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.

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:

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).

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.

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.

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.

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.