Your genes are inherited from both parents

Genotypes as a combination of maternal and paternal alleles

A genotype is the combination of an individual’s maternal and paternal alleles for a particular SNP or chromosome segment. Recall that the 23 pairs of chromosomes located in the nuclei of your cells are made up of long chains of DNA nucleotides. In effect, you inherited two copies of the genetic blueprint necessary to create a human, one copy from your mother (one chromosome from each pair) and one copy from your father (the other chromosome from each pair).

Your maternally and paternally inherited alleles

Consider a particular part of your genetic blueprint, for example, a segment of chromosome or a single DNA nucleotide. We call this a locus, that is, a location in your genome. For this locus, you have a maternally inherited version and a paternally inherited version, in other words, your two alleles. Let’s use as an example, a specific DNA nucleotide on chromosome number 11, that affects the color of your eyes. For this locus, you might have inherited an A from your mother and a G from your father. Your two alleles for this locus are therefore A and G. Your genotype for this locus is simply the combination of your maternally and paternally inherited alleles, in this case, A/G.

mtDNA and Y chromsomes

The vast majority of our 25,000 genes are found within the nucleus of a human cell, but some 37 of them are found in the cytoplasm that surrounds the nucleus. This type of DNA is called mtDNA or mitochondrial DNA. Like the y chromosome passed down from father to son, mitochondrial DNA is always passed down almost unchanged from a mother to all her offspring and is an important factor in tracing our ancestry.

Genome – the DNA found in a typical cell from your body

Your entire genetic blueprint, or genome, is made up of 23 pairs of chromosomes and mitochondrial DNA that are found in the nuclei of cells throughout your body. Recall that each chromosome is in fact a long chain of DNA nucleotides. The 46 chromosomes found in just one of your cells are made up of about six billion (6,000,000,000) DNA nucleotides. This means that the 23 chromosomes (one from each pair) you inherited each of your two parents are made up of approximately three billion DNA nucleotides.

How unique is your genome?

The sequence of DNA nucleotides in your chromosomes is very similar to that of any other human, but quite different from that of organisms from most other species. Such close genetic similarity is typical of organisms that belong to the same species. When we speak of the human genome, we are referring to the basic organization of DNA nucleotides into chromosomes that is characteristic of humans. Despite being very similar, there are of course differences between your genome and that of any other member of our species. A comparison of the genomes of two randomly selected humans would be expected to reveal on average one difference every 1000 DNA nucleotides. It is these small genetic differences that account for the variation we find in human physical characteristics, such as appearance and disease risk. The unique sequence of DNA nucleotides in your chromosomes constitutes your personal genome.

This content was last reviewed on February 18, 2010.