Lab Manual Exercise #6
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Population Genetics

  1. Genetic Drift: Blood Type Populations
  2. Genetic Drift: The Founder Effect
  3. Genetic Drift: Population Bottlenecks
  4. Genetic Drift: Classroom Demonstration
  5. Natural Selection: Not Genetic Drift
  6. Selection Against A Recessive Allele
  7. Genotype Percentages In A Population
  8. Recessive Albino & Himalayan Genes
  9. Argentine Ants In Southern California


1. Genetic Drift In Blood Type Populations

The classic Hardy-Weinberg Law states that the relative frequencies of genoytpes and phenotypes in large, randomly mating populations tend to remain constant from generation to generation. The law was written by a physician and a mathematician, and is based on a set of ideal conditions. In order for this law to hold true, you must exclude selection against unfavorable genes, mutations (creation of new genes), and interbreeding with genetically different populations (including immigration and emigration). Another factor that can significantly change the genotype ratios in relatively small populations is genetic drift. If it weren't for these exceptions to the Hardy-Weinberg model, there would be no genetic change and evolution would not occur.

Genetic drift is a gradual shift in the gene frequencies of small populations (less than 10,000 individuals) resulting in different genotypic ratios. The different genotypes may be expressed as different appearances (phenotypes), such as facial features or bark characteristics in California cypress trees (Cupressus). Genetic drift may also result in populations with different blood type percentages compared with other populations. Genetic drift is a plausible explanation for how racial differences may have first emerged in small, isolated populations when the earth was sparsely inhabited by tribes of people. Races may be defined as populations which differ in their gene frequencies. The frequencies of nonadaptive traits, such as hair color and blood types, may have gradually changed in small, isolated populations during prehistoric times. Today, these genetic differences are reflected in different racial populations. For example, the frequency of type A blood in the American white population is 42 percent, compared with 76 percent for type A blood in the Blackfoot Indian population of the northern United States and Canada. Another example of genetic drift is the marked difference in percentages of positive and negative blood types in Basque people of the Pyrenees Range of Europe compared with people of Asian countries. People from the isolated Pyrenees Range between France and Spain have predominantly Rh negative blood, while this blood type is relatively uncommon among Asian people.

Race
A
B
O
AB
 United States 
   42%   
   10%   
   45%   
   3%   
Chinese
31%
28%
34%
7%
Blackfoot
76%
-----
24%
-----
Navajo
24%
-----
76%
-----

Table 1. Genetic drift in the A-B-O Blood Types


2. Genetic Drift: The Founder Effect

The common cocklebur (Xanthium strumarium) has spread across North America with its unique hitchhiking, seed-bearing bur. Different populations of cockleburs throughout its range exhibit morphological variations that some botanists have classified as different varieties. There is some disagreement among botanists as to exactly how many varieties of common cocklebur exist, and precisely where is their native (indigenous) habitat. There are several named varieties listed in botanical literature, including var. canadense and var. glabratum; however, some authorities believe Xanthium strumarium is one cosmopolitan species with many highly variable populations around the world. Since cockleburs can colonize new areas quite easily (particularly disturbed areas), they are good examples of the "founder effect." The founder effect is genetic drift that occurs when a small number of individuals, representing a fraction of the gene pool, establish (found) a new colony and only certain alleles (genes) of the original population are passed on to the next generation. The founding colony does not have the genetic variability of the main population, and the frequency of certain traits may increase greatly by genetic drift compared with the much larger ancestral population.

Left: Cocklebur (Xanthium strumarium)
bearing prickly, hitchhiking burs. Right: An
assortment of seed-bearing cockleburs. Like
VelcroŽ fasteners, the cockleburs attach to
the fur of animals and articles of clothing.

Go To Article About The Cocklebur Plant
See The Remarkable Invention Of VelcroŽ

A classic example of the founder effect is the "Dunkers," a politically incorrect name for a German Baptist religious sect that settled in Franklin County, Pennsylvania between 1719 and 1729. Since the original families (who did not marry outside their religious sect) settled in Pennsylvania, there has been a dramatic change in some of their gene frequencies. For example, the frequency of type A blood in the "Dunkers" is now 60 percent, compared to 42 percent for the United States and 45 percent for West Germany. "Dunkers" also have fewer individuals with certain recessive traits, such as hitchhiker's thumb and attached ear lobes, compared with the U.S. population as a whole. The founder effect also explains the high frequency of dwarfism and polydactylism (extra fingers) in the Amish of Lancaster Pennsylvania, a colony begun by a few individuals (at least one of whom carried these traits). There is some evidence that the first humans to reach North America (across the Bering Straits land bridge) brought with them gene frequencies not representative of the Asiatic population they left. The unusual variation in bark, foliage and growth characteristics in isolated groves of cypress (Cupressus species) throughout coastal and mountainous regions of California may also be due (in part) to the founder effect; however, some traits, such as glandular (resinous) foliage, are more drought resistant and probably evolved by natural selection in the hot, dry interior regions of the state.

See Genetic Drift & Selection In California Cypress
See Article About Cypress Groves In California


3. Genetic Drift: Population Bottlenecks

Genetic drift can also result in the loss of genes from a population. For example, the B allele was apparently not passed on to subsequent generations of Blackfoot people, because present populations are deficient in genotypes that contain the B allele (BB, BO and AB). When populations become greatly reduced in size, some genes may not be passed on to the next generation. This phenomenon is referred to as a "genetic bottleneck." As a result, genetic variability may be severely reduced in succeeding generations.

Examples of "genetic bottlenecks" include the elephant seal (mercilessly hunted to near extinction in Baja California) and the endemic Torrey pine (Pinus torreyana) of San Diego County. The elephant seal was hunted almost to extinction in the 1800s, and by the end of the 1890s only about 20 survived. Because elephant seals breed harem-style, with a single male mating with a harem of females, one male may have fathered all the offspring at the extreme bottleneck point. The population today has expanded to about 30,000, but biochemical analysis shows that all of the elephant seals are almost genetically identical. The Torrey pine has also reached a genetic bottleneck in the small grove of trees that survives to this day at Torrey Pines State Park. Protein analysis of the pines shows practically no variability between individual trees. This lack of variability could seriously threaten their survival if the climate changes significantly. The population may not contain the genetic variability (gene pool) to adapt to a significant environmental change.

Simplified diagram of a genetic bottleneck. If the population of heterogeneous red and blue genetic members is reduced to a small number of individuals, the gene pool is greatly reduced. In this diagram, only a few red individuals survive the bottleneck. The few surviving red members pass their genes on to the new generation; however, this new homogeneous red population has a drastically reduced genotypic and phenotypic variability because they are all descendents of the few red individuals that survived the bottleneck. The northern elephant seal and Torrey pine have passed through genetic bottlenecks in the recent past, resulting in an almost total loss of genetic variability. As a result, the ability of these populations to adapt to changing environments may be very limited. This diagram could also be explained with an original population composed of red and blue alleles. When the population reached a very low level or bottleneck, only the red allele was passed on and the blue allele was lost. This simple analogy might help to explain how the B allele was lost from the indigenous Blackfoot people of North America.


4. Demonstration Of Genetic Drift

For a simplified demonstration of genetic drift, place 24 red beads and 24 white beads into an empty container. Make 24 random matings from this container of 24 red beads and 24 white beads. Assume that red (R) is dominant over white (r). Shake the container several times to make sure the beads are thoroughly mixed. Each random mating consists of reaching into the container and drawing out two beads. Each pair of beads essentially represents a pair of red or white genes carried by a pair of sex cells (egg and sperm). In order that you don't discriminate on the basis of color, make each draw without looking at the beads. When you draw two beads record your result in Table 3 below. If you get two red beads, place a tabulation mark under RR. If you get a red and a white bead, place a tabulation mark under Rr. If you get two white beads, place a tabulation mark under rr. Make a total of 24 draws, always replacing the beads back into the original container before the next draw. [Note: This exercise works best if the total number of beads in your container is exactly twice the number of draws (e.g. 48 beads for 24 draws, 60 beads for 30 draws, or 80 beads for 40 draws). Also use an even number of beads, such as 48, 60 or 80. The Biology 100 Laboratory Manual says to use 50 beads, but use 48 instead (24 red and 24 white).

Although this is a population problem involving a cross between the males and females of an entire population, the mathematical result comes out the same as a monohybrid cross involving one pair of heterozygous genes from each parent (Rr x Rr). In this type of cross we would expect to have the following genotype percentages in the offspring: 25% RR, 50% Rr and 25% rr. Since red (R) is dominant over white (r), 75% of the population will be red and 25% will be white. This cross is shown in the following Table 2:

  Gametes  
R
r
R
RR = 25%
   Rr = 25%   
r
   Rr = 25%   
rr = 25%

Table 2. A simple monohybrid cross involving one pair of genes from each parent. The ratio of offspring is 75% red (RR & Rr) and 25% white (rr).


If you draw exactly six double red beads (RR), twelve red and white beads (Rr) and six double white beads (rr) out of your 24 draws, the results of your offspring will be 6/24 or 25% homozygous red (RR), 12/24 or 50% heterozygous red (Rr) and 6/24 or 25% homozygous white (rr). This is the expected ratio if thousands of draws are made and there is no genetic drift. In other words, the 1:2:1 ratio of genotypes remains the same after all the draws are tallied. The following Table 3 shows the result of 24 draws with no genetic drift.

Genotype
RR
Rr
rr
  Tabulation  
||||| |
||||| ||||| ||
||||| |
Genotype
Frequency
6/24 = 25%
12/24 = 50%
6/24 = 25%
Phenotype
Frequency
Red: 18/24 = 75%
White: 6/24 = 25%

Table 3. No Genetic Drift After Many Crosses With Red & White Beads.

The following calculations show that the ratio of red and white beads from the above 24 draws in Table 3 is the same as your original number of red and white beads. In other words the ratio is still 24 red and 24 white because there is no genetic drift.

Number Of Red Beads: 6 x 2 (from RR) + 12 (from Rr) = 24

Number Of White Beads: 6 x 2 (from rr) + 12 (from Rr) = 24

The 6 RR and 6 rr must be multiplied by 2 because these are double reds and double whites. The 12 Rr represent 12 reds and 12 whites.


But the odds of getting exactly six double red beads (RR), twelve red and white beads (Rr) and six double white beads (rr) in only 24 draws is very unlikely. Therefore, the ratio of genotypes in the offspring may come out quite different from the 1:2:1 genotypic ratio in Table 3 above. This change in the expected genotype percentages (such as a 1:2:1 ratio) that occurs when a small population breeds (or when only 24 draws are made) is called genetic drift. The following Table 4 shows the results of 24 draws in which significant genetic drift has occurred. Compare the ratio of genotypes and phenotypes with Table 3 above.

Genotype
RR
Rr
rr
  Tabulation  
|||||
||||| ||
||||| ||||| ||
Genotype
Frequency
5/24 = 20.8%
7/24 = 29.2%
12/24 = 50.0%
Phenotype
Frequency
Red: 12/24 = 50%
White: 12/24 = 50%

Table 4. Genetic Drift After 24 Crosses With Red & White Beads.


The following calculations show that the ratio of red and white beads from the above 24 draws in above Table 4 are different from the original 24 red beads and 24 white beads:

Number Of Red Beads: 5 x 2 (from RR) + 7 (from Rr) = 17

Number Of White Beads: 12 x 2 (from rr) + 7 (from Rr) = 31

Since the number of white beads has increased to 31 and the number of red beads has decreased to 17, genetic drift has occurred.

Another 24 draws using 17 red beads and 31 white beads (a total of 48) resulted in the following Table 5. Now there is even more genetic drift because the white phenotype has increased to 62.5 % and the red phenotype has decreased to 37.5 %.

Genotype
RR
Rr
rr
  Tabulation  
||||
|||||
||||| ||||| |||||
Genotype
Frequency
4/24 = 16.7%
5/24 = 20.8%
15/24 = 62.5%
Phenotype
Frequency
Red: 9/24 = 37.5%
White: 15/24 = 62.5%

Table 5. More Genetic Drift After 24 Crosses With Red & White Beads.

The following calculations show that the ratio of red and white beads from the above 24 draws in Table 5 are even more different from the original 24 red beads and 24 white beads:

Number Of Red Beads: 4 x 2 (from RR) + 5 (from Rr) = 13

Number Of White Beads: 15 x 2 (from rr) + 5 (from Rr) = 35

Since the number of white beads has increased to 35 and the number of red beads has decreased to 13, more genetic drift has occurred. Note: Additional draws might also result in these ratios shifting back to more red beads and fewer white beads, possibly resulting in the same ratio of beads that you started out with in Table 3 (24 red and 24 white).


5. Natural Selection: Not Genetic Drift

Some genetic differences in populations are a result of natural selection working on favorable genetic combinations. For example, the adaptive advantage of darker skin pigmentation (more melanin) in racially distinct human populations of equatorial regions has been clearly demonstrated. The epicanthic fold of skin over the corners of the eyes in Asian people is an interesting racial characteristic that may have provided an adaptive advantage among ancestors of present-day Asian populations. It has been suggested that the fat-lined fold of skin above the eyelid may have protected the eye in bitterly cold weather of Mongolia. Although this characteristic doesn't appear to offer any environmental survival advantage in modern populations, it is undoubtedly a favorable phenotypic trait in mate selection. It is interesting to note that individuals with Down's syndrome (with three #21 chromosomes) also have the epicanthic fold.

See Polygenic Inheritance Of Human Skin Color


6. Complete Selection Against A Recessive Allele

Even if a recessive gene is lethal, it is very difficult to remove it from the population by natural selection. The recessive allele is passed on by the heterozygous people and continues to show up in the population after many generations. The following table shows complete selection against the innocuous nontaster allele (t). Although this is only a hypothetical example, the nontaster allele continues to be expressed in homozygous recessive individuals. After 99 generations the t gene still shows up in the sperm and eggs of the population with a frequency of 0.01 (1/100) or one percent:

Generation
Number (n)
Interval of
24 years.
Decimal value of T & t
alleles in gametes
of the parents.
t = 1/n + 1)
T = 1
- (1/n + 1)
Fractional ratio
of the t allele in
parental gametes.
1/n + 1
Homozygous
recessive (tt)
nontasters.
(1/n + 1)
2
  Year  
1
0.5 T     0.5 t
1/2 or 0.5
0.25 or 25%
1983
2
0.67 T     0.33 t
1/3 or 0.33
0.11 or 11%
2007
3
0.75 T     0.25 t
1/4 or 0.25
0.06 or 6%
2031
4
0.8 T     0.2 t
1/5 or 0.2
0.04 or 4%
2055
49
0.98 T     0.02 t
1/50 or 0.02
0.0004 or 0.04%
3135
99
0.99 T     0.01 t
1/100 or 0.01
0.0001 or 0.01%
4335


7. Calculating Fractional Genotype Ratios In A Population

A metabolic disease in humans called phenylketonuria (PKU) is the result of a recessive gene (a). The gene occurs in approximately one out of every 100 eggs and 100 sperm with a homozygous recessive frequency of about 1/10,000 in the general population. Homozygous recessive infants are unable to convert the amino acid phenylalanine into tyrosine. Consequently, phenylalanine is converted into phenylpyruvic acid which accumulates in the blood and causes brain damage. It is interesting to note that the popular herbicide Roundup® blocks a key enzyme so that the plants cannot synthesize aromatic amino acids containing a benzene ring, including phenylalanine, tryptophan and tyrosine.

In marriages between normal parents who produce a PKU child, the parents must be carriers (heterozygous) for the recessive gene causing this disease. If the recessive gene is represented by (a), then the normal parents of a PKU child would be Aa X Aa. The probability of this couple having a PKU child (aa) can be shown with a simple Mendelian monohybrid cross resulting in 1/4 AA, 2/4 Aa and 1/4 aa; however, to calculate the total probability of a normal couple having a PKU child, you must also calculate the probability of each parent being heterozygous (Aa), and then multiply these two values by the 1/4 chance of having a PKU (aa) baby.

The following Punnet Square (genetic checkerboard) shows the fractional genotypic ratios of heterozygous (Aa) people and homozygous recessives (aa) in a population. [Punnett square comes from R.C. Punnett who devised this method.] When multiplying the fractional ratios of sperm and eggs together to calculate the heterozygous and homozygous recessives, the value one is used for 99/100. The chance of a heterozygous woman (Aa) carrying this recessive gene is 1/100 + 1/100 = 2/100 or one in 50.


Tay-Sachs disease is another genetic disorder of infants caused by a recessive gene (n). Since the disease is fatal by three to five years of life, it is only passed on by heterozygous carriers. If both parents are heterozygous, their chances are one in four with each pregnancy that a Tay-Sachs child will be born. The frequency of Tay-Sachs births (homozygous recessive nn) in the Jewish population is about 1/3600. In the general population it is only about 1/160,000. The following Punnett Square shows the fractional genotypic ratios of heterozygous (Nn) people and homozygous recessives (nn) in a population. When multiplying the fractional ratios of sperm and eggs together to calculate the heterozygous and homozygous recessives, the value one is used for 59/60. Since the fractional value for Nn is 1/60, then the chance of a heterozygous Jewish man carrying this recessive gene is 1/60 + 1/60 = 2/60 or one in 30.

Tay-Sachs children lack a lysosomal enzyme called hexosaminidase (Hex-A) that is essential in breaking down a lipid (fatty material) called ganglioside (GM-2) in the brain cells. Because these children do not have the vital Hex-A enzyme, GM-2 accumulates in cells of the central nervous system (brain and spinal cord) causing cell deterioration and finally death. Genetic disorders involving an excess accumulation of lipids or sugars inside cells are called "storage diseases." The cells become bloated and vacuolated (with enlarged vacuoles).


8. Albinism & The Himalayan Gene

Albism is a genetic disorder caused by a recessive gene (b). Homozygous recessive individuals (bb) cannot synthesize the pigment melanin because they lack a key enzyme in a biochemical pathway that converts the amino acid tyrosine into melanin. Actually, albinism may result when any one of three enzymes involved in melanin production are deficient. Even though a person may inherit at least three sets of genes for melanin production in the skin, the presence of a homozygous albino genotype (bb) will block the action of these other genes on different chromosome loci. This phenomenon is a good example of epistasis, where the gene at one locus interferes with the function of genes at other loci. Approximately one person out of every 20,000 in the U.S. is an albino. The square root of 1/20,000 is approximately 1/141, the fractional ratio of recessive (b) alleles carried in the sperm and eggs of the population. The following Punnett square shows the fractional ratio of heterozygous people in a population and the probability that you may carry the gene for albinism:

The chance of a heterozygous man or a heterozygous woman carrying the recessive gene for albinism is 140/20,000 + 140/20,000 = 280/20,000 = 70/5,000 or 7 in 500. Like the previous examples for PKU and Tay-Sachs disease, you could have also used 1/141 for the heterozygous (Bb), since 140/141 X 1/141 is approximately 1/141. The chance of being a carrier is 1/141 + 1/141 = 2/141, approximately 1/70 or 7 in 490.


Himalayan rabbits and siamese cats have light colored fur with dark extremities, such as the ears, nose, paws and tail. The dark extremities are caused by the recessive Himalayan gene (h). Himalayan rabbits are known to be homozygous recessive (hh) for this gene, which is involved in the production of melanin. Experimental evidence suggests that the enzyme produced by the Himalayan gene is active only at low body temperatures. Therfore, the black fur only occurs at the extremities where the body heat is lower. [Extemities are colder because they lose more body heat to the environment.] Perhaps this is an ancestral trail where the dark extremities are less likely to freeze because they absorb light and solar heat. The activation of the temperature-influenced Himalayan gene has been demonstrated by shaving off the fur on the back of a Himalayan rabbit and applying an ice pack. The new fur that grew in was black instead of white, showing that the enzyme controlling melanin production is active only at low temperatures.


9. Argentine Ants In Southern California

The most common ant in southern California is the Argentine ant (Iridomyrmex humilis). It is a small, dark-colored ant about 3 mm (1/8 inch) long that invades homes in search of food and water. They are especially fond of sweets, but will feed on practically any food. They love the yolks of hard boiled eggs and carry minute yellow clumps of yolk back to their nest in endless ant columns. These ants are extremely well adapted to urbanized areas of the United States with mild climates and well-watered gardens. They pose a serious threat to native wildlife by upsetting delicate food webs. They are especially formidable due to their aggressive behavior and the enormous size of their colonies which can literally "team up" with other colonies.

The Argentine ant was introduced into the United States in the late 1890s, as coffee ships from Brazil unloaded their cargo in New Orleans. Being prolific breeders and constantly on the go, the original colony has moved across the southern half of the United States. Colonizing individuals often lose genetic variation because they do not carry all the genes from their original population. Usually this has harmful effects, but in Argentine ants it has led to a loss of clan warfare and the formation of supercolonies that overwhelm native species. Most ant colonies are very territorial, and will fight different colonies of the same species. Since Argentine ants in the United States originated from the original colonizers in Louisiana, perhaps from the original pregnant female who arrived there, they are all closely related with very similar DNA. They apparently will accept ants from different colonies as members of their gigantic family. In fact, Argentine ants from different colonies will actually "team up" and attack together in vast swarms. They simply outnumber and overpower their enemy. In their native homeland of Argentina, neighboring colonies (clans) fight each other, even though they are only 200 yards (200 m) apart. Also there are many native predators in Argentina, including fungal parasites and bacteria.

A single colony of Argentine ants may contain 10,000 female workers, and there may be hundreds of colonies around your home. According to entomologist David Faulkner, if you have a 10 x10 foot (3 x 3 m ) patio slab, you could have a million or more individuals and possibly 20 or 30 queens. They get along fine because they're all related to the original colonizers in Lousiana. Workers live a month or more as adults, but queens live up to 10 years or more. With other ants, when the queen dies, the one-queen colony dies because no more ants are being produced. With multi-queen Argentine ants, another queen simply moves in and takes over the role of the deceased queen. In fact, a queen from San Diego would probably be accepted in a colony elsewhere in California.

Argentine ants have become a serious threat to the coast horned lizard (Phrynosoma coronatum) in southern California. The primary food source for these endangered lizards are native harvester ants, particularly the California harvester ant (Pogonomyrmex californicus). I spent many years observing this fascinating red ant while growing up in San Gabriel Valley, and I can personally testify that it has a painful sting. As of 2006, this large red ant is seldom seen in urbanized areas of coastal southern California.

California harvester ant (Pogonomyrmex californicus), primary diet of coast horned lizard.

Urbanization has certainly been a factor in the demise of California harvester ants, but an even greater factor resulting in the elimination of native ants and coast horned lizards is the aggressive Argentine ant. Apparently the horned lizard is not fond of Argentine ants, and is actually attacked by them in enormous swarms. Colonies of Argentine ants need a damp area to survive, and have not invaded some of the dry habitats where native harvester ants and desert horned lizards (P. platyrhinos) still live. Of course, they can readily colonize urbanized desert areas inhabited by people. Well-watered gardens with stepping stones and concrete slabs provide the idea living requirements for these ants. In their native Argentina they live under rocks.

Coast horned lizard (Phrynosoma coronatum).

Argentine ants are a nuisance in gardens and orchards because they tend and protect scale insects and aphids. They even carry aphids to the tender buds of your prized roses. In return, the ants consume a sweet secretion from the aphids called "honeydew." In addition, swarms of these ants will invade orchard trees, destroying the fruit crop. The narrow genetic variability that has kept all the California populations of Argentine ants on friendly terms may eventually backfire due to excessive inbreeding. Perhaps some day these ants may not have the genetic variability to adapt to a changing environment or viral infection.

See Wayne's Word Article About Argentine Ants


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