polygenic inheritance

- (c) Multiple alleles

- (d) Codominance

**Answer: (a)** — A single phenylalanine-hydroxylase gene producing multiple effects is pleiotropy (one gene → many traits).

**Q12.** The Punnett square was developed by:

- (a) Walter Sutton

- (b) Gregor Mendel

- (c) T.H. Morgan

- (d) Reginald Punnett

**Answer: (d)** — Reginald C. Punnett, a British geneticist, devised the square to predict offspring genotype probabilities.

### NEET Previous Year Questions (PYQs)

*Real NEET previous-year questions on this chapter, with explanations in our own words.*

**Q13.** ABO blood grouping in humans is an example of: (NEET PYQ)

- (a) Both codominance and multiple alleles

- (b) Incomplete dominance

- (c) Only codominance

- (d) Only multiple alleles

**Answer: (a)** — Three alleles (I^A,I^B,i) = multiple alleles; I^A and I^B both expressing in AB = codominance. ABO shows both.

**Q14.** In Antirrhinum, a cross between red (RR) and white (rr) flowers gives a pink F1. This is: (NEET PYQ)

- (a) Codominance

- (b) Complete dominance

- (c) Pleiotropy

- (d) Incomplete dominance

**Answer: (d)** — Neither allele is fully dominant, so the heterozygote (Rr) is an intermediate pink — incomplete dominance.

**Q15.** A man of blood group AB marries a woman of blood group O. The possible blood groups of children are: (NEET PYQ)

- (a) A, B, AB and O

- (b) Only AB

- (c) AB and O

- (d) A and B

**Answer: (d)** — AB (I^A I^B) × O (ii) → children are I^A i (d) or I^B i (c). Only A and B are possible.

**Q16.** The number of different types of gametes produced by a plant of genotype RrYy is: (NEET PYQ)

- (a) Eight

- (b) Four

- (c) Two

- (d) Sixteen

**Answer: (b)** — RrYy produces four gamete types — RY, Ry, rY, ry — by independent assortment.

**Q17.** Which of Mendel's laws is based on the separation of alleles during gamete formation? (NEET PYQ)

- (a) Law of linkage

- (b) Law of dominance

- (c) Law of segregation

- (d) Law of independent assortment

**Answer: (c)** — The Law of Segregation: paired alleles separate so each gamete gets only one (purity of gametes).

**Q18.** A single gene affecting multiple phenotypic traits is termed: (NEET PYQ)

- (a) Pleiotropy

- (b) Epistasis

- (c) Polygeny

- (d) Codominance

**Answer: (a)** — Pleiotropy — one gene, many effects (e.g. phenylketonuria, sickle-cell). Polygeny is the reverse (many genes → one trait).

**Q19.** The phenotypic ratio of a Mendelian dihybrid cross is: (NEET PYQ)

- (a) 9:3:3:1

- (b) 1:1:1:1

- (c) 3:1

- (d) 1:2:1

**Answer: (a)** — 9:3:3:1 in F2 — the product of two independent 3:1 monohybrid ratios.

**Q20.** Test cross is a cross between F1 hybrid and: (NEET PYQ)

- (a) Another F1

- (b) Homozygous dominant parent

- (c) Homozygous recessive parent

- (d) F2 individual

**Answer: (c)** — Crossing with the homozygous recessive parent reveals whether the dominant individual is homozygous or heterozygous.

**Q21.** Mendelian genetics fails to explain continuous variation such as human height because such traits are: (NEET PYQ)

- (a) Pleiotropic

- (b) Polygenic

- (c) Sex-linked

- (d) Codominant

**Answer: (b)** — Continuous traits like height and skin colour are polygenic — many genes with additive effects plus environment.

**Q22.** If both parents are heterozygous for blood group (I^A i and I^B i), the chance of an O-group child is: (NEET PYQ)

- (a) 25%

- (b) 0%

- (c) 50%

- (d) 75%

**Answer: (a)** — I^A i × I^B i → I^A I^B, I^A i, I^B i, ii — one of four (25%) is ii = group O.

**Q23.** Walter Sutton and Theodore Boveri are associated with the: (NEET PYQ)

- (a) Lac operon

- (b) Theory of natural selection

- (c) Law of dominance

- (d) Chromosomal theory of inheritance

**Answer: (d)** — They noted chromosome behaviour parallels gene behaviour and proposed the chromosomal theory of inheritance (1902).

**Q24.** In incomplete dominance, the F2 ratio differs from a normal monohybrid cross in its: (NEET PYQ)

- (a) Genotypic ratio only

- (b) Neither ratio

- (c) Both ratios

- (d) Phenotypic ratio only

**Answer: (d)** — Genotype stays 1:2:1, but the phenotype becomes 1:2:1 (not 3:1) because the heterozygote looks different.

### Active Recall Prompt

Write everything you can recall, naming each topic first. Cover: **Mendel** (pea, seven traits, true-breeding); the **monohybrid cross** (3:1, 1:2:1, Punnett, test cross); the **Law of Dominance** and **Law of Segregation**; **incomplete dominance** (Antirrhinum) and **codominance** (ABO blood groups, genotypes); the **dihybrid cross** (9:3:3:1, independent assortment); and **polygenic inheritance** and **pleiotropy** (PKU). Begin each fact with its topic and end it with a full stop.