Alleles

Alleles are different versions of a gene, representing variations in DNA sequences. Individuals inherit one allele from each parent, influencing their genotype and the expression of specific traits. Dominant and recessive alleles play roles in determining the observable characteristics of an organism.

• Homozygous and Heterozygous: 

If an individual has two identical alleles for a gene, it is said to be homozygous (e.g., AA or aa). If it has two different alleles, it is heterozygous (e.g., Aa).

• Dominance and Recessiveness: 

Alleles may be dominant or recessive. The dominant allele typically masks the effects of the recessive allele in a heterozygous individual (having two different alleles).

Think of alleles as different versions of a gene, like flavours of ice cream. Now, imagine you have a gene for “eye colour,” and there are two flavours: one for brown eyes and one for blue eyes. If you have two scoops (alleles) of the same flavour—let’s say, both for brown eyes—you’re “homozygous” for brown eyes (AA). If you have one scoop of brown and one scoop of blue—meaning you have different alleles—you’re “heterozygous” (Aa). So, being homozygous means having two identical scoops, and being heterozygous means having two different scoops. This idea helps explain how we inherit traits and why people can have similar or different features.

Dominant and Recessive Alleles

In a monohybrid cross, organisms inherit one allele from each parent for a specific trait.

Dominant Alleles mask the expression of recessive alleles. If an organism inherits one dominant allele and one recessive allele, the dominant allele’s trait will be expressed.

Genotype and Phenotype

• Genotype: 

The genetic makeup of an organism, representing the combination of alleles for a particular trait. Genotypes can be homozygous dominant, heterozygous, or homozygous recessive.

• Phenotype: 

The observable physical or biochemical expression of the genotype. It includes characteristics such as colour, height, or any trait influenced by genetic makeup.

• Homozygous: 

When an organism has two identical alleles for a particular gene (homozygous dominant or homozygous recessive).

• Heterozygous:

When an organism has two different alleles for a particular gene.

Punnett Square

A tool used to predict the possible genotypes of offspring in a genetic cross.

• Law of Segregation: 

States that each individual has two alleles for each gene, and these alleles segregate (separate) during the formation of gametes.

The 1:1 inheritance

The term “1:1 inheritance of genes” generally refers to the Mendelian principle of segregation, a fundamental concept in genetics formulated by Gregor Mendel in the 19th century. This principle explains how genetic information is passed from one generation to the next.

During the process of gamete formation (sperm and egg cells), alleles (variants of a gene) segregate in a 1:1 ratio. Mendel’s experiments with pea plants helped explain this concept. 

• Segregation: 

In an individual, there are two alleles for each gene (one inherited from each parent). During the formation of gametes (sperm and egg cells), these alleles segregate or separate, so that each gamete carries only one allele for a particular gene.

• 1:1 Ratio: 

The segregation of alleles results in a 1:1 ratio of alleles in the gametes. This means that half of the gametes carry one allele, and the other half carry the other allele. This ensures that each parent equally contributes to the genetic makeup of the offspring.

For example, consider a gene with two alleles, A and a. If an individual is heterozygous (has one A allele and one a allele), during gamete formation, half of the gametes will carry the A allele, and the other half will carry the a allele. This results in a 1:1 ratio of A:a alleles in the gametes.

When these gametes combine during fertilisation, the resulting offspring inherit one allele from each parent, restoring the diploid state.

Single-factor inheritance

Single-factor inheritance, also known as Mendelian inheritance, refers to a type of genetic inheritance pattern in which the presence or absence of a single gene determines a specific trait. This gene, in turn, may have different forms known as alleles. The inheritance of the trait follows the principles elucidated by Gregor Mendel in his experiments with pea plants in the 19th century.

In a single-factor inheritance scenario, each individual possesses two alleles for a particular gene, with one allele inherited from each parent. These alleles may be either dominant or recessive. The interaction between dominant and recessive alleles determines the expression of the trait in the individual.

The two main types of alleles in a single-factor inheritance are:

• Dominant Allele:

This allele expresses its trait even if only one copy is present in the individual. It “dominates” over the recessive allele.

• Recessive Allele:

The trait associated with this allele is expressed only when two copies of the recessive allele are present, meaning the individual is homozygous for the recessive trait.

The Punnett square is often used to depict the possible combinations of alleles and predict the outcomes of genetic crosses in single-factor inheritance. Examples of traits governed by single-factor inheritance include Mendel’s classic pea plant traits, such as seed color (yellow or green) and flower colour (purple or white).

Imagine there’s a gene that determines how tall you’ll be. This gene comes in two versions, or alleles: Tall (T) and Short (t).

• Tall (T) allele: This allele makes you tall.

• Short (t) allele: This allele makes you short.

Now, you inherit one allele from your mom and one from your dad because everyone has two copies of each gene.

• If you get a Tall allele from one parent and a Tall allele from the other (TT), you’ll be tall.

• If you get a Short allele from both parents (tt), you’ll be short.

• If you get one Tall allele and one Short allele (Tt), the Tall allele is dominant, so you’ll be tall because it takes charge.

This is the basic idea of single-factor inheritance for height. The gene for height has two versions, and whether you end up tall or short depends on which alleles you inherit from your parents. It’s like having a simple genetic rule for how tall you’ll be based on the combination of Tall and Short alleles.

The 3:1 inheritance

To explain the three-to-one ratio using the example of height, let’s consider a hypothetical scenario where we are looking at the inheritance of the trait of height in pea plants. For simplicity, we’ll use the traits of tall (T) and short (t), with tall being dominant over short.

We start with two pea plants, one tall (TT) and one short (tt).

According to Mendel’s Law of Segregation, the alleles (gene variants) for each trait segregate or separate during the formation of gametes (reproductive cells), so each parent produces gametes with only one allele for the trait.

• The tall plant (TT) produces gametes with the allele for tallness (T).

• The short plant (tt) produces gametes with the allele for shortness (t).