2.7) Understanding Variation in Populations
Let’s discuss more in depth how genetic variations move through populations. Remember that the goal of this course is to give you the tools you need to use DNA to optimize diet, fitness and health. We want you to be comfortable with the research that has gone into the genetic reports. However, when reading diet, fitness and health genetic reports this research has been done for you and simplified into an easy-to-read format. This is to help you understand what goes behind the results you will be reading.
Genotype! Your genetic makeup
Genotype is the genetic makeup of an individual or group of individuals. It can refer to a single locus, a particular trait, set of traits, or their complete genetic makeup.
Phenotype! Your observable or biochemical characteristics
Phenotype is the combination of traits and features that results after an organism’s genetic code interacts with its environment; how a genetic “blueprint” is actually expressed. Relates to the characteristics of an individual. These characteristics can include physical form, developmental processes, biochemical and physiological properties, behavior, and the products of behavior. Most commonly, we think of traits and behaviour. Some simple examples of your phenotype include your height, weight, blood type, eye color and hair color.
Alleles are alternative forms, or variants
Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between different people. These slightly different versions of the same gene are called alleles.
Alleles are one of two or more alternative forms at the same locus, variants, of the same gene with small differences in their sequence of DNA. These alleles can be the result of different mutations, most often a SNP where a letter has changed. For example, sickle cell anemia arises from an allele of the beta-globin gene which has had a SNP change from A to T. The ancestral allele (also known as the wild type allele) contains A at on the short arm of chromosome 11, more specifically 11p15.5. while the mutant (variant) allele contains T in place of A.
These small allelic differences contribute to each individual’s uniqueness and make you different from everyone else.
Figure 2-14 In this example the alleles for purple and white flowers are two alternative forms, variants, of the same gene with small differences in their sequence of DNA. These variations arise by mutation, they are found at the same place on the chromosome.
Genotype: The genetic makeup of an individual or group of individuals. It can refer to a single locus, a particular trait, set of traits, or their complete genetic makeup. For example, you might have the CC, CT or TT genotype at a locus.
Phenotype: The combination of traits and features that results after an individual’s genetic code interacts with its environment. Relates to the characteristics of an individual. Most commonly, we think of traits and behaviour. For example, your height, weight and eye color are all part of your phenotype.
Allele: One of two or more alternative forms at the same locus, var7iants, of the same gene with small differences in their sequence of DNA. For example, the two different base pairs looked at for a SNP. For example, you might have the C or T allele at a particular locus.
Variants: The genetic variations or differences that make up the 0.2% to make each person unique.
Genotype frequency
Genotype frequency is the proportion of individuals in a population that possess a given genotype. It is how often we see each allele combination.
Figure 2-15 The genotype frequency is the proportion of individuals in a population that possess a given genotype. It is how often you see that allele combination. In this example the genotype frequency of W/W is (6/9) 67%, W/w is (1/9) 11%, and w/w is (2/9) 22%. Note that the genotype frequencies at a particular locus in a population should always add up to 100%.
Question 1. If there are two alleles at a particular locus (W and w) what are the three possible genotypes? Hint: W/w is considered the same as w/W.
Answer 1. The possible genotypes at this locus are W/W, W/w, w/w.
Question 2. In the example figure 4-3 what are the possible phenotypes?
Answer 2. The possible phenotypes are purple or white.
Question 3. In the example figure 4-3 what are the possible alleles?
Answer 3. The possible alleles are W and w.
Frequency can vary in different populations. For example, the frequency of individuals that have red hair is higher in the North American population than in the Korean population.
Let’s look at some of the common reasons that genotype frequencies are different within different sub-populations. These include mating patterns, genetic drift, physical distribution and migration.
Figure 2-16 Four examples of selection that can cause genotype frequencies to differ between populations.
Mating patterns (sexual selection)
Sexual Selection arises through preference by one sex for certain characteristics in individuals of the other sex. Strong sexual selection typically results in two distinct forms that are exaggerated, or more elaborate, in the sex with highest variability in reproductive success.
For example, sexual selection in humans appears to be biologically driven to produce fertile offspring. Research shows that males tend to prefer more feminine women’s faces and voices (O’Connor, 2013) and evaluate symmetry and apparent health when selecting a reproductive partner.
Figure 2-17 In peacocks the male (left) is much more elaborate than the female (right). This is an example of sexual selection.
Genetic drift (random forces)
Genetic drift is common when a significant number of individuals in a population die or are otherwise prevented from breeding, resulting in a drastic decrease in the size of the population. Genetic drift can result in the loss of rare alleles and can decrease the size of the gene pool.
This would have occurred for humans during intense global changes such as ancient pandemics, genocide and wars.
Physical distribution
A species with a broad distribution rarely has the same genetic makeup over its entire range. Physical distribution is one way that genetic variation can be preserved in large populations over wide physical ranges, as different forces will shift relative allele frequencies. If the individuals at either end of the range reconnect and continue mating, the resulting genetic intermixing can contribute to more genetic variation overall.
For example, large mountain ranges and oceans in the past created sources of physical distribution in humans.
Migration
Migration is movement of individuals into or out of a defined population. If the migrating individuals stay and mate with the destination individuals, they can provide a sudden influx of alleles, that can alter the existing proportion of alleles in the destination population.
This is the most relevant for humans today. With international migration our populations are ever changing. This has made genetic testing so interesting, because most people, perhaps more in colonized countries such as North America, now have a variety of different ancestry that they can trace using DNA.