#1 A-Z: ALLELES AND CHROMOSOMES

People tend to notice that brothers and sisters (or even cousins) look alike. Because you are a smart human being, you know that your physical appearance is controlled by a masterful molecule called DNA. And you know that as siblings share the same parents, they share DNA, and hence look similar. 

DNA resides within the nucleus of every cell, forming a complex network called chromatin when paired with with storage proteins called histones. During cell division, this chromatin condenses into the familiar structures we call chromosomes ¹. 

What lies within these chromosomes, and how do they influence our traits? 

The answer lies in genes- the fundamental units of inheritance. Genes are sections of DNA located at specific positions on the chromosome ², called loci. Genes are passed on from parents to offspring and contain the necessary information to make you, you. 

An excellent diagram I found illustrating the concept ³.

Genes code for characters, and a trait is a form of a character. For example, if the character I am studying is 'height', then the traits could be 'tall' or 'short'. If 'colour' was the character, traits could be 'purple', 'yellow', 'red', 'green', 'magenta' and so on.

But hey, you may ask, what's all this gotta do with an allelle?

An allele is responsible for a trait! Alleles (or allelomorphs) are variant forms of a gene. Taking the previous example, a gene would be the 'height' gene and its alleles are 'tall' and 'short'. These alleles are expressed as physical traits, giving rise to your phenotype, the observable physical characteristics that define you. But under the surface, your genotype, your actual genetic makeup, is a unique combination of alleles inherited from both parents. 

Homologous chromosomes are pairs of chromosomes that have similar structures and carry alleles coding for a particular character. One chromosome in each pair comes from the father, and the other from the mother. Humans are called diploid organisms because we have two sets of alleles, with one set inherited from each parent. 

An illustration of homologous chromosomes, with flower colour as the gene/character ⁴. 

Heredity is basically just that- the transfer of traits or alleles from parent to offspring. 

For centuries, people sort of just assumed our parents' traits blended in us. The early ideas about how heredity occurs were sometimes really off the mark. Aristotle, for example, suggested an organism was a mixture of its parents' traits- which transmitted through semen (he considered it to be a purified form of blood) and the mother's menstrual blood. (You can blame Aristotle for pretty much inventing the term 'bloodlines'.)

The real breakthrough in our understanding of heredity came with the work of Mendel (but that's a story for another blogpost).

Coming back to alleles. 

Alleles occupy specific gene loci. Most gene loci are highly polymorphic, meaning they have multiple alleles (such as in the colour example stated earlier). The majority of polymorphisms are silent, meaning they do not alter the expression of a gene- thus, a great deal of genetic variation is hidden in the form of alleles that do not produce obvious phenotypic differences. 

In populations such as Drosophila, the term 'wild type' describes alleles associated with the typical phenotype in "wild" populations.  It is historically regarded as leading to a dominant, common, and normal phenotype, in contrast to "mutant" alleles that lead to recessive and rare phenotypes.

The TLDR version: In conclusion, alleles are the variations that underlie the rich tapestry of traits that make each individual unique. They occupy specific positions on chromosomes, influencing our physical characteristics, and are the vehicle through which heredity operates. Understanding the intricacies of alleles allow us to unravel the fascinating story of genetic inheritance. 

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¹ The majority of eukaryotic genes are stored on a set of large, linear chromosomes. Prokaryotes, on the other hand, store their genomes on a single circular chromosome. 

² In 1902-03, Walter Sutton and Theodore Boveri published independent papers proposing what we now call the "chromosomal theory of inheritance". This theory states that individual genes are found at specific locations on particular chromosomes, and that the behavior of chromosomes during meiosis can explain why genes are inherited according to Mendel’s laws. And our favourite Drosophilist, Thomas Hunt Morgan, confirmed it. 

³ Picture credits to https://my.clevelandclinic.org/

⁴ Picture credits to https://jackwestin.com/