Ultraviolet radiation from the Sun stimulates melanocytes in the skin to produce more melanin, which in turn makes the skin browner. This seasonal tanning is most noticeable among light complexioned people. It has the effect of shielding the inner skin.

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

There is a selective advantage for people with dark complexions in regions of intense solar radiation.  

Urea cycle disorders lead to buildup of ammonia. The mainstay of current treatment is to use substances such as sodium benzoate and sodium phenylacetate to complex with ammonia, forming water soluble compounds that can be excreted in the urine. In some cases, liver transplantation is used successfully as well.

The child is expected to have a new mutation given that the disorder displays complete penetrance and both parents are unaffected. Therefore, one would predict that the mutation would not be found in either parent (although germline mosaicism would be possible).

Incest apparently does not increase the likelihood of mutations occurring. However, it does usually increase the likelihood of children inheriting recessive conditions if the recessive allele for them already exists in the family line.

Lactose intolerance is 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 their production often later gets turned off.

Milk sugar or lactose, is difficult for many adults to digest due to an inability of their bodies to produce enough of a particular chemical to break it down. Also, there are other possible causes of these symptoms, including allergies.

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.

Much of their diet consisted of animals fats. This provide great numbers 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.

This cultural practice had the effect of permanently altering the shape, size and position of foot bones so that their feet were smaller and subsequently, more attractive to the Chinese. This is a classic example of developmental adjustment.

Most humans are capable of tanning or producing more melanin in their skin in response to ultraviolet radiation. In the winter, usually there is less ultraviolet radiation and our bodies respond by producing less melanin.

This woman has experienced a permanent change in stature from what would likely have been the case if nutrition had been poor during her growing years. Therefore, this is an example of developmental adjustment.

Natural selection is most likely to result in the genetic change of a population when a stress is severe, constant and long term. Selection usually will favour the successful adaptation resulting in its increased frequency in subsequent generations.

The microscopic organisms that cause malaria are parasites on normal red blood cells. Full-blown sicklers produce abnormal and deformed red cells. As a result, they are poor hosts for this disease and are immune. However, they are frequently anemic.

If individuals can respond more successfully, they will usually be more healthy and vigorous. As a result, they are more likely to live longer and to reproduce. The net result will be that their genes will be more common in subsequent generations.

Using fire for warmth is a cultural response to the environmental stress of a cold climate. In contrast, developmental adjustment and the two kinds of acclimatization are biological responses.

Natural selection has resulted in genetic differences between populations that allow them to be more or less successful in adapting to particular stresses. Populations that have been exposed to a stress generally have the best adaptation.

In order for the fetus to be Rh positive, the father must also be Rh positive. As a result, when the mother is Rh negative and the father is Rh positive (or his Rh type is unknown), doctors usually assume that there will be a serious medical problem.

The Rh blood factor is a dominant trait. Therefore, both homozygous dominant (DD) and heterozygous (Dd) people have Rh antigens on the surface of their red cells, which make them Rh positive.

Pleiotropy is a genetic inheritance pattern in which a single gene causes or affects an entire series of traits.

Europeans have the highest frequency of Rh negative people (40%), which puts them at the highest risk for this problem. About 13% of newborn Europeans are at risk, but we can now prevent the life threatening complications in most cases.

If the father is homozygous dominant (DD) the chances of the fetus being Rh positive and an incompatibility problem occurring will be 100%. If the father is heterozygous (Dd), the chances are 50%. There will be no problem if the fetus is Rh negative.

This hypothetical example would be an example of the intermediate expression of a trait. Apparently, this is rare. For most genes, it is expected that individuals with heterozygous genotypes will have phenotypes like homozygous dominant individuals.

Modifying genes alter the effect of companion genes in the phenotype of an individual. A real life example is a gene that modifies the effect of a dominant cataract allele.