Why the numerical half is where marks are lost
Ask for the probability that a child of two carriers is affected and the answer is one quarter. Ask for the probability that the child is a carrier and it is one half. Ask for the probability that an unaffected child is a carrier and it is two thirds, because the affected quarter has been excluded from consideration. Three closely related questions with three different answers.
That pattern - small differences in wording producing different calculations - is what makes Genetics demanding. The biology underneath is not difficult; reading the question precisely is.
How NEET actually asks Genetics
NTA publishes no chapter-wise weightage for NEET, so figures circulating online are coaching estimates from past papers. Check the official NTA information bulletin for the current paper structure.
The recurring types are: compute a ratio or probability from a described cross, interpret a pedigree, apply Hardy-Weinberg, identify a mode of inheritance from a description, and recall the classic experiments establishing DNA as the genetic material. The molecular chapters are more recall-based than the Mendelian ones.
Key concepts, compressed
- ✓Mendel's laws describe how alleles segregate and assort, and they hold for genes on different chromosomes.
- ✓Deviations from Mendelian ratios - incomplete dominance, codominance, epistasis, polygenic inheritance - do not contradict his laws but reflect more complex gene action.
- ✓Linkage occurs when genes lie on the same chromosome and therefore do not assort independently; recombination frequency measures how far apart they are.
- ✓In humans, sex is determined by the XX-XY system, and X-linked genes show a distinctive inheritance pattern because males carry only one X.
- ✓DNA is the genetic material in almost all organisms, established by a sequence of classic experiments.
- ✓The Hardy-Weinberg principle describes allele frequencies in an idealised non-evolving population.
Ratios and relations you need before the questions
| Situation | Result | Note |
|---|---|---|
| Monohybrid F2 phenotypic | 3 : 1 | genotypic 1 : 2 : 1 |
| Dihybrid F2 phenotypic | 9 : 3 : 3 : 1 | requires independent assortment |
| Incomplete dominance F2 | 1 : 2 : 1 | phenotypic equals genotypic |
| Test cross | cross with homozygous recessive | reveals the genotype |
| Two carriers, affected child | 1/4 | |
| Two carriers, carrier child | 1/2 | not 1/4 |
| Two carriers, unaffected child is a carrier | 2/3 | affected quarter excluded |
| Gamete types | 2ⁿ for n heterozygous pairs | |
| Hardy-Weinberg | p² + 2pq + q² = 1 | p + q = 1 |
| Affected frequency | q² | homozygous recessive |
| Carrier frequency | 2pq | heterozygous |
| X-linked recessive | males far more often affected | sons inherit from mother |
The five mistakes that cost the most marks
- ✓Answering the affected probability when the question asked for the carrier probability. One quarter versus one half - read which is wanted.
- ✓Applying 9:3:3:1 to linked genes. Independent assortment is a precondition, and its failure is precisely how linkage is detected.
- ✓Confusing incomplete dominance with codominance. The first blends the phenotypes; the second expresses both fully and separately.
- ✓Getting X-linked transmission backwards. An affected male received the allele from his mother, since his single X came from her.
- ✓Starting a Hardy-Weinberg calculation anywhere other than q². It is the value the question supplies, and everything else follows from it.
Practice set 1: Mendelian inheritance
1. What phenotypic and genotypic ratios appear in the F2 of a monohybrid cross?
Phenotypic 3:1 and genotypic 1:2:1. Crossing two heterozygotes gives one homozygous dominant, two heterozygotes and one homozygous recessive. Since the heterozygotes look like the dominant homozygote, three of four show the dominant phenotype. The two ratios differ precisely because dominance masks the heterozygote.
2. What is the F2 phenotypic ratio of a dihybrid cross?
9:3:3:1 - nine showing both dominant traits, three each showing one dominant and one recessive, and one showing both recessive. This arises from two independent 3:1 ratios multiplied together, which is exactly why the ratio requires the genes to assort independently.
3. State Mendel's law of dominance.
When two different alleles are present, only the dominant one is expressed in the phenotype, while the recessive one remains hidden. This explains why the F1 generation of a cross between two pure lines resembles only one parent, and why the recessive trait reappears in the F2.
4. State the law of segregation.
The two alleles of a gene separate during gamete formation so that each gamete receives exactly one. It is the most universally valid of Mendel's laws, because it reflects the physical separation of homologous chromosomes during meiosis I - a mechanism Mendel could not have known about.
5. State the law of independent assortment and its limitation.
Alleles of different genes assort independently during gamete formation. Its limitation is that it holds only for genes on different chromosomes, or far apart on the same one. Genes close together on the same chromosome are linked and tend to be inherited together, which is the subject of practice set 3.
6. What is a test cross and what is it for?
A cross between an individual showing the dominant phenotype and a homozygous recessive one, used to determine whether the first is homozygous or heterozygous. If any offspring show the recessive trait, the tested parent must be heterozygous. It is the standard method for revealing a genotype that the phenotype conceals.
7. How does a back cross differ from a test cross?
A back cross is between an offspring and either parent or a genetically identical individual. A test cross is the specific case where that parent is homozygous recessive. So every test cross is a back cross, but not every back cross is a test cross - a distinction NEET asks about directly.
Practice this now
Practice set 2: deviations from Mendelian inheritance
8. What is incomplete dominance and what ratio does it give?
Neither allele is fully dominant, so the heterozygote shows an intermediate phenotype - as in pink snapdragons from red and white parents. The F2 phenotypic ratio is 1:2:1, which equals the genotypic ratio, because each genotype now has its own distinct appearance.
9. What is codominance, and how does it differ from incomplete dominance?
Both alleles are expressed fully and separately in the heterozygote, as in AB blood group where both A and B antigens appear. Incomplete dominance blends the phenotypes into an intermediate; codominance shows both simultaneously without blending. Both give a 1:2:1 ratio, so the ratio alone cannot distinguish them - the phenotype description must.
10. What are multiple alleles, and what is the standard example?
More than two allelic forms of a gene existing in a population, though any individual carries only two. The ABO blood group system is the standard example, with alleles I^A, I^B and i. Note that I^A and I^B are codominant with each other while both are dominant over i, which is what produces four phenotypes from three alleles.
11. What is pleiotropy?
A single gene affecting multiple, apparently unrelated phenotypic traits. Phenylketonuria is the standard example, where one enzyme defect causes mental impairment, reduced hair and skin pigmentation, and other effects. Pleiotropy is the reverse of polygenic inheritance, where many genes affect one trait.
12. What is polygenic inheritance?
A trait controlled by several genes, each contributing a small additive effect, producing continuous variation rather than discrete categories. Human skin colour and height are standard examples. The phenotype also depends on the environment, which is why polygenic traits show a bell-shaped distribution in a population.
13. What is epistasis?
One gene masking or modifying the expression of another gene at a different locus. It alters the expected dihybrid ratio - 9:3:3:1 may become 9:7, 12:3:1 or 13:3 depending on the type. Distinguish it from dominance, which is an interaction between alleles of the same gene rather than between different genes.
Practice set 3: linkage and sex determination
14. What are linkage and recombination?
Linkage is the tendency of genes on the same chromosome to be inherited together, violating independent assortment. Recombination is their separation by crossing over during prophase I. Recombination frequency increases with the distance between genes, which is what makes genetic mapping possible.
15. What did Morgan's experiments establish?
That genes are located on chromosomes and that linked genes do not assort independently. Working with Drosophila, he found dihybrid crosses giving ratios far from 9:3:3:1, with parental combinations over-represented. He also showed that the degree of linkage varies with distance, which allowed the first genetic maps.
16. What are the main sex determination systems?
XX-XY in humans and Drosophila, where the male is heterogametic. XX-XO in grasshoppers, where males have one fewer chromosome. ZZ-ZW in birds and some reptiles, where the female is heterogametic. Identifying which sex is heterogametic is the key point, since that sex determines the offspring's sex.
17. What is characteristic of sex-linked inheritance?
The trait's expression depends on sex, because the gene lies on a sex chromosome. X-linked recessive conditions appear far more often in males, who have only one X and therefore express whatever allele it carries. Y-linked traits pass exclusively from father to son.
18. Give two examples of X-linked recessive conditions in humans.
Haemophilia and red-green colour blindness. Both appear predominantly in males, and both pass from a carrier mother to her sons. A female needs the allele on both X chromosomes to be affected, which is far less likely, so females are usually carriers rather than sufferers.
19. Why are males more often affected by X-linked recessive conditions?
Because a male has only one X chromosome, so a single recessive allele is expressed with no second copy to mask it - he is hemizygous. A female needs two copies. It follows that an affected male must have inherited the allele from his mother, since his only X came from her, and this is the point most often reversed.
Practice this now
Practice set 4: molecular basis of inheritance
20. Which experiments established DNA as the genetic material?
Griffith's transformation experiment with pneumococcus showed a transforming principle existed. Avery, MacLeod and McCarty identified it as DNA by eliminating proteins and RNA enzymatically. Hershey and Chase confirmed it using radioactively labelled bacteriophage, showing DNA and not protein entered the bacterial cell.
21. Describe the Watson-Crick model of DNA.
Two antiparallel polynucleotide strands wound into a right-handed double helix, with the sugar-phosphate backbone outside and paired bases inside. Adenine pairs with thymine through two hydrogen bonds and guanine with cytosine through three. The uniform helix diameter follows from always pairing a purine with a pyrimidine.
22. What is Chargaff's rule?
In any double-stranded DNA, the amount of adenine equals that of thymine and the amount of guanine equals that of cytosine, so purines equal pyrimidines overall. The rule was an empirical observation that base-pairing later explained, and it was a crucial clue for Watson and Crick.
23. What does semi-conservative replication mean?
Each daughter DNA molecule contains one parental strand and one newly synthesised strand. Meselson and Stahl demonstrated it using nitrogen isotope labelling and density gradient centrifugation, ruling out the conservative and dispersive alternatives. The parental strand serves as the template that guarantees accuracy.
24. What is the difference between transcription and translation?
Transcription copies a DNA segment into RNA, occurring in the nucleus in eukaryotes and using RNA polymerase. Translation converts the messenger RNA sequence into a protein at the ribosome, with transfer RNA delivering amino acids. The overall flow - DNA to RNA to protein - is the central dogma.
25. What are the key features of the genetic code?
It is a triplet code, so three bases specify one amino acid; degenerate, since most amino acids have several codons; unambiguous, as each codon specifies only one amino acid; non-overlapping and commaless; and nearly universal across all organisms. There are also specific start and stop codons.
Practice set 5: disorders and population genetics
26. Give three Mendelian disorders in humans.
Sickle cell anaemia, an autosomal recessive condition caused by a single amino acid substitution in haemoglobin; thalassaemia, caused by reduced synthesis of globin chains; and phenylketonuria, an autosomal recessive enzyme deficiency. Each results from a mutation in a single gene, which is what makes them Mendelian.
27. Give three chromosomal disorders and their karyotypes.
Down syndrome is trisomy of chromosome 21, giving 47 chromosomes. Turner syndrome is 45 with a single X, producing a sterile female. Klinefelter syndrome is 47 with XXY, producing a sterile male. All three arise from non-disjunction during meiosis rather than from a gene mutation.
28. State the Hardy-Weinberg principle.
Allele and genotype frequencies remain constant across generations in a large, randomly mating population with no mutation, migration or selection. Expressed algebraically, p² + 2pq + q² = 1 with p + q = 1. Its real value is as a null model - deviation from it indicates that evolution is occurring.
29. In a population, 9% show a recessive condition. What proportion are carriers?
42%. Start from q² = 0.09, so q = 0.3 and p = 0.7. Carriers are the heterozygotes, 2pq = 2 × 0.7 × 0.3 = 0.42. Always work in that order - q² to q to p to 2pq. Note that carriers vastly outnumber affected individuals, which is generally true for rare recessive conditions.
30. How do you identify the mode of inheritance from a pedigree?
Look for specific signatures. If affected individuals appear in every generation, suspect dominant inheritance; if the trait skips generations, suspect recessive. If affected individuals are overwhelmingly male and the trait passes through unaffected mothers, suspect X-linked recessive. If two unaffected parents have an affected child, the trait must be recessive.
How to study this chapter efficiently
- ✓Write parental genotypes first, then gametes, then the cross. Working backwards from an expected ratio is where errors start.
- ✓Read carefully whether a question asks for affected, carrier or unaffected. The three answers are one quarter, one half and three quarters respectively.
- ✓For Hardy-Weinberg, always begin at q². Every other quantity follows from it in a fixed order.
- ✓Learn the classic experiments as a sequence - Griffith, then Avery, then Hershey-Chase - since each answered what the previous one left open.
- ✓Make one table of chromosomal disorders with karyotype and consequence.
- ✓For pedigrees, check first whether two unaffected parents have an affected child. That single observation establishes recessive inheritance immediately.
Turn this into active practice
The distinctive errors in this unit come from misreading rather than misunderstanding. A student who can explain carrier status perfectly will still answer one quarter when the question asked for carriers, and that only becomes visible when the answer is marked wrong.
The NEET Genetics quiz on QUFF generates fresh questions across Mendelian ratios, deviations, linkage, molecular genetics and population genetics, marks them instantly and explains each answer. Do mixed sets and, for every error, check whether you solved the wrong question rather than solved it wrongly.