We'll begin our exploration of prenatal development by looking at some of the ways in which heredity helps to shape the way we are. We will look at what happens genetically during conception and take a brief look some genetic abnormalities (or non-typical traits). Before going into these topics, however, it is important to emphasize the interplay between heredity (genetics) and the environment. Why are you the way you are? As you consider some of your features (height, weight, personality, being diabetic, etc.), ask yourself whether these features are a result of heredity (nature) or environmental factors (nurture) - or both. Chances are, you can see the ways in which both heredity and environmental factors (such as lifestyle, diet, and so on) have contributed to these features. For decades, scholars have carried on the “nature/nurture” debate. For any particular feature, those on the “nature” side would argue that heredity plays the most important role in bringing about that feature. Those on the “nurture” side would argue that one’s environment is most significant in shaping the way we are. It is difficult to isolate the root of any single behavior as a result solely of nature or nurture and most scholars believe that even determining the extent to which nature or nurture impacts a human feature is difficult to answer. In fact, almost all human features are polygenic (a result of many genes) and multifactorial (a result of many factors, both genetic and environmental). It’s as if one’s genetic make-up sets up a range of possibilities, which may or may not be realized depending upon one’s environmental experiences. For instance, a person might be genetically predisposed to develop diabetes, but the person’s lifestyle may help bring about the disease.
Gottlieb (1998, 2000, 2002) suggests an analytic framework for the nature/nurture debate that recognizes the interplay between the environment, behavior, and genetic expression. This bidirectional interplay suggests that the environment can affect the expression of genes just as genetic predispositions can impact a person’s potential. And environmental circumstances can trigger symptoms of a genetic disorder. For example, a person who has sickle cell anemia, a recessive genetic disorder, can experience a sickle cell crisis under conditions of oxygen deprivation. Someone predisposed genetically to type two diabetes can trigger the disease through poor diet and little exercise. In both instances, the individual's genes combine with the environment to determine the ultimate outcome.
The Human Genome Project
The Human Genome Project is an internationally funded effort to map the locations of human genes and understand the role these genes play in development, health, and illness.
Figure \(\PageIndex{1}\): (Pixabay License; Arek Socha via Pixabay)
Chromosomes consist of genes. When we discuss inherited conditions, we will explore those that are caused a discrete gene or genes (genetic conditions that involve some subset of a chromosome) and those that involve an entire chromosome (chromosomal disorders). There are an estimated 25,000 to 30,000 genes on each chromosome; a number far below the estimate of 100,000-150,000 held before the work of the Human Genome Project.
Understanding the role of genes in health and illness can bring about both harm and good (Weitz, 2007). A person who knows that they are at risk for developing a genetic disorder may be able to adopt lifestyle practices that minimize the risk and a person who discovers that they are not at risk may find comfort in knowing that they do not have to fear a particular disease. However, a person who finds out that they are at risk and there is nothing that can be done about it may experience years of fear and anxiety. And the availability of genetic testing may be more widespread than the availability of genetic counseling which can be very expensive. The possible stigma and discrimination that those with illness or at risk for illness must also be considered. In light of the high costs of health insurance, many companies are starting to offer benefits contingent on health assessments and lifestyle recommendations; and continued coverage depends on an employee following these recommendations. So a person who smokes nicotine products may have to pay a higher premium than a non-smoker or a person who is overweight may be required to engage in a program of exercise and be monitored for improvement. What if a person finds out that they carry the gene for Huntington’s disease (a neurological disorder that is ultimately fatal) which may surface when a person reaches their 40s? The impact this knowledge will have on health care still remains unknown. Who should know what is on your genome? Do you think this information should be shared between mates? What about employers? What would be the advantages and disadvantages?
Genes and Chromosomes
Our genetic information is stored in 23 pairs of chromosomes that vary widely in size and shape. Chromosome 1 is the largest and is over three times bigger than chromosome 22. The 23rd pair of chromosomes are two special chromosomes, X and Y, that determine our biological sex. Females have a pair of X chromosomes (46, XX), whereas males have one X and one Y chromosomes (46, XY). Chromosomes are made of DNA, and genes are special units of chromosomal DNA. Each chromosome is a very long molecule, so it needs to be wrapped tightly around proteins for efficient packaging.
The first step in the development of a new human life is the combination of DNA from the male gamete (sperm) and the female gamete (egg). While most of our cells contain 23 pairs of chromosomes, gametes contain only 23 unpaired chromosomes. During the process of meiosis, unique gametes are created that carry just half of the directions needed to create a new life. The uniqueness of those gametes is important to note as it is one of the factors that ensures that you and your siblings are not carbon copies of one another. Identical twins are carbon copies, sharing all of their genes. Identical twins are also known as "monozygotic" as they form from one zygote (fertilized egg) that then splits. In contrast, fraternal or dizygotic twins are no more genetically alike than other siblings. We'll now move on to explore the process of conception.
Figure \(\PageIndex{2}\): Identical or monozygotic twins share all of their genes. (Pexels License; Derek Dolro via Pexels)
