Heredity: Laws, Processes and Mendel's Work on Inheritance

The process by which the features of one organism are passed from one generation to another and so on is called Heredity. The process is done through genes, which defines the characters in the organism. Heredity refers to the genetic heritage passed down by one’s biological parents.

Read Also: How do Organisms Reproduce?

The study of heredity is Genetics.

Inheritance refers to the transmission of traits or information. This occurs from one generation of species to the next.

Mendel and his work on Inheritance

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Gregor Johann Mendel began his experiments with plant breeding and hybridization. He postulated the inheritance laws in living organisms.

• Mendel, also known as the “father of Genetics”, worked on pea plants to understand the concept of heredity.
• His work laid the foundation of modern Genetics.
• He proposed the three basic laws of inheritance 

1. The Law of Dominance

2. The Law of Independent Assortment

3. The law of Segregation 

Read More About Rules for the Inheritance of Traits - Mendel's Contributions

Laws of Mendel

• Law of Dominance – This law says that a gene has two contrasting alleles and one always expresses itself in the organism.
• Law of Segregation – The Law states that traits get segregated completely during the formation of gametes without any mixing of alleles.
• Laws of Independent assortment – The law states that the traits can segregate independently of different characters during gamete formation.

Variation

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The difference in traits between the parent and offspring is called variation.

Types of variation:

• Somatic Variation
• Gametic Variation

Somatic Variation:

• It takes place in the body cell.
• It is neither transmitted nor inherited.
• It is also known as an acquired trait, for example, cutting tails of dogs.

Gametic Variation:

• It takes place in gametes/reproductive cells.
• It is both transmitted and inherited.
• It is also known as inherited traits, for example skin color.

Accumulation of Variation During Reproduction

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Whether the organisms multiply asexually or sexually, some variation always occur during reproduction. 

Variations in Asexual Reproduction:

Variations are fewer and occur due to inaccuracies in DNA copying which is known as Mutation.

Variations in Sexual Reproduction

Variations are large and occur due to crossover, separations of chromosomes, mutation.

Read More About Sexual and Asexual Reproduction

Importance of variation

• Variation helps to increase an organism’s chances of survival.
• Different individuals gain different advantages due to various types of variation.

Dominant and Recessive Traits

The traits that express themselves in an organism in every possible combination and can be seen are called Dominant traits and the traits which are not expressed in the presence of dominant allele are called recessive.

Monohybrid Cross

• When only one trait is considered while crossing two organisms, then such a cross is known as monohybrid cross.
• The ratio of characters that are produced due to this cross is known as the monohybrid ratio.
• If a tall plant (TT) is crossed with a dwarf plant(tt), we get 3 tall: 1 short plant at the end of F2 generation. Monohybrid ratio = 3:1

Dihybrid Cross

• When two characters are taken into account while crossing two organisms. This is known as a dihybrid cross.
• From this cross, characters arise in the ratio of F2 generation, called dihybrid ratio.
• The first plants would have round green peas.
• On crossing the same for F2 generation, we would observe four combinations characters in a ratio of 9:3:3:1 which is a dihybrid ratio.

Sex Determination

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• The process of determining the sex of an individual, based on the composition of genetic material is called sex determination.
• In humans, sex determination happens on the basis of the presence or absence of Y chromosomes.
• XX is female and XY is male
• An ovum always contains X chromosomes.
• An ovum fused with Y chromosome gives rise to a male child and upon fusion with X chromosome gives rise to a girl child.

Speciation is the process in which a new species is formed from existing ones due to several evolutionary forces like natural selection, genetic drift, isolation of populations, etc.

Gene Flow is the flow of genes from one population to the next.

A population is a group or community of any living organism that can reproduce amongst each other and have fertile offspring.

Evolution is a tangible transformation in the heritable characteristics of a species over several generations. These changes can give rise to new species or species might change themselves to become better adapted to the surrounding environment.

Charles Darwin is famously regarded as the “father of Evolution”. In 1859 Darwin published a book “Origin of Species”. In this he described his theory of evolution.

Evolutionary Relationships

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Evolutionary relations of animals can be deduced by studying the homologous organs and analogous organs.

Homologous organs are those which have a similar structure but different function whereas Analogous organs are vice versa.

Evolution by Stage

• Evolution is a slow process and doesn’t happen overnight.
• There are multiple stages of evolution that can be observed around us.

Artificial selection

• Sometimes a species evolves into various different types of species due to artificial selection.
• For example, the Cabbage family. A single ancestor in the cabbage family can give rise to multiple species. This is due to the selection of different traits.

Human Evolution

• Humans belong to the primate family.
• Humans nowadays have a very intimate genetic connection to primates such as chimps.
• Migration is an important factor responsible for the evolution of humans.