Genetics and Heredity
Why do children so often resemble their parents?
Why do some brothers and sisters share similar traits, while others are very different?
To a large degree, it's a function of the genes. A gene is a portion of DNA that determines characteristics. Through meiosis and reproduction these genes can be transmitted from one generation to another. Genes are the basic units of heredity — they have in common.
How does this happen?
Our body is made up of trillions of cells. In some ways, these cells can be very different from each other. For example, they can specialize in a particular function, such as carrying oxygen (red blood cells), absorbing food (intestinal cells), or sensing light (cells in your eyes). In other ways, these cells have a lot in common. For instance, at the center of almost all of your cells is a ball-shaped structure called the nucleus, inside of which are 46 thread-like structures called chromosomes. These chromosomes contain the estimated 35,000 genes that, in many ways, make us who we are.
To understand how we end up with a given set of genes, we need to learn more about chromosomes.
A chromosome is a long strand of DNA, packaged together with proteins and other kinds of molecules. Each chromosome has a centromere, which plays an important role during cell division and also divides each chromosome into a short arm and a long arm. Scientists can tell different chromosomes apart based on their size, the relative lengths of their arms, distinctive staining patterns, and other characteristics.
Humans have two types of chromosomes: sex chromosomes and autosomes. Two sex chromosomes determine the sex of an individual, and they are called the X chromosome and the Y chromosome.
If you are female, you have two Xs, and if you are male, you have one X and one Y (although there are genetic conditions in which this varies). The autosomes comprise the other 22 chromosomes. The...