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Mendel's Laws Dominance, Segregation, Independent Assortment

Mendel's Laws of  Dominance, Segregation and Independent Assortment

It is a matter of common observation that all the individuals belonging to one species are alike. Humans resemble one another and so do the dogs, cats, tigers, mango plants, wheat or pea. This is natural because every living organism in this universe maintains its continuity by producing progeny similar to them by the process of reproduction. We are going to discuss about Mendel's Laws - Mendel's Law of Dominance, Mendel's Law of Segregation, Mendel's Law of Independent Assortment.
Mendel's Laws
Whenever a child is born, the family members usually start comparing the child's appearance as to whom he/she resembles more - Father or Mother. However, offsprings show some variations or differences from their parents and also among themselves so that each one of them has its own identity.

Have you seen any two human beings exactly alike except the identical twins? These differences between the young ones of the parents or various individuals belonging to one species are called variations.

Genetics

The term 'genetics' was used for the first time by W. Bateson in 1905. Genetics is the study of transmission of body features (both similarities and differences) from parents to offspring and the laws relating to such transmission. Gregor Johann Mendel (1822-1884) is very - appropriately known as the "father of genetics."

Heredity

Eggs laid by a sparrow hatch into sparrows but never into any other bird. Seeds of wheat always produce wheat plants but never any other plant. Similarly, dogs always produce pups which grow into dogs.

In aspect, heredity is the "like begets like" in its immediate phenomenon of successive acts of reproduction. Heredity is a process by which transmission of genetically based characteristics are transmitted from parents to offspring.

Variations

Like begets like', yet the young ones of same parents are never exactly similar (except identical twins) to each other and to the parents. Similarly, various members belonging to one species are never exactly similar. These differences found between the young ones of same parents or various individuals belonging to one species are called variations.

These variations are very small between the various members of one family, more between various families of one population and are very pronounced between different races or tribes of one species that inhabit distant areas. These variations and similarities are clearly observed amongst the offspring produced in sexual reproduction while asexual reproduction tends to preserve the similarities among all the individuals belonging to a given line of descent.

PHYSICAL CARRIERS OF HEREDITY 

The only bridge between the parents and offspring are the gametes. These are sperms (in animals) and pollen grains (in plants) from the male parent and ova or eggs (in animals) and ovules (in plants) from the female parent. It means whatever traits or characters are transmitted from the parents to the progeny, are physically carried by way of gametes.

MENDELISM

Mendel's Laws are the most acceptable theories which explains the mechanism of heredity. This theory was postulated by Gregor Johann Mendel (1822 - 1884). He was an Austrian monk. He had great interest in hybridization in plants. He noticed certain contrasting characters in the garden sweet pea (Pisum sativum) plant in the garden of the church.
Mendel's Law of Dominance

He performed certain experiments of hybridization for about 7 years. He maintained a proper record of his results and then made certain generalizations and put forward certain laws of inheritance in 1866. To understand the mechanism transmission of characters from generation to generation one must understand the terminology used in heredity.

Why Mendel selected pea plant for his experiments ? 

Mendel had selected garden pea plant for the following
1. Many varieties were available having contrasting traits
2. Varietles were available in pure forms that bred true, i.e. produced the same type generation after generation. reasons
3. Peas are normally sell-pollinated because pea flower is bisexual.
4. Self-pollination could be prevented by removing corresponding reproductive parts of the flower.
5. Cross-pollination couid be done artificially because the size of the flower is very convenient.
6. Reproductive span of pea plant is very small and two crops can be produced in one year.
Mendel took different varieties of this plant showing seven pairs of contrasting characters.

Mendel's Laws

Mendel after a tireless and ceaseless experimentation on sweet pea (Pisum sativum) plant made certain observations and postulated the following three  Mendel's Laws :
1. Mendel's Law of Dominance
2. Mendel's Law of Segregation
3. Mendel's Law of Independent Assortment

 Mendel's Law of Dominance (First Law among Mendel's Laws) : 

Mendel's Law of Dominance states that in heterozygous (hybrid) condition, out of the two contrasting alleles, one expresses itself morphologically and the other remains unexpressed. The factor or allele which expresses itself phenotypically in the presence of its contrasting allele is called dominant and the other which remains unexpressed is called recessive. e.g., when a pure tall plant is crossed with a dwarf plant, the whole of the progeny produced is Tall. The law named as Mendel's Law of Dominance. Mendel's Law of Dominance was first law among Mendel's Laws.

Mendel's Law of Segregation (Second Law among Mendel's Laws): 

Mendel's Law of Segregation states that the two factors or genes controlling one character separate or segregate without blending or influencing each other during the formation of gametes so that each gamete receives one factor or gene for each character. It means gametes are always pure, hence, it is also known as law of purity of gametes together with Mendel's Law of Segregation.
Mendel's Law of Segregation

Explanation : In Mendel's Law of Segregation, Mendel postulated this law based upon his monohybrid crosses. For example, he crossed a pure breeding tall plant with a dwarf plant. The F1 generation produced was all tall. On selfing the F1 plants he raised F2 generation. He found that the F2 generation produced consisted of 75% tall and 25% dwarf plants i.e., in the ratio of 3 : 1.
In other monohybrid crosses for different contrasting characters the results are same.

From the above F, generation which is produced on selfing of F, generation, it is clear that 25% progeny is dwarf. These dwarf plants are possible from the F, tall plants only if ;
(i) the gene for dwarfness segregates or separates from the gene for tallness
(ii) gene for tallness does not blend or influence the gene for dwarfness who present simultaneously
(iii) phenotypically dominant individual may be homozygous (TT) or heterozygous (Tt). But recessive plants äre always homozygous i.e., tt for that character. It was all about Mendel's Law of Segregation.

Mendel's Law of Independent Assortment (Third Law among Mendel's Laws): 

Mendel's Law of Independent Assortment takes place when there are two or more pairs of contrasting character. Mendel's Law of Independent Assortment states that the factors or genes controlling different characters assort independently without influencing each other during the formation of gametes. The law named as Mendel's Law of Independent Assortment.

Explanation : Mendel explained this law on the basis of his dihybrid crosses. In such cases Mendel considered the inheritance of two different characters simultaneously. For example, shape of the seed may be round (dominant) and wrinkled (recessive) while the colour of the seed may be yellow (dominant) or green (recessive).

He took two pure breeding plants, one having round and yellow (RRYY) seeds and the other wrinkled and green (rryy) seeds. On cross-pollination F1 generation was raised in which all the plants produced had round and yellow seeds.

By selfing the F1 plants when F2 generation was raised he found four combinations in the following ratios :
(i) Round and yellow = 9
(ii) Round and green = 3
(iii) Wrinkled and yellow = 3
(iv) Wrinkled green = 1

Thus, you must remember that the two kinds of ratios in the F2 generation for monohybrid and dihybrid crosses are : Monohybrid ratios in F2 generation
(i) Phenotypic ratio – 3: 1
(ii) Genotypic ratio - 1:2: 1

Dihybrid ratios in F2 Generation
(i) Phenotypic ratio - 9:3:3:1
(ii) Genotypic ratio - Very Complex

INHERITANCE OF SEX 

The human species has 23 pairs of chromosomes. Each of the chromosome pairs numbered 1-22 have identical chromosomes and these are categorized as autosomes. But the 23rd pair is different. It is called sex chromosomes which are designated - as X and Y. XX pair with similar partners is found in females whereas XY pair with dissimilar partners is found in males.

During fertilisation there are equal chances that an ovum with X-chromosome of cell will be fertilised by an X bearing sperm or Y bearing sperm. The resulting zygote has its sex determined at the moment of fertilization.
If XX chromosomes are present, the child will be a girl.
If XY chromosomes are present, the child will be a boy.

Sex-Linked Inheritance 

It is the appearance of a trait which is due to the presence of an allele exclusively either on the X chromosome or on the Y chromosome. X-linked inheritance-Genes of certain characters are present on the X-chromosomes but their alleles are absent on the Y-chromosome.

That is why, they pass on from offspring alongwith the to parents X-chromosome. For example : Certain inherited defects, such as, colour blindness (inability to distinguish between certain colours especially red and green) and haemophilia (bleeders disease) (blood lacks clotting factor) are far more common in males than in females. Both the defects named above are due to recessive genes and both genes occur on the X-chromosomes.

The female with two X-chromosomes is unlikely to suffer from colour- blindness or from haemophilia because both the X-chromosomes may not carry the abnormal gene. The gene which may be abnormal on one X-chromosome being recessive, its influence will be hidden by the normal gene on the other X-chromosome.

The male with only one X-chromosome has only one gene for colour blindness or clotting factor. If that is the abnormal gene, then there is nothing on the Y-chromosome to mask it and colour-blindness or haemophilia results.

Consider the following three cases :
(a) Colour blind Male ( XcY) married with a woman with a normal vision (XX). Then all the daughters of such a couple will be carriers of the defect. All the son will have normal colour vision.
(b) Male with a normal colour vision (XY) married with a colourblind female (XcXc). Then all the son will be colour blind but, all the daughter will have normal vision but carriers.
(c) Colour blind Male ( XcY) married with a woman with one abnormal gene i.e. carrier (XXc). Then 25 % of daughters and 25 % of sons will be colour blind, 25 % of the daughters will be carriers and 25 % of sons will have normal  vision.

MUTATIONS 

A mutation is a sudden change in one or more genes. It alters the hereditary material of an organism's cells, thereby causing changes in certain traits.

For example:
(i) A gene mutation is caused by slight chemical changes in DNA. Sickle cell anemia is blood disease caused by a gene mutation. The mutation causes minor change in the DNA of a gene that controls the production of a person's red blood cells.
(ii) A chromosome mutation occurs if the number or arrangements of chromosome changes. Downs's syndrome also called Mongolism, is a mental and physical disorder which occurs if a person is born with one extra 21st chromosome i.e., 47 chromosomes.

Genetic engineering

Criss-cross Inheritance : This inheritance can be observed in X-linked crosses either in haemophilia or colour blindness. It is a type of cross in which a son inherits the disease from the mother and the daughter inherits the disease from her father. For example, a cross between normal carrier mother of the disease haemophilia and a normal father gives a son, he may suffer from haemophilia (mother to son).

In other case if a normal carrier mother of the disease haemophilia and a haemophilic father is crossed and a daughter is produced, she may suffer from haemophilia (father to daughter). This is known as criss-cross inheritance.

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