Human Genetics Research Methods: Pedigrees and Population Genetics

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  • 0:06 Why Humans Are Poor…
  • 1:19 How We Study Human Genetics
  • 3:26 Multifactorial Traits
  • 5:13 Human Population Genetics
  • 6:53 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
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.

Multifactorial Traits

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.

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