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CHAPTER - 9 HEREDITY AND EVOLUTION
The transfer of characters from parents to offspring is known as heredity and the process through which characters or traits pass from one generation to another is called inheritance.
Accumulation of Variations During Reproduction
The difference in the characters among the individuals of a species is termed as variations. These variations are accumulated by the process of sexual reproduction.
Heredity (Inheritance of Traits)
Traits or characteristics, which are passed on from parents to their offspring (generation to generation) are controlled by genes.
A gene is a unit of DNA which governs the synthesis of one protein that constants a specific character of an organism. e.g., Inheritance of free or attached earlobes.
Traits (Characteristics)
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Inherited Traits
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Acquired Traits
Inheritance Traits
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Transferred from parents to offspring.
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Traits include height, skin, colour, hair.
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These are genetic variations caused in reproductive tissues.
Acquired Traits
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Developed throughout the life time of the individual.
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Learning some Skill, knowledge we raised muscular development.
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These are somatic variations caused in non-reproductive tissue.
Some Important Terms and Definitions
Chromosome
A long thread-like structure in the nucleus. It appears during cell division and carries genes.
Gene
A functional unit of heredity. It is present on chromosome. It is a piece of DNA that codes for one protein that in turn determines a particular character (phenotype).
Character
The feature or characteristic of an individual like height, colour, shape. etc.
Trait
An inherited character. i.e., feature, which is normally inherited and has its detectable variant too, e.g., tall and dwarf are traits of a character, i.e., height.
Allele
One of the different forms of a particular gene, occupying the same position on a chromosome.
Dominant allele
An allele, whose phenotype will be expressed even in the presence of another allele of that gene. It is represented by a capital letter, e.g., T.
Recessive allele
An allele, which gets masked in the presence of a dominant allele and can only affect the phenotype in the absence of a dominant gene. It is represented by a small letter, e.g., t.
Genotype
Genetic composition of an individual.
Phenotype
The expression of the genotype, which is an observable or measurable characteristic.
Father of Inheritance
The Austrian monk, Gregor Johann Mendel is known as Father of Genetics. He performed many experiments on pea (Pisum sativum) plant related to crossbreeding.
He studied seven pairs of contrasting characters in pea plants and only one character at a time.
Reasons for selecting pea plant
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Easy to grow
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Short life cycle (i.e., it is possible to study many generations.)
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It has 7 pairs of contrasting characters.
Table of contrasting characters, seven pairs
CHARACTER
Flower Colour
Flower Position
Seed Colour
Seed Shape
Pod Shape
Pod Colour
Height Of Plant
DOMINANT TRAIT
Violet
Axial
Yellow
Round
Inflated
Green
Tall
RECESSIVE TRAIT
White
Terminal
Green
Wrinkled
Constricted
Yellow
Dwarf/Short
Monohybrid Cross (Study of single trait)
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Mendel took pea plants with different characteristics such as height (tall and short plants).
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The progeny produced from them (F1 - generation plants) were all tall. Mendel then allowed F1 progeny plants to undergo self-pollination.
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In the F2 - generation, he found that all plants were not tall, three quarter were tall and one quarter of them were short. This observation indicated that both the traits of shortness and tallness were inherited in F2 -generation. But only the tallness trait was expressed in F1 -generation.
Thus, the phenotypic ratio is 3: 1 and the genotypic ratio is 1: 2: 1 for the inheritance of traits for one contrasting character, i.e., monohybrid cross.
Dihybrid Cross (study of two traits)
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Mendel took pea plants with two contrasting characters, i.e., one with a green round seed and the other one with a yellow wrinkled seed.
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When the F1 progeny was obtained, they had round and yellow seeds, thus establishing that round and yellow are dominant traits.
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Mendel then allowed the F1 progeny to be self-crossed
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(self-pollination) to obtain F2 progeny. He found that seeds were round yellow, round green, wrinkled yellow and some were wrinkled green.
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The ratio of plants with above characteristics was 9: 3: 3: 1, respectively (Mendel observed that two new combinations had appeared in F2).
Mechanism of Inheritance
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Both the parents contribute a copy of the same gene to their progeny. Each gene cell thus, has one set of gene, present as chromosome. Each cell of the body will have two copies of each chromosome, one inherited from each parent.
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When two gene cells combine, they restore the normal number of chromosomes in the progeny. This ensures the stability of the DNA of species.
Sex determination in human
In human beings, there are 23 pairs of chromosomes, out of which 22 pairs are autosomes and one pair are sex-chromosomes.
Females have a perfect pair of sex chromosome (homogametic), but males have a mismatched pair (heterogametic) in which one is X (normal sized) and the other is Y-chromosome (short in size).
Hence, an egg fertilized by X-chromosome carrying sperm results in a zygote with XX, which becomes a female and if an egg is fertilized by Y-chromosome carrying sperm, it results in a XY zygote that becomes male.
Thus, the sex of the children will be determined by what they inherit from their father. A child who inherits an X-chromosome will be a girl and one who inherits a Y-chromosome will be a boy.
