Introduction

Eukaryotic organisms organize their DNA content into physical segments called chromosomes.  Humans have a total of 46 chromosomes, 22 pairs of homologous chromosomes along with 2 sex chromosomes.

Let us learn a little bit more about the organizations of genes along chromosomes.

Learning

Along the sequence of DNA which makes up the chromosome, there are discrete segments which provide specific functions; these segments are called ‘genes’ and they can encode the information required to produce proteins.  It is important to note that while the exact number and sequence of genes will be the same on each homologous pair of chromosomes, the genes may be slightly different by the virtue of having differences in their actual nucleotide sequences.  Different versions of the same gene are called alleles.  You can think of alleles as being like your two hands – both serve similar functions, look alike, have similar features, however, they are in fact distinct (left vs. right).

Where do different versions of the same gene (alleles) come from?  The answer has more to do with chance than anything else.  Alterations of a gene’s nucleotide sequence can be introduced through errors in DNA replication.  When an error is made (i.e. an incorrect nucleotide is introduced into a gene sequence), this is referred to as a ‘mutation’.  Mutations can be caused by a variety of mechanisms, including through exposure to environmental factors such as radiation, but in all cases result in a change in the DNA sequence of a chromosome.  When these alterations occur within a gene, a new allele may result.  It is important to note that not all mutations are ‘bad’, and in fact new, beneficial versions of a gene can be created randomly through mutation.

For the purposes of beginning our discussion on the discovery of genetics, we need to introduce two new terms.  Recall that eukaryotic organisms like humans contain two copies of each chromosome, and therefore contain two copies of each gene present, one on each chromosome.  If two copies of an individual’s gene/allele are identical on the two chromosomes, the person would then be considered homozygous (from Greek term ‘same pair’).  In contrast, if the two alleles are different, the person would be considered heterozygous (from the Greek term ‘different pair’).  We will refer to homozygous and heterozygous states throughout this module.

Summary

In this section, we have learned the following:

• Genes are discrete segments of DNA which code for specific proteins in the cell.
• Alleles are alternate forms of the same gene.
• Mutations result when changes occur to the DNA sequence of a gene.  Mutations are the ultimate source of all new alleles.
• Homozygous – two alleles of a gene are identical between the two chromosomes.
• Heterozygous –  two alleles of a gene are different between the two chromosomes.

Sources:

“Cell Division.” By OpenStax Biology 2e. Retrieved from https://openstax.org/books/biology-2e/pages/10-1-cell-division/ Licensed under: CC-BY: Attribution