Key Concepts
Vocabulary: chromosomes, genes, DNA, genetic disorders, amniocentesis, chorionic villi sampling, gene therapy
Heredity and Genetics
Heredity
Main Idea: Heredity is the passing of physical traits from parents to their children.
Each one of us inherits traits such as hair and eye color, as well as the shape of your earlobes, from your parents. Inherited traits, however, can also be influenced by the environment. For example, height is an inherited trait, but poor nutrition may limit growth during childhood. Other inherited traits that can be impacted by the environment include body size and the tendency for certain diseases, like diabetes. Most of the cells in the human body contain a nucleus, or the control center of a cell. Inside each nucleus is a set of chromosomes (KROH-muh-sohmz), thread-like structures found within the nucleus of a cell that carry the codes for inherited traits. Most of the cells in the human body contain 46 chromosomes that are arranged in 23 pairs. Sections of chromosomes, called genes, are the basic units of heredity. Genes occur in pairs, just like chromosomes. One gene from each pair is inherited from each parent. You have thousands of genes in every cell of your body.
DNA
The chemical unit that makes up chromosomes is called DNA, or deoxyribonucleic (dee-AHK-si-ry-boh-noo-KLEE-ik) acid. All living things are made of DNA. DNA is made up of chemical building blocks arranged along a single molecule. Several of these molecules are linked together in a strand to form a DNA sequence, known as the genetic code.When a child is born, that child carries a combination of DNA sequencing from both parents. This DNA contains different proteins that result in individual traits. All the characteristics you have, such as your eye color, the amount of curl in your hair, and your height, are determined by your genetic code. This unique code is a combination ofthe DNA ofboth your parents. Only identical twins share the same DNA pattern.
Genetics and Fetal Development
Main Idea: Chromosomes from a sperm and an egg unite to carry the hereditary traits from parents.
Passing on traits from parent to child involves genetics. Most human cells have 46 chromosomes, or 23 pairs. However, egg and sperm cells have half that number-23 chromosomes. When a sperm and egg unite during fertilization, the resulting zygote will have 46 chromosomes, 23 from each parent. These chromosomes carry the hereditary traits of the parents, which are passed on to their child. Heredity and the environment can affect human growth and development.
A zygote divides many times, ultimately producing the trillions of cells that make up the human body. Between each cell division, each chromosome in the nucleus of the cell copies itself, producing two sets of the 46 chromosomes. The cell the divides, and the two sets of chromosomes separate. Each new cell then contains one complete set of the 46 chromosomes that are identical to the ones found in the first cell of the zygote.
Dominant and Recessive Genes
Each human trait is determined by at least one pair of genes. Some genes are dominant, while others are recessive. The traits of the dominant genes generally appear in the offspring when they are present. The traits of recessive genes usually appear only when the dominant genes are not present. For example, suppose an individual receives two genes for eye color, one for brown eyes and one for blue eyes. This individual will have brown eyes because the gene for brown eyes is dominant and the gene for blue eyes is recessive. If an individual has blue eyes, that means he or she has two recessive genes for blue eye color.
Genes and Gender
One pair of chromosomes determines gender. Females have two chromosomes that look exactly alike; these are calledX chromosomes. Males have two different chromosomes, one shorter than the other. The shorter chromosome is theY chromosome. The longer one is the X chromosome.
Since sperm and egg cells contain only half the chromosomes of other cells, these cells have only one sex cell, not two. Because females have only X chromosomes, their egg cells contain only an X chromosome. Sperm, because they come from a male, contain either an X or a Y chromosome. Thus, the sperm from the male determines the gender of a child. I f a sperm cell carries the X chromosome, the child will be a girl. If the sperm carries a Y chromosome, the child will be a boy.
Genetic Disorders
Main Idea: Genetic disorders are caused by defects in genes.
Genetically Engineered Drugs
Genes used to treat diseases are not inserted directly into human beings. They are instead placed into other organisms, causing that organism to produce substances that can be used to treat human diseases and disorders. Through genetic engineering, some vaccines that can prevent disease have been produced.
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