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 the 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.
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 ) .
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 ) .
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 .