Pedigree Analysis, Mendelian Disorders & Chromosomal Disorders
NCERT-Based NEET Biology Notes for NEET 2027
📚 Quick Summary
Pedigree analysis, Mendelian disorders and chromosomal disorders are important topics from Class 12 Biology and the NCERT chapter Principles of Inheritance and Variation.
Pedigree analysis helps us understand how a particular trait or genetic condition is transmitted through generations. Mendelian disorders are generally caused by changes in a single gene, whereas chromosomal disorders result from abnormalities involving chromosome number or structure.
These notes are designed for NCERT revision, NEET Biology preparation and quick MCQ practice.
🎯 Preparing for NEET Biology?
Read the NCERT chapter carefully first. Then revise inheritance patterns, disorder examples, chromosome numbers and important genetic terms. Regular MCQ practice can improve recall and accuracy.
Explore NEET Biology Resources →Pedigree Analysis, Mendelian Disorders and Chromosomal Disorders
Pedigree analysis is a useful method for studying the inheritance of traits in families. It uses standard symbols to represent individuals and relationships across generations.
In human genetics, pedigree analysis can help determine whether a trait shows an autosomal dominant, autosomal recessive or sex-linked pattern.
Mendelian disorders are usually caused by alterations in a single gene, while chromosomal disorders involve abnormalities in chromosome number or chromosome structure.
1. Pedigree Analysis: Reading Family Trees
A pedigree is a diagram that represents the inheritance of a particular trait through several generations of a family.
Common Pedigree Symbols
- Square: Male
- Circle: Female
- Unshaded symbol: Usually represents an unaffected individual
- Shaded symbol: Usually represents an affected individual
- Horizontal line: Mating relationship
- Vertical line: Relationship between parents and offspring
- Arrow: Proband or individual through whom the family was identified
Learn the basic pedigree symbols first. Correctly reading the pedigree is the first step in identifying an inheritance pattern.
2. Autosomal Dominant Inheritance
In an autosomal dominant condition, a disease-causing allele located on an autosome can express itself when present in a single copy.
Typical Features
- Often appears in successive generations.
- Males and females can be affected.
- Both sexes can transmit the condition.
- An affected heterozygous parent may transmit the allele to approximately 50% of offspring in each pregnancy.
3. Autosomal Recessive Inheritance
In an autosomal recessive condition, the disorder is expressed when an individual has two disease-associated recessive alleles.
Typical Features
- May skip generations.
- Males and females are affected equally.
- Unaffected carrier parents can have an affected child.
- Consanguineous marriage can increase the likelihood of expression of some recessive disorders.
4. X-Linked Recessive Inheritance
In an X-linked recessive condition, the responsible gene is located on the X chromosome.
Important Features
- More commonly expressed in males.
- There is no father-to-son transmission of an X-linked trait.
- An affected father passes his X chromosome to all daughters.
- A carrier mother can transmit the altered allele to her sons.
5. Common Mendelian Disorders
Mendelian disorders generally result from mutations or alterations in a single gene. NCERT highlights several important examples.
| Disorder | Inheritance | Important NEET Fact |
|---|---|---|
| Haemophilia | X-linked recessive | Defective blood clotting |
| Colour blindness | X-linked recessive | Difficulty distinguishing certain colours |
| Sickle-cell anaemia | Autosomal recessive | Abnormal haemoglobin HbS |
| Phenylketonuria | Autosomal recessive | Defect in phenylalanine metabolism |
| Thalassemia | Autosomal recessive | Reduced or absent synthesis of globin chains |
6. Haemophilia
Haemophilia is an X-linked recessive disorder in which the blood-clotting process is impaired.
Because the responsible gene is located on the X chromosome, the condition is much more commonly expressed in males.
Haemophilia → X-linked recessive → Blood clotting problem
7. Colour Blindness
Colour blindness is generally inherited as an X-linked recessive condition and involves difficulty in distinguishing certain colours.
It is more commonly expressed in males because males possess only one X chromosome.
8. Sickle-Cell Anaemia
Sickle-cell anaemia is an autosomal recessive genetic disorder associated with an abnormal form of haemoglobin called HbS.
A mutation in the beta-globin gene results in a change in the haemoglobin protein. Under low-oxygen conditions, red blood cells may assume a characteristic sickle shape.
Sickle-cell anaemia → Autosomal recessive → HbS
9. Phenylketonuria
Phenylketonuria (PKU) is an inherited metabolic disorder associated with impaired metabolism of the amino acid phenylalanine.
It is inherited as an autosomal recessive disorder.
10. Thalassemia
Thalassemia is an inherited blood disorder in which the synthesis of one or more globin chains of haemoglobin is reduced or absent.
It is an autosomal recessive disorder. Depending on the affected globin chain, thalassemia may be classified as alpha or beta thalassemia.
🧠 Mendelian Disorder Memory Hook
H = Haemophilia
C = Colour blindness
S = Sickle-cell anaemia
P = Phenylketonuria
T = Thalassemia
Remember the inheritance pattern separately for each disorder.
11. Major Chromosomal Disorders
Chromosomal disorders may result from abnormalities in chromosome number or chromosome structure. Numerical abnormalities commonly arise because of non-disjunction during meiosis.
| Disorder | Karyotype | Cause | Key Feature |
|---|---|---|---|
| Down syndrome | 47, +21 | Trisomy 21 | Intellectual disability and characteristic facial features |
| Klinefelter syndrome | 47, XXY | Extra X chromosome in a male | Tall stature, small testes and possible infertility |
| Turner syndrome | 45, XO | Monosomy X | Female with short stature and ovarian dysfunction |
| Edwards syndrome | 47, +18 | Trisomy 18 | Multiple developmental abnormalities |
| Patau syndrome | 47, +13 | Trisomy 13 | Severe developmental abnormalities |
12. Down Syndrome
Down syndrome is caused by trisomy of chromosome 21. The usual chromosome number is therefore 47.
It is associated with characteristic facial appearance, developmental and intellectual disability, and other physical features.
Down → 21 → Trisomy 21 → 47 chromosomes
13. Klinefelter Syndrome
Klinefelter syndrome occurs in males who usually have an additional X chromosome, giving the karyotype 47, XXY.
- Tall stature may occur.
- Small testes are characteristic.
- Infertility is common.
- Gynaecomastia may occur.
14. Turner Syndrome
Turner syndrome is associated with monosomy X and has the karyotype 45, XO.
- Occurs in females.
- Short stature is common.
- Ovarian dysfunction occurs.
- Infertility is common.
15. Edwards Syndrome
Edwards syndrome is caused by trisomy of chromosome 18 and is represented by 47, +18.
16. Patau Syndrome
Patau syndrome is caused by trisomy of chromosome 13 and is represented by 47, +13.
🧠 Trisomy Memory Trick
Down = 21
Edwards = 18
Patau = 13
Think: 21 – 18 – 13
17. Mendelian vs Chromosomal Disorders
| Feature | Mendelian Disorder | Chromosomal Disorder |
|---|---|---|
| Main cause | Alteration in a single gene | Chromosome number or structure abnormality |
| Inheritance | May follow Mendelian patterns | Often associated with chromosomal abnormalities |
| Examples | Haemophilia, sickle-cell anaemia, PKU | Down, Turner, Klinefelter |
🔥 NEET High-Yield Points
- Pedigree analysis represents inheritance through generations.
- Square represents male and circle represents female.
- Autosomal dominant traits often appear in successive generations.
- Autosomal recessive disorders may skip generations.
- X-linked traits do not show father-to-son transmission.
- Haemophilia is X-linked recessive.
- Colour blindness is X-linked recessive.
- Sickle-cell anaemia is autosomal recessive.
- Phenylketonuria is autosomal recessive.
- Thalassemia is autosomal recessive.
- Down syndrome → Trisomy 21.
- Edwards syndrome → Trisomy 18.
- Patau syndrome → Trisomy 13.
- Turner syndrome → 45, XO.
- Klinefelter syndrome → 47, XXY.
- Non-disjunction can lead to abnormal chromosome numbers.
18. NEET Practice MCQs
1. In a pedigree, a square generally represents:
A. Female
B. Male
C. Carrier only
D. Affected female
✅ Answer: B. Male
2. Which inheritance pattern does not involve father-to-son transmission?
A. Autosomal dominant
B. Autosomal recessive
C. X-linked
D. Y-linked
✅ Answer: C. X-linked
3. Haemophilia is generally inherited as:
A. Autosomal dominant
B. Autosomal recessive
C. X-linked recessive
D. Y-linked dominant
✅ Answer: C. X-linked recessive
4. Sickle-cell anaemia is:
A. X-linked dominant
B. Autosomal recessive
C. Y-linked
D. Autosomal dominant
✅ Answer: B. Autosomal recessive
5. Down syndrome is caused by:
A. Monosomy X
B. Trisomy 13
C. Trisomy 18
D. Trisomy 21
✅ Answer: D. Trisomy 21
6. The karyotype 47, XXY represents:
A. Turner syndrome
B. Down syndrome
C. Klinefelter syndrome
D. Patau syndrome
✅ Answer: C. Klinefelter syndrome
7. Turner syndrome is associated with:
A. 47, XXY
B. 45, XO
C. 47, +21
D. 47, +13
✅ Answer: B. 45, XO
8. Edwards syndrome involves:
A. Trisomy 13
B. Trisomy 18
C. Trisomy 21
D. Monosomy X
✅ Answer: B. Trisomy 18
9. Patau syndrome is associated with:
A. Trisomy 13
B. Trisomy 18
C. Trisomy 21
D. XXY
✅ Answer: A. Trisomy 13
10. A disorder caused by alteration in a single gene is generally called:
A. Chromosomal disorder
B. Mendelian disorder
C. Polygenic disorder only
D. Numerical abnormality
✅ Answer: B. Mendelian disorder
📌 One-Minute Revision
Pedigree → Family inheritance diagram
Square → Male
Circle → Female
Autosomal dominant → Often successive generations
Autosomal recessive → May skip generations
X-linked → No father-to-son transmission
Haemophilia → X-linked recessive
Colour blindness → X-linked recessive
Sickle-cell anaemia → Autosomal recessive
Down → Trisomy 21
Edwards → Trisomy 18
Patau → Trisomy 13
Turner → 45, XO
Klinefelter → 47, XXY
🌿 NEET Biology for Odisha Students
Students preparing for NEET in Odisha can use these NCERT-based Genetics notes for systematic revision of Class 12 Biology.
For better NEET preparation, combine NCERT reading with chapter-wise revision, previous-year questions and regular MCQ practice.
Explore NEET Biology for Odisha Students →📖 Related Biology Resources
NCERT-focused Biology notes, diagrams, MCQs and NEET preparation resources for Class 11, Class 12 and NEET Biology students.