Principles of Inheritance and Variation
Welcome to HSLC Guru! This study guide is designed for Class 12 ASSEB Biology students preparing for the board examination. Chapter 5, “Principles of Inheritance and Variation,” is one of the most scoring and conceptually rewarding units in the syllabus. It introduces you to the fascinating science of genetics, how traits are passed from parents to offspring, and why siblings often differ from each other despite sharing the same parents. From Mendel’s classical experiments on garden pea to modern understanding of chromosomal disorders, this chapter forms the foundation of heredity and molecular biology.
Chapter Summary
Genetics is the branch of biology that deals with the study of heredity and variation. Heredity is the transmission of characters from parents to offspring, while variation refers to the differences shown by individuals of a species. Gregor Johann Mendel, the Father of Genetics, conducted hybridization experiments on garden pea (Pisum sativum) between 1856 and 1863 and proposed the laws of inheritance. He selected seven pairs of contrasting traits and performed monohybrid and dihybrid crosses. In a monohybrid cross between tall (TT) and dwarf (tt) pea plants, the F1 generation was all tall, and the F2 generation showed a phenotypic ratio of 3:1 (tall:dwarf) with a genotypic ratio of 1:2:1 (TT:Tt:tt). This led to the Law of Dominance and the Law of Segregation. In a dihybrid cross, the F2 generation produced a phenotypic ratio of 9:3:3:1, which gave rise to the Law of Independent Assortment.
Several patterns of inheritance deviate from Mendel’s principles. In incomplete dominance, neither allele is completely dominant; for example, in snapdragon (Antirrhinum) and four o’clock plant, a cross between red (RR) and white (rr) flowers produces pink (Rr) F1, and the F2 ratio is 1:2:1. In codominance, both alleles express themselves equally, as seen in the ABO blood group system in humans, controlled by multiple alleles (IA, IB, i). Pleiotropy is the phenomenon where a single gene controls multiple phenotypic traits, such as in phenylketonuria (PKU) and sickle-cell anaemia. Polygenic inheritance involves multiple genes contributing to a single trait, exemplified by human skin colour and height, which show continuous variation.
The chromosomal theory of inheritance, proposed independently by Walter Sutton and Theodore Boveri in 1902, established that chromosomes are the carriers of hereditary factors and behave parallel to Mendel’s factors. Thomas Hunt Morgan worked on Drosophila melanogaster (fruit fly) and discovered the phenomena of linkage (genes located close together on the same chromosome are inherited together) and recombination (exchange of genetic material between homologous chromosomes during meiosis). Sex determination varies across species: in humans, the XX-XY system operates, where females are XX and males are XY; in grasshoppers, the XX-XO system is found; in birds, moths, and reptiles, the ZZ-ZW system is observed; and in honey bees, the haplo-diploid system functions, where males develop from unfertilized eggs (haploid) and females from fertilized eggs (diploid).
Mutation is a sudden, heritable change in the DNA sequence. Chromosomal mutations involve changes in the structure (deletion, duplication, inversion, translocation) or number (aneuploidy, polyploidy) of chromosomes, while gene mutations involve changes at the nucleotide level. Genetic disorders are broadly classified into Mendelian disorders and chromosomal disorders. Mendelian disorders include haemophilia (X-linked recessive bleeding disorder), sickle-cell anaemia (autosomal recessive, HbS allele), phenylketonuria (PKU, inability to metabolize phenylalanine), and thalassaemia (defective synthesis of haemoglobin chains). Chromosomal disorders include Down’s syndrome (trisomy of chromosome 21), Klinefelter’s syndrome (XXY, additional X in male), and Turner’s syndrome (XO, missing X in female). Pedigree analysis helps trace the inheritance pattern of these disorders across generations.
Short Questions and Answers (1 Mark)
Q1. Who is known as the Father of Genetics?
Answer: Gregor Johann Mendel is known as the Father of Genetics for his pioneering work on inheritance patterns in garden pea plants.
Q2. What is the scientific name of garden pea used by Mendel?
Answer: The scientific name of garden pea is Pisum sativum.
Q3. What is the phenotypic ratio of a monohybrid cross in F2 generation?
Answer: The phenotypic ratio of a monohybrid cross in the F2 generation is 3:1 (dominant:recessive).
Q4. What is the dihybrid phenotypic ratio in F2?
Answer: The phenotypic ratio of a dihybrid cross in F2 is 9:3:3:1.
Q5. Name the disease caused by trisomy of chromosome 21.
Answer: Down’s syndrome is caused by trisomy of chromosome 21.
Q6. What is the genotype of a person with Klinefelter’s syndrome?
Answer: A person with Klinefelter’s syndrome has the genotype 44 + XXY (47 chromosomes).
Q7. Define pleiotropy with one example.
Answer: Pleiotropy is the phenomenon where a single gene controls multiple phenotypic traits. Example: phenylketonuria (PKU).
Q8. Which organism did Thomas Hunt Morgan use for his experiments?
Answer: Thomas Hunt Morgan used the fruit fly Drosophila melanogaster for his experiments on linkage and recombination.
Q9. What is the sex determination mechanism in honey bees?
Answer: Sex determination in honey bees follows the haplo-diploid mechanism, where males develop from unfertilized eggs (haploid) and females from fertilized eggs (diploid).
Q10. What is a test cross?
Answer: A test cross is a cross between an individual with an unknown genotype and a homozygous recessive individual to determine the unknown genotype.
Short Answer Questions (2-3 Marks)
Q1. State Mendel’s Law of Segregation.
Answer: Mendel’s Law of Segregation, also known as the Law of Purity of Gametes, states that during the formation of gametes, the two alleles of a gene segregate (separate) from each other so that each gamete carries only one allele of a gene. This separation occurs during meiosis. The alleles unite again during fertilization, and no allele blends or contaminates the other. This law explains the reappearance of recessive traits in the F2 generation despite their absence in F1.
Q2. Differentiate between incomplete dominance and codominance.
Answer: In incomplete dominance, neither allele is completely dominant, and the heterozygote shows an intermediate phenotype between the two parents. Example: in snapdragon, red (RR) crossed with white (rr) gives pink (Rr) flowers in F1. In codominance, both alleles express themselves fully and independently in the heterozygote, producing a distinct phenotype that shows traits of both parents. Example: in the ABO blood group system, individuals with IAIB genotype have AB blood group, expressing both A and B antigens.
Q3. Explain the chromosomal theory of inheritance.
Answer: The chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Boveri in 1902. According to this theory, chromosomes are the physical carriers of Mendel’s hereditary factors (genes), and the behaviour of chromosomes during meiosis explains Mendel’s laws. Like Mendel’s factors, chromosomes occur in pairs, segregate during gamete formation, and combine independently during fertilization. This theory provided a cellular basis for Mendelian inheritance.
Q4. What is haemophilia? Why is it called the “royal disease”?
Answer: Haemophilia is an X-linked recessive genetic disorder in which the affected individual lacks the ability to clot blood normally due to a deficiency of clotting factors. Even a minor cut may lead to non-stop bleeding. It mainly affects males because males have only one X chromosome, while females, being carriers, rarely express the disease. It is called the “royal disease” because Queen Victoria of England was a carrier, and her descendants in several European royal families were affected.
Q5. Describe the XX-XO type of sex determination.
Answer: In the XX-XO type of sex determination, found in insects like grasshoppers and roundworms, females have two X chromosomes (XX) and are homogametic, producing only one type of egg with an X chromosome. Males have only one X chromosome (XO) and are heterogametic, producing two types of sperm: half with an X chromosome and half without any sex chromosome. The sex of the offspring is determined by the sperm.
Q6. What is sickle-cell anaemia? Mention its inheritance pattern.
Answer: Sickle-cell anaemia is an autosomal recessive disorder caused by a single point mutation in the beta-globin gene of haemoglobin, where glutamic acid is replaced by valine at the sixth position. It results in sickle-shaped red blood cells that block blood vessels. Inheritance is autosomal recessive: HbAHbA individuals are normal, HbAHbS are carriers (with sickle-cell trait), and HbSHbS individuals are affected.
Long Answer Questions (5-7 Marks)
Q1. Explain Mendel’s monohybrid cross in detail with a suitable example. State the laws derived from it.
Answer: A monohybrid cross is a genetic cross between two parents that differ in only one pair of contrasting characters. Mendel performed a classical monohybrid cross between pure tall (TT) and pure dwarf (tt) garden pea plants.
Procedure: Mendel selected pure-breeding (homozygous) tall (TT) and dwarf (tt) plants as parents. The pollen from tall plants was transferred to the stigma of dwarf plants and vice versa.
F1 Generation: All offspring (Tt) were tall, showing dominance of the tall character over dwarf.
F2 Generation: When F1 plants were self-pollinated, the F2 generation showed both tall and dwarf plants in the ratio of 3:1 (phenotypic ratio) and 1:2:1 (genotypic ratio: 1 TT : 2 Tt : 1 tt).
Laws derived: (i) Law of Dominance: One factor in a pair masks the expression of the other; the masked factor is recessive. (ii) Law of Segregation: The two factors of a pair separate during gamete formation, and each gamete receives only one factor.
Q2. Describe Mendel’s dihybrid cross and explain the Law of Independent Assortment.
Answer: A dihybrid cross is a cross between two parents differing in two pairs of contrasting characters. Mendel crossed pea plants having round yellow seeds (RRYY) with plants having wrinkled green seeds (rryy).
F1 Generation: All offspring were round yellow (RrYy), heterozygous for both traits.
F2 Generation: Self-pollination of F1 plants produced four phenotypes in the ratio 9:3:3:1 — 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. This ratio indicates the appearance of two new combinations (round green and wrinkled yellow) along with the parental types.
Law of Independent Assortment: When two pairs of traits are considered together, the alleles of one pair segregate and assort independently of the alleles of the other pair during gamete formation. Hence, the inheritance of one trait does not affect the inheritance of another trait. This explains the appearance of new combinations in the F2 generation.
Q3. Explain the inheritance of ABO blood groups in humans. Why is it considered an example of multiple alleles and codominance?
Answer: The ABO blood group in humans is controlled by a single gene (I) located on chromosome 9. This gene has three alleles: IA, IB, and i. Since more than two alleles control a single trait at the same locus, this is an example of multiple alleles. However, in any individual, only two of these alleles are present.
Antigens and phenotypes: IA produces antigen A, IB produces antigen B, and i produces no antigen. IA and IB are codominant over each other, while both are dominant over i.
Genotype and phenotype: IAIA or IAi → blood group A; IBIB or IBi → blood group B; IAIB → blood group AB (codominance, both A and B antigens expressed); ii → blood group O.
It illustrates codominance because in IAIB individuals, both alleles express themselves equally and independently, producing both A and B antigens on red blood cell surfaces.
Q4. Describe the major chromosomal disorders in humans with their characteristic features.
Answer: Chromosomal disorders are caused by changes in chromosome number or structure due to non-disjunction during meiosis.
(i) Down’s Syndrome: Caused by trisomy of chromosome 21 (21+21+21), giving 47 chromosomes instead of 46. First described by Langdon Down (1866). Features include short stature, small round head, broad flat face, partially open mouth, protruding tongue, palm crease, and intellectual disability.
(ii) Klinefelter’s Syndrome: Caused by an additional X chromosome in males (44 + XXY = 47 chromosomes). Features include tall stature, gynaecomastia (development of breasts), feminine body characteristics, sterility (no sperm production), and underdeveloped male organs.
(iii) Turner’s Syndrome: Caused by absence of one X chromosome in females (44 + XO = 45 chromosomes). Features include short stature, webbed neck, sterility (rudimentary ovaries), absence of secondary sexual characters, and underdeveloped breasts.
Q5. Explain sex determination in human beings with the help of a suitable diagram description.
Answer: In humans, sex determination follows the XX-XY pattern. Out of the 23 pairs of chromosomes, 22 pairs are autosomes, and 1 pair is sex chromosomes. Females have two similar X chromosomes (XX) and are homogametic, producing only one type of egg containing an X chromosome. Males have one X and one Y chromosome (XY) and are heterogametic, producing two types of sperm — 50% carrying an X chromosome and 50% carrying a Y chromosome.
Cross: Mother (XX) × Father (XY) → eggs are all X type, while sperms are X or Y type. Fertilization: X (egg) + X (sperm) → XX (female); X (egg) + Y (sperm) → XY (male). Therefore, the sex of the child is determined by the type of sperm that fertilizes the egg, and the father is responsible for the sex of the offspring. The probability of a male or female child is 50% in each pregnancy.
Multiple Choice Questions (MCQs)
Q1. Mendel performed his experiments on which plant?
(a) Sweet pea
(b) Garden pea
(c) Wild pea
(d) Soybean
Answer: (b) Garden pea
Q2. The phenotypic ratio of a dihybrid cross in F2 is:
(a) 3:1
(b) 1:2:1
(c) 9:3:3:1
(d) 1:1:1:1
Answer: (c) 9:3:3:1
Q3. Down’s syndrome is caused by trisomy of which chromosome?
(a) 18
(b) 21
(c) X
(d) Y
Answer: (b) 21
Q4. The genotype of a person with Turner’s syndrome is:
(a) XXY
(b) XO
(c) XYY
(d) XXX
Answer: (b) XO
Q5. Incomplete dominance was first observed in:
(a) Garden pea
(b) Snapdragon
(c) Maize
(d) Drosophila
Answer: (b) Snapdragon
Q6. Sex determination in honey bees is:
(a) XX-XY
(b) XX-XO
(c) ZZ-ZW
(d) Haplo-diploid
Answer: (d) Haplo-diploid
Q7. Haemophilia is a:
(a) Autosomal dominant disorder
(b) Autosomal recessive disorder
(c) X-linked recessive disorder
(d) Y-linked disorder
Answer: (c) X-linked recessive disorder
Q8. Linkage was discovered by:
(a) Mendel
(b) Sutton
(c) Morgan
(d) Bateson
Answer: (c) Morgan
Q9. The blood group AB is an example of:
(a) Incomplete dominance
(b) Codominance
(c) Pleiotropy
(d) Epistasis
Answer: (b) Codominance
Q10. Skin colour in humans is an example of:
(a) Polygenic inheritance
(b) Codominance
(c) Incomplete dominance
(d) Pleiotropy
Answer: (a) Polygenic inheritance
Fill in the Blanks
Q1. Mendel proposed his laws of inheritance based on experiments on __________.
Answer: garden pea (Pisum sativum)
Q2. The phenotypic ratio of incomplete dominance in F2 is __________.
Answer: 1:2:1
Q3. Klinefelter’s syndrome has the genotype __________.
Answer: 44 + XXY
Q4. The chromosomal theory of inheritance was proposed by Sutton and __________.
Answer: Boveri
Q5. In ZZ-ZW sex determination, the female is the __________ sex.
Answer: heterogametic
True or False
Q1. Mendel formulated three laws of inheritance.
Answer: True
Q2. The ABO blood group is controlled by two alleles.
Answer: False (it is controlled by three alleles — multiple alleles)
Q3. Sickle-cell anaemia is an autosomal recessive disorder.
Answer: True
Q4. In humans, the mother is responsible for the sex of the child.
Answer: False (the father is responsible because he produces both X- and Y-bearing sperm)
Q5. Drosophila melanogaster has 4 pairs of chromosomes.
Answer: True
Glossary
| Term | Definition |
|---|---|
| Allele | An alternative form of a gene located at the same locus on homologous chromosomes. |
| Genotype | The genetic constitution of an organism for a particular trait. |
| Phenotype | The observable physical characteristic resulting from gene expression. |
| Homozygous | An individual having two identical alleles for a gene (e.g., TT or tt). |
| Heterozygous | An individual having two different alleles for a gene (e.g., Tt). |
| Dominant | An allele that masks the expression of another allele in heterozygous condition. |
| Recessive | An allele whose expression is masked by a dominant allele. |
| Monohybrid cross | A genetic cross involving a single pair of contrasting traits. |
| Dihybrid cross | A genetic cross involving two pairs of contrasting traits. |
| Test cross | A cross between an individual of unknown genotype and a homozygous recessive. |
| Incomplete dominance | Inheritance where heterozygote shows an intermediate phenotype. |
| Codominance | Inheritance where both alleles express themselves fully in the heterozygote. |
| Multiple alleles | Existence of more than two alleles of a gene at the same locus. |
| Pleiotropy | A single gene controlling multiple phenotypic traits. |
| Polygenic inheritance | Inheritance of a trait controlled by multiple genes. |
| Linkage | Tendency of genes located close on the same chromosome to be inherited together. |
| Recombination | The formation of new combinations of alleles due to crossing over during meiosis. |
| Mutation | A sudden, heritable change in the DNA sequence. |
| Aneuploidy | Loss or gain of one or more chromosomes from the normal set. |
| Polyploidy | Increase in the number of complete sets of chromosomes. |
| Down’s syndrome | Chromosomal disorder caused by trisomy of chromosome 21. |
| Klinefelter’s syndrome | Chromosomal disorder in males with genotype 44 + XXY. |
| Turner’s syndrome | Chromosomal disorder in females with genotype 44 + XO. |
| Haemophilia | An X-linked recessive disorder of blood clotting. |
| Sickle-cell anaemia | An autosomal recessive disorder caused by mutation in the beta-globin gene. |
| Phenylketonuria (PKU) | An autosomal recessive disorder due to lack of phenylalanine hydroxylase enzyme. |
| Thalassaemia | An autosomal recessive disorder of haemoglobin synthesis. |
| Pedigree analysis | The study of inheritance pattern of a trait across generations of a family. |