Mendelian law of inheritance

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Mendelian law of inheritance
(All 5 images from wikimedia.commons.org (All public domain))

Mendel's two laws of inheritance, the law of inheritance and the law of independent assortment, form the basis of genetics. This has led to his characterisation as the "Father of Genetics." These laws describe basic and important rules for how traits are passed from parent to child. Many human traits, from eye colour to sickle cell anaemia, are inherited following patterns conforming to Mendel's laws. Modern genetics has done a great deal to expand on this original work, but the core observations describe phenomena relevant today.

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Facts

Mendel's work showed that individuals have two copies of every trait. One trait can mask a second trait. The dominant trait doesn't destroy the recessive trait, however. This allows a parent to pass on a trait that they do not exhibit. One example of this is in humans is eye colour. A parent with one copy each of the trait for blue eyes and brown eyes will have brown eyes but can pass the blue trait to her child.

Mendelian law of inheritance
The punnet square is a common visualisation of Mendel's laws.

Types

Mendel's first law is the law of inheritance. It says that when an individual makes a gamete (an egg or sperm), it gets only one of the parent's copies of any particular trait. Mendel's second law is the law of independent assortment. It says that when gametes are formed, which copy of one trait the gamete receives does not influence which copy of a second trait it receives. For example, a gamete getting a copy of the blue eye trait does not impact which copy of the hitchhiker's thumb trait it gets.

Mendelian law of inheritance
The dihybrid cross illustrates both of Mendel's Laws.

Exceptions

Since Mendel's work with pea plants, scientists have discovered many traits that do not follow these two laws. Not all genes independently assort. Genes that do not are called linked genes. This happens when genes are located close together on chromosomes. There are also violations of the first law, where a gamete gets both copies of a gene or entire chromosome from a parent. An example of this in humans is Down syndrome.

Mendelian law of inheritance
We can't see genes under a microscope, but we can see chromosomes.

History

Gregor Mendel spent his life as a monk studying how traits were passed from parents to offspring in pea plants. He could only see obvious physical differences between plants, so he examined characteristics such as flower colour and plant height. He began with purebred plants that when self-pollinated always produced a consistent result, like tall or short. By crossing these purebred varieties, he saw traits disappear in one generation of plants and return in the next.

Mendelian law of inheritance
Mendel's seven traits

Modern Considerations

Since Mendel's time, the language describing inheritance has changed to reflect understanding of the physical basis for his concepts. Mendel's term "trait" has been replaced with the term "gene." We now know that genes are located on chromosomes. The word "allele" refers to which copy of a gene is on a particular chromosome. For example, a person can carry an allele for brown eyes and an allele for blue eyes.

Mendelian law of inheritance
The location of genes on chromosomes gives physical grounding to Mendel's laws.

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