In this topic you will learn how studying about heredity may save lives.

Some common traits in people follow simple patterns of dominant and recessive traits. Freckles, long eyelashes, and free earlobes are dominant traits. Their opposite forms—no freckles, short eyelashes, and attached earlobes—are recessive.

Some inherited traits in people are not as simple as dominant or recessive. One example is the shape of red blood cells. Normal red blood cells are round. Sometimes red blood cells are curved like the blade of a sickle. The genes for round cells do not hide the appearance of the genes for sickle-shaped cells.

Human chromosomes carry genes that control a variety of traits. Humans have 23 pair of chromosomes. One of those pairs, the sex chromosomes, determines the sex of a person. There are two kinds of sex chromosomes, the X chromosome and the Y chromosome. Females inherit two X chromosomes, one from each parent. Only males have a Y chromosome. Males inherit one X chromosome from one parent and one Y chromosome from the other. The sex of the child is determined by the chromosome received from the father. There is always a 50% probability that a child will be a male child and a 50% probability that a child will be female.

The sex chromosomes carry genes for many traits. Some of the genes carried on the X chromosome are not found on the smaller Y chromosome. These are sex-linked genes. Color blindness and hemophilia are human traits determined by sex-linked genes. These are called sex-linked traits.

The process of crossing plants or animals to produce offspring with certain desirable traits is called selective breeding. People have used selective breeding for centuries to produce better food crops, hardier domestic animals, and prettier flowers. Farmers use selective breeding to produce drought-resistant wheat, sweeter corn, or crops that are resistant to damage from insects. Cattle are bred to produce more milk or leaner meat. Selective breeding developed most of the dog breeds we know today.

Scientists have recently developed a new method of controlling inherited traits in offspring. Genetic engineering is a way of changing the DNA sequence in a gene so that the gene will produce a particular trait. One type of genetic engineering involves moving a gene from one organism to another.

Today's researchers have made it possible to "splice" genes. Gene splicing is a process in which scientists can take the genes from one organism and attach them to the genes in another organism.

A clone is a living thing that receives all of its DNA from just one parent.