Principles of Inheritance and Variation: Genetics, Mendelian Laws, Crosses and Genetic Disorders for NEET Preparation

The Principles of Inheritance and Variation explain how genetic information is transmitted from parents to offspring and why individuals of the same species can show different characteristics. Inheritance deals with the transmission of genetic characters, while variation refers to the differences observed among individuals.

This topic begins with Gregor Mendel’s experiments on pea plants and the basic laws of inheritance. It then connects these principles with genetic crosses, incomplete dominance, codominance, multiple allelism, polygenic inheritance, pleiotropy, chromosomes, linkage, recombination, mutations, sex determination, pedigree analysis, and genetic disorders.

For your preparation, understanding the relationship between genes, alleles, chromosomes, and inheritance patterns is important. Mendelian crosses provide the foundation, while later discoveries explain inheritance patterns that cannot be described by simple dominant and recessive relationships.

Basic Concepts in Genetics

Before studying Mendelian inheritance, you should understand some basic genetic terms.

• Inheritance: The transmission of genetic information or characters from parents to offspring.

• Variation: Differences in characteristics among individuals of the same species or between parents and their offspring.

• Heredity: The transmission of characters from one generation to the next.

• Homologous chromosomes: A pair of chromosomes in which one chromosome is inherited from each parent. They carry genes for the same characters at corresponding loci.

• Character: A general feature or observable property, such as plant height.

• Trait: A specific form of a character, such as tall or dwarf plant height.

• Gene: A unit of heredity that influences a particular character.

• Allele: An alternative form of a gene.

• Dominant allele: An allele that expresses its effect in a heterozygous condition.

• Recessive allele: An allele whose effect is expressed when present in the homozygous recessive condition.

• Homozygous: A condition in which an individual has two identical alleles, such as TT or tt.

• Heterozygous: A condition in which an individual has two different alleles, such as Tt.

• Genotype: The genetic constitution of an individual.

• Phenotype: The observable characteristics of an individual.

During gamete formation, the two alleles of a gene separate. As a result, each gamete receives only one allele. During fertilisation, the allelic pair is restored.

Mendel's Hybridisation Experiments

Gregor Johann Mendel conducted his famous experiments on pea plants between 1856 and 1863. His work established the basic principles of inheritance and earned him the title Father of Genetics.

Mendel selected pea plants because they had several clear contrasting characters, a relatively short generation time, and could be self-pollinated as well as cross-pollinated under controlled conditions.

He studied seven pairs of contrasting characters:

These experiments helped Mendel understand how hereditary factors are passed from one generation to another.

Monohybrid Cross

A monohybrid cross studies the inheritance of one pair of contrasting characters.

Mendel studied plant height using tall and dwarf pea plants. A pure tall plant can be represented as TT, while a pure dwarf plant can be represented as tt.

Parental Cross

TT × tt

The F1 generation contains:

All Tt — Tall

When F1 plants are self-pollinated:

Tt × Tt

The F2 generation contains the following genotypes:

1 TT : 2 Tt : 1 tt

The corresponding phenotypic ratio is:

3 Tall : 1 Dwarf

The monohybrid cross shows that alleles do not permanently blend with each other. Instead, they remain distinct and separate during gamete formation.

Test Cross

A test cross is used to determine the genotype of an individual showing a dominant phenotype.

The individual with the unknown genotype is crossed with a homozygous recessive individual.

For example:

TT × tt → All Tt

If the dominant individual is heterozygous:

Tt × tt → 1 Tt : 1 tt

Therefore, a 1:1 phenotypic ratio in a test cross indicates that the individual showing the dominant phenotype is heterozygous.

Mendel's Laws of Inheritance

Mendel proposed three fundamental laws to explain the inheritance of characters.

1. Law of Dominance

The Law of Dominance states that when two contrasting alleles are present together in a heterozygous condition, one allele expresses itself and is called dominant, while the other is recessive.

For example:

Tt → Tall

Here, T is dominant over t.

2. Law of Segregation

The Law of Segregation states that the two alleles of a gene separate from each other during gamete formation. Therefore, each gamete receives only one allele.

This law is also called the Law of Purity of Gametes.

For example, a Tt plant produces two types of gametes:

T and t

The two alleles come together again during fertilisation.

3. Law of Independent Assortment

The Law of Independent Assortment states that alleles of different genes assort independently during gamete formation when the genes are independently inherited.

This principle is demonstrated by Mendel’s dihybrid cross.

Dihybrid Cross and 9:3:3:1 Ratio

A dihybrid cross studies the inheritance of two pairs of contrasting characters simultaneously.

Mendel studied seed shape and seed colour in pea plants. The two characters were:

• Seed shape: Round and Wrinkled

• Seed colour: Yellow and Green

A typical Mendelian dihybrid cross produces the following phenotypic ratio in the F2 generation:

9 : 3 : 3 : 1

The four phenotypic combinations represent different combinations of the two characters.

The dihybrid cross provided evidence for the Law of Independent Assortment.

Exceptions to Mendelian Inheritance

Not all inheritance patterns follow a simple dominant-recessive relationship. Several genetic phenomena produce different patterns of inheritance.

Incomplete Dominance

In incomplete dominance, neither allele completely dominates the other. As a result, the heterozygous individual shows an intermediate phenotype.

For example:

Red × White → Pink

The F2 generation generally shows:

1 Red : 2 Pink : 1 White

Unlike complete dominance, the heterozygote has a phenotype different from both homozygous conditions.

Codominance

In codominance, both alleles are expressed fully in the heterozygous condition.

The ABO blood group system provides an example. The gene has three alleles:

Iᴬ, Iᴮ and i

Iᴬ and Iᴮ are codominant, while both are dominant over i.

Therefore:

IᴬIᴮ → AB blood group

Both Iᴬ and Iᴮ are expressed in an individual with the AB blood group.

Multiple Allelism

Multiple allelism occurs when a gene has more than two alternative alleles in a population.

The ABO blood group system is an example because the gene has three alleles:

Iᴬ, Iᴮ and i

Although a population can have multiple alleles, an individual still carries only two alleles for a particular gene.

Polygenic Inheritance

In polygenic inheritance, a character is controlled by multiple genes. Such inheritance can produce a range of phenotypes rather than only a few distinct categories.

Examples include human skin colour and height.

Pleiotropy

Pleiotropy occurs when a single gene influences more than one phenotypic character.

Phenylketonuria (PKU) is an example associated with pleiotropic effects.

Chromosomal Theory of Inheritance

The Chromosomal Theory of Inheritance connected Mendel’s hereditary factors with chromosomes.

Walter Sutton and Theodor Boveri independently proposed the idea that chromosomes play a major role in heredity.

The theory states that:

• Genes are located on chromosomes.

• Chromosomes occur in homologous pairs in diploid organisms.

• Homologous chromosomes separate during gamete formation.

• Chromosome pairs can assort independently during meiosis.

The behaviour of chromosomes during meiosis provides a cellular basis for Mendel’s laws.

T.H. Morgan and Linkage

Thomas Hunt Morgan used Drosophila melanogaster, commonly known as the fruit fly, to provide experimental evidence supporting the chromosomal basis of inheritance.

Drosophila was useful for genetic experiments because it has a short life cycle and produces many offspring.

What Is Linkage?

Linkage refers to the tendency of genes located on the same chromosome to be inherited together.

Genes that are located close to each other on a chromosome generally show stronger linkage and have a lower chance of being separated by crossing over.

Genes that are farther apart generally show a higher recombination frequency.

Recombination

Recombination produces new combinations of genes. It commonly occurs through crossing over between homologous chromosomes during meiosis.

Morgan’s experiments showed different recombination frequencies for different gene pairs. For example:

• Eye colour and body colour: 1.3% recombination

• Eye colour and wing size: 37.2% recombination

These differences showed that genes have specific positions on chromosomes.

Alfred Sturtevant used recombination frequencies to construct genetic maps and estimate the relative positions of genes on chromosomes.

Sex Determination

Different organisms use different mechanisms for determining sex.

1. XX-XO Type

In this system:

• Female: XX

• Male: XO

This mechanism occurs in some insects, including grasshoppers.

2. XX-XY Type

In this system:

• Female: XX

• Male: XY

Humans and Drosophila follow the XX-XY type of sex determination, although the mechanism differs between the two organisms.

3. ZW-ZZ Type

In this system:

• Female: ZW

• Male: ZZ

This mechanism occurs in birds.

Haplodiploid Sex Determination

In honey bees, females are diploid and develop from fertilised eggs, while males are haploid and develop from unfertilised eggs.

In the commonly studied honey bee chromosome number:

• Female: 32 chromosomes

• Male: 16 chromosomes

Thus, sex is associated with whether the egg is fertilised.

Mutation and Mutagens

A mutation is a change in genetic material. Mutations can contribute to genetic variation and may occur spontaneously or due to exposure to mutagenic agents.

Point Mutation

A point mutation involves a change affecting a single nucleotide or base pair in DNA.

Sickle cell anaemia is an example of a genetic disorder associated with a point mutation in the gene coding for the beta-globin chain.

Frameshift Mutation

A frameshift mutation occurs when nucleotides are inserted or deleted in a number that is not a multiple of three. This changes the reading frame of the genetic message and can alter the resulting protein.

Mutagens

Mutagens are physical or chemical agents that can increase the frequency of mutations.

Examples include certain types of radiation and chemical substances.

Pedigree Analysis

Pedigree analysis is used to trace the inheritance of a character or genetic disorder through several generations of a family.

Common pedigree symbols include:

• Square: Male

• Circle: Female

• Horizontal line: Mating

• Vertical line: Offspring

• Shaded symbol: Affected individual

• Unshaded symbol: Unaffected individual

• Double horizontal line: Consanguineous mating

• Diamond: Sex unspecified

Pedigree analysis can help determine whether a trait follows an autosomal or sex-linked pattern and whether it may be dominant or recessive.

Genetic Disorders

Genetic disorders can result from changes in individual genes or abnormalities in chromosome number or structure.

1. Mendelian Disorders

Mendelian disorders result from alterations in individual genes and may show autosomal or sex-linked inheritance patterns.

Colour Blindness

Colour blindness is commonly inherited as an X-linked recessive disorder. It affects the ability to distinguish certain colours.

Haemophilia

Haemophilia is an X-linked recessive disorder associated with impaired blood clotting.

It is more commonly expressed in males because males have only one X chromosome.

Sickle Cell Anaemia

Sickle cell anaemia is an autosomal recessive disorder associated with a point mutation in the beta-globin gene.

The mutation leads to the production of abnormal haemoglobin and can cause red blood cells to acquire a characteristic sickle shape under certain conditions.

Phenylketonuria

Phenylketonuria is an autosomal recessive metabolic disorder associated with deficiency of the enzyme phenylalanine hydroxylase.

It is also an example of pleiotropy because the altered gene can influence multiple phenotypic characteristics.

Thalassemia

Thalassemia refers to inherited disorders involving reduced production of haemoglobin chains.

The genes associated with alpha and beta thalassemia are located on different chromosomes:

• Alpha thalassemia: Associated with genes on chromosome 16

• Beta thalassemia: Associated with the beta-globin gene on chromosome 11

2. Chromosomal Disorders

Chromosomal disorders can result from changes in chromosome number or chromosome structure.

Aneuploidy

Aneuploidy involves the gain or loss of one or more individual chromosomes.

The major types include:

• Nullisomy: 2n − 2

• Monosomy: 2n − 1

• Trisomy: 2n + 1

• Tetrasomy: 2n + 2

These changes can arise due to errors in chromosome separation during cell division.

Down Syndrome

Down syndrome is caused by trisomy of chromosome 21. Individuals generally have 47 chromosomes due to the presence of an additional chromosome 21.

Klinefelter Syndrome

Klinefelter syndrome generally occurs in males with an additional X chromosome, giving the chromosome constitution XXY.

It may be associated with features such as reduced fertility or sterility and other physical and developmental changes.

Turner Syndrome

Turner syndrome generally occurs in females with a missing X chromosome. The typical chromosome constitution is 45, XO.

It may be associated with features such as underdeveloped ovaries and infertility.

Principles of Inheritance and Variation connects Mendel’s experiments with the modern understanding of genes, chromosomes and genetic variation. Monohybrid and dihybrid crosses explain basic inheritance patterns, while incomplete dominance, codominance, polygenic inheritance, linkage and recombination show why inheritance can be more complex. Mutations, sex determination, pedigree analysis and chromosomal abnormalities further explain how genetic changes are inherited and expressed.