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Introduction
Genetics is the study of heredity, or how certain features pass from biological parents to their offspring, or young. Every kind of plant and animal produces young of its own species, or type. The young resemble their biological parents. But offspring are not usually exactly the same as their biological parents. For example, their hair color or height may be different. Genetics explains how offspring get some of their bioloical parents’ features, or traits, but not others.
Genes are the basic units of heredity. They carry information about a living thing, including its traits. Genes exist inside the cells that make up living things. Threadlike structures called chromosomes carry the genes. Each chromosome is made up mainly of a substance called deoxyribonucleic acid, or DNA. The genes are short sections of DNA.
In human beings most cells have 23 pairs of chromosomes, or 46 in all. The sex cells—sperm cells and egg cells—each have only one set of 23 chromosomes. This is because of the way reproduction works. Every human baby begins as an egg cell from the mother. First, though, a sperm cell from the father must enter the egg. When this happens, the sperm’s chromosomes unite with the chromosomes from the egg. Together they form 23 pairs of chromosomes. The egg then begins to divide over and over to grow into a baby. As the baby grows, each of its cells gets a copy of the original chromosomes.
DNA is the material that carries all the information about how a living thing will look and function. For instance, DNA in humans determines such things as what color the eyes are and how the lungs work. Each piece of information is carried on a different section of the DNA. These sections are called genes. DNA is short for deoxyribonucleic acid. It is in every cell of every living thing.
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