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Genetics Basic Of Inheritance

Updated on October 14, 2014

Introduction

Basis of continuity of life is reproduction. Reproduction can be sexual or asexual. Sexual reproduction besides creating a new individual also combines the traits of parents into its offspring. Now the question arise how

  • How come these traits are transferred from parents to offspring??
  • Why they are not changing it completely??

This all questions were answered by the branch of biology called GENETICS. Genetics is branch of biology which deals with the study of the genes. Genes are the information carrier which possess information about the traits. Each trait may have its genes.

Before reaching to the forms of genes, there were many theories regarding the inheritance. People believed in inheritance are suggested by many wall painting of ancient world. Even they knew about the cross breeding of the species for better result, but nothing was documented nor any scientific researches were available to prove the point. Few of the theories which were discussed widely and were supported for few years till they were replaced by other. Some of the theories were:-

  1. Charles Darwin proposed that every body part produces gemmules which are collected in semen by blood circulation and through these gemmules, the traits are transferred to offspring. This theory was called hypothesis of pangenesis
  2. Theory of Pangenesis was discounted by August Weismann in year 1892 and proposed the theory of Somatoplasm and Germatoplasm. According to Weismann, body is made of Somatoplasm and Germoplasm is responsible for transfer of trait from one generation to another. According to him, Germoplasm is immortal and is never destroyed, whereas Somatoplasm gets destroyed when body gets destroyed.

Gregor Johnn Mendal – Father of Genetics

Born in year 18222, Mendel studied philosophy and then started its work on inheritance in year 1856, at University of Vienna.

His work being too simple, as compared to its companion and the mathematical approach he had adopted, his work could not get recognized in his lifetime, but it later year 1900, few other botanist started the work in his fashion and when his work was recognized after about 35 years. So, he was then rightly called “FATHER OF GENETICS”.

Mendal’s Experiment

Mendel started his experiment on inheritance in year 1856 at University of Viennna. He reported his result in year 1965. He choose garden pea for his experiment. The main advantage behind this were that this plant small, easy to grow and they cross breed easily. Additionally, they have large number of offspring and they have too many various traits upon which he can experiment upon.

He selected seven different traits. Simplest of crosses conducted, in which only one trait was considered, was called monohybrid.

Similarly for crosses in which two trait were considered, was called dihybrid and for three traits together it was trihybrid.

Crosses were made by collecting pollen grains from one type of plant to fertilize, the egg of other plants. And then offspring in form of seeds were collected. There results of crosses were not affected by the source of the pollen grains or egg, test were also carried out by interchanging source of the egg and pollen grain. Such crosses were called reciprocal crosses, the results showed no change.

The Original parents in the cross were called Parental Generation or P generation.

Offspring of P Generation were called First Filial Generation or F1 generation.

Offspring of F1 Generation were called Second Filial Generation or F2 generation.

Seven Pair Of Contrasting Traits

sr. no
Characters
Contratsing Traits
1
Stem Height
Tall/dwarf
2
Flower Colour
Violet/white
3
Flower Position
Axial/terminal
4
Pod Shape
Full/constricted
5
Pod Colour
Green/yellow
6
Seed Shape
Round/wrinkled
7
Seed Colour
Yellow/green

Results

In typical monohybrid cross between tall and dwarf stem plant. When true tall plant was crossed with dwarf plant, F1 generations, were all tall plant. When F1 generation were selfed. Mendel Observed that, out of 1064 F2 progeny about 787 were tall and remaining were dwarf. Dwarfness was not present in F1 generation but it reappeared F2 generation.

Similar observation was made with the other traits too. Typically analyzing the results of all the traits, it was found that one of the contrasting trait disappeared in the F1 progeny but reappeared in F2 progeny in Ration of 3:1.

Mendel called the trait which disappeared in the F1 progeny as Recessive Trait and the one which remain in all generation as Dominant Trait.

While considering dihybrid cross round and yellow coloured seed plant was crossed with a wrinkled and green seed plant. F1 progeny was round and yellow coloured seed plant. When this plant was selfed, Out of 16 Plants, 9 were yellow and round seed, 3 were yellow and wrinkled one, 3 were green and round one and 1 was green and wrinkled seed plant.

Sr no
Cross
F1
F2
Ratio
1
Tall*dwarf Stem
Tall stem
787 tall, 277 dwarf
2.84:1
2
Violet*white flower
Violet flower
705 violet, 224 white
3.15:1
3
Axial*terminal pod
Axial pod
651 axial, 207 terminal
3.14:1
4
Full*constricted pod
Full pod
882 full, 299 constricted
2.95:1
5
Green*yellow pod
Green pod
428 green, 152 yellow
2.82:1
6
Round * wrinkled seed
Round Seed
5474 round, 1850 wrinkled
2.96:1
7
yellow * green seed
Yellow Seed
6022yellow, 2001green
3.01:1

Mono Hybrid Cross Result

Principles of Inheritance

Based on the results of the test carried out by Mendel, he proposed four postulates of In heritance which are popularly known as Principles of Inheritance.

1) Each genetic character is controlled by pair of unit factor now commonly known as alleles or allomorphic pair. There are two unit factors for tallness and two for dwarfness, similarly for other traits too.

2) Based on result obtain in monohybrid cross, Mendel proposed that when two dissimilar unit factor join together, then only one can express itself. The one which express is called dominant unit factor and one which does not is called recessive unit factor. This explains the disappearance of one of the trait in F1 progeny. Now we call this unit factor as Genes.

3) During Gamete formation each unit factor segregate randomly, so that each gamete receives one or other unit factor with equal probability. This explains the re-appearance of recessive trait in F2 progeny. This can be explained as, in F1 progeny all the F1 were with dissimilar unit factor. During Gamete formation in F1 progeny out of four, two gametes were with recessive unit factor and two were with dominant factor, when they selfed, two F2 progeny formed were with dissimilar unit factor, forming generation with dominant trait. One was with only dominant factor hence it was also a dominant trait plant and one was with both unit factor of recessive trait which formed recessive dominant plant. Hence out of 4 progeny of F2 generation one was of recessive trait and other three were of dominant trait. This Principle is popularly known as Principle of Segregation or Purity of Gametes.

4) Fourth postulate was based on observations made from the dihybrid crosses. This postulates states that, when two events occur simultaneously then combined probability of two outcomes is equal to the product of their individual probability of occurrence.

We already know that if yellow and green seeds are crossbreed, there is probability that there will be 3 yellow and one green seeded plant in F2 progeny, similarly for round and wrinkled seed it will be 3 rounds and 1 wrinkled seed in F2 progeny.

Now, two event are combined i.e. yellow and round seeded plant is cross breed with wrinkled and green seeded plant, then according to this postulate

Out of 16(4*4) F2 progeny,

a) 9(3*3) will be yellow and round

b) 3(1*3) will be green and round

c) 3(3*1) will be yellow and wrinkled

d) 1(1*1) will be green and wrinkled

Di-Hybrid Cross Result

Di-Hybrid Cross Results
Di-Hybrid Cross Results

Post Mendalian Patterns Of Inheritance

After Mendel showed the path, the scientist after 1900 started experiment on same line on other species of plants and animals. More type of traits was checked upon and more interesting results and patterns of inheritance were found in subsequent years.

  • Incomplete Dominance

In experiment with two plants namely four-o-Clock and snapdragon, when red and white plant were cross breed, the F1 progeny was a pink plant, which means that in hetrozygote, recessive allele also expressed it self and hence different phenotype was formed.


  • Multiple Alleles

From the experiments carried out after 1900, it became clear that there can be more than two allele of gene. The simplest of Example being human blood group. It has three alles IA, IB, and IO, which are responsible for four type of blood group, A, B, AB and O group. This was called multiple allelism.

  • Epistasis

There are examples in which one gene mask or modifies the expression of another non-allegic gene, this is called epistasis. The gene which override the expression is called epistatic gene.

  • Polygenic Trait

Some traits are controlled by more than two gene. Such trait are called polygenic or quantitative. Here more than two may be three or four genes are involved and based of number of genes the traits is decided. Example is human height or skin colour.

  • Pleiotropy

There are example where one gene controls more than one trait such a phenomena is called pleiotropy. Such genes are called pleiotrophic gene. For example, white eye mutation in Drosophila lead to dipigmentation in many other parts of body, hence gene which controls eye colour is also responsible for pigmentation of some other body parts.

Conclusion

Inheritance was very much interesting subject of human study. Mendel started his work, and later on extension of his work lead to many more discoveries and hence Mendel was Called FATHER OF GENETICS.

Genetics not only lead to know how we inherit but also helped us to find the cure to many genetics disorders and also ways were found so that many disorders can be prevented from being inherited to next generation.

So genetics the latest branch of biology has given many things to human and is to deliver more in times to come.

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