Genetics

Understanding Inheritance Patterns in Genetics



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Heredity is the transfer of specific traits and characteristics from parents to their offspring. Since Gregor Mendel's famous pea-plant experiments, we have come to understand genetic inheritance down to specific genes with immense accuracy. Some traits are as simple as a single gene passed from parent to child through basic Mendelian inheritance. However, there are many traits that exhibit far more complicated inheritance patterns.

Between 1856 and 1863, Gregor Mendel cultivated close to 30,000 pea plants and studied the appearance of a variety of characteristics. The study showed that over all of his plants, a quarter showed recessive traits, half showed hybrid traits and another quarter were pure-bred dominant. This led to the development of the Law of Dominance, Law of Segregation and the Law of Independent Assortment, all of which would be later known as principles of Mendelian Inheritance.

The Law of Dominance holds that if an organism has two different versions of the same trait, like brown eyes and blue eyes for instance, one will be dominant. This tends to hold for the simplest traits, thought there are exceptions and these are explained below.

The Law of Segregation explains how traits are separated to be given to each of the offspring. The two copies of the inheritable trait, one from each parent, are separated and a copy is given to each gamete for reproduction. Each is equally likely to be the gamete passed on to the offspring. In turn, offspring generally receive one copy of the genome from each parent, resulting in hybrids of the traits of each parent.

The Law of Independent Assortment relates specifically to different traits. It says that each inheritable unit, later known as an allele of a particular gene, is assorted independently of the alleles of every other gene. As a result, offspring are a random assortment of all the possible alleles from each parent combined.

Before we can dive deeper into these inheritance laws and explain the weaknesses and limitations to them, we must explore some basic terminology. First have the gene, which is the basic unit of heredity encoded by our DNA. Next we have alleles, which are the possible variations of a given gene. We receive one set of alleles from each parent. For instance, when determining eye color, we may receive an allele for brown eyes from one parent and another allele for blue eyes from the other parent. If an organism has two of the same allele of a gene, they are considered "homozygous". An organism with two different alleles is "heterozygous".

In the world of simple genetic inheritance, you have two kinds of alleles: dominant and recessive. Dominant alleles are usually denoted by an uppercase letter (B would be brown eyes, the dominant allele for eye color) while a recessive allele is usually indicated by a lowercase letter (b would blue eyes, the recessive allele). If an individual receives one dominant allele and one recessive allele, they are heterozygous and will express the dominant characteristic. They will only display the recessive characteristic with two copies of the recessive allele.

The genotype of an organism is its complete genetic makeup, defined by all the genes that are encoded in your DNA. For instance, we could say you are heterozygous for the eye-color gene and your genotype would be Bb. The resulting phenotype is the expression of those genes into specific traits or characteristics. In this instance, your phentotype would be "brown eyes".

However, all of these cases are simple in that they have only one gene, with very clear dominant and recessive alleles. Most genetic traits are not so easy to characterize. Instead, there are a number of different kinds of dominance as well as a host of other ways that genes can be carried over to offspring beyond simple independent assortment.

The first exception to these rules is "incomplete dominance". In this case, a heterozygous individual does not simply express the dominant allele. Instead, there is only partial expression. For example, in primroses and similar plants, one might have a copy of the red allele and a copy of the white allele for the flower. However, when expressed the resulting plant will be pink in color.

Another is called "codominance". Here we have heterozygous individuals that express the both alleles simultaneously. If you can imagine a plant with a red allele and a white allele, codominance would be the instance where they are spotted red and white.

Beyond just how dominance appears with combinations of alleles, there are also exceptions to the rules of independent assortment. We now know that independent assortment exists because the limited number of human chromosomes are separated and a set is given to each of the two resulting gametes in the cell division known as meiosis. An instance where two genes are on the same chromosome could exist where these two traits are unlikely to show true independent assortment. Hair color and eye color are a good example of this. The two major genes for these traits are on the same chromosome, so the traits tend to move together. This explains why blonde hair is frequently seen together with blue eyes and brown hair is frequently seen with brown eyes.

Also, many traits are encoded by many genes and these are called "polygenic". While one gene encodes for brown versus blue in eye color, there are actually many genes that determine eye color. That is why eye color is such a wide spectrum, with many shades of brown, blue and green. Skin color is another great example. No one gene determines the total pigmentation of the skin. A whole array of genes determine different pigments and how they are expressed, creating the final skin tone of the individual.

Inheritance patterns are complicated and vary from trait to trait. Mendel's work highlight some basic concepts of inheritance that have been added to and altered as our understanding of genetics and inheritance has rapidly evolved.


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