Law of Segregation and Law of Dominance

Introduction

Gregor Johann Mendel (1822-1884) laid the foundation for the development of genetics as we know it today. He performed several monohybrid and dihybrid crosses using seven different qualitative traits of pea plants. What he observed from his rigorous experiments with the pea plant helped deduce the laws of inheritance, which are the basis of heredity for sexually reproducing diploid organisms.

Experiments

Mendel conducted rigorous hybridization experiments (1856-1863) with the pea plant, Pisum sativum in which he crossed different varieties of pea plants with each other. Mendel chose to work with following characters:

Characters	Dominant	Recessive
Stem length	Tall	Dwarf
Seed form	Round	Wrinkled
Seed color	Yellow	Green
Flower color	Violet	White
Flower position	Axial	Terminal
Pod shape	Inflated	Constricted
Pod color	Green	Yellow

Monohybrid Crosses

A monohybrid cross involves breeding betweenl two individuals that bear contrasting traits for a particular character.

• Mendel crossed two pea plants (the P generation) that bear contrasting traits for a particular character. For example, he crossed a pea plant that produces yellow seeds with one that produces green seeds.

• The offspring produced was called the F1 generation. Self-fertilization of F1 produced the F2 generation which yielded both the parental characters.

According to Mendel, the offspring inherits two discrete “factors”, one from each parent, but expresses only the more dominant factor of the two. The expressed character was termed the dominant trait (represented by a capital letter), while the other trait which didn’t show up in F1 was termed recessive (represented by a small letter).

Dihybrid Crosses

The breeding of two individuals with contrasting characters is called a dihybrid cross

• Mendel crossed true-breeding pea plants homozygous for two contrasting characters with each other.

  For example − Mendel crossed a pea plant that produces wrinkled and green seeds with one that bears round and yellow seeds.

• The F2 generation produced after selfing of the F1 generation produced all the combinations of the two parental characters.

  For example − round and yellow, wrinkled and yellow, wrinkled and green and round and green seeds.

He repeated both crosses with all the other characteristics as well. Mendel carefully recorded the observable phenotypes and the frequency of their occurrences in the F1 and F2 generations.

Results

Monohybrid Crosses

• The genotypic ratio and phenotypic ratio in the F1 generation were 1, i.e., all plants uniformly expressed only one characteristic.

• In the F2 generation, the genotypic ratio was 1: 2: 1; (AA) : (Aa) : (aa).

• The phenotypic ratio of the F2 generation was 3:1 (dominant trait: recessive trait).

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Dihybrid Crosses

• The genotypic and phenotypic ratio produced by F1 was 1

• The F2 generation produced a genotypic ratio of 1: 2 : 2: 4 : 1: 2: 1 : 2: 1.

• The phenotypic ratio of this cross was 9: 3: 3: 1 (round, yellow: wrinkled, yellow: round, green: wrinkled, green).

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Mendel's Laws of Inheritance

Over a long period of seven years of rigorous experimentation on various varieties of pea plants, Mendel observed some specific patterns of inheritance, which were put forward as Mendel’s Laws of Inheritance.

The Law of Dominance

• From his monohybrid crosses, Mendel observed that only one trait from either parent is expressed phenotypically (i.e., is observable) in the offspring.

• Mendel proposed that a diploid organism carries two versions (alleles) of a particular character. These versions (or copies) may be the same (homozygous) or contrasting (heterozygous).

According to Mendel’s law of dominance,

In a heterozygous organism, the expression of one allele is always repressed due to the presence of another allele which is termed the “dominant allele”.

The Law of Segregation

Mendel deduced the following law from his monohybrid crosses when the F2 generation produced both parental traits.

During the formation of gametes in diploid organisms, each of the two alleles determining a particular character is segregated into individual gametes. Each gamete receives only one allele.

The Law of Independent Assortment

Considering the new combinations of traits different from the parental phenotypes that were produced in the F2 generation of his dihybrid crosses, Mendel put forth the following law −

In a diploid organism, the segregation of the alleles encoding one character is not influenced by the presence of alleles encoding another character (i.e., segregation of alleles is purely independent).

Rediscovery of Mendel's Theory

• Mendel’s efforts hardly gained any recognition when he first published them.

• This changed in 1900 with the rediscovery of his work by Hugo de Vries, Carl Correns, and Erich von Tschermak.

• William Bateson, the famous English biologist was a great advocate of Mendel's work. He studied his experiments and coined the term “genetics”, “allelomorphs”, “homozygote” and “heterozygote” to explain and give current scientific relevance to Mendel’s work.

• The most significant contributor to the rediscovery was Wilhelm Johannsen, who in 1909 coined the terms “gene”, “phenotype” and “genotype” for the factors/ traits of Mendel.

• Mendel’s work was republished in 1901, in the journal “Flora”

• The significance of Mendel’s work is profound, and upon eventual recognition, he was crowned the Father of Genetics.

Significance

• Mendel’s factors are now known as genes, which are the basic units of heredity.

• His idea that a diploid organism carries two copies of a factor has now translated into alleles, which are the alternative forms of a particular gene.

• Mendelian laws have made the study of patterns of hereditary diseases easier. The three laws have also been applied to the study of family histories, by conducting pedigree analysis.

General Mendelian Deviation

Mendel’s laws are not universal. These laws fail in the cases of codominance and incomplete dominance, linked genes, non-allelic gene interactions, multiple alleles, and sex-linked traits.

Conclusion

Mendel's hybridization experiments with pea plants helped deduce the three laws of inheritance dealing with the principles of dominance, allele segregation, and their independent assortment into germ cells. However, his contemporaries failed to acknowledge the importance of his experiments. In 1900, finally, Mendel gained recognition and was eventually crowned the Father of Genetics.

The significance and application of these laws are profound. However, there are some exceptional cases where these laws don’t apply, such as incomplete dominance, codominance, cases of sex-linked inheritance, etc.

FAQs

Q1. Why did Mendel choose to work with Pisum sativum?

Ans. The pea plant offered various advantages such as easy cultivation, short life cycle and quick growth in just one season, predominant self-fertilization, easy hybridizations, and a large number of varieties, allowing the plant to be tested for various traits.

Q2. Does the dominance of alleles have any impact on the inheritance of the genes?

Ans. No. In a heterozygous condition, both, the dominant and the recessive alleles will be inherited. The dominance of alleles only determines the phenotype of an individual.

Q3. What does the term “incomplete dominance” imply?

Ans. It is a condition, wherein a heterozygote exhibits a phenotype that falls in between the phenotypes of the parents, i.e., the phenotype that does not resemble either of the parents fully.

Q4. How is a test cross performed?

Ans. A test cross is performed between the F1 generation (unknown genotype, dominant phenotype) and an organism with a homozygous recessive genotype.

Q5. Under what condition will a recessive trait be expressed?

Ans. A recessive trait is expressed only under homozygous conditions, i.e., aa.