Principles of Inheritance and Variation

Principles of Inheritance and Variation

Table of Content

Definition of Heredity

“Heredity is the transfer of genetic information from parents to offspring. Their heredity characters are present on the chromosomes in the form of genes and the combination of these genes expresses characters which may be similar to one of the parents.”

Variation in the characters of offspring arise due to a unique process called genetic recombination during crossing over events of meiosis.

Definition of Variation

“Variation is the degree by which the progeny differ from their parents.”

Human are aware since 8000 – 1000 BC that one of the reason of variation was hidden in sexual reproduction and they tried to exploit the variations that were naturally present in wild population of animals and plants. Therefore, they try to selectively breed the organism and try to get the desired characters.

Example of Variation

With the help of domestication and artificial selection from ancestral cows we are able to produce Indian breed, i.e. Sahiwal cows found in Punjab.

Mendel Law of Inheritance

Gregor Mendel conducted hybridization experiments on garden peas for 7 years and proposed the law of inheritance. He selected the large sample size with greater credibility of collecting data. He investigated garden peas with contrasting traits, i.e. yellow or green seeds, tall or dwarf, etc. This helped in establishing the basic frame work of inheritance. During the experiment, Mendel also carried out artificial pollination via different true – breeding pea lines. He selected 14 true – breeding pea plant varieties and selected different contrasting traits.  Some selected traits were smooth or wrinkled seeds, tall or dwarf, yellow or green seeds, etc.

Following table shows seven contrasting traits selected by Mendel in peas for experiment:

Mendel in peas

Reason of selecting Garden Peas

Mendel selected garden peas (Pisum sativum) for his experiment because of several reasons –

  1. Presence of several contrasting characters that can be studies easily.
  2. Short life span.
  3. Pollination of pea flowers is easier and therefore, hybrids produced were fertile.
  4. Flowers show self – pollination and reproductive whorls being enclosed by corolla.


Working method

Mendel carried out his experiment with proper planning and his success depends on the working method he adopted.

  1. He studied single character at a time.
  2. Mendel used all available techniques in order to avoid cross pollination by undesirable pollen grains.
  3. He adopted concepts of statistics and mathematics so as to analyze the results obtained by him.

Inheritance of One Gene

Mendel carried out hybridization experiment, whereby he crossed tall and dwarf pea plant to study the inheritance of one gene. He collected the seeds produced as a result of this cross and grew first hybrid generation referred as first filial or Filial1 progeny or generation. Mendel observed that all F1 plants were tall and none were dwarf. Similar observations were found in other characters as well.

Then, Mendel self – pollinated the tall F1 plants and found some of the offspring were dwarf in F2 generation, i.e. the character that was hidden in F1 was now expressed in F2. The portion of dwarf plants were 1/4th of the F2 plants while 3/4th of the F2 plants were tall.

Following diagram shows the outcome of first hybrid generation, where at F1 Mendel observed all tall pea plants. Simultaneously, we can also see the dwarf character existed in F2 stage:

first hybrid generation

Based on these observations, Mendel proposed that something stably passed down, from parent to offspring, unchanged via gametes, over successive generations and called them as genes. Genes are the unit of inheritance and consist of information required to express the particular trait in an organism.

If we alphabetically represent each gene, whereby capital letter for tall and small letter for short, then the traits will be expressed as follows:

alphabetically represent each gene

In the above image, T is used for tall trait and t is used for dwarf trait. Thus, T and t are allele to each other. Mendel also concluded that in case of true breeding, dwarf or tall pea variety of allelic pair are identical or homozygous (represented as TT or tt). On the other hand, Tt is considered as heterozygous.

Mendel’s work and result

After experimentation, Mendel proposed several laws that are referred as “Laws of heredity.”

  1. Law of dominance – “This law states that when two contrasting factors for two characters come together in an organism, only one is expressed externally and shows visible effect.” The character which is visibly present is called dominant while which remains hidden or do not express in recessive.” Thus, according to it –
    1. Factors are the discrete units that control characters.
    2. These factors occur in pair.
    3. In case of dissimilar pair, one pair is dominant while other is recessive.
  2. Law of segregation or Purity of Gametes – “This states that alleles separate or segregate during gamete formation, and randomly unite at fertilization.” When F2 generation was produced by allowing F1 hybrid to self - pollinate, to find out segregation or separation it was observed that both dominant and recessive plants appeared in 3:1 ratio.” In other words, alleles do not show any blending; rather both the characters are recovered in F2. Although, parents have both alleles but the gamete receive only one of the two factors. It is important to note that, homozygous produce all similar gametes while heterozygous produce two kinds of gametes.
  3. Law of independent assortment – “This law states that when two or more characteristics are inherited, individual hereditary factors assort independently during gamete production, giving different traits an equal opportunity of occurring together.”  To prove this, he did a dihybrid cross.  He crossed homozygous dominant smooth and yellow seeded (YYRR) with homozygous recessive wrinkled and green seeded (yyrr) plants. The F1 hybrid was self - pollinated and F2 generation was obtained with the phenotypic ratio of 9:3:3:1 and genotypic ratio of 1:2:1:2:4:2:1:2:1.”

Inheritance of Two Genes

Mendel also researched with inheritance of two genes, but crossing over pea plant with two contrasting traits, such as plant with seeds with round and green color and plant with seeds with yellow and wrinkled shape and found that the seeds resulted from this crossing over was yellow colored and round shaped. He thus, concluded that yellow color is dominant over green and round shape is dominant over wrinkled shape.

Now let us consider several genotype symbols –

Y = dominant yellow seed color

y = recessive green seed color

R = round shaped seeds

r = wrinkled seed shape

The genotype of parents is written as follows – RRYY and rryy.

Following figure shows the cross between these two parents produced the following result:

4-cross between these two parents produced

Result of the Dihybrid Cross

Phenotype Ratio =

Round Yellow: Round Green: Wrinkled Yellow: Wrinkled Green

9          :           3          :           3          :           1

In the above figure, the gametes RY and ry unite on fertilization and produce RrYy hybrid at F1. When Mendel self - hybridized F1 plants, he found 3/4th of F2 plants had yellow seeds while 1/4th had green. Thus, the yellow and green color segregated in a ratio 3:1. In the similar manner, round and wrinkled seeds also segregated in the ratio 3:1.

Law of Independent Assortment

Referring the above image of dihybrid cross, the phenotypes round yellow: wrinkled yellow: yellow round: wrinkled and green appeared in the ration 9:3:3:1.

According to the law of independent assortment “when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.”

Chromosomal Theory of Inheritance

Chromosomal Theory of Inheritance was proposed by Boveri and Sutton in 1902. Sutton described Mendel principle of Inheritance on cytological basis. According to him, during meiosis, one member of the pair of homologous chromosome goes to one daughter cell and second to another daughter cell. The principle of Independent Assortment (proposed by Mendel) found cytological proof from the fact that member of one pair of homologous chromosomes independently move to poles towards another pair. Sutton calculated the number of combinations of chromosomes in same manner as gametes was calculated by Mendel. He also found that the number of chromosomes combinations were same as Mendel postulated during crosses of pea plant. During the independent assortment of chromosomes, four types of allelic combinations are made which are in the phenotype ratio 9:3:3:1 at F2

Following figure shows Chromosomal Theory of Independence. It is clear that the genes and chromosomes arrange themselves in homologous manner and separate in two different ways during meiosis. This results in four types of allelic combination and at F2 the phenotypic ratio is 9:3:3:1.

 Chromosomal Theory of Independence

Arguments of Sutton and Boveri for Chromosomal Theory of Inheritance

  1. Since eggs and sperm cells are the one bridge that is transferred from one generation to another. It implies that all the heredity characters are included in them.
  2. During maturation, the sperm cell practically loses all its cytoplasm. But sperm contributes heredity similar to eggs; therefore the heredity factors are carried in nucleus.
  3. Similar to Mendelian factors, chromosomes are also found in pairs.
  4. Union of egg and sperm re – establishes new organism with two sets of chromosomes previously seen in the body cells of parent organism.
  5. Chromosomes divide accurately during cell division and this gives an idea that genes are carried on chromosomes.
  6. Chromosomes segregate during meiosis.
  7. Member of chromosome pair also segregate independently of other chromosome pairs. Genes, proposed by Mendel also segregate independently.

Sex Determination

Definition of Sexual differentiation in Humans: “Sexual Differentiation in Humans is the process of development of sex differences in humans.” It is the process of development of different genitalia and the internal genital tracts, body hair, breasts, etc. play the significant role in sex determination.

The development of sexual differences in human is due to the presence of sex chromosome. It begins with XY sex – determination system followed by the complex mechanism for the development of phenotypic differences between female and male humans. Female has two X chromosomes while male have one X chromosome and one Y chromosome. At the early developmental stage of an embryo, both sexes possess equivalent internal structures, referred as mesonephric ducts and paramesonephric ducts.

In humans, sex determination mechanism is referred as XY type. Out of 23 pair of chromosomes, 22 pairs are exactly same in case of both male and female. These chromosomes are called autosomes. A pair of X chromosomes is present in female; sex determination in male and femalewhile X and Y chromosome are the determinant of male characteristics. During spermatogenesis, two types of gametes are produced in which 50% of sperm carry X chromosome while another 50% of the sperm carry Y chromosome. There is equal probability of fertilization of the ovum with sperm carrying X or Y chromosome. Thus, it is evident that sex of the child depends on the sperm and during pregnancy; there is always 50% probability of either a female or a mal child.

Following image shows the sex determination in male and female. Here an egg comprises of XX chromosome and sperm consist of XY chromosome. 


“Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in genotype and the phenotype of an organism.” Mutation is also the phenomenon that results to variation in DNA.

Following image shows mutation where the DNA is changed to mutant copy rather than original one:

mutation where the DNA is changed to mutant copy

Genetic Disorders

  1. Pedigree Analysis – In pedigree analysis, the inheritance of a particular trait is represented in the family tree over generations. It is the strong tool, which helps in studying the inheritance of specific trait, disease or abnormality. It is important to note that each and every feature of an organism is controlled by one or other gene which is located on DNA present on chromosome. However, alteration or changes takes place occasionally and such an alteration is called mutation. Number of disorders has been found that are associated with the inheritance of altered or changed genes.
  2. Mendelian Disorders – These disorders are determined by mutation in a single gene. The pattern of inheritance of such disorders are traced and studied by Pedigree Analysis. Most prevalent Mendelian disorders are Sickle – cell anemia, hemophilia, cystic fibrosis, thalassemia, color blindness, phenylketonuria, etc. These disorders can be dominant or recessive as well.
  • Hemophilia – It is sex linked recessive disease which is transmitted from unaffected carrier female to some of the male progeny. In this disease, a simple cut results in non – stop bleeding in an affected individual. Possibility of female of becoming hemophilic is extremely rare because mother needs to be carrier in this case and father needs to be hemophilic. Following figure shows the condition of hemophilia in an individual. The blood of such person is not able to clot:

condition of hemophilia in an individual

  • Sickle cell anemia – It is autosome linked recessive trait which is transmitted when both the parents are carrier for the gene. This disease is controlled by single pair of allele, HbA and HbS.  Heterozygous individual (HbAHbS) are apparently unaffected but they are the carrier of the diseases. Following figure shows the condition of sickle cell anemia in an individual where sickle cells block the flow of blood while normal red blood cells results in free flow of blood vessels:

condition of sickle cell anemia

  • Phenylketonuria – It is inherited as autosomal recessive trait and is an inborn error of metabolism. The individual suffering from phenylketonuria lacks an enzyme that transform amino acid phenylalanine into tyrosine, due to which phenylalanine is accumulated and converted into phenyl pyruvic acid and other derivatives. These accumulate in brain and thereby result in mental retardation.
  1. Chromosomal Disorders – These disorders are caused due to lack of excess or abnormal arrangement of one or more chromosomes. “Failure of segregation of chromatids during cell – division results in the gain or loss of chromosome(s), called aneuploidy. For instance, Down’s syndrome results in the gain of extra copy of chromosome 21.           In the same way, Turner’s syndrome is resulted from the loss of an X chromosome in human females. “Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and this phenomenon is called polyploidy.” This condition is often observed in plants.
  • Down’s syndrome or Trisomy 21 – It was first described by Langdon Down in the year 1866 and occurs due to the presence of additional copy of chromosome number 21. The person suffering from this disorder is short statured with furrowed tongue, short round head and partially open mouth. Psychomotor, mental and physical development is retarded in such individuals.
  • Klinefelter’s syndrome or 47, XXY or XXY – This disorder results due to the presence of additional copy of X chromosome. Such individuals have masculine development; followed by expression of feminine development as well (It includes development of breast). The individuals suffering from this disorder are sterile.
  • Turner’s syndrome or 45,X – This disorder occurs due to the absence of one X chromosome. Such females are sterile and lack secondary sexual characters.

Frequently Asked Questions

Q1: What are true – breeding lines?

Answer: “A true – breeding line is one that having undergone continuous self – pollination, shows the stable trait inheritance and expression for several generations.”


Q2: What are alleles?

Answer: “Alleles are the alternate pair of the same gene which is present on the homologous pair of chromosome.”


Q3: What is the difference between homozygous and heterozygous?




It gives rise to similar homozygous individuals and is usually represented as tt or TT.

It produces offspring of different genotype and is represented as Tt or tT.

Both the alleles have similar traits.

Both the alleles have contrasting traits.

Homozygous individuals carry either recessive or dominant allele but not both.

Heterozygous individuals carry both dominant and recessive alleles.

It produces one type of gametes.

It produces two types of gametes.

It does not display extra vigour.

It displays extra vigour and is referred as heterosis or hybrid vigour.


Q4: What is incomplete dominance?

Answer: It is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele.” It results in third phenotype in which the expressed trait is the combination of both the trait of parents. It is also referred as semi dominance or partial dominance.

Example of Incomplete Dominance: Pink roses are the example of incomplete dominance.

Following figure shows incomplete dominance, whereby RR is red rose and rr is white rose. At F1, they produce Rr and all offspring are of pink color.

incomplete dominance 

Q5: What is the difference between incomplete dominance and co dominance?

Answer: Incomplete dominance occurs when the alleles received by parents are neither recessive nor dominant rather blend together and produce new trait that is somewhere between the two traits. On the other hand, co dominance is also the similar phenomenon where neither dominant nor recessive trait is displayed, rather both the alleles mix up and difference between incomplete and co dominanceshows in the offspring. For instance, red and while flowered rose plant may produce red or white flowers but white flower with freckles of red spots.

Following diagram shows the difference between incomplete and co-dominance:


Q6: What is point mutation?

Answer: Point Mutation is a change in one or a few base pairs in a gene.” There are two Types of point mutation –

  1. Base substitution:– It occurs when one base is switched out with another base.
  2. Frame shift mutation:– It occurs when one base is added or removed.

Following figure shows two types of point mutation, i.e. base substitution and base shift mutation.

two types of point mutation

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