HEREDITY
Heredity
Heredity is the process of transferring the parental characters from parents to offspring.
It is the process in which the parental characters pass from generation to generation.
It is also called inheritance.
Though similarity is found among the members of a family, they are not exactly alike i.e. there are some dissimilarities among them which is called variation.
Those characters which are able to transfer from generation to generation are called heredity characters.
They are also called parental characters.
The branch of biology which deals with the mechanism of heredity and variation is called Genetics
It is the study of various aspects of genes like their structure, location, function, etc.
The small portion of chromosome which is responsible to transfer the parental characters from one generation to another is called gene.
Gregor Johann Mendel is known as the father of genetics.
His work is known as Mendelism or law of Inheritance or law of genetics.
He did various kinds of crosses in sweet pea (Pisum sativum) and analyzed the results obtained from the crosses.
Key Terms related with heredity
Gene
It is the structural unit of chromosome made up of nucleic acids governing a particular character of the living organism
Offspring
They are the individuals produced from sexual reproduction.
They are also called filial generation.
Unit character
Definite external character of an individual for a cross is called its unit character.
Each organism consists of its own type
of leaf, flower, stem etc. which acts as a unit character.
Contrasting character
The characters which always appear in two opposing conditions are called contrasting characters.
For example, tall and dwarf character are the contrasting character for height.
Monohybrid cross
If a single unit character is studied during an experiment, then it is called monohybrid cross.
E.g. A cross is made between tall and dwarf pea plants.
The character involved in this cross is the height of the plant.
Dihybrid cross
If two unit characters are studied during an experiment, then it is called dihybrid cross.
E.g. a cross is made between a round yellow seeded plant and a wrinkled green seeded plant.
Two characters involved in this cross are shape and colour of the seed.
Dominant characters
Those characters which are often expressed during a cross are called dominant characters.
These characters express themselves by covering other characters.
Dominant characters are represented by the first letter of the word of the dominant character in the capital letter
E.g. Tall is a dominant character which is represented by the letter T.
Recessive characters
Those characters which are less prominent and are present in gene in hidden form are and less expressed are called recessive characters.
These characters are present but cannot express themselves due to the presence of dominant character.
Recessive characters are represented by the first letter of the word of the contrasting dominant character in the small letter
E.g. Dwarf is a recessive character which is represented by the letter t.
Alleles or allelomorphs
A pair of gene controlling a pair of contrasting characters is usually known as an allelic pair.
The members of allelic pair are said to represent alleles of each other.
For example, in the allelic pair Tt, T is the allele of t and vice versa.
Phenotypic character
Those characters which are expressed externally in the living organism so that are seen with naked eyes are called phenotypic characters.
They are the physical or observable characters or morphological appearance.
Example TT
Genotypic characters
Those characters that are expressed through gene but cannot be seen with the naked eye are called genotypic characters.
They are the genetic make-up of an organism
Example Tt
Homozygous pair
A pair of gene which are responsible to the same contrasting character is called homozygous pair.
The pair of gene represented by TT is responsible for tallness in pea.
Heterozygous pair
A pair of gene, which is responsible for combining different contrasting characters, is called heterozygous pair.
The pair represented by Tt; T represents the tallness and t represents dwarfness and the plant is hybrid (may be tall but not as tall as TT).
F1-generation
The offspring produced by crossing true-breeding parental forms are called F1 generation.
It refers to 1st filial generation.
F2-generation
It refers to 2nd filial generation produced by the self-crossing of F1-generation.
Hybrid
Those organisms which have heterozygous pair of genes are called hybrids. Eg. Hybrid tall [Tt]
Hybridization
It is process of crossing or breeding experiment two contrasting character to obtain hybrid.
Punnet square
It is a checker board, which helps to study all the possible results of various crosses.
Symbols used to denote gene
Genes are represented by the letters of the English alphabets.
The dominant gene is symbolized by the capital letter and the dominant gene is symbolized by the small letter.
For example: Tt where T [Tall] is dominant and t[ dwarf] is recessive characters respectively.
Why did Mendel select a pea plant for his experiment?
Pea plants are naturally self-pollinated and artificially cross-pollinated. It helps to maintain purity of character in off springs.
It has numbers of different numbers of distinguishable contrasting characters like tall and dwarf, red and white flowers. Therefore, the characters were easily distinguishable.
The hybrid plants, resulting from cross pollination, are also fertile. In many cases it is found that cross pollination gives infertile offspring.
A large number of progeny (off springs) can be obtained from a single plant.
The life cycle is of short duration. So results can be seen in short period.
Pea plants are very easy to handle no special care is required.
Seven characters studied by Mendel
Any two probable causes that Mendel did not do his experiment in frog instead of pea plant are: -
(i) Breeding cannot be controlled easily in frog because of external fertilization whereas breeding can be controlled easily in pea plant because of the structure of flowers.
(ii) Frog can't be handled easily as it can't be cultivated easily and it is difficult to study different stages of frog being long life cycle whereas pea plants can be cultivated easily within few years and they have short life cycle. So, it is easy to study several generations within a few years.
Any two difficulties Mendel would have faced if he had carried his experiment on human instead of pea plant are: –
(i) He would have faced some problems to cross between any two contrasting characteristics since in human beings (men and women) do not have distinguishable contrasting characteristics like that in pea plants.
(ii) Human beings live in society and have to follow certain social norms. Therefore, in practical life it would be difficult for him to cross between selected men and women for any two contrasting characteristics.
Mendel’s law
On the basis of the results obtained from the Mendel’s experiment, he proposed the following three laws
Law of dominance
Law of segregation or purity of gametes
Law of independent assortment
Law of Dominance [First law]
This law of applicable for Monohybrid cross.
Law of dominance states that “when a cross is made in between a pair of pure contrasting characters, only one of them is able to express phenotypically while the other remains hidden in the F1 generation”.
Such expressed character is called dominant character and the masked/hidden character is called recessive character.
For example, when the cross is made between a pure black mouse (BB) and pure white mouse(bb), in F1 generation, all the black mouse are seen.
In these black mouse, the traits B and both are found but only Black(B) can express itself as it is a dominant character and the white (b) remains hidden.
This hidden character might be expressed in the F2 generation.
Law of purity of gametes/law of segregation [ second law]
This law is applicable for monohybrid cross.
This law explains about the results obtained from the cross of F1 generations.
Law of segregation states that “Though the genes, which control recessive character, are hidden in the hybrid organism it is present in the separate location of the chromosome, which later reappear while gametes are formed”.
It states that “The two members of a pair of factors separate during the formation of gametes. They do no blend with each other but segregate out into different gametes. Thus, any gamete is not impure”.
This is also called the law of segregation as the heterozygous pair will separate during gamete formation.
Let the genotypes for black be BB and for white bb.
First the parent plants are cross pollinated to provide first filial generation (F1) and then F1 plants were self-pollinated to give second filial generation (F2)
In first filial generation all the off springs are black (i.e. only dominant character is seen, but all plants are hybrid)
Then in second filial generation there are 3 black and one white (Only phenotypically but genotypic ally the plant is one pure black two hybrids and one white).
Phenotypic ratio 3:1 (Dominant: Recessive). (Black: white)
Genotypic ratio 1:2:1 (Dominant: Hybrid: Recessive) (Pure black: hybrid: white)
F1 Parents Bb Bb
F1 Gametes B b B b
Fertilization
F2 Generations BB Bb Bb bb
Pure black Hybrid black Hybrid black Pure white
Phenotypic ratio (Black and white) = 3:1
Genotypic ratio (Pure black, hybrid black and pure white) = 1:2:1
As shown above in F1 generation we have 100% hybrid tall
In F2 generation we have Phenotype ratio: 3:1 [ Tall=75%: Dwarf=25%]
Genotype ratio: 1:2:1 [ Pure tall:25%, Hybrid tall:50% and Dwarf:25%]
Prepare a cross showing the results obtained in the F2 generation when a pure tall pea plant is crossed with a pure short pea plant and write its phenotypic and genotypic ratio.
Let TT be pure tall pea plant and tt be pure short pea plant.
Pure tall Pure short
Parents TT tt
Gametes T T t t
Fertilization
F1 Generations Tt Tt Tt Tt
100% Hybrid
F1 Parents Tt Tt
F1 Gametes T t T t
Fertilization
F2 Generations TT Tt Tt tt
Pure tall Hybrid Tall Hybrid tall Pure short
Phenotypic ratio = 3:1
Genotypic ratio = 1:2:1
Q. Prepare a cross showing the results obtained in the F2 generation when a pure red flowered pea plant is crossed with a pure white flowered pea plant and write its phenotypic and genotypic ratio
Some of dominant and recessive characters in human:
Any two differences between dominant character and recessive character are:-
Any two differences between genotype and phenotype are:-
# When mating of black and white mouse takes place and all the offspring produced in first generation are black why is there no white offspring? Show by drawing chart.
When mating of black and white mouse takes place and all the offspring produced in first generation are black because according to law of dominance only one character is able to express itself phenotypic ally when the crossing is done between one pair of contrasting character. Here black is dominant character whereas white is recessive character. Therefore, in F1-generation all the hybrid black mouse are produced.
As shown in above chart, in F1-generation the entire mice are black because black colour of the mouse suppressing the white colour of the mouse and express phenotypically.
# The result obtained by crossing pure red flowering pea plant (R) and pure white flowering pea plant (r) is shown in the given checker board.
(i) Which filial generation does this result belong to?
Ans: This result belongs to F2-generation.
(ii) What is the ratio of the plant having dominant and recessive character?
Ans: The ratio of the plant having dominant and recessive character is 3: 1.
(iii) Which one is pure red flowering plant?
Ans: 'RR' is a pure red flowering plant.
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