Principles of Chromosomal theory & Interpretation of Mendel’s Laws according to chromosomes theory

Principles of chromosome theory

Chromosomes are found in the somatic cells as homologous pairs ( 2n ) , Sex cells ( gametes ) contain half of chromosome number ( n ) due to meiosis where homologous pairs of chromosomes are segregated into two equal sets of chromosomes assorted into gametes .

Each pair of chromosomes behaves independently at its transmission in gametes , During fertilization ( fusion of male gamete with the female one ) the diploid number of chromosomes returns back again .

Genes are located on the chromosomes  and the single chromosome may carry hundreds of genes , The two scientists Sutton and Boveri had reached to the principle of the chromosome theory in 1902 .

Interpretation of Mendel’s Laws according to chromosomes theory :

In 1860 , after his experiments on pea plant Gregor Mendel had arrived into the following :

Each trait is controlled by one pair of genetic factors ( which later named genes ) that may be dominant or recessive .

Each pair of contrasting traits ( dominant and recessive ) is called allelomorphic characters ( alternative characters ) .

Mendel's first law

Mendel’s first law

Law of segregation of genes ( factors ) ( Mendel’s first law ) :

When crossing two homozygous ( pure ) individuals , one of them bears the dominant character and other bears the recessive character :

The dominant character appear at 100 % in first generation .

Both of the dominant and recessive characters appear in members of second generation in ratio 3 : 1 and these characters ( genetic pattern ) are called Mendelian characters ( traits of complete dominance ) .

In meiosis , The genes carried on the chromosome pairs are segregates into the gametes and During fertilization , The chromosomes return back again in pairs .

Law of independent assortment of genes ( Mendel’s second law ) :

When crossing two individuals bear two pairs or more of different allelmorphic characters , each pair of characters is inherited independently where : 

The two dominant characters appear at 100 % in members of first generation .

Both the two dominant and the two recessive characters appear in ratio 9 : 3 : 3 : 1 in members of the second generation .

Assortment of genes that carried on the chromosomes in gametes is independent because each gene is located on a separate chromosome .

 

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