Mendal's Laws of Inheritance: Law of Segregation and Law of Independent Assortment

Mendel used two types of crosses to prove his Law of Segregation and Law of Independent Assortment.

Monohybrids and monohybrid cross:

The offspring produced by the cross fertilization between two plants differing in one character are called monohybrids. The cross in which one trait is followed at a time is called monohybrid cross.

Mendel formed 14 pure-breeding lines of seven characters. Then he cross-fertilized plants that differed in only one character. The offspring of such a cross were called monohybrids. He crossed a true breeding round seed male plant with a true breeding wrinkled seed female plant. Mendel used monohybrid cross to prove his law of segregation.

Dihybrids and dihybrid cross:

The offspring produced by the cross fertilization between two plants differing in two characters are called dihybrids. The cross in which two traits are followed at a time is called dihybrid cross. Mendel used dihybrid cross to prove his law of independent assortment.

LAW OF SEGREGATION

The law states that the alleles of gene present on the homologous chromosomes segregate during meiosis in such a way that each gamete get one allele not both.

There are two colours of seed of pea plant. Yellow (YY) and green (yy). Yellow colour is dominant to green. Mendel used true breeds for his experiments.

F₁ Cross: The yellow colour seeds are crossed with the green colour seeds. The offspring of this cross have yellow colour seeds in F₁ generation.

F₂ Cross: The F₁ seeds are crossed with each other. The progeny of F₂ generation has:

• One fourth are green colour seeds
• Three-fourths are yellow colour seeds

The green colour characteristic disappears in the F₁ generation. But it reappears in the F₂ generation. The ratio of yellow colour seeds to green colour seeds in the F₂ generation is approximately 3:1.

Gene: A unit of heredity which is transferred from a parent to offspring and have a particular characteristics or function.

Alleles: The alternative forms of gene are called alleles.

Dominant allele: The allele that hides the expression of recessive allele. For example, the allele Y (for yellow colour) is dominant to allele y (for green colour).

Recessive allele: The allele whose expression can be masked or hidden. For example, green colour allele (y) is recessive.

Representation of alleles: Crosses are expressed by letter or letters. These letters are descriptive of the trait in question. The first letter of the description of the dominant allele is commonly used. In fruit flies, the mutants are compared with a wild-type. The symbol is taken from the allele that was derived by a mutation from wild condition. A superscript "+" is written next to the symbol. It represents the wild-type allele. A capital means the mutant allele is dominant. A small letter means the mutant allele is recessive.

Phenotype: The observable characteristics or physical expression of a gene is called phenotype. For example, seed colour yellow or green is phenotype.

Genotypes: The genetic makeup or genes combination is called the genotype. For example, the phenotype yellow seed colour has genotype YY or Yy and phenotype green seed colour has genotype yy.

Homozygous: An individual having two identical alleles of a particular gene. For example, homozygous dominant (YY) or homozygous recessive (yy).

Heterozygous: An individual having two different alleles of a particular gene. For example, heterozygous (Yy).

LAW OF INDEPENDENT ASSORTMENT

          It states that when alleles of more than one trait are followed together in cross, the alleles of these traits assort independently to each other during gamete formation.

In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.

Example: Pea seed colour and pea seed shape genes

Let's look at a concrete example of the Law of Independent Assortment. Imagine that we cross two pure-breeding pea plants: one with yellow, round seeds (YYRR) and one with green, wrinkled seeds (yyrr). Because each parent is homozygous, the law of segregation tells us that the gametes made by the wrinkled, green plant all are ry, and the gametes made by the round, yellow plant are all RY. That gives us F₁ offspring that are all RrYy.

The allele specifying yellow seed color is dominant to the allele specifying green seed color, and the allele specifying round shape is dominant to the allele specifying wrinkled shape, as shown by the capital and lower-case letters. This means that the F₁ plants are all yellow and round. Because they are heterozygous for two genes.

When Mendel did this cross and looked at the offspring, he found that there were four different categories of pea seeds: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. These phenotypic categories appeared in a ratio of approximately 9:3:3:1.

This ratio was the key clue that led Mendel to the Law of Independent Assortment. That's because a 9:3:3:1 ratio is exactly what we'd expect to see if the F₁ plant made four types of gametes (sperm and eggs) with equal frequency: YR, Yr, yR, and yr.

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