Principles of Inheritance and Variation CBSE Notes for Chapter 4

Inheritance is the term given to the process by which characters are passed from parents to offspring which forms the basis of heredity. Heredity is the process of passing down genetic traits from parents to offspring. The degree of difference in characters between a parent and offspring is called variation. All these are studied under genetics which deals with the study of inheritance and variation of characters passed from parents to offspring. The first proper documented study of this inheritance and variation was done by G.J. Mendel which is why he is considered the father of genetics.

In a living cell, the chromosomes of a particular gene are present in a diploid manner and each one is called an allele. Genotype is the genetic make-up of an organism i.e. the TT or tt. Whereas phenotype is the external morphological appearance or expression of the genotype i.e. TT phenotype will be tall and for tt phenotype will be dwarf.  

Mendel’s Laws of Inheritance

Mendel used garden peas (Pisum sativum) to perform his hybridization experiment to study genetics for seven years i.e. from 1856 to 1863 which led him to propose the laws of inheritance. He first prepared self-pollination of the pea lines to obtain true breeds of particular characters and then performed cross-pollination/artificial pollination of those pea lines having contrasting characters. He selected seven pairs of contrasting characters i.e. 14-true breeding pea plant varieties. The contrasting pairs of characters are listed below;

Inheritance of One Gene

Mendel crossed tall and dwarf pea plants collected their seeds, and grew them to produce the first hybrid generation called the first filial progeny or F1 generation. Then he crosses the seeds to obtain the second filial progeny or F2 generation. Similarly, he crossed other contrasting characters containing plants to obtain F1 and F2 generations.   After these crossings, he concluded that;

• F1 generations always resemble one of their parent i.e. the dominant one.
• F2 generation has offspring that represents both dominant and recessive parents i.e. 3/4th of the offspring resembles the dominant parent whereas 1/4th offspring resembles the recessive parent. 
• The typical genotypic ratio after the F1 generation is usually 1:2:1 and the phenotypic ratio is 3:1.
• All the offspring in both generations resemble their parent’s type and no intermediate characters or blending of characters were seen. 
• These results were obtained for all the traits. 

Monohybrid Cross

The above type of crossing that Mendel performed with only true-breeding pea line studying only one particular character is called monohybrid cross i.e. crossing of only one selected character. E.g. Homozygous tall (TT) crossed with homozygous dwarf (tt). 

• Genotypic ratio: 1 TT: 2Tt: 1tt = 1:2:1
• Phenotypic ratio: 3 Tall: 1 Dwarf = 3:1

Based on this monohybrid cross he proposed two out of three laws from laws of inheritance. The two laws were;

1. Law of dominance: It states that among two contrasting characters the dominant one will prevail in the offspring. It explains why only one character was expressed in the F1 generation and a 3:1 proportion of character was obtained in the F2 generation.
2. Law of segregation: It states that during gamete formation only one allele from the pair of chromosomes will get transferred from each parent. This law is also known as the “Law of Purity of Gametes”.

The law of segregation is universal whereas there are certain exceptions to the law of dominance. These exceptions are explained by the law of incomplete dominance as dicussed below:

Law of Incomplete Dominance

It states that in some cases there is no dominant or recessive gene present i.e. both the parent will be expressed in the offspring but in an intermediate or mixed manner. E.g. snapdragon or dog flower (Antirrhinum sp.) where the red flower is dominant over white. When expressed as a heterozygous condition an intermediate expression occurs. 

• Genotypic ratio: 1 RR: 2 Rr: 1 rr = 1:2:1
• Phenotypic ratio: 1 Red: 2 Pink: 1 white = 1:2:1

Here, in incomplete dominance, the phenotypic ratio changes from 3:1 to 1:2:1.

Co-dominance

It states that in some cases there is no dominant gene or recessive gene, both genes act dominant and express themselves simultaneously in heterozygous conditions. E.g. ABO blood grouping in humans. It is controlled by the I-gene and has three types of alleles; I^A, I^B, i. I^A and I^B are dominant whereas i is recessive. When I^A and I^B are present together they are both expressed. 

Blood Group (Phenotype)	Genotypes
A	IAIA, IAi
B	IBIB, IBi
AB	IAIB
O	ii

Punnett Square Method

British geneticist R.C. Punnett developed a graphical representation called Punnett square to calculate the possibility of all possible genotypes of offspring in a genetic cross. E.g. the cross between TT(tall) and tt (dwarf) can be represented as; 

Back Cross and Test Cross

The back cross is the cross in which the offspring from F1 progeny is crossed with either one of its parents. 

A test cross is the crossing of the F1 progeny with its recessive parent only to determine the unknown genotype of the individual. 

Inheritance of Two Genes

Mendel also worked and crossed pea plants that differed into two characters and got a typical phenotypic ratio of  9 : 3 : 3: 1 and genotype ratio  1: 2: 2: 4: 1: 2: 1: 2: 1.

For example; let’s see the cross between homozygous round and yellow pea with homozygous wrinkled and green pea. 

Here, the Phenotypic ratio = 9:3:3:1 and the Genotypic ratio = 1:2:1:2:4:2:1:2:1

Based on the dihybrid cross Mendel proposed the third law from the three laws of inheritance;

Law of Independent Assortment

It states that “in the combination of two pairs of traits in a hybrid, segregation of one pair of characters is independent of the other pair of characters”.

Chromosomal Theory of Inheritance

Mendel’s work was not accepted at that time because due to a lack of communication between the scientific communities, the use of maths in his work was not acceptable to some biologists, lack of physical proof of his work on the genes, etc. reasons. Then after years of his death, his work was rediscovered by three scientists namely Carl Correns, Hugo De Vries, and Von Tschermrk independently. They were able to do so due to the development of a microscope so that cell division and its chromosomes were observable. 

Based on the work done by the above three scientists to rediscover Mendel’s work Walter Sutton and Theodore Boveri noted that the behavior of chromosomes was parallel to the behavior of genes and they used chromosome movement to explain Mendel’s Laws. Based on this development they proposed the Chromosomal Theory of Inheritance. The salient features of this theory were;

• Chromosomes are responsible for the transmission of heredity.
• Two identical chromosome forms a homologous pair.
• During gamete formation, this homologous pair segregates. 
• Independent pairs of homologs segregate independently from each other.
• The behavior of chromosomes is parallel to the behavior of genes because genes are located on chromosomes.

Experimental verification of this theory was done by T.H. Morgan, who worked with fruit flies (Drosophila melanogaster). He chose fruit flies because;

• They can grow easily on a simple synthetic medium with very basic nutrients.
• They have a very short life span i.e. around 2 weeks.
• A single mating produces a large number of progenies.
• There is a clear difference between males and females i.e. females are larger than males.
• Many types of hereditary variation can see n with a low-power microscope.

Linkage and Recombination

The term linkage and crossing over is introduced by TH Morgan. The physical association of parental genes in a chromosome is called linkage, such genes are called Linked genes and recombination is used to describe non-parental gene combinations. When two genes in a dihybrid cross were situated on the same chromosome, the proportions of parental gene combinations were much higher than in the non-parental type. Linked genes are exceptions to the law of independent assortment (3rd law). 

Later Morgan’s student Alfred Sturtevant used the frequency of recombination between genes on the same chromosome as a measure of distance between genes and mapped their position on chromosomes. Genetic maps are used in human genome projects (HGP).

Pleiotropy and Polygenic Inheritance

Both these phenomena are exceptions to Mendel’s laws of Inheritance. In pleiotropy, a single gene can exhibit multiple phenotypic expressions. It is the effect of a gene on metabolic pathways which contributes to different phenotypes. E.g. White-eye mutation in Drosophila results in changes in body color, and starch grain size in pea seed and seed size where a single gene control both starch grain size and seed shape.

Whereas polygenic inheritance is the condition in which a character’s expression is controlled by the number of genes. E.g. height of a human, human skin color which is controlled by three genes A, B, and C where AABBCC leads to the darkest skin color, AaBbCc leads to intermediate skin color, and aabbcc leads to the lightest skin color or albino condition. 

Sex Determination

The chromosome involved in sex determination is called the sex chromosome (Allosome). Whereas the chromosome in an organism other than the sex chromosome is called somatic chromosomes (Autosomes). Henking (1891) studied spermatogenesis in some insects in which he observed that 50% of sperm received a nuclear structure after spermatogenesis, other 50% of sperm did not receive it, he named them as ‘X-body’ is now called X-chromosome. There are various mechanisms of sex determination, these are;

• XX-XY type sex determination: Male is heterogametic (X & Y) and female is homogametic (X only). The numbers of chromosomes are the same and the male determines the sex of the offspring. If sperm containing the Y chromosome fertilizes the egg then the offspring will be a male and if sperm containing the X chromosome fertilizes the egg the offspring will be female. E.g. Human & Drosophila.
• ZZ-ZW type sex determination: The male is homogametic (with two Z) and the female is heterogametic (Z & W) i.e. the female determines the sex of the offspring. If the female egg contains a Z chromosome then the offspring will be a male and if the egg contains a W chromosome the offspring will be a female. E.g. Birds, butterflies, reptiles, some fishes, etc.
• XX-XO type sex determination: Here, the male is heterogametic, i.e. XO (produce gametes with X and gametes without X) and the female is homogametic, i.e. XX (all gametes are with X chromosomes). The male determines the sex of the offspring i.e. if the male sperm contains an X chromosome then the offspring will be female and if the male sperm lacks any sex chromosome then the offspring will be a male. E.g. Many insects such as grasshoppers, cockroaches, spiders, bedbugs, etc.
• Haplo-Diploidy type sex determination: Here females are diploid (32 chromosomes) and males are Haploids (16 chromosomes). The males (Drone) produce sperm by mitosis thus they do not have fathers and thus cannot have sons, but have grandfathers and grandsons. Seen in Honey Bees. 

Mutation

The mutation is a sudden heritable change in DNA sequences resulting in changes in the genotype (and sometimes phenotype) of an organism. It is the alteration in chromosomes that result in abnormalities or aberrations that can be easily observed in cancer cells. The substance that causes mutations are called Mutagens and these are of two types; 

1. Physical factors: X-rays, gamma rays, UV rays, etc. 
2. Chemical factors: Mustard gas, phenol, formalin, etc. 

Mutations are of two types; 

Pedigree Analysis

The analysis of a trait or traits in several generations of a family is called pedigree analysis. It is represented in the form of a hierarchical chart that is called the pedigree tree. It helps to trace the inheritance of a specific trait or abnormality or disease.

Genetic Disorders

The disorders that occur due to defect in the genetic structure of a person is called a genetic disorder. These disorders can get transmitted from one generation to another. There are mainly 2 types of genetic disorders Mendelian disorders and Chromosomal disorders.

Mendelian Disorders

It occurs due to mutation or alteration in a single gene. These are transmitted to the offspring and can be traced in a family using pedigree analysis.

1. Color blindness: It is a sex-linked recessive disorder that affects either the red or green cone of the eye failing to discriminate between red and green color. Occurs due to mutation in certain genes present on the X-Chromosome that occurs in 8% of males and only about 0.4% of females. The son of a woman who carries the gene has a 50% chance of being color blind. A daughter will not normally be color blind unless her mother is a carrier and her father is color blind. It shows the following genotypes; normal male XY, normal female XX, color-blind male X^CY, color-blind female X^CX^C, and color-blind carrier (Female) X^CX. 
2. Haemophilia: It is also called bleeder’s disease or royal disease. It is a sex-linked recessive disorder that affects a single protein that is part of a cascade of proteins involved in blood clotting is affected. The heterozygous female (carrier) for hemophilia may transmit the disease to sons. The possibility of a female becoming hemophilic is rare because the mother has to be at least a carrier and the father should be hemophilic. It shows the following genotypes; normal male XY, normal female XX, hemophilic male X^hY, hemophilic female X^hX^h, and hemophilic carrier (Female) XX^h.
3. Sickle-Cell anemia:  It is an autosome-linked recessive disorder that is controlled by single pair of alleles HB^A and Hb^S. It is transmitted from parents to the offspring when both partners are carriers of the gene. The defect is caused by the substitution of Glutamic acid (Glu) for Valine (Val) at the sixth position of the beta globin chain of the hemoglobin molecule that results in the change in the shape of the RBC from a biconcave disc to an elongated sickle-like structure. It shows the following genotypes; normal Hb^A Hb^A, carrier Hb^AHb^S, and affected Hb^SHb^S.
4. Phenylketonuria (PKU): It is an autosomal recessive disorder in which an inborn error in the metabolic pathway results in an individual lacking the enzyme phenylalanine hydroxylase which converts phenylalanine to tyrosine. As a result, phenylalanine accumulates in the brain resulting in mental retardation and skin pigmentation. It is transmitted from parents to the offspring when both parents are carriers. It shows the following genotypes; normal AA, carrier Aa, and affected aa.
5. Thalassemia: It is an autosomal recessive disorder due to mutation or deletion that results in the reduced rate of synthesis of one of the globin chains (α and β chains) that leads to the formation of abnormal Hb. Blood disease is transmitted from parents to the offspring when both partners are carriers. It shows the following genotypes; normal AA, carrier Aa, and affected aa It has two types;
   1. α-thalassemia: Controlled by two closely linked genes HBA1 and HBA2 on Chromosome 16.
   2. β-Thalassemia: Controlled by single gene HBB on Chromosome 11.

Chromosomal Disorders

They are caused due to absence or excess or abnormal arrangement of one or more chromosomes that results due to the failure of segregation of chromatids during the cell-division cycle resulting in the gain or loss of a chromosome(s), which is called Aneuploidy.

1. Down’s Syndrome: It occurs due to an additional copy of chromosome number 21 (trisomy of 21= 45+XX or 45A+XY). It shows the following symptoms; short stature, small round head, furrowed tongue, partially open mouth, broad palm with characteristic palm crease, and physical, psychomotor, and mental development is retarded.
2. Klinefelter’s Syndrome: It occurs due to the presence of an additional copy of the X-chromosome resulting in a karyotype of 47A + XXY that is a result of fertilization of an abnormal egg (Containing XX) with sperm containing the ‘Y’ chromosome. It shows the following symptoms; an individual has overall masculine development, however, the feminine development (development of breast, i.e., Gynaecomastia) also occurs making the individual sterile.
3. Turner’s syndrome: It is due to the absence of one of the X chromosomes, i.e., 45 autosomes with X0. It shows the following symptoms; females are sterile as ovaries are rudimentary and they lack other secondary sexual characteristics.

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FAQs on Principles of Inheritance and Variation

Who Coined the Term Genetics? 

William Bateson an English geneticist coined the term genetics.  

How many Types of Genotypes and Phenotypes are Found in the Human ABO Blood Type?

There are 4 different phenotypes and 6 different genotypes found in the human ABO system. 

How many Different Types of Genotypes and Phenotypes are Found in the F2 Generation of a Dihybrid Cross?

In a dihybrid cross there are 4 different phenotypes and 9 different genotypes are found.

Why Hemophilia is Called Royal Disease?

Because this disease was found in the royal family of England. Queen Victoria was a carrier of the disease. Her pedigree shows several hemophilic descendants.