Our bodies are mainly composed of cells and our DNA is contained within the nucleus of each cell. DNA is an immensely large and complex molecule comprising over three billion units called nucleotides. Genes are segments of DNA and these are responsible for all of our features (colour of eyes, skin, hair etc, shape of nose, head, body etc size of hands, feet etc). Even though DNA is a very long molecule, it is present in distinct packages called chromosomes. Humans have 23 pairs giving a total of 46 chromosomes, apes and monkeys have 24 pairs, other life forms have different numbers of chromosomes and only creatures with the same number of chromosomes can produce viable offspring. As well, DNA in each chromosome is present as two strands in the form of a double helix. See diagram.
As stated, almost all cells contain DNA in the form of two of each chromosome, except males have an X and Y chromosome and females have two Xs; the male sperm cell and the female ovum (egg cell) however, contain only one set each. The 23 single chromosomes from the ovum combine with the 23 from the sperm, forming a complete set of 23 pairs (called a zygote); and at this moment, the life of a new and unique human being begins. This singular genetic blueprint, defining every trait of this new person, has been uniquely fixed among a billion possibilities, and can never be changed; nor will it ever be repeated.
God’s judgement on mankind
When God had finished His work of creation, He pronounced it to be very good. However, God told Adam that he would surely die if he ate the fruit from the Tree of Knowledge of Good and Evil. Adam and Eve did eat the fruit and they died spiritually (spiritual death is separation from God) and the process leading to physical death was activated in their bodies. Mutations, which are faults or defects that occur in DNA as it is being copied began to occur in strands of DNA in all forms of life. Amazingly there are processes within the cell to correct these coping errors but some still get through. Mutations build up in each species over time and during its life. For human beings, mutations have given us over one thousand disorders and the list is increasing.
This accumulation of mutations may have been a causal factor in the reduction in life spans, particularly in the period after the flood. Sometime later, God gave the Civil Law through Moses. Part of this is a prohibition against sexual relations with close members of a family. This prohibition was not necessary initially, as the deleterious effects of mutations had not built up to the point of being expressed in people.
As shown above, when a child is conceived, a complete set of the mother’s chromosomes (23) combine with a complete set of the father’s chromosomes (23). Both sets of chromosomes now containing mutated genes and at conception, if the baby receives only one mutated gene from one parent and one non-mutated gene from the other parent, normally the mutated gene will not be expressed in the infant. However, if both forms of a particular gene are mutated, the mutation will be expressed in the infant and this is far more likely to happen if both parents are close relatives.
As mutations accumulate, genetic disorders caused by intrafamily marrying are becoming common, not so much in western societies (presumably because our laws and practices were based on the Bible), but in groups who have not had the benefit of scripture. Howard Schneider writes in reference to Saudi Arabia’s alarming increase in genetic disorders such as thalassemia, sickle cell anemia, hydrocephalus and diabetes due to intrafamily marriages:
Nevertheless, even in a country hesitant to discuss its most intimate problems with the outside, a public dialog is developing…. Religious sheiks counsel young men to “choose a wife carefully” and with an eye towards health.
A heading in the Digital Journal was quite explicit: ‘British Muslim Inbreeding Causing Genetic Disorders.’
God gave the Law, including marriage restrictions to Moses about 3,500 years ago, the biological reason did not become apparent until the relatively recent science of genetics revealed it. Again God’s word, the Bible, has been shown to be reliable.
 Genesis 1:31.
 Genesis 3:6.
 Leviticus 18:6-18; 20:17; Deuteronomy, 27:22.
 Washington Post Foreign Service, January 16, 2000, page A01.
 digitaljournal.com/article/250164, February 11, 2008.