Genetics: Mendelian genetics, Heredity, principles of inheritance and variations
What is Genetics?
Genetics is the branch of biology concerned with the study of genes, Genetic variations and heredity in organisms.
The word “Genetics” is derived from the ancient Greek word “Genetikos” meaning generative/genitive, which in turn derives from “genesis” meaning ‘origin’.
Genetic processes work in combination with an organism’s environment and experiences to influence development and behaviour, often to as nature v/s nurture.
Trait inheritance and molecular inheritance mechanism of genes are still primary principles of Genetics in the 21st century, but modern genetics has expanded beyond inheritance to studying the function and behaviour of genes.
Gregor Johan Mendel is the father of genetics.
Definition of gene:
A gene is a molecular or large chemical, radical (the DNA) of carbon, Oxygen, Hydrogen, Nitrogen and Phosphorus attached to an undifferentiated protein thread, the chromonema and passed from one cell to another generation without ordinarily changing its form or constitution.
- Gene is the basis unit of inheritance for a given character.
- It is the ultimate unit of recombination.
- It is referred to as the ultimate unit of physiological activities.
- It is also the ultimate unit of reproduction.
Heredity is derived from the Greek word “Hereditas”, meaning inheritance. Heredity also known as inheritance or biological inheritance is the passing of traits from parents to offsprings either through asexual or sexual reproduction. The study of heredity in biology is genetics.
The process by which character or traits pass from one generation to the next is called inheritance.
Variations are the morphological, physiological, cytological, and behavioural differences amongst the individuals of the same species and the offsprings of the same parents.
They are found in all characters and in every conceivable direction. Therefore no two individuals are the same.
G.J Mendel, the father of genetics, considered as a pioneer of modern genetics. The first quantitative study of inheritance was carried out by Mendel.
Mendel started experimenting on pea plants (pisum sativum) in the monastery garden. He conducted the experiments and analysed the data statistically to identify the general rules of heredity.
- Selection of material:
- Mendel selected green pea for his experiment Material, because:
- it is an asexual plant and the lifecycle is very less.
- It has perfectly bisexual flowers, and the flowers are self pollinating.
- It is easy to cross to produce the desired plant.
- He found 7 pairs of contrasting characters or alleles.
Mendel’s experiments were based on cross breeding of two plants of pea, differing with each other in certain contrasting traits, which are listed below:
Character Dominant recessive
1. Stem height Tall Dwarf
2. Flower color Violet White
3. Flower position Axial Terminal
4. Pod shape Inflated Constricted
5. Pod color Green Yellow
6. Seed shape Round Wrinkled
7. Seed color Yellow Green
So above are the 7 pairs of contrasting traits in pea used by Mendel in his experiment.
Mendel has chosen seven pairs of characters. Each character has an alternative trait. So the plants with alternative characters were taken as male and other as female. Reciprocal crosses were also made. F1 generation (first filial generation) was obtained from this cross. F1 generation plants were self crossed and an F2 generation progeny was obtained. In the same way other generations (F3 and F4) etc were also obtained. This result clearly shows that the F1 plants are hybrids showing the dominant character; if the plants were tall and dwarf, the F1 progeny will be all tall. The trait appeared in F1 generation is called dominant, and others which are not expressed are called recessive.
Monohybrid Cross Experiment:
Mendel cross pollinated two varieties of pisum sativum . He took pure homozygous tall and pure homozygous dwarf as parent.
* To produce homozygous parents, Mendel self pollinated pea plants for several generations.
* So, he cross pollinated homozygous tall (TT) with homozygous dwarf(tt).
* The mature seeds produced were germinated. All became (Tt) plants.
* He named it the First filial generation (F1 generation).
* Now these F1 plants are taken as parents are taken as parents and self pollinated.
* The plant raised from these seeds belong to the second filial generation or F2 generation.
* In this generation both the tall and dwarf plants are found.
The phenotype ratio is 3:1
The genotype ratio is
Homozygous tall : Heterozygous tall:
Dihybrid Cross Experiment:
In the dihybrid Cross Experiment, Mendel studied two pairs of contrasting characters at a time.
In this experiment, the two characters :
1. Seed color : Yellow (YY), green(GG)
2. Seed shape : Round (RR), Wrinkled(rr)
He looked parent generation is pure or homozygous yellow round and homozygous green Wrinkled.
In the F1 generation, all the plants become yellow round. These F1 plants were taken again as parents and are self pollinated and in the F2 generation, he found four types of plant.
Yellow Round: 9
Yellow Wrinkled: 3
Green Round: 3
Green Wrinkled: 1
Thus, the phenotypic offspring in F2 generation is 9:3:3:1, which he called a Dihybrid ratio.
Hence, phenotypic ratio is 9:3:3:1
Genotypic ratio is 1:2:2:1:4:2:1:2:1
Laws of inheritance:
1. Law of unit factor
2. Law of Dominance and Recessive
3. Law of segregation or purity of gametes
4. Law of independent Assortment
1. Law of unit factor:
Mendel described that each character is controlled by a pair of unit factors. This pair is known as allele or allelomorphic pair and is defined as a pair of contrasting characters.
2. Law of Dominance and Recessive:
When a pair of contrasting characters is present together, only one of them expresses itself and the remaining get suppressed or hidden. The character which is expressed is called dominant and the character which remains hidden is recessive.
When a homozygous tall (TT) is crossed with homozygous dwarf (tt) in F1 generation, a heterozygous (Tt) is produced but the tall character (T) is expressed out which is called as dominant and the dwarf character (t) remain hidden is called recessive.
3. Law of Segregation or Purity of Gametes:
All allelic factors or genes present together in the hybrid (heterozygous condition) segregate or separate from one another and are placed in different gametes in full potency for the next generation.
Tall(T) and dwarf (t) character is present in a hybrid (heterozygous condition) when producing gametes, the two allelic factors (T) and (t) segregate or separate with full potency for next generation.
Law of independent Assortment:
When two or more pairs of contrasting characters are taken into consideration in a cross, each factor assorts or places itself independently of the other during its passage from one generation.
When Yellow Round (YYRR) is crossed with (yyrr) green Wrinkled in F2 generation, four different types of plants are produced.
1. Yellow Round_ 9
2. Yellow Wrinkled_ 3
3. Green Round_ 3
4. Green Wrinkled_ 1
If we check each pair of contrasting characters separately, the phenotype ratio of Yellow: Green
= 9+3 : 3+1
And Round : Wrinkled
=9+3 : 3+1
= 12: 4
= 3: 1
So, it implies that each pair assort independently of the other.