Principles of Inheritance Notes PDF for NEET Class 12 Biology

→ Introduction: Unlocking the Code of Life

Hello future doctors! Have you ever wondered why an elephant always gives birth to a baby elephant and never a horse? Or why mango seeds only grow into mango trees? Or more personally, why do you share your mother's eye color but your father's nose, yet you look completely distinct from both of them? The scientific answers to all these fascinating questions form the very foundation of Genetics.

Genetics is the branch of biology that deals with two fundamental pillars:

• 1. Inheritance (Heredity): This is the process by which characters are passed on from parent to progeny. It is the solid basis of heredity.
• 2. Variation: This is the degree by which progeny differ from their parents. Nature loves variation because it is the raw material for evolution!

Humans knew as early as 8000-1000 B.C. that one of the causes of variation was hidden in sexual reproduction. They used artificial selection and domestication from ancestral wild populations to create improved breeds. A classic example mentioned in NCERT is the Sahiwal cows in Punjab, developed from wild cows. However, our ancestors had no idea about the scientific basis of these phenomena.

→ Gregor Johann Mendel: The Father of Genetics

The systematic study of inheritance began with Gregor Johann Mendel, an Austrian monk. He conducted extensive hybridization experiments on garden peas (Pisum sativum) for seven long years (1856-1863) and subsequently proposed the laws of inheritance.

Mendel's Experimental Material: The Mighty Pea

Mendel selected 14 true-breeding pea plant varieties as pairs. A true-breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.

The 7 Contrasting Characters (Traits) studied by Mendel:
1. Stem height: Tall / Dwarf
2. Flower color: Violet / White
3. Flower position: Axial / Terminal
4. Pod shape: Inflated / Constricted
5. Pod color: Green / Yellow
6. Seed shape: Round / Wrinkled
7. Seed color: Yellow / Green

The Technique: Emasculation and Bagging

To perform his crosses, Mendel had to ensure no unwanted pollen fertilized his plants. He opened the flower buds of the female parent and removed the anthers before they matured. This is called Emasculation. He then covered the emasculated flowers with a bag (usually made of butter paper) to prevent contamination by unwanted pollen. This is called Bagging. Once the stigma became receptive, he dusted the desired pollen on it.

→ Inheritance of One Gene (Monohybrid Cross)

Let us examine one of Mendel's classic experiments where he crossed a pure tall pea plant with a pure dwarf pea plant. He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the Filial 1 progeny or the F1 generation.

Mendel observed that ALL the F1 progeny plants were Tall! There were absolutely no dwarf plants. He made similar observations for all the other pairs of traits—the F1 always resembled either one of the parents, and the trait of the other parent was not seen in them.

Mendel then self-pollinated the tall F1 plants. To his massive surprise, in the F2 generation, some of the offspring were dwarf! The trait that was hidden in the F1 generation reappeared. The proportion of plants that were dwarf was exactly 1/4th, while 3/4th of the F2 plants were tall. The tall and dwarf traits were identical to their parental type and did not show any blending.

Genes, Alleles, and Terminology

Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes. He called these things 'Factors'. Today, we call them Genes. Genes are the fundamental units of inheritance.

Genes which code for a pair of contrasting traits are known as Alleles. They are slightly different forms of the same gene.

• Genotype: The genetic makeup of an organism (e.g., TT, Tt, tt).
• Phenotype: The physical expression or observable characteristic (e.g., Tall, Dwarf).
• Homozygous: An individual possessing identical alleles for a trait (TT or tt).
• Heterozygous: An individual possessing different alleles for a trait (Tt).

[Image of a monohybrid cross Punnett square]

The Punnett Square and Mathematical Binomial

The Punnett Square was developed by British geneticist Reginald C. Punnett. It is a graphical representation used to calculate the probability of all possible genotypes of offspring in a genetic cross.

For a monohybrid cross (Tt x Tt), the results mathematically follow the binomial expression (ax + by)², where 'a' and 'b' are the frequencies of the gametes.
Expansion: (1/2 T + 1/2 t)² = 1/4 TT + 1/2 Tt + 1/4 tt.
Phenotypic Ratio: 3:1 (3 Tall : 1 Dwarf)
Genotypic Ratio: 1:2:1 (1 TT : 2 Tt : 1 tt)

→ Mendel's Postulates (The First Two Laws)

Based on his observations on monohybrid crosses, Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses.

1. Law of Dominance

(i) Characters are controlled by discrete units called factors.
(ii) Factors occur in pairs.
(iii) In a dissimilar pair of factors (heterozygous state), one member of the pair dominates (dominant) the other (recessive).
This law is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2.

2. Law of Segregation

This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation, though one of these is not seen at the F1 stage.
During the formation of gametes, the factors or alleles of a pair segregate (separate) from each other such that a gamete receives only one of the two factors. A homozygous parent produces all similar gametes, while a heterozygous parent produces two kinds of gametes in equal proportions.
Note: This law is universally accepted without any exceptions!

→ Post-Mendelian Discoveries: Exceptions to the Rules

When experiments were repeated on other plants, scientists found that F1 phenotypes did not always perfectly match either parent. Mendel's Law of Dominance wasn't universal.

Incomplete Dominance

In incomplete dominance, the F1 phenotype does not resemble either of the two parents and is in between the two. The best examples are flower color in dog flower (Snapdragon or Antirrhinum majus) and the 4 O'clock plant (Mirabilis jalapa).

When a true-breeding Red-flowered plant (RR) is crossed with a true-breeding White-flowered plant (rr), the F1 generation (Rr) produces Pink flowers!
When F1 is self-pollinated, the F2 generation shows:
1 Red (RR) : 2 Pink (Rr) : 1 White (rr).
NEET Key Point: In Incomplete Dominance, the Phenotypic ratio (1:2:1) is exactly the same as the Genotypic ratio (1:2:1).

Codominance & Multiple Alleles

In codominance, the F1 generation resembles BOTH parents side-by-side. A classic example is the ABO blood grouping in human beings. ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface, and the kind of sugar is controlled by this gene.

The gene I has three alleles: I^A, I^B, and i.
Alleles I^A and I^B produce a slightly different form of the sugar, while allele i does not produce any sugar. Because humans are diploid, each person possesses any two of the three I gene alleles.

• I^A and I^B are completely dominant over i.
• But when I^A and I^B are present together, they both express their own types of sugars! This is because of Codominance. Red blood cells have both A and B types of sugars, resulting in blood group AB.

Since there are three different alleles, there are six different genotypes possible, resulting in four different phenotypes (A, B, AB, and O). Because more than two alleles govern the same character, ABO blood grouping is also a perfect example of Multiple Alleles. (Note: Multiple alleles can only be studied in a population, not in a single individual).

→ Inheritance of Two Genes (Dihybrid Cross)

Mendel crossed pea plants that differed in two distinct characters: a plant with seeds that were yellow in color and round in shape, crossed with a plant that had green color and wrinkled shape.

The F1 plants all had Round and Yellow seeds. This showed that round shape is dominant over wrinkled, and yellow color is dominant over green color.
When he self-pollinated these F1 plants (RrYy), he found that in the F2 generation, the phenotypes appeared in a beautiful mathematical ratio of 9:3:3:1.
(9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green).

3. Law of Independent Assortment

Based on his dihybrid crosses, Mendel proposed his second set of generalizations: The Law of Independent Assortment. It states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters during gamete formation.

→ Polygenic Inheritance and Pleiotropy

Nature is complex. Sometimes traits aren't controlled by just one gene, and sometimes one gene doesn't control just one trait.

→ Chromosomal Theory of Inheritance

Mendel published his work in 1865, but it remained entirely unrecognized till 1900. Why?
1. Communication was poor.
2. His concept of stable, unblending "factors" wasn't accepted by contemporaries who believed in continuous variation.
3. His mathematical approach to biology was considered bizarre.
4. He couldn't provide physical proof for the existence of these factors.

In 1900, three brilliant scientists—de Vries, Correns, and von Tschermak—independently rediscovered Mendel's results. By this time, microscopy had advanced, and scientists could clearly observe cell division and chromosomes (colored bodies).

In 1902, Walter Sutton and Theodor Boveri noted that the behavior of chromosomes was strictly parallel to the behavior of genes. They used chromosome movement during meiosis to explain Mendel's laws. They united the knowledge of chromosomal segregation with Mendelian principles and called it the Chromosomal Theory of Inheritance.

→ Linkage and Recombination (T.H. Morgan)

Experimental verification of the chromosomal theory of inheritance was done by Thomas Hunt Morgan and his colleagues. They worked with the tiny fruit flies, Drosophila melanogaster.

Why did Morgan choose Drosophila?

• They could be grown on simple synthetic medium in the laboratory.
• They complete their life cycle in just about two weeks.
• A single mating could produce a large number of progeny flies.
• There was a clear differentiation of the sexes (males are smaller than females).
• It has many types of hereditary variations that can be seen with low power microscopes.

Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. He observed that when two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type. This proved that Mendel's Law of Independent Assortment fails when genes are located close to each other on the exact same chromosome!

Morgan coined the term Linkage to describe this physical association of genes on a chromosome and the term Recombination to describe the generation of non-parental gene combinations.

NEET Data Point: Morgan found that genes for yellow body and white eyes were very tightly linked and showed only 1.3% recombination. Meanwhile, genes for white eyes and miniature wing showed 37.2% recombination. His student, Alfred Sturtevant, famously used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome. Today, genetic maps are extensively used as a starting point in the sequencing of whole genomes!

→ Sex Determination Mechanisms

The mechanism of sex determination has always been a puzzle. The initial clue came from insects. A scientist named Henking (1891) traced a specific nuclear structure all through spermatogenesis in a few insects. He observed that 50% of the sperm received this structure, whereas the other 50% did not. Henking named this structure the X body, but he could not explain its significance. Later, scientists proved that the 'X body' was actually a chromosome, and hence it was named the X-chromosome.

• XX-XO Type (Male Heterogamety): Seen in many insects like grasshoppers. Males have only one X chromosome (XO), while females have a pair of X chromosomes (XX). Males produce two types of sperms: 50% with an X chromosome and 50% without any X chromosome. Hence, the male sperm determines the sex of the offspring.
• XX-XY Type (Male Heterogamety): Seen in humans and Drosophila. Males have one X and one Y chromosome (XY), while females have a pair of X chromosomes (XX).
• ZZ-ZW Type (Female Heterogamety): Seen in birds. Here, the situation is completely reversed. The females have one Z and one W chromosome (ZW), while the males have a pair of Z chromosomes (ZZ). In this case, the female ovum determines the sex of the chick!

→ Mutation: The Source of New Alleles

Mutation is a phenomenon which results in the alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. Besides recombination, mutation is another phenomenon that leads to massive variation in DNA.

• Point Mutation: A mutation arising due to a change in a single base pair of DNA. Example: Sickle-cell anemia.
• Frameshift Mutation: Deletion or insertion of base pairs of DNA alters the entire reading frame from the point of insertion/deletion.
• Chemical and physical factors that induce mutations are referred to as mutagens. UV radiation is a powerful physical mutagen.

→ Genetic Disorders

1. Pedigree Analysis

Since we cannot perform controlled crosses in humans (like Mendel did with peas), the study of family history about inheritance of a particular trait provides an alternative. The analysis of traits in several generations of a family is called the pedigree analysis. It helps in tracing the inheritance of a specific trait, abnormality, or disease.

2. Mendelian Disorders

These are mainly determined by the alteration or mutation in a single gene. They follow Mendel's principles of inheritance.

• Haemophilia: A sex-linked recessive disease. A single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. In an affected individual, a simple cut will result in non-stop bleeding. The heterozygous female (carrier) transmits the disease to sons. The possibility of a female suffering from haemophilia is extremely rare because the mother has to be at least a carrier and the father should be haemophilic. Queen Victoria was a famous carrier of this disease.
• Sickle-cell Anaemia: An autosome-linked recessive trait. The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta-globin chain of the haemoglobin molecule. This happens due to a single base substitution at the sixth codon of the beta-globin gene from GAG to GUG. The mutant haemoglobin molecule undergoes polymerization under low oxygen tension, causing the change in the shape of the RBC from a biconcave disc to an elongated sickle-like structure.
• Phenylketonuria (PKU): Inborn error of metabolism. Autosomal recessive trait. The affected individual lacks an enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine into tyrosine. Phenylalanine accumulates and converts into phenylpyruvic acid and other derivatives, leading to severe mental retardation.
• Thalassemia: Autosomal recessive blood disease. Unlike sickle-cell anemia (which is a qualitative problem of synthesizing an incorrectly functioning globin), Thalassemia is a quantitative problem of synthesizing too few globin molecules. Alpha Thalassemia is controlled by two closely linked genes HBA1 and HBA2 on chromosome 16. Beta Thalassemia is controlled by a single gene HBB on chromosome 11.
• Colour Blindness: A sex-linked recessive disorder due to defect in either red or green cone of the eye, resulting in failure to discriminate between red and green color. This defect is due to a mutation in genes present on the X chromosome. It occurs in about 8% of males and only about 0.4% of females.

3. Chromosomal Disorders

These are caused due to the absence, excess, or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called Aneuploidy. Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes, known as Polyploidy (often seen in plants).

→ Final Wrap Up for Genetics Excellence

Genetics is fundamentally a game of probabilities, logic, and distinct patterns. To truly master this chapter for the NEET examination, make sure you practice Punnett squares thoroughly, understand the exact chromosomal difference between Klinefelter's and Turner's, and memorize the specific nucleotide mutation for Sickle Cell Anemia. Keep revising, practice those pedigree charts, and remember—your genetic potential to succeed is absolutely limitless!