Milk sugar (lactose) is difficult for many adults to digest due to inability of their bodies to produce enough of a particular chemical to break it down. 

 Cells become cancerous by accumulation of genetic damage.

Precancerous cells have an abnormal shape, their nuclei become unusually large and a typically shaped and they contain increased amount of DNA.

If a healthy cell detects damage, it stops the cell cycle, assesses the damage and begins repair.

When the p53 gene mutates, cell division is no longer regulated.

The function of Tumor-suppressor genes is to control the damage. Some tumor-suppressor genes detect the damaged DNA and other tumor-suppressor genes coordinate the activities of other genes that repair the damage.

Tumor cells grow uncontrollably into an undifferentiated mass that sometimes metastasize.

There are differences between individuals and population in terms of calorie requirements and the ability to digest particular kind of food. These differences are due to body size, age, gender, and peculiar variations in digestive systems.

Cell death is a mechanism that normal cells use when cell damage cannot be repaired.

Lactase deficiency has a non-random distribution. Generally, it is the least common among Europeans. Asians and Native Americans have very high frequencies. In Africa, there are regions of extremely high frequency and others of low frequency.

Lactose intolerance occurs due to an inadequate production of lactase. This enzyme breaks down or digests lactose in the digestive tract. While most babies produce sufficient amount of lactase, the gene for its production often later gets turned off.

Keep in mind that the transitions between regions of lighter and darker complexions were mostly gradual.

Ultraviolet radiation must penetrate the outer skin layer in order to produce the vitamin D. This is important because vitamin D is vital for the intestine to absorb calcium and phosphorus from food for bone growth and repair.

Today the rate of evolutionary change for skin colour is much slower than the homogenizing effect of human migration. This process of interregional movement of societies has been accelerating, particularly since the time of Columbus.

We are mostly the product of the combination of genes that we inherited from our parents. However, environmental influences also have an effect. For example, severely undernourished children are not likely to reach their full potential stature.

Much of their diet consisted of animals fats. This provide great number of calories to keep their bodies warm. Dairy products were essentially non-existent for them, and vegetable foods were very scarce in their Arctic environment.

People with Turner syndrome are females who inherit only one X chromosome, their genotype is usually written as X0.

There is a selective advantage for people with dark complexion in regions of intense solar radiation. Similarly, light complexion is an advantage in areas with significantly less sunlight. This is due to the fact that melanin is a protective shield.

The gradual, more or less continuous change in hair colour from one group of primitives to another is not random or haphazard. There probably is a progressive gradation because there are different evolutionary pressures from one region to another.

This distribution pattern is most likely the result of migrations of group of red-haired people into areas where other hair colours predominated. However, it could also result from discriminatory marriage partner selection.

There are no distinct biological races of humans today. However, we still often misuse the concept to label each other. Commonly defined races usually are groups of people who share ethnic identity and some broad physical similarities.

Because sex chromosome abnormalities rarely result in an early death and their symptoms are often not severe, comparatively large number of people with these abnormalities survive to old age.

Women with Turner syndrome are essentially postmenopausal from early childhood and are sterile because their ovaries do not develop normally and they do not ovulate.

This progressive disease characterized by degeneration of brain cells resulting in severe memory loss usually occurs in old age for normal people if it occurs at all. It is far more common among Down syndrome sufferers and it occurs at an earlier age.

The feminizing effects of Klinefelter syndrome can be diminished if boys are treated regularly with testosterone injections from the age of puberty. As a result of this treatment, most become sufficiently ordinary in appearance to live in society without notice.