Have you ever wondered how people study human genetics? Do you know what a pedigree or a complex disease is? In this lesson you'll learn about some of the techniques that human geneticists use and what pedigrees and complex diseases have to do with human genetics.
Why Humans Are Poor Model Organisms
So, let's talk about human genetics. Studying human genetics is unlike studying the genetics of any other organism. In many ways, humans are very poor model organisms for genetics. To begin with, human generation times are very long. Gregor Mendel would have been waiting around a very long time if pea plants had a generation time as long as humans. Long generation times make for slow progress when doing genetic crosses, which brings us to another problem with human genetics: The inability to make controlled crosses.
So, any human geneticist that tried to make controlled human crosses would most likely be considered a very disturbed criminal and not a brilliant scientist. Besides, humans usually only have one child at a time, which makes it really difficult to generate numbers of offspring that can achieve statistical significance. On top of all this, there's the issue of genetic manipulation. Key genetic techniques, like mutation screening and transgenics, are completely off-limits to human geneticists, even though they are popular among science fiction writers and Hollywood.
How We Study Human Genetics
So what does that leave for human geneticists to study? Basically, they have to study the human genome in its natural context without manipulation. Instead of mutant screens, human geneticists must study naturally occurring mutations. Instead of doing controlled crosses, human geneticists must study how genes and phenotypes are passed along to individuals within existing families by analyzing pedigrees, which are charts of family histories that show the phenotypes and family relationships of the individuals. Just from gathering family histories and creating pedigrees, we can often determine a lot about a genetic condition, like whether it is dominant or recessive. We can also determine whether a condition is caused by a gene on the X chromosome or an autosome.
If a condition is caused by a gene on an autosome, which in humans means any of the numbered chromosomes one to twenty-two, the condition is said to be autosomal. However, if a condition is caused by a gene on the X chromosome, it is said to be sex-linked. Most sex-linked conditions, like red/green color-blindness, are recessive. So if a person that has a copy of the color-blind allele has another X chromosome with the dominant wild type allele, then the person will not be color-blind.
Instead, this person will only be a carrier of the color-blind allele. This happens very frequently in females, because they have two X chromosomes. Only very rarely will a female have two recessive alleles for color-blindness and be affected by it. However, because males have only one X chromosome, there is no second X chromosome to provide a dominant allele. Because of this, when a human male receives a single copy of the color-blind allele, the single copy will determine his phenotype and cause him to be color-blind.
So far in this lesson, we've only talked about traits and conditions that are controlled by genes at a single locus in the human genome. However, we all know that not every trait is determined by genes at one locus. In fact, many traits are determined by a combination of genes at more than one location in the human genome, as well as non-genetic factors.
Multifactorial traits are traits that are caused by multiple factors. In most cases, multifactorial traits have both genetic and non-genetic factors that contribute to the phenotype. Height in humans is an example of a multifactorial trait. However, most human geneticists aren't particularly concerned about human height. Instead, human geneticists are mostly concerned with human disorders and diseases.
Some of the most common and devastating human diseases are multifactorial, including diabetes, heart disease, Alzheimer's disease, rheumatoid arthritis and cancer, just to name a few. All of these diseases are thought to have contributions from multiple gene loci and multiple non-genetic factors as well.
A geneticist might call them multifactorial; however, these diseases that are caused by multiple factors are usually called complex diseases. A common feature of complex diseases is that their severity and age of onset are often quite variable. Adding to this complexity is the fact that some of these diseases actually contribute to each other. For instance, diabetes itself is a risk factor for heart disease.
Human Population Genetics
Not surprisingly, the genetics of complex diseases are much more difficult to understand than a simple autosomal condition, and the methods used to study them are also more complicated. Because of the multifactorial nature of these diseases, each gene that contributes to a complex disease has only a partial effect on the disease.
Because no gene has the ability to single-handedly cause a complex disease, we can't track an allele through a family alongside the disease. Instead, human geneticists have to use population genetics when studying complex diseases. This usually requires the collection of thousands of DNA samples and medical histories from both affected and unaffected individuals within the same population.
In addition, because the geneticists are trying to isolate the genetic factors, they try to minimize the impact of non-genetic factors. Of course, there is no way to completely eliminate these factors from the equation. Instead, geneticists do the next best thing and try to match the non-genetic factors of the affected and unaffected individuals.
If 50% of the affected individuals are smokers, then about 50% of the unaffected individuals should also be smokers. If the average age of the affected individuals is 65, then the average age of the unaffected individuals should also be about 65. The more similar that the two groups are, then the more likely it is that the differences between them are due to their genetics.
So, to review, human genetics is a unique field that requires unique approaches to unravel the human genome. Genetic techniques like mutant screens and controlled crosses just can't be done with human subjects, so instead, human geneticists must rely on naturally occurring mutations and existing family histories. Pedigrees are charts of family histories, which show the phenotypes and family relationships of individuals.
Just from gathering family histories and creating pedigrees, we can often determine a lot about a genetic condition, like whether it is dominant or recessive. We can also determine whether a condition is caused by a gene on the X chromosome or an autosome. An autosomal condition is caused by a gene on an autosome. In contrast, a sex-linked condition is caused by a gene on the X chromosome. Sex-linked conditions, like red-green color-blindness, usually affect males because they only have one X chromosome, and therefore, the trait is controlled by only one gene.
While some traits are determined by genes at one locus, many others are determined by a combination of multiple genes and non-genetic factors. Multifactorial traits are traits that are caused by multiple factors. Some of the most common and devastating human diseases are multifactorial. These diseases that are caused by multiple factors are usually called complex diseases. A common feature of complex diseases is that their severity and age of onset are often quite variable. In order to study these complex diseases, human geneticists will often use population genetics.
When you have finished this lesson, you should be aware that human genetics is a very complicated study due to long generational times. It's also a vital study of complex diseases and family relationships.