We'll now take a closer look at how life begins and then transition to exploring the terminology that relates to what genes we carry (our genotype) as compared to those traits that we express (our phenotype). As we have already noted, all that we are (our phenotype) is, ultimately, a product of nature (our genotype) interacting with nurture (our environment).
Conception
The Female Reproductive System
Gametes
There are two types of sex cells or gametes that are involved in reproduction: the "male" gametes or sperm and "female" gametes or ova. The "male" gametes are produced in the testes in a process called spermatogenesis which begins at about 12 years of age. The "female" gametes or ova which are stored in the ovaries are present at birth but are immature. Each ovary contains about 250,000 (Rome 1998) but only about 400 of these will become mature eggs (Mackon and Fauser 2000). Beginning at puberty, one ovum ripens and is released about every 28 days, a process called oogenesis.
After the ovum or egg ripens and is released from the ovary, it is drawn into the fallopian tube and in 3 to 4 days, reaches the uterus. It is typically fertilized in the fallopian tube and continues its journey to the uterus. At ejaculation (or insemination, depending), millions of sperm are released into the vagina, but only a few reach the egg and typically, only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the male, unites with the nucleus of the egg. As a result, a new cell is formed. This cell, containing the combined genetic information from both genetic contributors, is referred to as a zygote.
Chromosomes contain genetic information from each parent. While other normal human cells have 46 chromosomes (or 23 pair), gametes contain 23 chromosomes. In a process called meiosis, segments of the chromosomes from each "parent" form pairs and genetic segments are exchanged as determined by chance. Because of the unpredictability of this exchange the likelihood of having offspring that are genetically identical (and not twins) is one in trillions (Gould and Keeton, 1997).
Determining the Sex of the Child
Twenty-two of those chromosomes from each "parent" are similar in length to a corresponding chromosome from the other "parent" (these are in quotes because there are many variations to this process including anonymous donors from sperm banks and those not considered to be a parent to the child). However, the remaining chromosome looks like an X or a Y. Half of the male’s sperm contain a Y chromosome and half contain an X. All of the ova contain two X chromosomes. If the child receives the combination of XY, the child will usually be termed genetically male. If it receives the XX combination, the child will usually be termed genetically female.
Monozygotic and Dizygotic Twins
Monozygotic twins occur when a single zygote or fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate but genetically identical offspring. About one-third of twins are monozygotic twins. Are you an identical twin?
Sometimes, however, two eggs or ova are released and fertilized by two separate sperm. The result is dizygotic or fraternal twins. About two-thirds of twins are dizygotic. These two individuals share the same amount of genetic material as would any two children from the same mother and father. Older mothers are more likely to have dizygotic twins than are younger mothers and couples who use fertility drugs are also more likely to give birth to dizygotic twins. Consequently, there has been in increase in the number of fraternal twins in recent years (Bortolus et. al., 1999).
Genotypes and Phenotypes
The word genotype refers to the sum total of all the genes a person inherits. The word phenotype refers to the features that are actually expressed, visible and not. Look in the mirror. What do you see, your genotype or your phenotype? What determines whether or not genes are expressed? Actually, this is quite complicated (Berger, 2005). Some features follow the additive pattern which means that many different genes contribute to a final outcome. Height and skin tone are products of an additive pattern. In other cases, a gene might either be turned on or off depending on the gene with which it is paired or in response to something in the environment. Some genes are considered dominant because they will be expressed if they are present (only one gene is needed for the phenotype to be impacted). Others, termed recessive, are only expressed in the absence of a dominant gene. Some characteristics which were once thought of as dominant-recessive, such as eye color, are now believed to be a result of the interaction of several genes (McKusick, 1998). Dominant traits include curly hair, facial dimples, normal vision, and dark hair. Recessive characteristics include red hair, pattern baldness, and nearsightedness. Sickle cell anemia is a recessive disease; Huntington disease is a dominant disease. Other traits are a result of partial dominance or co-dominance in which both genes are influential. For example, if a person inherits both recessive genes for sickle cell anemia, the disease will occur. But if a person has only one recessive gene for the disease, the person may experience effects of the disease only under circumstances of oxygen deprivation such as high altitudes or physical exertion (Berk, 2004).
