Understanding Inheritance, Cell Theory, and the Origin of Life

Understanding Inheritance and Continuous Variation

This type of inheritance refers to continuous characteristics regulated by genes that are inherited independently.

Sometimes, continuous variation does not only depend on one quantitative inheritance but also on the influence environmental factors have on the genotype. The height and weight of a population are classic examples of continuous characteristics that are affected by environmental factors.

Mendel's Laws of Inheritance

Mendel's First Law: Law of Uniformity

When two pure-bred individuals are crossed, all offspring of the first filial generation (F1) have the same genotype and phenotype.

Mendel's Second Law: Law of Segregation

When two individuals from the first filial generation are crossed, the second filial generation (F2) have two types of phenotypes.

Mendel's Third Law: Law of Independent Assortment

When two individuals that have two or more different characteristics are crossed, each characteristic is transmitted independently from the rest.

Redi's Experiment: Disproving Spontaneous Generation

In 1668, the Italian doctor Francesco Redi demonstrated that the maggots found in rotting meat came from flies' eggs.

1. Redi placed pieces of the same type of meat in three jars.
2. The first jar he left open, the second one he covered with a thin piece of fabric, and the third one he closed with an airtight material (cork).
3. The three jars were left in the same place and, after a few days, he saw that there were maggots in the first two jars, but not the third one.

Pasteur's Experiment: Further Refuting Spontaneous Generation

The French chemist Louis Pasteur demonstrated that microorganisms were not created from spontaneous generation. The theory was finally rejected.

1. Pasteur placed meat soup in two flasks and bent their necks into an S-shape.
2. He sterilized the liquid by heating it. After several weeks he noticed that the soup had not decomposed.
3. He cut the neck of one of the flasks and after a few days he observed that the soup in this flask had decomposed.
4. He observed that the soup in the other flask remained unchanged.

Conclusion: The S-shaped neck let air into the flask, but microorganisms could not enter and cause decomposition.

Miller's Experiment: Simulating Early Earth Conditions

The North American scientist Stanley Miller put a mixture of gases, which were believed to be present in the primitive atmosphere, into a container and applied high-voltage electric shocks. The reaction produced several organic compounds, including amino acids.

However, nowadays we know that the primitive atmosphere was not made up of the gases described by Oparin. This is why the majority of experts think his hypothesis is unable to explain the origin of life.

Oparin's Theory: Abiogenic Origin of Life

The Russian biochemist Alexander Ivanovich Oparin developed an abiogenic theory about the origin of life on Earth. His theory is accepted by most scientists today.

1. The components of the Earth's primitive atmosphere reacted due to the energy of storms, solar radiation, and continuous volcanic eruptions. Simple organic molecules were created.
2. As the Earth cooled down, intense continuous rain was produced and the primitive seas were formed, where the organic molecules accumulated.
3. Organic molecules started to join, creating bigger molecules. Some of them were insulated from the water inside structures called coacervates.
4. Some of the coacervates developed molecules (nucleic acids) in their interior that were capable of self-replicating. These were the first organisms and the origin of all cells.