Dihybrid Cross and Inheritance of Two Genes

Introduction

Gregor Johann Mendel was the first person who discovered the basic principles of heredity during the mid-19th century. Hence, he is known as the father of modern genetics. He conducted experiments in his garden on pea plants and observed their pattern of inheritance from one generation to the next generation.

Dihybrid cross

Visualisation of dihybrid cross using a Punnett square

Gregor Mendel’s Experiment

Mendel laid the basic groundwork for the field of genetics, and he eventually proposed the laws of inheritance: Law of Segregation, Law of Independent Assortment and Law of Dominance. These laws came into existence from his experiments on pea plants with a variety of traits.

Mendel first studied the inheritance of one gene in the plant through monohybrid cross. He considered only a single character (plant height) on pairs of pea plants with one contrasting trait. Later, he studied the inheritance of two genes in the plant through dihybrid cross.

Mendel studied the following seven characters with contrasting traits:

  • Stem height: Tall/dwarf
  • Seed shape: Round/wrinkled
  • Seed colour: Yellow/green
  • Pod colour: Green/yellow
  • Pod shape: Inflated/constricted
  • Flower colour: Violet/white
  • Flower position: Axial/terminal

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What is Dihybrid Cross?

A dihybrid cross is a breeding experiment between two organisms which are identically hybrid for two traits. In other words, a dihybrid cross is a cross between two organisms, with both being heterozygous for two different traits.

Mendel took a pair of contradicting traits together for crossing; for example colour and the shape of seeds at a time. He picked the wrinkled-green seed and round-yellow seed and crossed them. He obtained only round-yellow seeds in the F1 generation. This indicated that round shape and yellow colour of seeds are dominant in nature. Meanwhile, the wrinkled shape and green colour of seeds are recessive traits. Then, F1 progeny was self-pollinated. This resulted in four different combinations of seeds in the F2 generation. They were wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green in the phenotypic ratio of 9:3:3:1.

During monohybrid crosses of these traits, he observed the same pattern of dominance and inheritance. The phenotypic ratio 3:1 of yellow and green colour and of round and wrinkled seed shape during monohybrid cross was retained in dihybrid cross as well.

Consider “Y” for yellow seed colour and “y” for green seed colour, “R” for round shaped seeds and “r” for wrinkled seed shape. Thus, the parental genotype will be “YYRR” (yellow-round seeds) and “yyrr” (green-wrinkled seeds).

The dihybrid cross of both will give F1 progeny YyRr (yellow-round seeds) as shown in the figure above.

Further Reading:

Frequently Asked Questions

1. Who is known as the father of modern genetics?

Gregor Mendel is known as the father of modern genetics. He was awarded this honour for his experiments which laid the groundwork for genetics and inheritance.

2. What is an example of a Dihybrid cross?

Mendel’s experiment with peas is a classic example of a dihybrid cross. The experiment was done to highlight if any relationship exists between various pairs of alleles.

3. How to find the genotype of a Dihybrid cross?

  1. The first step would be to establish a parental cross (P).
  2. Next, make a 4×4 (or 16 square) Punnett Square for the chosen traits to be crossed.
  3. Ascertain the parents’ genotype and assign letters to represent the alleles – use lower case letters for recessive traits and upper case letters for dominant traits.
  4. Arrange the traits on the square – the logic is that recessive traits emerge only if both the parents have recessive traits. For example, if both the parents have the trait “f“, which is recessive, the emerging trait will be (“ff”). However, if one of the parents have “F“, then the resulting trait will be “Ff“, but never “fF.

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