Genetic Structure of Human Populations: Definition & Concept

Instructor: Erika Steele

Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.

Human beings are extremely diverse in appearance and susceptibility to diseases such as cancer, diabetes and obesity -- yet our genomes our 99.9% identical. How are we so different if our genome says we are nearly all the same? Read on to learn more.

Nearly Identical, but Totally Unique

Humans are an intelligent and inquisitive species. We are driven to understand the world around us, including ourselves. One thing we seek to explain is what makes us unique as individual human beings, and what defines us as members of the species Homo sapiens.

All human beings have a genome that is 99.9% identical, and yet we are all genetically unique. We certainly look different from one another, most of the time. Some of us have probably looked at our own family members and wondered how we are related to those people. And our differences go beyond appearances. We also have different susceptibilities to various diseases and differences in how our bodies respond to treating diseases. For this reason, we study the genetic structure of human populations.

Diversity of Human Features
Human Diversity

The Science Words

Before we talk about what makes humans similar -- and different -- let's first go over some basic terms of the field. Genetics is the study of genes, gene variation, and heredity in organisms. A genome is the all the genetic material found on the chromosomes of an organism. The genome will include DNA sequences that code for a gene and sequences that do not code for a gene. DNA stands for deoxy-ribonucleic acid and is the molecule found in chromosomes.

A gene is a short stretch of DNA that codes for a polypeptide (protein) or RNA that has a function in an organism. Humans inherit two copies of the human genome; one from the paternal or male parent and the other from the maternal or female parent in the form of chromosomes.

Genes can have different versions, called alleles. The genetic structure of a population is a pattern in the genetic make-up within individuals of that population.

Exploring the Genetic Structure

Mendelian Genetics

We can examine the genetic structure differences between people by looking at alleles. Traits like hair color, skin color, and eye color are polygenic, meaning that two or more alleles are inherited from each parent. Hopefully, that helps explain why you probably are related to those people even if you are nothing like them.

Most traits have two alleles. You get one from your mother and one from your father. Those alleles will be expressed or show up as an observable trait depending on what type of alleles inherited. Think of gene expression as the way your cells express how beautiful you are -- this is called a phenotype. The genes you have are the instructions telling your cells what to sing about, or the genotype.

Some alleles will sing louder than others. These are called dominant alleles. They will always be heard. You only need to inherit one copy from one parent to have dominant trait expressed. If you have two dominant alleles, you will be homozygous for the trait. If you have one dominant allele and one recessive allele, you are heterozygous for the allele. This is illustrated in Figure 1 below. The father is affected because he has one dominant allele. The mother is unaffected and has two recessive alleles. Half of their children will have the trait.

Figure 1: A dominant allele will always be expressed when present.
Dominant Trait

Other alleles are quiet. These alleles are recessive. The only way you will see them expressed is if you inherit two copies of them. Chances are that if you are nothing like those people, your parents were carriers of a recessive trait. Please keep in mind that even though alleles may be dominant or recessive, most human traits are not determined by a single gene; they are polygenic. In eye color, for instance, brown is dominant over blue; having multiple copies of the gene gives us shades of blue or brown (Green eyes are another story altogether).

For this lesson, let's examine eye color as a single gene condition. For instance, let's say your parents both have brown eyes, but you have blue eyes. How did that work? Well, in this instance, blue eyes are the recessive trait. If your parents each were heterozygous for the trait, or had one dominant (brown) and one recessive (blue) allele, then they would both have the phenotype of brown eyes. If you received both recessive alleles, you would be homozygous for the recessive trait, and would have blue eyes. This is shown in Figure 3.

Figure 3: Recessive alleles require two copies to be expressed.
Recessive Trait

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