Genetics is the study of heredity — how traits pass from parents to offspring. Mendel’s pea plant experiments gave us the first quantitative laws. CBSE Class 12 and NEET both test this heavily. Expect three to four NEET questions a year.
Core Concepts
Mendel’s laws
Law of dominance — in a heterozygote, one allele masks the other. Law of segregation — alleles separate during gamete formation so each gamete gets one. Law of independent assortment — different gene pairs segregate independently (when on different chromosomes).
Law of segregation is the most fundamental. During meiosis, homologous chromosomes separate into different gametes. Since alleles are on homologous chromosomes, they also separate. A heterozygous Tt plant produces equal numbers of T and t gametes.
Law of independent assortment applies only when genes are on different chromosomes. Genes on the same chromosome are linked and do not assort independently — they tend to be inherited together (Morgan’s work with Drosophila).
Monohybrid cross
Tall (TT) × dwarf (tt) gives all tall F1 (Tt). Selfing F1 gives F2 with 3:1 phenotypic ratio and 1:2:1 genotypic ratio. This is the classic Mendel experiment.
The Punnett square for Tt × Tt:
| T | t | |
|---|---|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (dwarf) |
Phenotypic ratio: 3 tall : 1 dwarf. Genotypic ratio: 1 TT : 2 Tt : 1 tt.
The 3:1 ratio holds only when: (1) dominance is complete, (2) sample size is large enough, (3) all genotypes are equally viable, and (4) there is random mating.
Dihybrid cross
Two traits tested together — round yellow (RRYY) × wrinkled green (rryy). F1 all round yellow. F2 gives 9:3:3:1 phenotypic ratio, confirming independent assortment.
Breaking down the 9:3:3:1:
- 9/16 round yellow (R_Y_)
- 3/16 round green (R_yy)
- 3/16 wrinkled yellow (rrY_)
- 1/16 wrinkled green (rryy)
The 9:3:3:1 can be derived as the product of two independent 3:1 ratios: .
Modified dihybrid ratios arise when genes interact:
| Interaction | Ratio | Example |
|---|---|---|
| Complementary | 9:7 | Flower colour in sweet pea |
| Recessive epistasis | 9:3:4 | Coat colour in Labrador |
| Dominant epistasis | 12:3:1 | Fruit colour in squash |
| Duplicate genes | 15:1 | Kernel colour in wheat |
| Inhibitory | 13:3 | Feather colour in fowl |
Test cross
Crossing a dominant phenotype with a homozygous recessive tells you if the dominant parent is homozygous or heterozygous. All dominant offspring → homozygous; 1:1 ratio → heterozygous.
The test cross is the geneticist’s diagnostic tool. It works because the homozygous recessive parent contributes only recessive alleles — so the offspring phenotype directly reveals the unknown parent’s genotype.
Incomplete dominance and co-dominance
Not every trait is simple dominance. Snapdragon red × white gives pink F1 (incomplete). ABO blood groups show co-dominance — both A and B antigens are expressed in AB.
Incomplete dominance: The heterozygote has an intermediate phenotype. Red (RR) × White (rr) → Pink (Rr). Selfing pink: 1 Red : 2 Pink : 1 White. The phenotypic ratio equals the genotypic ratio (1:2:1) because the heterozygote is distinguishable.
Co-dominance: Both alleles are fully expressed. In blood group AB (II), both A and B antigens are present on RBC surfaces — not a blend but both simultaneously.
Multiple alleles: ABO blood group has three alleles (I, I, i) but any individual has only two. I and I are co-dominant to each other, both dominant over i.
Linkage and crossing over
Genes on the same chromosome tend to be inherited together — this is linkage (Morgan, 1910). During meiosis, crossing over between homologous chromosomes can separate linked genes, producing recombinant offspring.
Recombination frequency = (recombinant offspring / total offspring) × 100%. This is used to create genetic maps — 1% recombination = 1 centiMorgan (1 map unit) of distance between genes.
Sex-linked inheritance
Genes on the X chromosome show a distinctive pattern: males (XY) express all X-linked alleles (hemizygous), while females (XX) can be carriers for recessive traits. Haemophilia and red-green colour blindness are classic X-linked recessive traits.
Pattern: An affected male gets the allele from his carrier mother. He passes it to all his daughters (who become carriers) but to none of his sons (who get Y from him).
Genetic disorders
| Disorder | Type | Inheritance |
|---|---|---|
| Sickle cell anaemia | Autosomal | Recessive |
| Phenylketonuria (PKU) | Autosomal | Recessive |
| Thalassemia | Autosomal | Recessive |
| Haemophilia A | X-linked | Recessive |
| Colour blindness | X-linked | Recessive |
| Down syndrome | Chromosomal | Trisomy 21 |
| Turner syndrome | Chromosomal | 45, XO |
| Klinefelter syndrome | Chromosomal | 47, XXY |
Worked Examples
Cross RrYy × RrYy. Each parent makes four gamete types (RY, Ry, rY, ry). A 4×4 Punnett square gives 16 combinations in the 9:3:3:1 ratio. The 9 are both dominant, the 1 is both recessive.
If observed ratios deviate slightly from 3:1 or 9:3:3:1, use to test significance. This is how Mendel’s data was later statistically validated.
For colour blindness: Mother XX × Father XY.
Daughters: XX (carrier, 50%) and XX (affected, 50%). Sons: XY (normal, 50%) and XY (affected, 50%).
So half the daughters are affected and half the sons are affected. This is unusual — normally we expect only sons to be affected in X-linked recessive conditions. An affected father changes the pattern.
A tall pea plant (T?) is crossed with a dwarf (tt). If all offspring are tall → parent was TT. If half are tall and half dwarf → parent was Tt.
Results: 52 tall, 48 dwarf. Ratio is approximately 1:1. Parent is Tt (heterozygous).
In a cross, 400 offspring were obtained: 180 parental type 1, 170 parental type 2, 25 recombinant type 1, 25 recombinant type 2.
Recombination frequency = (25 + 25) / 400 × 100 = 12.5%. The genes are 12.5 cM apart on the chromosome.
Common Mistakes
Writing that Mendel’s third law applies to genes on the same chromosome. It does not — linked genes do not assort independently.
Confusing genotype and phenotype. Genotype is genetic make-up; phenotype is expressed trait.
Saying a 3:1 ratio is genotypic. It is phenotypic; the genotypic ratio in a monohybrid F2 is 1:2:1.
Confusing co-dominance and incomplete dominance. In co-dominance, both alleles are fully expressed (AB blood group has both A and B antigens). In incomplete dominance, the heterozygote shows a blended phenotype (pink flower from red × white).
Thinking that sex-linked means sex-limited. Sex-linked genes are on sex chromosomes. Sex-limited traits (like beard growth) are autosomal but expressed only in one sex due to hormonal influence.
Exam Weightage and Revision
This topic is a repeat performer in board papers and entrance exams. NEET typically asks one to two questions on the core mechanisms, CBSE boards give three to six marks, and state PMT papers often include a diagram-based long answer. The PYQs cluster around a small set of facts — lock those and you clear the topic.
NEET 2023 asked about modified dihybrid ratios and a pedigree analysis question. NEET 2022 tested ABO blood group genotypes. CBSE boards ask five-mark questions on Mendel’s laws with Punnett squares. Genetics carries 10-15 marks across NEET papers every year.
When a question gives a scenario, identify the core mechanism first, then match it to the concepts above. Most wrong answers come from reading the scenario too quickly.
Memorise four ratios — 3:1, 1:2:1, 9:3:3:1, 1:1. Every Mendelian question reduces to one of these.
Practice Questions
Q1. In a cross between two pink snapdragon plants, what phenotypic ratio is expected?
Pink × pink = Rr × Rr (incomplete dominance). Offspring: 1 RR (red) : 2 Rr (pink) : 1 rr (white). Phenotypic ratio: 1 red : 2 pink : 1 white. In incomplete dominance, the phenotypic ratio equals the genotypic ratio.
Q2. A man with blood group A marries a woman with blood group B. Can they have a child with blood group O?
Yes, if both are heterozygous. Father: Ii. Mother: Ii. Offspring possibilities: II (AB), Ii (A), Ii (B), ii (O). Probability = 1/4.
Q3. What is the difference between a test cross and a back cross?
A test cross is specifically a cross with a homozygous recessive individual (used to determine the genotype of the dominant parent). A back cross is a cross with either parent. All test crosses are back crosses, but not all back crosses are test crosses — only the one involving the homozygous recessive parent qualifies.
Q4. Why does independent assortment not apply to linked genes?
Independent assortment requires genes to be on different chromosomes. Linked genes are on the same chromosome and move together during meiosis I. They can only be separated by crossing over, which occurs at a frequency less than 50%. The closer the genes are on the chromosome, the less frequently they recombine.
Q5. A dihybrid cross gives a 9:7 ratio. What type of gene interaction is this?
Complementary gene interaction. Both dominant alleles (A and B) must be present together to produce the trait. The 9/16 class (A_B_) shows the trait; the remaining 3+3+1 = 7/16 all lack it. Example: flower colour in sweet pea — both genes C and P are needed for purple colour.
FAQs
Did Mendel know about DNA? No. Mendel published his work in 1866, long before DNA was identified as the genetic material (Avery in 1944, Watson and Crick in 1953). Mendel called his hereditary units “factors.” The term “gene” was coined by Johannsen in 1909.
Why did Mendel’s laws work so perfectly? Mendel was fortunate (or shrewd) in his choice of traits. The seven traits he studied happened to be on different chromosomes (or far apart on the same chromosome), so they assorted independently. He also used large sample sizes and applied statistical analysis. Later criticism suggested his data was “too good” — possibly due to unconscious bias in counting.
What is epistasis? Epistasis is when one gene masks or modifies the expression of another gene at a different locus. In recessive epistasis (9:3:4), one homozygous recessive genotype hides the effect of the other gene entirely. This is different from dominance, which is an interaction between alleles at the same locus.
Can a recessive trait skip generations? Yes. A recessive allele can be carried by heterozygous individuals who do not express the trait. When two carriers mate, 1/4 of offspring express the recessive trait — appearing to “skip” a generation. This is the hallmark pattern of autosomal recessive inheritance in pedigrees.
Genetics rewards clean logic. Draw Punnett squares neatly, label gametes clearly, and the ratios fall out.