Principles of Inheritance and Variation — Study Notes

1. Introduction

Genetics is the branch of biology that deals with heredity and variation. Heredity is the transmission of traits from parents to offspring, while variation is the degree by which offspring differ from their parents and from each other. The science of genetics began with the work of Gregor Johann Mendel, an Augustinian monk who conducted experiments on garden pea (Pisum sativum) in a monastery garden in the mid-nineteenth century. Mendel's experiments, published in 1865, laid the foundation of modern genetics and earned him the title of the Father of Genetics.

Mendel chose the garden pea because it has contrasting, easily distinguishable traits, is self-pollinating with a short generation time, produces a large number of seeds, and can be cross-pollinated artificially. He followed a scientific approach with statistical analysis of his results, counting the progeny in each generation. Although Mendel's work was not appreciated during his lifetime, it was rediscovered independently in 1900 by Hugo de Vries, Carl Correns and Erich von Tschermak. This chapter covers Mendel's laws, deviations from them, chromosomal theory of inheritance, sex determination, mutations, genetic disorders and the modern understanding of heredity.

2. Mendel's Experiments and the Laws of Inheritance

Mendel conducted artificial hybridisation experiments on pea plants, crossing plants with contrasting traits. He selected seven pairs of contrasting traits, such as tall and dwarf plants, round and wrinkled seeds, yellow and green seeds, violet and white flowers, etc. He first produced pure lines by self-pollination, then performed monohybrid and dihybrid crosses.

2.1 Monohybrid Cross and Law of Dominance

In a monohybrid cross, parents differing in only one trait are crossed. For example, a pure tall plant (TT) crossed with a pure dwarf plant (tt) produces all tall plants (Tt) in the F1 generation. The trait that appears in the F1 generation is called the dominant trait (tallness), and the one that does not appear is the recessive trait (dwarfness). This led to the first law, the law of dominance: when two contrasting factors are brought together in a hybrid, one factor (dominant) expresses itself, while the other (recessive) is masked.

2.2 Law of Segregation

When the F1 tall plants (Tt) are self-pollinated, the F2 generation shows tall and dwarf plants in a ratio of 3:1. This shows that the alleles segregate (separate) during gamete formation, so each gamete carries only one allele for each trait. This is the law of segregation: alleles of a pair do not blend but separate into different gametes during gametogenesis.

2.3 Dihybrid Cross and Law of Independent Assortment

In a dihybrid cross, parents differing in two traits are crossed. For example, a plant with round yellow seeds (RRYY) crossed with a plant with wrinkled green seeds (rryy) produces all round yellow (RrYy) plants in the F1 generation. The F2 generation shows four phenotypes in the ratio 9:3:3:1 (round yellow, round green, wrinkled yellow, wrinkled green). This led to the law of independent assortment: when two pairs of traits are combined in a hybrid, the factors of each pair segregate independently of the factors of the other pair, so all possible combinations of factors can occur in the gametes.

2.4 The Punnett Square and Back Cross

A Punnett square is a graphical representation used to calculate the possible genotypes and phenotypes of offspring in a cross. A test cross is crossing an organism of unknown genotype with a homozygous recessive individual to determine its genotype. A back cross is crossing the F1 hybrid with either of its parents.

3. Chromosomal Theory of Inheritance and Post-Mendelian Concepts

The chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Boveri in 1902, stating that genes are located on chromosomes and that the behaviour of chromosomes during meiosis parallels the behaviour of genes. However, later discoveries showed that not all traits follow Mendel's laws strictly, leading to the concept of deviations from Mendelian inheritance.

3.1 Incomplete Dominance

In incomplete dominance, the F1 hybrid shows a phenotype that is intermediate between the two parents because neither allele is completely dominant. For example, in Mirabilis jalapa (four o'clock plant), a cross between red (RR) and white (rr) flowers produces pink (Rr) flowers in the F1 generation, and the F2 shows red, pink and white in the ratio 1:2:1. This is because the allele R is not completely dominant over r.

3.2 Co-dominance

In co-dominance, both alleles express themselves fully in the heterozygote, so both phenotypes appear simultaneously. The best example is the ABO blood group system in humans. The ABO blood groups are controlled by the gene I, which has three alleles: IA, IB and i. IA and IB are co-dominant, while i is recessive. Thus, an individual with genotype IAIB has blood group AB, where both A and B antigens are expressed. The possible blood groups are A (IAIA or IAi), B (IBIB or IBi), AB (IAIB) and O (ii).

3.3 Multiple Alleles

When a gene has more than two allelic forms, they are called multiple alleles. The ABO blood group gene I is the classic example, with three alleles (IA, IB and i). The genotype of a person for blood group is determined by the combination of any two of these alleles.

3.4 Pleiotropy

When a single gene controls more than one trait, it is called pleiotropy. For example, in pea plants the gene for flower colour also affects the colour of the seed coat and the presence of red spots in leaf axils. Sickle cell anaemia is a pleiotropic condition where the mutant gene affects multiple organs.

3.5 Polygenic Inheritance

When a trait is controlled by more than one pair of genes (polygenes), each having a small additive effect, it is called polygenic inheritance. Human skin colour and height are examples. The phenotypic distribution is continuous and shows a bell-shaped curve, and the F2 ratio in a two-gene model is 1:4:6:4:1.

4. Sex Determination

The mechanism by which the sex of an individual is determined is called sex determination. In humans and most other animals, sex is determined genetically at the time of fertilisation.

4.1 In Humans

Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). During oogenesis, all ova carry one X chromosome. During spermatogenesis, half the sperms carry an X chromosome and half carry a Y chromosome. If a sperm carrying X fertilises the ovum, the zygote is XX (female); if a sperm carrying Y fertilises the ovum, the zygote is XY (male). Thus the sex of the child depends on the father, and there is a 50:50 chance of a male or female child.

4.2 In Birds, Insects and Other Organisms

In birds and some insects (like butterflies), the female is heterogametic (ZW) and the male is homogametic (ZZ); this is called the ZZ-ZW system. In grasshoppers, males have one X chromosome (XO) and females have two (XX); this is the XO type. In some insects like Drosophila, sex is determined by the X:A ratio.

5. Mutation

A mutation is a sudden, stable and heritable change in the genetic material. Mutations occur in nature and can also be induced artificially by mutagens such as X-rays, ultraviolet rays and certain chemicals. A point mutation is a change in a single base pair of DNA. The most common example is sickle cell anaemia, which results from a single base pair change in the beta-globin gene, replacing glutamic acid with valine at the sixth position of the beta chain of haemoglobin.

5.1 Types of Mutations

• Gene mutations (point mutations): changes in a single gene.
• Chromosomal mutations: changes in the structure or number of chromosomes, e.g., deletion, duplication, inversion, translocation.
• Mutations can be in germ cells (heritable) or somatic cells (not inherited).

Mutations are the raw material for evolution, but most mutations are harmful. Harmful mutations are selected against in nature, while beneficial mutations are preserved by natural selection.

6. Genetic Disorders

Genetic disorders are diseases caused by abnormalities in the genetic material. They can be of two types: Mendelian disorders (caused by alteration or mutation in a single gene) and chromosomal disorders (caused by the absence or excess of one or more chromosomes or abnormal chromosomal arrangement).

6.1 Mendelian Disorders

• Haemophilia: A sex-linked (X-linked) recessive disorder in which the blood does not clot, caused by the absence of a clotting factor. It is transmitted from carrier mothers to sons.
• Colour blindness: An X-linked recessive disorder in which the affected person cannot distinguish between red and green colours.
• Sickle cell anaemia: An autosomal recessive disorder in which the red blood cells become sickle-shaped due to a single base substitution in the beta-globin gene.
• Thalassaemia: An autosomal recessive blood disorder in which the production of globin chains of haemoglobin is impaired.

Mendelian disorders are transmitted from parents to offspring following Mendel's laws, and their pattern of inheritance can be studied with a pedigree analysis.

6.2 Chromosomal Disorders

• Down's syndrome: Caused by trisomy of chromosome 21. Affected individuals have an extra chromosome 21 (2n + 1 = 47). They show mental retardation, a flat face with a small nose, and often develop congenital heart disease and low immunity.
• Turner's syndrome: Caused by the absence of one X chromosome (45 with XO). Affected females are sterile, with short stature, rudimentary ovaries and lack of secondary sexual characters.
• Klinefelter's syndrome: Caused by an extra X chromosome in males (47 with XXY). Affected males are sterile, with low sperm count, and show gynaecomastia (development of breasts) and some female features.

Quick Revision Tables

Table 1: Mendel's Laws of Inheritance

Table 2: Post-Mendelian Inheritance Patterns

Table 3: Genetic Disorders

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students think the F1 in a monohybrid cross shows a 3:1 ratio; the 3:1 ratio appears only in the F2 generation, while F1 is uniformly dominant.
2. The law of segregation is confused with independent assortment; segregation applies to alleles of a single gene pair, while independent assortment applies to alleles of different gene pairs.
3. Incomplete dominance is said to give a 3:1 ratio; it gives a 1:2:1 phenotypic ratio in F2 (red, pink, white).
4. Co-dominance is confused with incomplete dominance; in co-dominance both alleles express fully (AB blood group), while incomplete dominance shows an intermediate phenotype.
5. The ABO blood group is said to be controlled by two alleles; it is controlled by three alleles (IA, IB and i), making it an example of multiple alleles.
6. Sickle cell anaemia is said to be caused by a deletion of a base; it is caused by a single base substitution in the beta-globin gene, replacing glutamic acid with valine.
7. Down's syndrome is said to result from an XO condition; it results from trisomy of chromosome 21, giving 47 chromosomes.
8. Sex of a child is said to depend on the mother; the sex of the child depends on the father, because the sperm determines whether the zygote is XX (female) or XY (male).

Exam Tips

1. Memorise Mendel's three laws with their ratios: dominance (F1 uniform), segregation (3:1), independent assortment (9:3:3:1).
2. Learn the seven pairs of contrasting traits Mendel studied and why pea was chosen (self-pollination, short generation time, many seeds).
3. Distinguish incomplete dominance (1:2:1 F2) from co-dominance (both alleles express), and remember Mirabilis jalapa and ABO blood groups.
4. The ABO gene with its three alleles and the six possible genotypes is a high-frequency question.
5. For chromosomal disorders, remember the chromosome count: Down's (47, trisomy 21), Turner's (45, XO), Klinefelter's (47, XXY).
6. Remember the pedigree analysis method for tracing Mendelian disorders, and that haemophilia and colour blindness are X-linked recessive.
7. Mutation leads to sickle cell anaemia through a point mutation; note that most mutations are harmful but are the raw material of evolution.

Conclusion

The principles of inheritance and variation, beginning with Mendel's pioneering experiments on the garden pea, form the bedrock of modern genetics. Mendel's laws of dominance, segregation and independent assortment explain the transmission of traits in simple Mendelian fashion, while post-Mendelian concepts such as incomplete dominance, co-dominance, multiple alleles, pleiotropy and polygenic inheritance reveal the rich complexity of real inheritance. The chromosomal theory of inheritance links genes to chromosomes, and the study of sex determination, mutations and genetic disorders completes the picture. Understanding these principles is essential for the next chapter, which explores the molecular basis of inheritance, where the physical and chemical nature of the gene itself is revealed, connecting the classical genetics of Mendel to the molecular genetics of DNA.