Concept of Genetics and Alleles – Law of Inheritance

Concept of Genetics and Alleles

Introduction:

Definition – Genetics is a branch of biology that deals with heredity, genetic variation, and transmission of traits from parent to offspring.

Genetics has opened a new field of life science. Scientists were eager to know how the traits are transmitted from one generation to another. Today scientists are studying human genes and their traits and diseases.

History of Genetics

The study of genetics is to understand heredity. The origin of genetics is the development of evolution by Darwin and Wallace, and later Gregor Johan Mendel worked on genetics in the 19th century. Mendel is known as the Father of Genetics.

Law of Inheritance by Gregor Mendel

During the 19th-century scientists started understanding inheritance. He selected a pea plant for his experiment. Garden pea (Pisum sativum), Mendel worked on it for 7 years and proposed laws of inheritances. Mendel’s investigation for the first time has done statistical analysis and mathematical logic was applied in biology.

Mendel choose seven distinct characteristics in the pea plant such as stem height, pod -shape, color, flower-color, position, seed color and shape.  This allowed him to prove the basic framework for inheritance.

Mendel Experiments

Mendel conducted 2 main experiments to determine the laws of inheritance. These experiments were:

• Monohybrid Cross Experiment- A monohybrid cross is defined between two individuals who have homozygous genotypes that are completely dominant or recessive alleles.
• A Monohybrid cross is carried out to study the offspring of homozygous individuals express the heterozygous genotype that inherits from the parent.
• For example – Mendel took opposite traits of the pea plant (tall-short) and crossed them. He observed that the first-generation offspring were tall and named as F1 progeny. He crossed F1 progeny, it results at a ratio of 3: 1.
• Another example is Huntington’s disease with genetic disorder. The homozygous dominant allele of the gene is paired with the homozygous recessive that is carried to the next generation.

• Dihybrid Cross Experiment- A dihybrid cross is carried out to follow the behavior of phenotypes of two genes through mating individuals which carry multiple alleles. When a homozygous dominant trait is crossed with a homozygous recessive trait in the first generation, a dominant allele hides the traits of recessive allele.
• He took two traits for example, wrinkled green seeds and round yellow seeds, and observed that a first generation F1 progeny were round yellow and concludes that dominant traits were round shape and yellow color. The two different generations were crossed, and it gives rise to four different phenotypes in F2 generations with ratio of 9: 3: 3: 1.

Alleles

Definition- alleles are a pair of genes that occupy a location on a chromosome and control the same trait.

In human chromosomes, alleles are present in pairs and maintain the same trait. Therefore, humans are diploid organisms. Two similar alleles are present in the genetic locus, where one allele is inherited from each parent.  An allele can be two or more variants of a gene at one genetic locus.

Dominant alleles are when the individual only has one copy of the allele (heterozygous). For example, the allele of brown eyes is dominant since you need one copy of the ‘brown eye’ allele to have brown eyes, although with two copies you will still have brown eyes.

When both alleles are dominant, then it is called as codominance. The characteristic is due to both alleles being expressed. For example, Blood group AB results as a codominance of A and B of dominant alleles.

Recessive alleles are when the individual has two copies of the allele (homozygous).  For example, the allele of blue eyes is recessive, to have blue eyes you need to have two copies of the ‘blue eye’ allele.

Multiple Alleles

Alleles or allelomorphs are the alternative forms of a gene present in the same location on the homologous chromosomes. Genes have more than two allelic forms, which are referred to as multiple alleles. Multiple alleles are situated on homologous chromosomes at the same locus. There is no crossing over between multiple alleles, crossing over is between two different genes. When multiple alleles are crossed, the phenotype is a mutant type and F2 generation form cross showing a monohybrid ratio.

Pseudoalleles

Pseudo alleles are genes located close to each other and they are similar in function and genetically linked. The Pseudo alleles term was proposed by Morgan and Lewis. Pseudo alleles are inherited together. The two genes will have a related function and act as a single gene.

Characteristic of Pseudo Alleles:

• These are closely linked allele within which crossing over occurs.
• They affect the same character.

Example – Rey eyed drosophila has different mutants like white.

Complementation Test

Multiple mutations of a single wild type phenotype has observed the simple test between two possibilities in the complementation test. The test is performed by two mutants that are crossed and the F1 is observed. The F1 is wild type phenotype where each mutation is in one of the two possible genes.

For example, eye color in drosophila is used to demonstrate the genes controlling eye color, each as X chromosome. The dominant wild-type allele of each gene produces deep red eyes. The mutant alleles produce a different color. If mutants from five genes are crossed, the F1 would express deep red eye color (wild type phenotype).