Heredity and Variation

# Heredity                                           

Transformation of characters from parents into offspring is called heredity.

# Variation

The dissimilarity between the individuals of the same species is called variation. The main causes of variation are:

• Different organisms have different sets of chromosomes.
  • Formation of gametes by meiosis cell division.
  • Transformation of characters from father and mother both.
  • Environmental influence or environmental cause.

# Genetics

The branch of biology concerned with the study of the mechanism of heredity and variation is called Genetics. Gregor Johann Mendel is called Father of Genetics.

# Monohybrid cross

In a cross, the character which can express itself is called dominant character and that which is hidden by dominant character is called recessive character. For example, if a cross is made between pure tall and pure short pea plants (monohybrid cross), in F₁ generation all the pea plants are found tall. Here tall is dominant character and short is recessive character.

# Genotype & phenotype

External appearance of an organism is called phenotype and genetic make up of an organism is called genotype of that organism. For example, a tall pea plant having trait. Tt has phenotype tall but genotype tall and short both.

# Hybrid

Hybrid is that individual which has different phenotype and genotype. For example by pea plant having traits Tt is not pure tall but hybrid tall because its phenotype is tall but genotype is both tall and short.

# Allele

A pair of genes controlling a pair of contrasting characters is known as an allelic pair and the members of an allelic pair are called allele of each other. For example, in Tt, T is allele of t and vice-versa.

# Mendel selected a pea plant for his experiment because the following characters are found in a pea plant (Pisum sativum)

• It is a naturally self pollinated and cross pollination also can be made artificially in it.
  • It has a number of distinguishable contrasting characters.
  • The hybrid plants obtained as a result of cross pollination are fertile.
  • A large number of progeny can be obtained from a single plant.
  • The life cycle is of short duration.
  • The plants are convenient to handle.

# Mendel’s Laws

# Law of Dominance: This law states that “When a cross is made between a pure contrasting characters, only one of them is able to express itself phenotypically where others remains hidden in F₁ generation. The one, which is expressed, is dominant and the other is recessive.”

# Law of segregation / Law of purity of gametes:

 This law stated that – “The two members of a pair of factors separate during the formation of gametes. They do not blend with each other but segregate out into different gametes.”

# Law of independent assortment:

This law states that “When two pairs of allele are brought together in the hybrid (F₁), they, at the time of gamete formation, segregate or assort independently at random and freely.”

Result of this cross is shown below

	Ry	rY	Ry	ry
Ry	RRyy Round-Green	RrYY Round-Yellow	RRyy Round-Green	Rryy Round-Green
rY	RrYy Round-Yellow	rrYY Wrinkle-Yellow	RrYy Round-Yellow	rrYy Wrinkle-yellow
RY	RRYy Round-Yellow	RrYy Round-Yellow	RRYy Round-Yellow	RrYy Round-Green
ry	Rryy Round-Green	rrYy Wrinkle-yellow	Rryy Round-Green	rryy Wrinkle-Green

DNA and RNA are two types of nucleic acids responsible for heredity.

# Differences between RNA and DNA are as the followings:

RNA	DNA
It is ribose nucleic acid.	It is deoxyribose nucleic acid.
It is single stranded.	It is double stranded.
Main function of it is to synthesize protein.	Main function of it is to transfer parental characters into offspring.
It has uracil base instead thyamine.	It has thyamine base instead of uracil.
It contains ribose sugar.	It contains deoxyribose sugar.
It may be found in or out of nucleus.	It is found in nucleus only.

• Mendel would not be succeeded in his experiment if he used frog instead of pea plant, because frogs reproduce by crossing in which purity of characters cannot be checked. Similarly it is also not convenient to have them.
• Blue eyed offspring may be formed from brown eyed parents. It is because the blue eyed character is as recessive character in the hybrid brown eyed parents. This recessive character becomes visible in F₂ generation.

# Differentiate between mutation and variation:

Differences between mutation and variation are as follows:

Mutation	Variation
A sudden heritable change in the genetic material of an organism is called mutation.	The structural difference that provides individually to each member of a species is called variation.
The various factors like ultra-violet rays, x-rays, gamma-rays and different chemicals are responsible for mutation.	The various factors like structural and functional differences of gene, environmental factors like food, light, temperature, humidity, etc. are responsible for bringing out variation.

REASONABLE FACTS

1.     Gregor Johann Mendel is considered as the father of genetics.

        Ans- Gregor Johann Mendel (1822-1884 AD.), an Austrian monk, is considered as the father of genetics because he introduced the concept of genes as the basic unit of heredity for the first time.

2.     Mendel selected pea plants of his study.

        Ans- Mendel selected pea plants for his stugy.

                Because for the following reasons:

        –       They have very short life cycle and can be easily cultivated.

        –       The pea flower is specially suited for controlling breeding.

        –       They produce large no. of offspring after each successive breeding.

        –       They are sexually reproducing.

3.     Mendel wouldn’t be succeeded in his experiment if he used frog instead of pea plants.

        Ans- Mendel wouldn’t be succeeded in his experiment if he used frog instead of pea plants because a frog reproduces by cross only. Thus, purity of characters cannot be checked in them. Similarly, they need special climatic conditions to survive so it was difficult to have.

4.     Blue eyed offspring may be formed from brown-eyed parents.

        Ans- because, if the brown eyed parents are not pure i.e. they are hybrid, they have blue-eyed character as recessive. The recessive character is expressed in next generation and the blue eyed offspring is formed.