Class 10 Science chapter Heredity and Evolution Biology

Mendel's Experimentation 

After obtaining plants pure for their characters Mendel started his experiments. Mendel did not consider a number of characters at the same time. He first conducted experiment by considering only a single character. This cross dealing with only a single character at a time is termed as monohybrid cross. Similarly he conducted experiments by taking two characters at a time. This was termed as dihybird cross.

Mendel's Monohybrid Cross 

In one of his experiments Mendel selected plants having contrasting characters for height, i.e. he selected long plants (above six feet) and dwarf plants (about one foot). Being pure for their characters, the plants were homozygous long and and homozygous dwarf. These plants represented the parental  (P) generation. He cross-pollinated the long and dwarf plants. He collected the seeds obtained from this cross and grew them. The plants obtained from this cross were long, none of them was dwarf or of intermediate size. This generation was called first filial generation (F¹). 

Mendel self-pollinated the plants of f¹ generation and the seeds obtained were grown again. This generation of plants obtained was called second filial generation (F²) . The F² plants included both along and dwarf plants in the ratio of 3 : 1. 

Mendel self pollinated the F2 plants. The seeds thus  obtained were raised to plants. These plants constituted the third filal generation F3. Mendel observe that dwsrf plants of F2 generation produced only dual plans. Of the F2 tall plants only one third produced only tall plants hence were cold plants or pure toll plants while the remaining to 3rd produced tall and dwarf plants in the ratio of (hence work old hybrid tall plants).

 INTERPRETATIONS MADE BY MENDEL 

On the basis of his observations, Mendel concluded that: 

• 'something' present in the gametes of parents makes the characters appear in the next generation. Mendel called this 'something' as factors (now called Gene's) .

• Each character is governed by a pair of factors height of pea plant is governed by two factors. One for longness and other for dwarfness. 

• F¹ offsprings exhibit only one of the parental form of a trade and not the other oggsprings show longness but not dwarfness.

• The factors of each character segregate during gamete formation so that each gamete receives only one factor for each character and is always pure.

Representing Crosses by Symbols 

In Genetics the various crosses are shown by using symbols. In the crosses are represented by using alphabets symbols. For example, T and t alleles may represent long and short stem height of pea plant. The dominant allele is represented by capital letter and the recessive allele by a small letter. For example, the dominant allele of plant height may be represented by T and the recessive allele by t. Similarly 'x' between two individuals in the present mating or crossing. Homozygosity for a Particular trait is represented by two capital letters or two small letters. For example, a pea plant homozygous for plant height may be represented by TT (long) or tt (dwarf). 

By using symbols we can represent the monohybrid cross conducted by Mendel for plant height as given below : 

The use of symbols in crosses helps us to know whether individual is homozygous or heterozygous  for a particular trit and which type of progeny can be obtained from it.