Wednesday, July 22, 2020

GENETICS 4

TEST CROSS
To test whether homozygous or hetero..

Unknown genotype offspring × recessive parent

If heterozygous, result is 1:1

DIHYBRID CROSS

Mendel also worked with two characters on pea plant. He chose color and shape of the seed to explain the inheritance of two genes.

Y – dominant yellow color

y – recessive green color

R – round shape of the seed

r – wrinkled shape of the seed


Phenotypic Ratio:

Round yellow : round green : wrinkled yellow : wrinkled green

        9  :   3    :    3    :   1

So, monohybrid ratio phenotype ratio 3:1 is followed in dihybrid cross also.

Both colour 3:1

Shape 3:1 


LAW OF INDEPENDENT ASSORTMENT

Segregate and then inheritated seperately

So, equal chance for each gamete to carry an allele, and then express itself in offspring.


CHROMOSOMAL THEORY OF INHERITANCE

  • Mendel published his work on inheritance of characters in 1865 but was unrecognized till 1900.
Why remain unrecognised?
1. Lack of communication, not published widely
2. Mendal- 1 gene- controls - 1 character
     1gene- 2 alleles- 2 different character
     Which do not blend with each other
   So, unacceptable.
3. Mathematics and statistics
4. Lack of proof, (regarding factor/gene)

  • In 1900, de Vries, Correns and von Tschermak worked independently and rediscovered Mendel’s results.

Imp reason of Rediscovery-- microscopy- cell division- chromosomes 

  • In 1902, Walter Sutton and Theodore Boveri studied the chromosomal movement during meiosis.
  • They observed- behaviour of chromosome is like the behaviour of mendal factors.
  • According to this theory, (combination of these 2)

  1. Genes are located at specific locations on the chromosomes.
  2. Chromosomes as well as gene both occur in pairs.
  3. Homologous chromosomes + genes separate during meiosis.
  4. Fertilization restores chromosome number to diploid condition.
  5. Chromosomes + genes segregate as well as assort independently.

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