You will need to know some genetics basics for the ASVAB. Someday you’re going to find yourself acting like your mother or father. Whether you like it or not, it happens because parents pass their traits on to their offspring. Understanding genetics — how traits are physically passed from parents to offspring and what happens when the process goes wrong — helps scientists pinpoint the causes of diseases and disorders and can help them develop treatments and cures.
In human genetics, a healthy person contains 23 pairs of chromosomes (the structure that contains the genes). The mother and the father each supply one chromosome per pair. Genes contained in the chromosomes determine many characteristics of the resulting child.
Copying genes
When body cells multiply to produce tissues and organs (and eventually a complete living thing), they reproduce their genetic material. Most cells reproduce by mitosis, in which the nucleus of a cell divides, forming two cells and two identical sets of chromosomes.
However, sex cells (eggs and sperm) reproduce differently. Through meiosis, each cell divides into four cells, each containing only half the number of chromosomes as a nonsex cell. This process takes place so the sex cells of one person (with 23 chromosomes) can hook up with the sex cells of another person (with 23 chromosomes) to produce 46 chromosomes, or 23 pairs. Otherwise, way too many chromosomes would be floating around.
Sometimes cells don’t copy themselves and divide perfectly, and a genetic mistake is made. This frequently results in a fetus who doesn’t live or in a fetus with a genetic disease or disorder. For example, Down syndrome is the result of a fetus’s having 47 instead of 46 chromosomes.
Determining your gender with two little letters
The genes on one pair of chromosomes, called the sex chromosomes, determine whether a child will be male or female. In females, the two sex chromosomes are alike, and they’re labeled XX. In males, the chromosomes are different and are labeled XY.
The child always receives an X chromosome from the mother (who only has XX chromosomes). The father (who has XY chromosomes) can contribute either an X or a Y chromosome, so Papa actually determines the sex of the child.
Knowing which genes get passed down the family line
Many characteristics that you possess (from the way your nose turns up at the end to the color of your eyes) are determined by a pair of genes (or multiple pairs of genes). These two genes may be alike, or they may not.
Some genes are dominant, and some genes are recessive. If you have two unlike genes, the characteristic that they produce comes from the dominant gene; the gene that doesn’t overshadow the other is called the recessive gene. If each parent has two unalike genes, both parents will have the dominant trait, but they can have a child with the recessive trait — because each parent contributes a gene to the offspring, each parent may contribute a recessive gene to the child. Whew!