The sequence of bases in the human genome is remarkably similar from person to person, but over hundreds of thousands of years of evolution SNPs and other mutations have been introduced into the human gene pool. Some of these mutations produce alterations in gene products that are fatal, and these mutations are extinguished. However, other mutations in germ cells (sperm and eggs) can be passed along from generation to generation, and they provide the basis for the many variations in phenotype that make each of us unique. Over time, mutations have created variants of genes that are responsible for differences in the color of our hair, our eyes, and our skin. Mutations influence our intelligence, our height, our weight, our personalities, our blood pressure, our cholesterol levels, and how fast we can run. Mutations have introduced gene variants that encode for slightly different proteins, which in turn, influence all aspects of our phenotype. It is important to emphasize an individual's phenotype is not solely the result of their genome; instead, phenotype is the result of the interaction between and individual's genome and their environment from the time of conception until death.

When SNPs and other mutations create variants or alternate types of a particular gene, the alternative gene forms are referred to as alleles. In other words, a given gene can have multiple alleles (i.e., alternate forms). Some genes have just a few alleles, but others have many.

Autosomes, and Sex Chromosomes

Recall also that chromosomes come in pairs. Humans have 22 pairs of autosomal chromosomes with the same gene in both members of a given pair) and one pair of sex chromosomes, which are designated XX in females and XY in males. The X and Y chromosomes are physically different from one another in that the Y chromosome is much shorter, and the Y chromosome only has about nine gene loci that match those on the X chromosome. This means that, except for the genes on an XY pair of chromosomes, we have two copies of each gene - one from each of our parents. The alleles that we receive from each parent might be the same (homozygous) or they might differ (heterozygous). The figure below schematically depicts a pair of chromosomes and shows three hypothetical genes: hair color, body height, and multiple lipoma formation.

Since there are two copies of each gene, there are two alleles, which may be the same or different. The figure below shows a hypothetical example in which there is an allele for red hair on one chromosome and an allele for brown hair on the other.

Two homologous chromosomes illustrating alleles for hair color, height, and formation of lipomas (fatty tumors). The alleles can be the same on the two homologous chromosomes (homozygous) or they can differ (heterozygous).

(Note that there may be many alleles for some genes, but normally we each have two alleles for each gene on our autosomes. Note also that in the hypothetical illustration to the right the alleles for the multiple lipoma trait are also different.

The obvious question that arises is, what happens when the two alleles that are present differ? What will the phenotype be? The answer depends on whether one allele is dominant over the other.

Dominant and Recessive Alleles

A dominant allele is one that is expressed to a greater degree than the other allele that is present. For example, one possible scenario for the differing lipoma alleles is shown below.

A mom who is homozygous for the dominant lipoma gene has children with a dad who is homozygous for not having lipomas. All of the children will be herozygous and will have lipomas because the lipoma gene is dominant.

What about another scenario in which the mom is heterozygous and the dad is homozygous recessive?

A heterozygous mom mating with a homozygous recessive dad will have (on average) half of their offspring who are homozygous recessive (no lipomas) and half who are heterozygous (have tumors because it is a dominant trait).

Mom is homozygous for the multiple lipoma trait (designated as "LL"), while Dad is homozygous for the absence of lipomas (designated "ll"). Mom can only contribute an "L" allele to her offspring, and Dad can only contribute the "l" allele, so all of their children will be heterozygous ("Ll"). In this particular case, heterozygous "Ll" individuals will all have multiple lipomas, because the multiple lipoma allele is dominant, while the alternate "l" allele is recessive.


For some alleles there is no dominance, and phenotype results from both alleles being expressed or from a blending of phenotype. The expression is an "average" or combination of the two traits.

Example: Major blood type in humans.

In humans, for example, there is a specific gene that codes for the protein that determines an individual's major blood type, which can be A, B, AB, or O. This is determined by a single gene that has three alleles that can code for:

 While there are three alleles, each of us has just two of them, so the possible combinations and the resulting blood types are those shown in the table below.


Phenotype (Blood Type)