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Name:___________________________ Chapter 20 – The Role of Genes in Inheritance Section 2 – Genetics – The Study of Inheritance A. Heredity - passing traits from parents to offspring. B. Genetics - is the study of how traits are passed from parents to offspring. C. A gene is a small section of DNA on a chromosome that has information about traits. 1. Thousands of genes are arranged on 23 pairs of chromosomes. 2. Humans should have a total of 46 chromosomes. D. What determines traits? 1. Each gene of a gene pair is called an allele. 2. When a pair of chromosomes contains different alleles for a trait, the trait is called hybrid. 3. When a trait has two identical alleles, the trait is called pure. E. Dominant and Recessive Alleles 1. A dominant allele will mask the other allele for a particular trait. 2. Recessive alleles show when two copies of the recessive allele are inherited. 3. To show a dominant allele a person can have 1 or 2 alleles for the trait. D. Expression of Traits 1. The environment can play an important role in the way a trait is shown. 2. Variations are the different ways that a certain trait appears. *examples are different hair color, eye color, and skin color. 3. A mutation is a change in a gene or chromosome. *mutations can be helpful, harmful, or neutral. 4. Selective breeding results in organisms with desired traits.