Chapter 3 — Chromosomal Basis of Inheritance Exercise Solutions
This chapter links Mendel's laws to chromosomal behaviour. Sex-linked inheritance problems require tracking X-chromosome alleles carefully.
How to Use This Page
1. Start with the Key Concepts table to revise every important term quickly. 2. Solve MCQs to test factual recall on sex determination and disorders. 3. Fill in the Blanks for precise terminology. 4. True or False to identify misconceptions — read corrections carefully. 5. Match the Following to reinforce associations between terms and descriptions. 6. Brief Answers (2-mark) for concise, exam-style responses. 7. Detailed Answers (5-mark) for structured responses with sub-points. 8. Common Mistakes to catch errors before the exam. 9. Exam Checklist for final revision.
Key Concepts (Quick Recap)
| Term | Definition |
|---|---|
| Chromosomal theory of inheritance | Genes are located on chromosomes; chromosomes are the carriers of hereditary information |
| Locus | Specific position of a gene on a chromosome |
| Sex determination | Mechanism by which the sex of an individual is decided (XX-XY, ZW-ZZ, etc.) |
| XX-XY system | Females XX (homogametic), males XY (heterogametic); found in humans and Drosophila |
| XX-XO system | Females XX, males XO (O = no second sex chromosome); found in grasshoppers |
| ZW-ZZ system | Females ZW (heterogametic), males ZZ (homogametic); found in birds and butterflies |
| Haplodiploidy | Females diploid (2n), males haploid (n); found in honeybees |
| Sex-linked inheritance | Traits controlled by genes located on sex chromosomes (X or Y) |
| X-linked recessive | Trait expressed in males with one affected X; females need two affected X alleles |
| Colour blindness | X-linked recessive disorder; inability to distinguish red and green colours |
| Haemophilia | X-linked recessive disorder; impaired blood clotting |
| Non-disjunction | Failure of chromosomes to separate during meiosis, causing aneuploidy |
| Down syndrome | Trisomy 21 (47 chromosomes); Mongoloid features, intellectual disability |
| Klinefelter syndrome | XXY (47 chromosomes); male, infertile, gynecomastia |
| Turner syndrome | XO (45 chromosomes); female, short stature, infertile |
I. Choose the Best Answer (MCQs)
1. Who provided experimental proof for the chromosomal theory of inheritance? (a) Sutton and Boveri (b) T.H. Morgan (c) Mendel (d) Watson and Crick
2. In the XX-XY system of sex determination, which parent determines the sex of the child? (a) Mother (b) Father (c) Both parents equally (d) Neither parent
3. In birds, the sex determination system is: (a) XX-XY (b) XX-XO (c) ZW-ZZ (d) Haplodiploidy
4. A colour-blind male (X^nY) marries a carrier female (X^NX^n). What fraction of their daughters will be colour blind? (a) 0% (b) 25% (c) 50% (d) 100%
5. Down syndrome is caused by: (a) Monosomy 21 (b) Trisomy 21 (c) Deletion of chromosome 21 (d) Translocation of chromosome 21
6. Klinefelter syndrome has the karyotype: (a) 45 + XO (b) 47 + XXY (c) 47 + XYY (d) 47 + Trisomy 21
7. In which organism is haplodiploidy found? (a) Humans (b) Drosophila (c) Honeybees (d) Birds
8. A female with Turner syndrome has the karyotype: (a) 47, XXX (b) 45, XO (c) 47, XXY (d) 46, XX
9. Which of the following is an X-linked dominant disorder? (a) Colour blindness (b) Haemophilia (c) Both are X-linked recessive, not dominant (d) Turner syndrome
10. Crossing over occurs during which stage of meiosis? (a) Prophase II (b) Pachytene of Prophase I (c) Metaphase I (d) Anaphase I
11. Recombination frequency between two genes is 5%. The map distance is: (a) 0.5 centimorgans (b) 5 centimorgans (c) 50 centimorgans (d) 100 centimorgans
12. In the ZW-ZZ system, which sex is heterogametic? (a) Male (b) Female (c) Both (d) Neither
II. Fill in the Blanks
1. The chromosomal theory of inheritance was proposed by Sutton and Boveri. 2. In humans, females have XX sex chromosomes and males have XY. 3. Colour blindness is an X-linked recessive disorder. 4. Down syndrome is also known as Trisomy 21 or Mongolism. 5. Turner syndrome is characterised by the karyotype 45, XO. 6. In haplodiploidy, males develop from unfertilised eggs (haploid) and females from fertilised eggs (diploid). 7. The exchange of chromosome segments between non-sister chromatids is called crossing over. 8. One map unit is equal to 1% recombination frequency. 9. Klinefelter syndrome results in 47, XXY karyotype. 10. The gene for haemophilia is located on the X chromosome.
III. True or False (with corrections for false ones)
IV. Match the Following
| Column A | Column B |
|---|---|
| 1. XX-XY system | e. Humans, Drosophila |
| 2. ZW-ZZ system | f. Birds, butterflies |
| 3. Haplodiploidy | a. Honeybees |
| 4. Down syndrome | g. Trisomy 21 |
| 5. Klinefelter syndrome | h. 47, XXY |
| 6. Turner syndrome | b. 45, XO |
| 7. Colour blindness | c. X-linked recessive |
| 8. Crossing over | d. Pachytene of Prophase I |
V. Answer Briefly (2-mark)
1. What is the chromosomal theory of inheritance? The chromosomal theory of inheritance was proposed by Sutton and Boveri (1902). It states that genes are located on chromosomes at specific positions called loci, and chromosomes are the carriers of genetic information. During meiosis, homologous chromosomes pair and separate, accounting for Mendel's laws of segregation and independent assortment. T.H. Morgan later provided experimental proof using Drosophila melanogaster.
2. Explain the XX-XY system of sex determination. In the XX-XY system, females have two X chromosomes (XX) and are homogametic, producing only X-bearing eggs. Males have one X and one Y chromosome (XY) and are heterogametic, producing two types of sperm — X-bearing and Y-bearing. The sex of the offspring is determined by the type of sperm that fertilises the egg: X sperm produces XX (female) and Y sperm produces XY (male). This system operates in humans, Drosophila, and many other organisms.
3. What is non-disjunction? Name two disorders caused by it. Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis (meiosis I or II). This results in gametes with an abnormal number of chromosomes (aneuploidy). Two disorders caused by non-disjunction are: (i) Down syndrome — trisomy 21 (47 chromosomes) caused by non-disjunction of chromosome 21. (ii) Klinefelter syndrome — XXY (47 chromosomes) caused by non-disjunction of sex chromosomes.
4. Differentiate between linkage and crossing over. Linkage is the tendency of genes on the same chromosome to be inherited together during meiosis. It reduces genetic variation by keeping parental combinations intact. Crossing over is the exchange of chromosome segments between non-sister chromatids during prophase I of meiosis. It breaks linkage and creates new recombinant gene combinations, increasing genetic variation. Linkage and crossing over are opposite phenomena — linkage keeps genes together, crossing over separates them.
5. Why are sex-linked recessive traits more common in males than females? Males have only one X chromosome (XY). A single recessive allele on the X chromosome is sufficient to express the trait because there is no second X chromosome to mask it. Females have two X chromosomes (XX) — they need two copies of the recessive allele (one on each X) to express the trait. With only one affected X chromosome, females are carriers but do not show the trait. This is why X-linked recessive conditions like colour blindness and haemophilia are more common in males.
6. Describe Klinefelter syndrome. Klinefelter syndrome is a chromosomal disorder in males caused by the presence of an extra X chromosome, resulting in a 47, XXY karyotype. It occurs due to non-disjunction of sex chromosomes during meiosis. Characteristics include: tall stature, gynecomastia (enlarged breasts), underdeveloped testes, infertility (azoospermia), and reduced facial and body hair. The condition is not inherited but arises from a random error in gamete formation. Testosterone replacement therapy can help manage some symptoms.
VI. Answer in Detail (5-mark)
1. Explain the different types of sex determination mechanisms with examples. Sex determination mechanisms vary across organisms:
- XX-XY type: Females are XX (homogametic, produce only X-bearing eggs) and males are XY (heterogametic, produce X-bearing and Y-bearing sperm). The father determines the sex of the offspring. Found in humans, Drosophila, and many mammals.
- XX-XO type: Females are XX and males are XO (O indicates absence of a second sex chromosome). The male produces two types of sperm — X-bearing and O-bearing. Found in grasshoppers, cockroaches, and some insects.
- ZW-ZZ type: This is the reverse of XX-XY. Females are ZW (heterogametic) and males are ZZ (homogametic). The mother determines the sex of the offspring. Found in birds, butterflies, fish, and some reptiles.
- Haplodiploidy: Females develop from fertilised eggs (diploid, 2n) and males develop from unfertilised eggs (haploid, n). Found in honeybees, ants, and wasps. The queen controls whether eggs are fertilised, thereby controlling the sex ratio.
- Environmental sex determination: In some turtles and lizards, the temperature during egg incubation determines sex. High temperatures produce females and low temperatures produce males (or vice versa, depending on species).
2. Explain sex-linked inheritance with reference to colour blindness. Show a cross between a carrier female and a normal male. Colour blindness is an X-linked recessive disorder where affected individuals cannot distinguish red and green colours:
- Gene location: The gene for colour vision is on the X chromosome. There is no corresponding gene on the Y chromosome.
- Genotypes: X^NX^N (normal female), X^NX^n (carrier female), X^nX^n (colour blind female), X^NY (normal male), X^nY (colour blind male).
- Cross: Carrier female (X^NX^n) × Normal male (X^NY)
- Offspring: X^NX^N (normal female), X^NX^n (carrier female), X^NY (normal male), X^nY (colour blind male)
- Ratio: 1 normal female : 1 carrier female : 1 normal male : 1 colour blind male
- Key points: A colour-blind father cannot pass the trait to his sons (sons receive Y chromosome). A carrier mother has a 50% chance of passing the affected X to each son. Colour-blind females are rare because they need two affected X chromosomes.
- Diagram description: Draw the Punnett square with X^N, X^n gametes on the female side and X^N, Y gametes on the male side. Label all four offspring genotypes and phenotypes.
3. Describe the chromosomal disorders — Down syndrome, Klinefelter syndrome, and Turner syndrome. Chromosomal disorders arise from abnormalities in chromosome number or structure:
- Down syndrome (Trisomy 21):
- Karyotype: 47 chromosomes (47, +21)
- Cause: Non-disjunction of chromosome 21 during meiosis, resulting in an extra copy of chromosome 21.
- Features: Intellectual disability, Mongoloid facial features (epicanthal folds, flat nasal bridge), short stature, single transverse palmar crease (simian crease), protruding tongue, congenital heart defects.
- Risk increases with maternal age (especially above 35 years).
- Life expectancy has improved with medical care but remains lower than average.
- Klinefelter syndrome (47, XXY):
- Karyotype: 47, XXY
- Cause: Non-disjunction of sex chromosomes producing XX egg or XY sperm.
- Features: Male with feminine characteristics — tall stature, gynecomastia (enlarged breasts), small testes, azoospermia (infertility), sparse body hair, reduced testosterone levels.
- Usually diagnosed at puberty when secondary sexual characteristics fail to develop properly.
- Testosterone replacement therapy can help.
- Turner syndrome (45, XO):
- Karyotype: 45, XO
- Cause: Non-disjunction of sex chromosomes producing an egg with no X chromosome.
- Features: Female with short stature, webbed neck, shield-shaped chest, ovarian dysgenesis (streak ovaries), infertility, absence of secondary sexual characteristics, cardiovascular defects.
- Growth hormone therapy and oestrogen replacement can help manage symptoms.
4. Explain linkage and crossing over. How is recombination frequency used to construct genetic maps? Linkage and crossing over are key concepts in understanding how genes are inherited:
- Linkage: Genes located on the same chromosome tend to be inherited together. This is because they are physically linked on the same DNA molecule. T.H. Morgan demonstrated this in Drosophila with body colour (b⁺/b) and wing size (vg⁺/vg) — parental combinations appeared more frequently than expected under independent assortment.
- Crossing over: During pachytene of prophase I in meiosis, non-sister chromatids of homologous chromosomes exchange segments. This creates recombinant chromosomes with new allele combinations. Crossing over is the physical basis of recombination.
- Recombination frequency (RF): RF = (Number of recombinant offspring / Total offspring) × 100%. It measures how often crossing over occurs between two genes. Higher RF means genes are farther apart on the chromosome.
- Genetic mapping: 1% recombination = 1 map unit = 1 centimorgan (cM). Genes are arranged linearly on chromosomes. If gene A-B RF = 5% and gene B-C RF = 10%, then A and C are 15 cM apart (if A, B, C are in that order).
- Limitations: RF approaches 50% maximum (equivalent to independent assortment for genes on different chromosomes). Double crossovers between distant genes can underestimate map distances.
Chromosomal Theory of Inheritance — Key Points
- Sutton and Boveri proposed chromosomal theory of inheritance (1902).
- Morgan provided experimental proof using Drosophila melanogaster.
- Genes are located on chromosomes at specific positions called loci.
- Alleles of a gene occupy the same locus on homologous chromosomes.
Sex Determination Types
| Type | Female | Male | Example |
|---|---|---|---|
| XX-XY | XX | XY | Humans, Drosophila |
| XX-XO | XX | XO | Grasshopper, cockroach |
| ZW-ZZ | ZW | ZZ | Birds, butterflies, fish |
| Haplodiploidy | Diploid | Haploid | Honeybees |
Sex-Linked Inheritance — Colour Blindness (X-linked recessive)
| Genotype | Phenotype |
|---|---|
| X^N X^N | Normal female |
| X^N X^n | Carrier female |
| X^n X^n | Colour blind female |
| X^N Y | Normal male |
| X^n Y | Colour blind male |
Cross: Carrier female × Normal male X^N X^n × X^N Y Offspring: X^N X^N (normal F), X^N X^n (carrier F), X^N Y (normal M), X^n Y (colour blind M) Ratio: 1:1 in females normal:carrier, 1:1 in males normal:affected
Chromosomal Disorders
| Disorder | Chromosomal Basis | Features |
|---|---|---|
| Down syndrome | Trisomy 21 (47 chromosomes) | Mental retardation, Mongoloid features |
| Klinefelter syndrome | XXY (47 chromosomes) | Male infertile, gynecomastia |
| Turner syndrome | XO (45 chromosomes) | Female, short stature, infertile |
| Patau syndrome | Trisomy 13 | Multiple abnormalities |
| Edward syndrome | Trisomy 18 | Multiple abnormalities |
Linkage and Crossing Over
- Linkage: Tendency of genes on the same chromosome to be inherited together.
- Crossing over: Exchange of chromosome segments between non-sister chromatids during prophase I.
- Morgan found genes for body colour and wing size were linked in Drosophila.
- Recombination frequency used to construct linkage maps (1 map unit = 1 cM = 1% recombination).
Worked Examples
Example 1 — ABO blood group inheritance
Can parents with blood groups AB and O have a child with group O?
Example 2 — Colour blindness (X-linked recessive)
A carrier mother (XᶜX) and a normal father (XY) have children. What fraction of sons are colour blind?
Example 3 — Sex determination (XX–XY)
Common Mistakes to Avoid
1. ❌ Forgetting that in ZW-ZZ type, males are homogametic (ZZ) and females are heterogametic (ZW) ✅ In ZW-ZZ, it is the OPPOSITE of XX-XY — females are ZW (heterogametic), males are ZZ (homogametic)
2. ❌ Writing just N and n without indicating X and Y chromosomes in sex-linked inheritance problems ✅ Always write X^N, X^n, Y to show that the gene is on the X chromosome
3. ❌ Confusing non-disjunction in meiosis I vs meiosis II ✅ Non-disjunction in meiosis I: homologous chromosomes fail to separate — affects all resulting gametes. Non-disjunction in meiosis II: sister chromatids fail to separate — affects only half the resulting gametes
4. ❌ Writing Turner syndrome as XX instead of XO ✅ Turner syndrome karyotype is 45, XO — only ONE X chromosome, not two
5. ❌ Saying a colour-blind father passes the trait to his sons ✅ A colour-blind father (X^nY) passes X^n only to daughters (they become carriers). Sons receive the Y chromosome and are unaffected by the father's X-linked condition
6. ❌ Confusing linkage with independent assortment ✅ Linked genes do NOT assort independently; only genes on different chromosomes assort independently
7. ❌ Stating that recombination frequency can exceed 50% ✅ RF ranges from 0% (complete linkage) to 50% (independent assortment). It cannot exceed 50%
8. ❌ Writing Klinefelter syndrome karyotype as XYY ✅ Klinefelter syndrome is 47, XXY; XYY is a different condition (Jacob syndrome)
Exam Checklist
- [ ] Can you state the chromosomal theory of inheritance and name its proposers?
- [ ] Can you describe all four sex determination mechanisms with examples?
- [ ] Can you explain why sex-linked recessive traits are more common in males?
- [ ] Can you set up and solve a cross for X-linked recessive inheritance (colour blindness)?
- [ ] Can you describe Down syndrome, Klinefelter syndrome, and Turner syndrome with karyotypes and features?
- [ ] Can you define non-disjunction and explain how it causes chromosomal disorders?
- [ ] Can you differentiate between linkage and crossing over?
- [ ] Can you calculate recombination frequency and use it for genetic mapping?
- [ ] Can you distinguish between XX-XY and ZW-ZZ sex determination systems?
- [ ] Can you explain haplodiploidy with reference to honeybees?
- [ ] Can you differentiate between meiosis I and meiosis II non-disjunction?
- [ ] Can you explain why colour-blind females are rare compared to colour-blind males?