Principles of Inheritance and Variation explains how traits pass from parents to offspring, starting with Mendel’s laws of dominance, segregation and independent assortment in pea plants. It then covers deviations like incomplete dominance, codominance, multiple alleles (ABO blood groups), gene interactions, linkage and recombination, sex determination, and human genetic disorders. It is a high-yield Class 12 genetics chapter that delivers several direct NEET questions every year and underpins the chapters on molecular genetics and evolution.
Table of Contents
- Key Concepts
- Weightage in Board & Entrance Exams
- Important Definitions
- Solved Examples
- Important Questions for Board Exams
- Quick Revision Points
Key Concepts
1. Mendel and the Garden Pea
Gregor Mendel worked on the garden pea (Pisum sativum) for seven years and gave the first scientific laws of inheritance. The pea was a smart choice: it has clear contrasting traits, is naturally self-pollinating, has a short life cycle, and produces many offspring, so ratios could be counted reliably.
He studied seven pairs of contrasting characters (for example tall vs dwarf plants, round vs wrinkled seeds, yellow vs green seeds, violet vs white flowers). Before results make sense, fix these terms:
- Gene: a unit of inheritance controlling a character. Allele: one of the alternative forms of a gene (T for tall, t for dwarf).
- Genotype: the genetic make-up (TT, Tt, tt). Phenotype: the visible trait (tall or dwarf).
- Homozygous: identical alleles (TT or tt). Heterozygous: different alleles (Tt).
- Dominant allele expresses in the heterozygote (T); the masked recessive allele (t) shows only when homozygous.
2. Monohybrid Cross and Mendel's First Two Laws
A monohybrid cross follows a single character. Crossing pure tall (TT) with pure dwarf (tt) gives an all-tall F₁ (Tt). Selfing the F₁ gives an F₂ in a 3 tall : 1 dwarf phenotypic ratio and a 1 TT : 2 Tt : 1 tt genotypic ratio.
Law of Dominance: in a heterozygote, one allele (dominant) expresses and masks the other (recessive). Law of Segregation (Law of Purity of Gametes): the two alleles of a pair separate during gamete formation, so each gamete carries only one allele. This law has no exception.
Test cross: to find whether a tall plant is TT or Tt, cross it with a recessive dwarf (tt). All tall offspring means TT; a 1:1 tall:dwarf ratio means the parent was Tt. A back cross is a cross of the F₁ with either parent.
3. Incomplete Dominance and Codominance
Incomplete dominance: the heterozygote shows an intermediate phenotype. In snapdragon (Antirrhinum) and the 4 o'clock plant, red (RR) × white (rr) gives pink (Rr). The F₂ ratio is 1 red : 2 pink : 1 white, so here the phenotypic and genotypic ratios are the same (1:2:1).
Codominance: both alleles express fully and independently in the heterozygote. Human ABO blood groups show this: allele Iᴬ and Iᵇ are codominant, so genotype IᴬIᵇ gives blood group AB (both A and B antigens appear).
4. ABO Blood Groups (Multiple Alleles)
The ABO gene has three alleles - Iᴬ, Iᵇ and i - a case of multiple allelism. Iᴬ and Iᵇ are dominant over i and codominant with each other.
| Blood group (phenotype) | Possible genotypes | Antigen on RBC |
|---|---|---|
| A | IᴬIᴬ, Iᴬi | A |
| B | IᵇIᵇ, Iᵇi | B |
| AB | IᴬIᵇ | A and B |
| O | ii | none |
5. Dihybrid Cross and the Law of Independent Assortment
A dihybrid cross follows two characters together, for example seed shape (round R dominant over wrinkled r) and colour (yellow Y dominant over green y). Crossing RRYY × rryy gives round-yellow F₁ (RrYy). Selfing gives an F₂ phenotypic ratio of 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green (9:3:3:1).
Law of Independent Assortment: during gamete formation the alleles of one gene pair segregate independently of another gene pair. This holds only for genes on different chromosomes (unlinked genes).
6. Epistasis, Pleiotropy and Polygenic Inheritance
- Epistasis: one gene masks the expression of another gene at a different locus (for example coat colour in Labradors; recessive epistasis can change the 9:3:3:1 ratio to 9:3:4).
- Pleiotropy: a single gene affects multiple traits. In sickle cell anaemia one mutated gene affects RBC shape, anaemia, spleen and more; in phenylketonuria one gene affects mental development and pigment.
- Polygenic inheritance: a trait controlled by many genes, each adding a small effect, giving a continuous range. Examples are human skin colour and height; the phenotype also depends on the environment.
7. Linkage and Recombination
Thomas Hunt Morgan, working on Drosophila, found that genes on the same chromosome tend to be inherited together and do not assort independently - this is linkage. Linked genes give far fewer recombinant types than expected.
The exchange of segments between homologous chromosomes during meiosis is crossing over, which produces recombinants. Genes that are close together are tightly linked (low recombination); genes far apart recombine more. Recombination frequency is used to map genes on a chromosome.
8. Chromosomal Theory and Sex Determination
Sutton and Boveri's chromosomal theory of inheritance linked Mendel's factors to chromosome behaviour during meiosis. Sex is decided by sex chromosomes:
- XX-XY (humans, Drosophila): female XX, male XY. Males are heterogametic.
- XX-XO (grasshopper): female XX, male XO (one X, no Y).
- ZZ-ZW (birds): male ZZ, female ZW. Here the female is heterogametic.
- Haplodiploidy (honeybee): females (diploid) develop from fertilised eggs, males (haploid drones) from unfertilised eggs by parthenogenesis.
9. Pedigree Analysis
A pedigree is a family tree that tracks a trait across generations. Squares are males, circles are females, and filled symbols are affected individuals. Pedigrees help predict whether a trait is dominant or recessive and whether it is autosomal or sex-linked.
Clues: an autosomal recessive trait can skip generations and appears in both sexes equally; an X-linked recessive trait (haemophilia, colour blindness) is far more common in males because they have a single X.
10. Mendelian and Chromosomal Disorders
Mendelian disorders arise from a change in a single gene and follow Mendelian inheritance.
| Disorder | Type | Key point |
|---|---|---|
| Haemophilia | X-linked recessive | Blood fails to clot; mostly affects males |
| Colour blindness | X-linked recessive | Cannot distinguish red and green |
| Sickle cell anaemia | Autosomal recessive | HbS due to Glu→Val at position 6 of the β-globin chain |
| Thalassemia | Autosomal recessive | Reduced synthesis of globin chains |
| Phenylketonuria | Autosomal recessive | Lacks enzyme to convert phenylalanine; causes mental retardation |
Chromosomal disorders arise from an extra or missing chromosome (aneuploidy), usually from non-disjunction during meiosis.
| Disorder | Chromosome change | Key features |
|---|---|---|
| Down syndrome | Trisomy 21 (extra 21) | Short stature, mental retardation, small round head |
| Klinefelter syndrome | 47, XXY (extra X in males) | Male with some feminine features, usually sterile |
| Turner syndrome | 45, X0 (one X missing) | Sterile female, short stature, underdeveloped ovaries |
Weightage in Board & Entrance Exams
| Exam | Typical Weightage | Most-Tested Areas |
|---|---|---|
| CBSE Board (Class 12) | 6–8 marks | Dihybrid ratio, pedigree problems, sex determination, blood groups |
| NEET | 2–3 questions | Mendelian disorders, linkage, pedigree, incomplete dominance vs codominance |
| Genetics numericals | High frequency | Ratios from crosses, probability of offspring, test cross outcomes |
Important Definitions
- Allele: an alternative form of a gene.
- Test cross: a cross of an organism showing the dominant trait with a homozygous recessive to reveal its genotype.
- Codominance: both alleles express fully in the heterozygote (blood group AB).
- Pleiotropy: a single gene controlling several phenotypic traits.
- Aneuploidy: gain or loss of one or more chromosomes due to non-disjunction.
- Recombination: the formation of new allele combinations through crossing over.
Solved Examples
Example 1
Q. A tall pea plant is crossed with a dwarf. The offspring are 50% tall and 50% dwarf. What is the genotype of the tall parent?
A. A 1:1 ratio is the result of a test cross, so the tall parent is heterozygous Tt (Tt × tt gives 1 Tt : 1 tt).
Example 2
Q. In snapdragon, red (RR) is crossed with white (rr). Give the F₁ and F₂ phenotypic ratios.
A. This is incomplete dominance. F₁ is all pink (Rr). F₂ is 1 red : 2 pink : 1 white.
Example 3
Q. A man of blood group AB marries a woman of blood group O. What blood groups can the children have?
A. IᴬIᵇ × ii gives Iᴬi (group A) and Iᵇi (group B). Children can be A or B, never AB or O.
Example 4
Q. A colour-blind man marries a normal homozygous woman. Describe the children.
A. Colour blindness is X-linked recessive. XᶜY × XᶜXᶜ (using Xᶜ = normal): all daughters are carriers, all sons are normal. No child is colour-blind.
Example 5
Q. What phenotypic ratio is expected in the F₂ of a dihybrid cross of two independently assorting genes?
A. 9:3:3:1. It comes from combining two independent 3:1 monohybrid ratios (3:1 × 3:1).
Important Questions for Board Exams
1-Mark Questions (VSA)
- Define codominance with one example.
- Why is the pea plant suitable for genetic studies? Give one reason.
- What is a test cross?
2–3-Mark Questions (SA)
- Differentiate between incomplete dominance and codominance with examples.
- Explain the Law of Segregation and why it is called the law of purity of gametes.
- How is sex determined in honeybees?
5-Mark Questions (LA)
- Work out a dihybrid cross and explain the Law of Independent Assortment with a Punnett square.
- Describe the inheritance and symptoms of any two Mendelian disorders in humans.
Quick Revision Points
- Monohybrid F₂: 3:1 phenotype, 1:2:1 genotype. Dihybrid F₂: 9:3:3:1.
- Law of Segregation has no exception; Law of Independent Assortment applies only to unlinked genes.
- Incomplete dominance gives 1:2:1 phenotype; codominance shows both alleles (blood group AB).
- ABO blood group is an example of multiple alleles and codominance.
- Linked genes are inherited together; crossing over produces recombinants.
- Humans are XX-XY; the sperm decides sex. Birds are ZZ-ZW; honeybees use haplodiploidy.
- Down = trisomy 21, Klinefelter = XXY, Turner = X0. Haemophilia and colour blindness are X-linked recessive.
Class 12 Biology · Botany – swipe through all 9 cards to understand the whole chapter.
Mendel & the Garden Pea
Genetics began with Gregor Mendel tracking how traits pass from parents to offspring in pea plants.
Pea chosen for easy self/cross-pollination and a short life cycle.
- Allele = an alternative form of a gene
- Homozygous (TT/tt) vs heterozygous (Tt)
- Dominant allele masks the recessive one in a heterozygote
Monohybrid Cross & First Two Laws
A one-character cross (Tall TT × dwarf tt) gives all-tall F1, and selfing reveals Mendel’s laws.
Law of Dominance + Law of Segregation (purity of gametes).
- Dominance: only the dominant allele shows in a heterozygote
- Segregation: the two alleles separate, so each gamete gets only one
- Alleles do not blend or contaminate each other
Dihybrid Cross & Independent Assortment
Following two characters at once gives the classic 9:3:3:1 F2 ratio.
9:3:3:1 is just (3:1) × (3:1) — each gene still 3:1.
- RrYy makes 4 gamete types: RY, Ry, rY, ry (equal)
- Independent assortment: one gene pair sorts independent of another
- RrYy × rryy test cross gives a 1:1:1:1 ratio
Incomplete Dominance & Codominance
Some heterozygotes are intermediate, or show both alleles together — breaking simple dominance.
Antirrhinum/Mirabilis pink; here F2 phenotype ratio = genotype ratio.
- Incomplete dominance: heterozygote is intermediate (pink)
- Codominance: both alleles fully expressed → blood group AB
- Codominant ABO alleles: Iᴬ and Iᴮ
ABO Blood Groups
One gene can have more than two alleles in a population, though each person carries only two.
Gene codes a sugar-adding enzyme on the RBC surface.
- Group A = Iᴬ Iᴬ or Iᴬ i; Group B = Iᴮ Iᴮ or Iᴮ i
- Group AB = Iᴬ Iᴮ (codominance); Group O = ii
- Iᴬ vs i shows complete dominance, Iᴬ vs Iᴮ shows codominance
Epistasis, Pleiotropy & Polygeny
Genes interact, modifying the classic Mendelian ratios in three distinct ways.
Epistatic gene masks; hypostatic gene is masked.
- Pleiotropy: one gene affects many traits (e.g. PKU)
- Polygeny: many genes add up → continuous variation (skin colour)
- Each affects the 9:3:3:1 dihybrid ratio differently
Linkage & Recombination
Genes on the same chromosome tend to be inherited together, violating independent assortment.
1% recombination = 1 centimorgan (1 map unit); Morgan in Drosophila.
- Tightly linked genes → low recombination; loosely linked → high
- Recombination comes from crossing over in meiosis I
- Chromosomal Theory (Sutton & Boveri): genes lie on chromosomes
How Sex Is Decided
Different organisms use different chromosome systems to determine sex.
In humans the father’s sperm (X or Y) decides the child’s sex.
- Human male is heterogametic (XY), female homogametic (XX)
- Grasshopper male is XO (one X, no Y)
- Birds: female is heterogametic (ZW), male is ZZ
Mendelian & Chromosomal Disorders
Genetic disorders come from single-gene defects or whole-chromosome number changes.
Down’s, Klinefelter’s, Turner’s arise from non-disjunction.
- X-linked recessive: haemophilia, colour blindness
- Autosomal recessive: sickle-cell anaemia, PKU, thalassemia
- Down’s = trisomy 21 · Klinefelter’s = XXY · Turner’s = XO
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Frequently Asked Questions
It states that the two alleles of a gene separate during gamete formation so that each gamete carries only one allele of a pair. Because the alleles stay pure and never blend, a recessive trait hidden in the F1 can reappear in the F2 generation.
A monohybrid cross gives an F2 phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1. A dihybrid cross gives a 9:3:3:1 phenotypic ratio, and a test cross of a dihybrid gives 1:1:1:1.
Yes, it is one of the highest-yield Class 12 Biology chapters and usually contributes around 2 to 4 questions in NEET. Pedigree analysis, blood group genetics, Mendelian ratios and genetic disorders are frequently tested.
In incomplete dominance the heterozygote shows an intermediate or blended phenotype, such as a pink flower from red and white parents. In codominance both alleles are fully and separately expressed in the heterozygote, as in the AB blood group where both A and B antigens appear.
Humans follow the XX-XY system where the female is XX (homogametic) and the male is XY (heterogametic). Since the mother always provides an X, it is the father’s sperm carrying either an X or a Y that decides the child’s sex.