Heredity — Detailed Notes

Chapter 8 | NCERT Class 10 Science

8.1 Accumulation of Variation During Reproduction

Inheritance gives the next generation a common basic body design plus subtle changes. When that generation reproduces, offspring get:

Differences inherited from parents
Newly created differences

Diagram — diversity tree:

Diversity tree — Accumulation of variation across generations

Asexual case: bacteria dividing produce very similar progeny; only minor DNA-copying inaccuracies.
Sexual case: greater diversity because inheritance rules combine material from two parents.

Not all variants survive equally. Environment selects advantageous variants (e.g. heat-tolerant bacteria in a heat wave). Selection of variants is the basis for evolutionary processes (developed further beyond this chapter’s core exam focus).

Better Method / Strategy

If trait A is rarer (10%) than trait B (60%) in an asexual population, B is likelier to have arisen earlier (had more time to spread), assuming similar transmission — classic NCERT thinking question.

8.2 Heredity

Heredity map — Heredity question decision map

Rules of heredity explain reliable inheritance of traits despite variation.

8.2.1 Inherited Traits

Children show human features yet are not exact copies of parents. Human populations show much variation (e.g. free vs attached earlobes — Activity idea).

Both parents contribute roughly equal genetic material → for each trait, a child has two versions (factors/genes).

8.2.2 Rules for Inheritance — Mendel’s Contributions

Mendel crossed pea plants with contrasting traits (tall/short, round/wrinkled seeds, flower colour, etc.) and counted progeny — combining biology with mathematics.

Monohybrid-style tall × short:

F1: all tall — no intermediate “medium” plants → traits do not blend; one trait is expressed
Selfing F1 tall plants → F2: about 3/4 tall, 1/4 short

Conclusion: F1 plants inherited both tallness and shortness factors, but only tallness was expressed.

Mendel proposed two copies of each factor (gene) in sexually reproducing organisms:

Genotype	Phenotype (tall dominant)
TT	Tall
Tt	Tall
tt	Short

Dominant trait: one copy enough (T).
Recessive trait: expressed only when both copies are recessive (tt).

Diagram — two generations (Mendel):

Mendel generations — Mendel tall × short — F1 and F2

Two traits together (e.g. seed shape and colour):

Example pattern: round green × wrinkled yellow.

Let round (R) be dominant to wrinkled (r); yellow (Y) dominant to green (y). Then:

Parent	Phenotype	Useful pure genotype
P1	Round green	RRyy
P2	Wrinkled yellow	rrYY

F1: all RrYy → phenotype round yellow (both dominant traits show).
Selfing F1 → F2 shows four phenotype classes (classic idea 9 : 3 : 3 : 1 when independent).

Parental combinations (same pairings as the original parents):

round green
wrinkled yellow

New combinations (not present as parental phenotypes):

round yellow
wrinkled green

(Do not swap these labels. Round yellow is the F1 look, but relative to the given parents it is a new combination in the dihybrid sense; the parental phenotype pair is round green & wrinkled yellow.)

Thus traits such as seed shape and seed colour are inherited independently.

Better Method / Strategy

To confirm F2 genotype ratio 1:2:1 (TT:Tt:tt), self each F2 class or cross with recessive (test ideas) — as NCERT Activity prompts.

Always state: F1 phenotype shows dominant; recessive reappears in F2.

8.2.3 How Do These Traits Get Expressed?

Gene: section of DNA providing information for one protein.

Example — plant height: growth depends on a plant hormone amount; hormone amount depends on efficiency of an enzyme in its pathway. If the gene for that enzyme is altered → less efficient enzyme → less hormone → short plant. So genes control traits via proteins.

Both parents contribute a copy of each gene → body cells have two sets. Germ-cells must have one set. If progeny inherited each parent’s genes only as one inseparable block, independent inheritance (Fig. 8.5 style) would fail. Instead, genes lie on separate chromosomes. Each cell has two copies of each chromosome (maternal + paternal). Each germ-cell takes one chromosome from each pair (maternal or paternal origin). Fertilisation restores the chromosome number — stable DNA content of the species.

Asexually reproducing organisms also follow inheritance rules in a simpler copying sense (single parent gene set transmitted with possible copying variation).

8.2.4 Sex Determination

Strategies differ among species:

Some reptiles: incubation temperature of eggs decides sex
Some snails: individuals can change sex (not fixed genetically)
Humans: largely genetic

Humans: 22 pairs of matched chromosomes + sex chromosomes:

Female: XX
Male: XY (Y is shorter)

Mother’s gametes all carry X. Father’s sperm carry X or Y (about half each).

Sperm	+ Egg (X)	Child
X	XX	Girl
Y	XY	Boy

So sex of the child depends on whether the paternal contribution is X or Y. All children get an X from the mother.

Diagram — sex determination:

Sex determination — XX / XY sex determination in humans

Better Method / Strategy

Sex determination answers must say father’s sperm decides, not the mother.

For “is this trait dominant?” — parent–child resemblance alone is often not enough without contrasting crosses/ratios (classic NCERT caution).

Chapter Closing Strategy

One monohybrid story + one dihybrid “new combinations” paragraph from memory.
Gene–protein–trait chain in three sentences.
XX/XY Punnett sketch in words until automatic.

Deep Dive Add-On — Mendel Worked Examples (Expanded)

Worked Example A — Monohybrid (tall × short)

Given: Pure tall (TT) × pure short (tt). Tall is dominant.

Generation	Cross / self	Genotypes	Phenotypes
Parents (P)	TT × tt	TT, tt	Tall × short
F1	All offspring	Tt	All tall
F2	Tt selfed (Tt × Tt)	1 TT : 2 Tt : 1 tt	3 tall : 1 short

Punnett (F2) in words:
Rows/columns gametes T and t → cells: TT, Tt, Tt, tt.

What to write in a 3-mark:
(i) F1 all tall → tallness dominant, no blending.
(ii) F1 must carry shortness hidden (Tt).
(iii) Short reappears in F2 (~25%) → recessive recovered when tt.

Worked Example B — Dihybrid parental vs new (fix these labels)

Parents given: round green × wrinkled yellow.

Assume: Round (R) > wrinkled (r); Yellow (Y) > green (y).

	Phenotype	Pure genotype
Parent 1	Round green	RRyy
Parent 2	Wrinkled yellow	rrYY
F1	Round yellow	RrYy

Self F1 (RrYy × RrYy) → F2 phenotype classes (independent assortment idea):

Phenotype class	Role relative to these parents	Classic ratio idea
Round yellow	New combination	9
Round green	Parental combination	3
Wrinkled yellow	Parental combination	3
Wrinkled green	New combination	1

Memorise this pair:

Parental combinations = round green and wrinkled yellow
New combinations = round yellow and wrinkled green

Why students lose marks: They treat “round yellow” as parental because it is the F1 look. Parental means “same trait pairing as the original parents,” not “most common F2 class.”

Independent inheritance conclusion: Because F2 shows combinations that were not the parental phenotype pairs, the two factors assort independently (not locked forever as one block).

Worked Example C — Alternate parental pairing (contrast only)

If instead parents were round yellow × wrinkled green (RRYY × rryy):

Parental combinations would be round yellow & wrinkled green
New combinations would be round green & wrinkled yellow

Same biology — labels follow whatever parents the question gives.

Worked Example D — Gene → protein → trait (height)

A gene codes for an enzyme in a hormone pathway.
Efficient enzyme → enough growth hormone → tall.
Altered gene → less efficient enzyme → less hormone → short.

So visible heredity is explained through proteins, not magic blending.

Worked Example E — Sex determination numbers

Parent	Chromosomes	Gametes
Mother	XX	all X
Father	XY	½ X , ½ Y

Zygote	Sex
XX	Girl
XY	Boy

Probability ≈ 1 : 1. Father’s sperm decides.

Quick self-check before exams

Can you write 3:1 and 1:2:1 without mixing names?
For round green × wrinkled yellow, can you list parental vs new without looking?
Can you say “gene → protein → trait” in one breath?
Can you refuse the myth “mother determines sex”?