Unit – II D

Mendel’s Experiment (Dihybrid Cross)

Dihybrid Cross:

  • In organism, there are many characters and each character is controlled by respective alleles. To study whether one pair of allele affects or influences the inheritance pattern of pair of other alleles, Mendel performed dihybrid cross experiments.
  • In dihybrid cross he considered two traits simultaneously. Further Mendel performed trihybrid crosses and then he proposed the third law called law of independent assortment.
  • A cross between two pure (homozygous) pattern in which inheritance pattern of two contrasting characters is studied is called dihybrid cross.
  • It is a cross between two pure (obtained by true breeding) parents differing in a two pairs of contrasting characters.

Procedure of Dihybrid Cross Experiment

Step – 1: Selection of parents and obtaining Pure lines:

  • For dihybrid cross Mendel selected pea plant having yellow and round seeds as female parent and pea plant having green and wrinkled seeds as male parent. He obtained pure line by selfing these plants for three generations. He confirmed that pea plant having yellow and round seeds are producing yellow and round seeds and pea plant having green and wrinkled seeds are producing green and wrinkled seeds.

Step – 2: Emasculation, Dusting and Raising F1 Generation:

  • Emasculation

    Emasculation is a process of removal of stamens before formation of pollen grains (anthesis). This is done in bud condition. The bud is carefully open and all stamens (9 + 1) are removed carefully.

  • Dusting and Rasing F1 Generation:

    The pollens from selected male flower are dusted on the stigma of emasculated female flower. This is artificial cross. Mendel crossed many flowers, collected seeds and raised F1 generation. Yellow and round are dominant alleles, hence all F1 Generation was with yellow and round seeds. All the plants produced in F1 generation with yellow and round seeds (YyRr), which are heterozygous for both the alleles and are called dihybrid.

  • Punnett Square for F1 Generation:

Step – 3: Selfing of F1 hybrids to produce F2 Generation:

  • Mendel allowed natural pollination in each F1 hybrid; collected seeds separately and F2 generation is obtained.
  • Punnett Square for F2 Generation:

Mendel's Experiment

Observations of Dihybrid Cross Experiment:

Expectations:

  • Mendel expected the ratio of yellow and round seeds to green and wrinkled seeds to be 3:1.

Outcome:

  • He found seeds of four types yellow round, yellow wrinkled, green round and green wrinkled in the ratio 9:3:3:1.
  • Out of these four types, two were parental combinations. Viz. yellow round and green wrinkled and two were new combinations yellow wrinkled and green round.
  • In all Mendelian dihybrid crosses the ratio in which four different phenotypes occurred was 9:3:3:1. This ratio is called dihybrid ratio.
  • Phenotypic ratio i.e. the ratio of yellow round, yellow wrinkled, green round and green wrinkled in the ratio 9:3:3:1.

Mathematical Explanation of Mendel’s Law ofIndependent Assortment:

  • Meaning of the word assortment is ‘randomly and freely’. Thus probability theory is applicable to the dihybrid cross experiment.
  • By the basic principle of probability, “Probability of two independent events occurring simultaneously is product of their individual probabilities”
  • The probability of the first trait is 3:1 while that of the second trait is also 3:1. Thus the dihybrid ratio should be (3:1) x (3:1) = 3 x 3 : 3 x 1 : 1 x 3 : 1 x 1 i.e. 9:3:3:1
  • Genotypic ratio YYRR: YYRr: YyRR: YyRr: Yyrr: Yyrr:yyRR:yyRr: yyrr is 1:2:2:4:1:2:1:2:1.
  • Mendel performed ample dihybrid crosses and reciprocal crosses with different combinations. Every time he got the same pattern of result. The uniform expression was both dominant in F1 generation.In F2 generation always he got both dominant in large number.

Mendel’s Third Law of Inheritance (Law of Independent Assortment):

  • When two homozygous parents differing in two pairs of contrasting traits are crossed, the inheritance of one pair is independent of other.
  • In other words, when a dihybrid (or polyhybrid) forms gametes, assortment (distribution) of alleles or different traits is independent of their original combinations in the parents. This law can be explained by help of dihybrid cross and dihybrid ratio.
  • It is immaterial whether both dominant characters enter the hybrid from the same or two different parents but the segregation and assortment remains the same.
  • The appearances of new combination proves the law.
  • The law is universally applicable.

Reasons of Mendel’s Success:

  • The choice of garden pea plant was excellent.
  • As Mendel has mathematical background he applied probability theory to the experiment. For this he counted the plants, seeds personally and kept complete record of every cross.
  • He started with one, then two traits and so on, while before him bilogists tried multiple traits simultaneously.
  • Fortunately, the phenomenon observed in modern genetics like linkage, co-dominance, incomplete dominance, gene interaction was not shown by the garden pea plant.
  • During emasculation and dusting he took utmost care that the no plant under observation is not getting contaminated with foreign pollen

Importance of Mendel’s Laws:

  • The concept of dominant and recessive factors is very important. This character is shown by many hereditary traits.
  • It gives an idea of new combinations of traits which are very useful in developing a desirable trait in a progeny.
  • This information is particularly used in the field of plant and animal breeding. Thus a new type of plants and animals can be produced by hybridisation.

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