Independent assortment of genes and linkage

Inheritance is possible because of chromosomes.

These chromosomes come in pairs - one from mom and one from dad - so they’re called homologous chromosomes.

Each chromosome has genes, which are segments of DNA that carry genetic information for a specific trait.

And different versions of the same gene are called alleles.

As an example, brown eye color and blue eye color are both alleles for the eye color gene.

And each parent offers one allele of a gene.

Now, these alleles can be either dominant often represented with a capital letter, or recessive, represented with the corresponding lowercase letter, the difference being that it only takes one dominant allele for its traits to be expressed, whereas it takes two recessive alleles for its traits to be expressed.

Human somatic cells - that is, all of the cells aside from the sperm and eggs, which are called gametes - have 23 pairs of chromosomes; 22 somatic pairs and one sexual pair - adding up to 46 chromosomes in total.

These chromosomes, along with the alleles they carry, segregate during meiosis - which is the process of making new gametes.

Gametes only carry half the genetic information of the parent - so 23 chromosomes.

Once the male and female gametes merge during fertilization, their alleles combine to make the genotype —or genetic information— of the new organism.

For every gene, alleles can combine to give rise to three possible genotypes, homozygous dominant - or AA, heterozygous - or Aa - and homozygous recessive - or aa.

This determines all of a person’s features —or phenotype— such as eye color, hair color, or even whether or not they’re color blind.

Now, independent assortment means that no matter which alleles an organism inherits for one gene that codes for a trait like eye color, it won’t affect the alleles it inherits for another gene that codes for a different trait, like hair color.

Let’s start with a simple example. Let’s represent the eye color gene with the letter “a” and the hair color gene with the letter “d”.

Now, the dominant allele for eye color - A - stands for brown eyes and the recessive allele - a - stands for blue eyes.

On the other side, the dominant allele for hair color - D - stands for dark hair while the recessive allele - d - stands for blond hair.

So let’s say we have a person with a heterozygous genotype —Aa— for eye color and heterozygous genotype —Dd— for hair color.

This person would have the dominant allele features - so brown eyes and dark hair, even though they still carry a recessive allele for blond hair and blue eyes.

Now, at the molecular level we know that the eye color gene is physically located on a pair of homologous chromosomes, and in this case, let’s say that the chromosome from mom carries the dominant allele —A—, and the chromosome from dad carries the recessive allele —a— .