What are alleles

Introduction

Alleles are different versions of the same gene. Genes are stretches of DNA that code for proteins (or sometimes RNAs), and alleles are the variations that cause differences in how these proteins are made or how they function.

• Think of a gene as a recipe, and alleles as variations of that recipe.
• The gene decides what protein is made; the allele decides how it's made or how it behaves.

Basic Genetic Structure: Where Alleles are found

In humans and most organisms, DNA is organized into chromosomes. You inherit:

• One chromosome from your mother
• One chromosome from your father

So for every gene, you usually have two alleles, one on each chromosome of the pair.

• If both alleles are the same, you're homozygous.
• If they are different, you're heterozygous.

As an Example let's take a look at your Eye Colour:

Let's say there's a gene that controls eye colour (this is simplified for clarity).

• One allele code for brown eyes.
• Another codes for blue eyes.

Brown is dominant, and blue is recessive.

• If you inherit one brown and one blue allele: you'll have brown eyes, because the dominant allele masks the recessive one.
• You'd be heterozygous in this case (Bb).
• If you have two blue alleles: bb = blue eyes.
• Two brown alleles: BB = brown eyes.

Inheritance Patterns that Follow Mendelian Inheritance in Humans

Mendelian inheritance refers to the way certain traits are passed down from parents to offspring based on the principles discovered by Gregor Mendel. These traits are controlled by a single gene with two alleles - one inherited from each parent - and follow predictable dominant and recessive patterns.

Let's explore some common human traits that follow this pattern:

Attached and Detached Earlobe -> The extent to which the earlobe is attached to the head follows a Mendelian pattern of inheritance.

• The attached earlobe is a recessive trait.
• The detached earlobe is a dominant trait.
• This means:
  • A person will only have attached earlobes if they inherit two recessive alleles (homozygous recessive).
  • If a person has detached earlobes, they may have one or two dominant alleles (heterozygous or homozygous dominant).
  • So, if someone has attached earlobes, they must have two identical recessive copies of the gene responsible for this feature.

Widow's Peak

A widow's peak is the distinctive 'V'-shaped point in the hairline in the centre of the forehead. This trait is also inherited in a Mendelian fashion.

• Widow's peak is caused by a dominant allele.
• The absence of a widow's peak (a straight hairline) is recessive.
• Therefore, anyone who has a widow's peak must have at least one dominant allele.
• Only individuals with two recessive alleles will have a straight hairline.

Dimples on the Cheeks

Dimples are small indentations that appear on the cheeks when a person smiles. They are caused by a shorter muscle pulling the skin inward, and are an example of a dominant trait.

• Having dimples is a dominant trait.
• Not having dimples is recessive.
• This means: If you have dimples, you likely carry at least one dominant allele for this trait. If you don't have dimples, you must carry two recessive alleles. Interestingly, having a dimple on only one cheek is rare - and if you do, that's quite unique!

If you're still unsure about this concept, I recommend checking out the lesson on 'Pedigree Analysis', where we've explained these same characteristics using simple and easy-to-follow pedigree charts.

Non-Mendelian Inheritance

Non-Mendelian Inheritance refers to inheritance patterns that do not follow Mendel's Laws of Inheritance. To easily identify Non-Mendelian Inheritance, you can look for Phenotypes that do not appear in ratios predicted by Mendelian Genetics. These patterns include codominance, incomplete dominance, multiple alleles, polygenic inheritance, mitochondrial inheritance, epistasis, and epigenetics. Let's explore each of them!

Codominance

• In certain traits, at heterozygote state, expression of both alleles contributes equally to the phenotype. Such a phenomenon is known as Codominance.
• For example, a person with AB-blood group type has both A and B carbohydrates on the surface of RBC at the same time. The enzymes encoded by I^A and I^B alleles of a single gene add the above-mentioned carbohydrates to the surface of RBC.
• So, if simply explained:

• I^AI^A alleles will carry A carbohydrates on RBC - A-Blood Group
• I^BI^B alleles will carry B carbohydrates on RBC - B-Blood Group
• I^AI^B alleles will carry both A and B carbohydrates on RBC - AB-Blood Group
• I^Ai will carry A carbohydrates on RBC - A-Blood Group
• I^Bi will carry B carbohydrates on RBC - B-Blood Group
• ii will carry no carbohydrates on RBC - O-Blood Group

Incomplete Dominance:

• The phenomenon of dominant allele completely masking the recessive phenotype, resulting in similar phenotypes for both homozygous dominant zygote as well as heterozygous zygote is called complete dominance.
• The phenomenon of expressing blend phenotypes from both alleles is called incomplete dominance.
• For example, let's take a plant with Red and White flowers, and let's assign C^R for Red flower and C^W for white flower. Now, consider we are just doing a general cross between purebred C^R and C^W plants. The First generation will always result in pink flowers shown as C^RC^W. This phenomenon where both blends to form a new phenotype is known as incomplete dominance.
• also, when we cross between pink flowered (C^RC^W) plants and red flowered (C^RC^R) plants. We can observe the first generation will produce pink flowers which have more red hue in them. This is influenced by the Incomplete Dominance of C^R allele.
• This can also be seen in humans in Hair Texture where C^SC^S is responsible for straight hair and C^CC^C is responsible for curly hair and C^SC^C is responsible for Wavy Hair (Incomplete Dominance).


• NOTE: We don't use R or W to show the alleles because neither of the allele is dominant so, we use superscripted C^R and C^W to show the alleles in incomplete dominance.

Multiple Alleles:

• Some genes have more than one two possible alleles, though an individual still inherits only two (one from each parent).
• Please observe the example on Codominance, where the ABO Blood group gene has three alleles, I^A, I^B, i. These alleles determine the blood group. So, the ABO blood group is a great example for Multiple Alleles.

Polygenic Inheritance

• Polygenic Traits are controlled by many genes working together, each contributing a small effect which leads to a continuous variation in the phenotype.
• A great example for this is Skin Colour of Humans. For simplicity let's consider three alleles that determine the skin colour. And let's call them A, B, C. Each allele contributes to one 'unit' of darkness. So, a person with AABBCC will have very dark skin. Whereas a person with aabbcc will have a very light skin and all the other genotypes will have colours between them.

Mitochondrial Inheritance (Maternal Inheritance)

• Unlike Nuclear DNA, mitochondrial DNA is only inherited from the mother because mitochondria in sperm cells are typically destroyed during fertilisation.
• Traits determined by this inheritance only travel through the maternal line. But both son and daughter can be affected but only daughters will pass the trait to their children.

Epistasis

• Epistasis is when one gene masks or modifies the expression of another gene at a different location. So, the phenotype you see is not just controlled by one gene but depends on how other genes behave.
For example, let's consider the coat colour of mice. Let's say Gene A determines pigment colour (B = Black, b = brown). And Gene B determines whether the pigment is deposited or not (C = pigment is deposited, c = pigment is not deposited). If a mouse has cc it will be albino regardless of Genotype at Gene A. So if Gene B is homozygous recessive, it will completely mask the Gene A. This is known as recessive epistasis.
• Let's look at the colour of plumage of house fowls. Let's say 'C' is responsible for producing colours in the feathers. 'c' will result in no pigmentation. And let's say there is a Gene called 'I' which suppresses the expression of colour (or Gene C). while 'i' is unable to prevent colouration. In this case, Dominant homozygous or Heterozygous Gene I will result in the fowl's plumage being white (regardless of the Genotype of Gene C). While the homozygous recessive Gene i will result in the fowl being coloured (only if Gene C is Homozygous Dominant or Heterozygous). This is known as Dominant Epistasis.

Epigenetics

• Epigenetic inheritance refers to heritable changes in gene expression that do not involve changes to the DNA sequence. This is due to switching on and switching off of certain genes by modifying nucleotides of a DNA sequence by histone modification, methylation and demethylation, where methyl groups are added to wild type DNA sequence or else removed from a methylated DNA sequence. The above random occasions result in different modified expressions for a single DNA sequence.
• Epigenetics is often influenced by environment, diet, parent-of-origin effects (e.g.: genomic printing), or stress.
• A perfect example for this is identical twins. They have the same DNA sequence, but they may show different traits due to different epigenetic modifications. In some cases where one Identical twin may get Schizophrenia and the other won't get it.

These are some patterns of Non-Mendelian Inheritance. That is commonly observed in humans and plants.

Written by Jathurshan Myuran