Principles of Inheritance — Part B (NEET Biology Class 12): Linkage, Sex Determination & Genetic Disorders
Chromosomal Theory, Linkage & Recombination
🎯 NEET priority: Highest-yield. Part B of the single biggest chapter in NEET Biology (8–10 Q/year across both parts). Genetic disorders and sex determination alone are near-guaranteed marks — the exact karyotypes and inheritance patterns are asked verbatim.
Chromosomal Theory of Inheritance
Mendel published in 1865, but his work went unrecognised until 1900 — for four reasons: poor communication of the era; his idea of discrete non-blending "factors" contradicted the observed continuous variation; his use of mathematics in biology was alien to contemporaries; and he had no physical proof of what factors were made of.
In 1900, three scientists — de Vries, Correns and von Tschermak — independently rediscovered his results. Meanwhile improved microscopy revealed chromosomes, and by 1902 their meiotic movement was worked out.
Walter Sutton and Theodore Boveri noticed that chromosome behaviour exactly parallels gene behaviour — both occur in pairs, both segregate at gamete formation with only one of each pair going to a gamete, and both assort independently of other pairs. Sutton united chromosomal segregation with Mendelian principles into the chromosomal theory of inheritance.
Thomas Hunt Morgan experimentally verified it using Drosophila melanogaster — chosen because it grows on simple synthetic medium, completes its life cycle in about two weeks, gives a huge progeny from one mating, has clearly distinguishable sexes, and shows many hereditary variations visible under low magnification.
Linkage & Recombination
Morgan crossed yellow-bodied, white-eyed females × brown-bodied, red-eyed males and intercrossed the F1. The F2 ratio deviated sharply from 9:3:3:1 — the two genes did NOT assort independently.
Linkage = the physical association of genes on the same chromosome, producing far more parental combinations than non-parental. Recombination = the generation of non-parental combinations. Both terms were coined by Morgan.
Linkage strength varies: white and yellow genes were tightly linked (only 1.3% recombination), while white and miniature wing were loosely linked (37.2% recombination). Alfred Sturtevant, Morgan's student, used recombination frequency as a measure of distance between genes to build the first genetic maps — the same principle later used in the Human Genome Project.
Trap: higher recombination frequency means genes are farther apart and more loosely linked. Low recombination = tight linkage = genes close together. Students routinely invert this.
Sex Determination — Four Systems
Sex Determination — the four systems
Henking (1891) traced a nuclear structure through insect spermatogenesis and found 50% of sperm received it, 50% did not. He named it the X body but couldn't explain its significance. Later work showed it was a chromosome — the X chromosome. Sex chromosomes were thus distinguished from autosomes.
System | Female | Male | Which sex is heterogametic | Example |
XO type | XX | XO (only one X, no Y) | Male | Grasshopper, many insects |
XY type | XX | XY | Male | Humans, Drosophila |
ZW type | ZW | ZZ | Female | Birds |
Haplodiploidy | Diploid (32 chromosomes) — queen/worker | Haploid (16 chromosomes) — drone | Neither (based on ploidy, not sex chromosomes) | Honey bee |
Sex determination in humans
Humans have 23 pairs of chromosomes: 22 pairs of autosomes (identical in both sexes) + 1 pair of sex chromosomes. Females are XX, males XY.
During spermatogenesis, 50% of sperm carry X and 50% carry Y; every ovum carries only X. So the sperm determines the sex of the child, and there is always a 50% probability of either sex in each pregnancy — which is why blaming women for the sex of a child is scientifically false.
Sex determination in honey bee — haplodiploidy
A fertilised egg → female (queen or worker), diploid, 32 chromosomes. An unfertilised egg → male (drone) by parthenogenesis, haploid, 16 chromosomes.
Three consequences NEET loves: drones produce sperm by mitosis (not meiosis — they're already haploid); they have no father and cannot have sons; but they do have a grandfather and can have grandsons.
Mutation, Pedigree Analysis & Mendelian Disorders
Mutation
Mutation alters DNA sequence, changing genotype and phenotype. Along with recombination, it is a major source of variation.
Chromosomal aberrations arise from deletion (loss) or insertion/duplication (gain) of a DNA segment — commonly observed in cancer cells.
Point mutation = change in a single base pair; the classic example is sickle-cell anaemia. Deletions and insertions of base pairs cause frame-shift mutations.
Mutagens are physical/chemical agents that induce mutation — e.g. UV radiation.
Pedigree analysis
Since controlled crosses are impossible in humans, pedigree analysis — tracing a trait through a family tree across generations — is the alternative tool for determining whether a trait is dominant/recessive and autosomal/sex-linked.
Mendelian disorders — caused by single-gene mutation
Disorder | Inheritance | Defect | Key facts |
Colour blindness | X-linked recessive | Defect in red or green cone of the eye | About 8% of males but only 0.4% of females affected — males have only one X |
Haemophilia | X-linked recessive | A single clotting-cascade protein is affected | A simple cut causes non-stop bleeding; carrier females transmit to sons; Queen Victoria's pedigree is the classic case |
Sickle-cell anaemia | Autosomal recessive | Glutamic acid → Valine at the 6th position of the β-globin chain (codon GAG → GUG) | Only HbˢHbˢ homozygotes are diseased; HbᴬHbˢ are unaffected carriers. RBC sickles under LOW oxygen tension |
Phenylketonuria | Autosomal recessive | Lacks the enzyme converting phenylalanine → tyrosine | Phenylalanine accumulates as phenylpyruvic acid, causing mental retardation; excreted in urine |
Thalassemia | Autosomal recessive | Mutation/deletion reducing the RATE of globin chain synthesis | α-thalassemia: genes HBA1 + HBA2 on chromosome 16. β-thalassemia: gene HBB on chromosome 11 |
The distinction NEET tests: thalassemia is a QUANTITATIVE problem — too few globin molecules made. Sickle-cell anaemia is a QUALITATIVE problem — a normal quantity of incorrectly-functioning globin.
Chromosomal Disorders & Why This Matters for NEET
Chromosomal disorders — caused by abnormal chromosome number/arrangement
Aneuploidy = gain or loss of chromosome(s), from failure of chromatid segregation during cell division. Trisomy = one extra copy; monosomy = one missing.
Polyploidy = gain of a whole extra chromosome SET, from failure of cytokinesis after telophase. Common in plants.
Disorder | Karyotype | Cause | Features |
Down's syndrome | Trisomy of chromosome 21 (47) | Extra copy of chromosome 21 | First described by Langdon Down (1866). Short stature, small round head, furrowed tongue, partially open mouth, broad palm with characteristic crease, retarded physical/psychomotor/mental development |
Klinefelter's syndrome | 47, XXY | Extra copy of X chromosome in a male | Overall masculine development BUT feminine features also expressed (gynaecomastia — breast development). Sterile |
Turner's syndrome | 45, XO | Absence of one X chromosome in a female | Sterile — ovaries are rudimentary; lacks other secondary sexual characters |
Why this matters for NEET
High-value one-liners: chromosomal theory = Sutton and Boveri, verified by Morgan on Drosophila; genetic maps = Sturtevant; grasshopper = XO, birds = ZW (female heterogametic), honey bee = haplodiploid; Down's = trisomy 21, Klinefelter = 47 XXY, Turner = 45 XO; sickle-cell = Glu→Val at position 6 of the β-chain.
Trap: Klinefelter (47, XXY) is a male with an extra X; Turner (45, XO) is a female missing an X — students swap these constantly. Also, aneuploidy comes from failed chromatid segregation while polyploidy comes from failed cytokinesis — different mechanisms entirely. And sickle-cell RBCs deform under LOW oxygen tension, not high.
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. The chromosomal theory of inheritance was proposed by:
(a) Mendel
(b) Sutton and Boveri
(c) Morgan and Sturtevant
(d) de Vries and Correns
Show answer
Answer: (b) — Walter Sutton and Theodore Boveri noted the parallel between chromosome and gene behaviour; Sutton formulated the chromosomal theory.
Q2. Mendel's work was rediscovered in 1900 by:
(a) Sutton, Boveri and Morgan
(b) de Vries, Correns and von Tschermak
(c) Henking, Down and Turner
(d) Morgan, Sturtevant and Bridges
Show answer
Answer: (b) — de Vries, Correns and von Tschermak independently rediscovered Mendel's results in 1900.
Q3. Thomas Hunt Morgan experimentally verified the chromosomal theory using:
(a) Pisum sativum
(b) Neurospora crassa
(c) Escherichia coli
(d) Drosophila melanogaster
Show answer
Answer: (d) — Morgan used the fruit fly Drosophila melanogaster — quick life cycle, large progeny, distinguishable sexes and many visible variations.
Q4. In Morgan's experiments, a higher recombination frequency between two genes indicates that they are:
(a) Tightly linked and close together
(b) Loosely linked and farther apart
(c) Not inherited at all
(d) On different chromosomes
Show answer
Answer: (b) — Higher recombination frequency means the genes are farther apart and more loosely linked; low recombination means tight linkage.
Q5. Genetic maps were first constructed using recombination frequency by:
(a) Thomas Hunt Morgan
(b) Henking
(c) Alfred Sturtevant
(d) Walter Sutton
Show answer
Answer: (c) — Sturtevant, Morgan's student, used recombination frequency between gene pairs as a measure of distance to map gene positions.
Q6. The 'X body' observed during insect spermatogenesis was named by:
(a) Sutton
(b) Henking
(c) Morgan
(d) Boveri
Show answer
Answer: (b) — Henking (1891) traced the structure and named it the X body, though he could not explain its significance.
Q7. In grasshoppers, the mechanism of sex determination is:
(a) Haplodiploid
(b) XY type
(c) XO type
(d) ZW type
Show answer
Answer: (c) — Grasshoppers show XO type — males have only one X besides the autosomes, females have a pair of X chromosomes.
Q8. In birds, the heterogametic sex is:
(a) Neither — sex is determined by ploidy
(b) Both are homogametic
(c) Male (ZZ)
(d) Female (ZW)
Show answer
Answer: (d) — In birds, females are ZW (heterogametic) and males are ZZ (homogametic) — female heterogamety.
Q9. A drone (male honey bee) has how many chromosomes?
(a) 46
(b) 8
(c) 16
(d) 32
Show answer
Answer: (c) — Drones develop from unfertilised eggs by parthenogenesis and are haploid with 16 chromosomes; females are diploid with 32.
Q10. Male honey bees produce sperm by:
(a) They do not produce sperm
(b) Mitosis
(c) Binary fission
(d) Meiosis
Show answer
Answer: (b) — Since drones are already haploid, they produce sperm by mitosis rather than meiosis.
Q11. A change in a single base pair of DNA is called:
(a) Chromosomal aberration
(b) Polyploidy
(c) Frame-shift mutation
(d) Point mutation
Show answer
Answer: (d) — A point mutation is a change in a single base pair; sickle-cell anaemia is the classic example.
Q12. Colour blindness occurs in approximately what percentage of males?
(a) 50 per cent
(b) 0.4 per cent
(c) 8 per cent
(d) 25 per cent
Show answer
Answer: (c) — Red-green colour blindness affects about 8 per cent of males but only about 0.4 per cent of females.
Q13. In sickle-cell anaemia, the amino acid substitution is:
(a) Glutamic acid replaced by valine at position 6
(b) Lysine replaced by serine at position 10
(c) Valine replaced by glutamic acid at position 6
(d) Glycine replaced by alanine at position 4
Show answer
Answer: (a) — Glutamic acid (Glu) is replaced by Valine (Val) at the sixth position of the beta globin chain, from codon GAG to GUG.
Q14. Which genotype shows the diseased sickle-cell phenotype?
(a) HbA HbS
(b) All three genotypes
(c) HbS HbS
(d) HbA HbA
Show answer
Answer: (c) — Only homozygous HbS HbS individuals show the disease; heterozygous HbA HbS individuals are unaffected carriers.
Q15. Aneuploidy results from:
(a) UV radiation only
(b) A single base substitution
(c) Failure of cytokinesis after telophase
(d) Failure of chromatid segregation during cell division
Show answer
Answer: (d) — Aneuploidy (gain or loss of a chromosome) results from failure of chromatid segregation; failed cytokinesis causes polyploidy.
Q16. The karyotype of Klinefelter's syndrome is:
(a) 45, XO
(b) 47, XXY
(c) 47 with trisomy 21
(d) 46, XY
Show answer
Answer: (b) — Klinefelter's syndrome is 47, XXY — an extra X chromosome in a male, producing gynaecomastia and sterility.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q17. Turner's syndrome is characterised by the karyotype: (NEET PYQ)
(a) 47, +21
(b) 45, XO
(c) 46, XX
(d) 47, XXY
Show answer
Answer: (b) — Turner's syndrome results from the absence of one X chromosome in a female — 45, XO. Such females are sterile with rudimentary ovaries.
Q18. Down's syndrome is caused by: (NEET PYQ)
(a) An extra Y chromosome
(b) Trisomy of chromosome 21
(c) Monosomy of chromosome 21
(d) Loss of an X chromosome
Show answer
Answer: (b) — Down's syndrome is caused by an additional copy of chromosome 21 (trisomy 21), first described by Langdon Down in 1866.
Q19. Thalassemia differs from sickle-cell anaemia in that thalassemia is: (NEET PYQ)
(a) A qualitative problem of incorrectly functioning globin
(b) Not inherited
(c) A quantitative problem of synthesising too few globin molecules
(d) Caused by a chromosomal aberration
Show answer
Answer: (c) — Thalassemia is a quantitative defect (too few globin chains made); sickle-cell anaemia is a qualitative defect (incorrectly functioning globin).
Q20. Haemophilia is transmitted: (NEET PYQ)
(a) From an unaffected carrier female to some male progeny
(b) From affected father to all daughters equally
(c) Only to female offspring
(d) Only through autosomes
Show answer
Answer: (a) — Haemophilia is an X-linked recessive disease shown transmitting from an unaffected carrier female to some of her male progeny.
Q21. In beta-thalassemia, the affected gene HBB is located on chromosome: (NEET PYQ)
(a) Chromosome 16
(b) Chromosome 11
(c) Chromosome 21
(d) The X chromosome
Show answer
Answer: (b) — Beta-thalassemia is controlled by the single gene HBB on chromosome 11; alpha-thalassemia involves HBA1 and HBA2 on chromosome 16.
Q22. Phenylketonuria results from the lack of an enzyme that converts: (NEET PYQ)
(a) Glucose to glycogen
(b) Tyrosine to phenylalanine
(c) Phenylalanine to tyrosine
(d) Glutamic acid to valine
Show answer
Answer: (c) — Affected individuals lack the enzyme converting phenylalanine to tyrosine, so phenylalanine accumulates as phenylpyruvic acid.
Q23. The genes for white eye and yellow body in Drosophila showed a recombination frequency of: (NEET PYQ)
(a) 1.3 per cent
(b) 50 per cent
(c) 0 per cent
(d) 37.2 per cent
Show answer
Answer: (a) — White and yellow were very tightly linked, showing only 1.3 per cent recombination; white and miniature wing showed 37.2 per cent.
Q24. Polyploidy, an increase in whole sets of chromosomes, results from: (NEET PYQ)
(a) Failure of cytokinesis after telophase
(b) Failure of chromatid segregation
(c) Crossing over
(d) Point mutation
Show answer
Answer: (a) — Failure of cytokinesis after the telophase stage increases whole chromosome sets — polyploidy, often seen in plants.
Active Recall Prompt
Write everything you can recall about Principles of Inheritance and Variation (Part B), naming each part first: the chromosomal theory of inheritance (why Mendel was ignored, the 1900 rediscovery, Sutton and Boveri, Morgan and Drosophila); linkage and recombination (what Morgan found, tight vs loose linkage, Sturtevant's maps); sex determination (XO, XY, ZW and haplodiploid systems, with examples, plus humans and honey bees); mutation (chromosomal aberration, point mutation, frame-shift, mutagens); pedigree analysis; Mendelian disorders (colour blindness, haemophilia, sickle-cell anaemia, phenylketonuria, thalassemia — inheritance pattern and defect for each); and chromosomal disorders (aneuploidy vs polyploidy; Down's, Klinefelter's and Turner's syndromes with their karyotypes). Begin each fact with its topic and end it with a full stop.