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SUMMARY

Genes are the foundation for all development, first instructing the living creature to form the body and brain and then influencing thought and behavior. Human conception occurs when two gametes (an ovum and a sperm, each with 23 chromosomes) combine to form a zygote, 46 chromosomes in a single cell.
Genes and chromosomes from each parent match up to make the zygote. The match is not always perfect because of genetic variations called alleles.
The sex of an embryo depends on the sperm: A Y sperm creates an XY (male) embryo; an X sperm creates an XX (female) embryo. Virtually every cell of every living creature has the unique genetic code of the zygote that began that life. The human genome contains about 20,000 genes in all.

Early duplication of the original one-celled zygote creates stem cells, each of which could become a person. Soon differentiation occurs, and cells develop into the placenta or part of the developing body.
Twins occur if a zygote splits into two separate beings (monozygotic, or identical, twins) or if two ova are fertilized in the same cycle by two sperm (dizygotic, or fraternal, twins).
Monozygotic multiples are genetically the same. Dizygotic multiples have only half of their genes in common, as do all siblings who have the same parents.
Assisted reproductive technology (ART), including drugs and in vitro fertilization (IVF), has led not only to millions of much-wanted babies but also to an increase in multiple births, who have a higher rate of medical problems. Several aspects of ART raise ethical and medical questions.

Genes interact in various ways—sometimes additively, with each gene contributing to development, and sometimes in a dominant–recessive pattern. Environmental factors influence the phenotype as well. Epigenetics is the study of all the environmental factors that affect the expression of genes, beginning at conception.
The environment interacts with the genetic instructions for every trait. Almost every aspect of a person is multifactorial and polygenic.
The first few divisions of a zygote are stem cells, capable of becoming any part of a person. Then cells differentiate, specializing in a particular function.
Combinations of chromosomes, interactions among genes, and myriad influences from the environment all ensure both similarity and diversity within and between species. This aids health and survival.

Environmental influences are crucial for almost every complex trait, with each person experiencing different environments. Customs and contexts differ markedly.
Genetic makeup can make a person susceptible to a variety of conditions; nongenetic factors also affect susceptibility. Examples include alcoholism and nearsightedness. Cultural and familial differences affecting both of these problems are dramatic evidence for the role of nurture.
Knowing the impact of genes and the environment can be helpful. People are less likely to blame someone for a characteristic that is inherited; realizing that a child is at risk of a serious condition may help with prevention.

Often a gamete has fewer or more than 23 chromosomes. Usually such zygotes do not implant, grow, or survive.
Infants may survive if they have three chromosomes at the 21st location (Down syndrome, or trisomy-21) or one, three, or more sex chromosomes instead of two. Affected individuals have lifelong physical and cognitive impairment but can live a nearly normal life.
Everyone is a carrier for genetic abnormalities. Genetic disorders are usually recessive (not affecting the phenotype unless inherited from both parents). If a disorder is dominant, the trait is usually mild, varied, or inconsequential until middle adulthood.
Genetic testing and counseling can help many couples. Testing usually provides information about possibilities, not actualities. Couples, counselors, and cultures differ in the decisions they make when risks are known.