Chapter 8

1. Chapter 8 Mendel and Heredity Section 1: The Origins of Genetics Section 2: Mendel’s Theory Section 3: Studying Heredity Section 4: Complex Patterns of Heredity

2. Section 1 The Origins of Genetics Objectives: • Identify the investigator whose studies formed the basis of modern genetics. • List characteristics that make the garden pea a good subject for genetic study. • Summarize the three major steps of Gregor Mendel's garden-pea experiments. • Relate the ratios that Mendel observed in his crosses to his data.

3. Section 1 The Origins of Genetics Mendel’s Studies of Traits • Mendel’s Breeding Experiments Gregor Mendel bred varieties of the garden pea in an attempt to understand heredity. • Useful Features in Peas The garden pea is a good subject for studying heredity for the following reasons: several traits show discrete forms, self-fertilization and cross-fertilization are possible, the garden pea is easy to cultivate, and the plants produces many offspring.

4. Section 1 The Origins of Genetics Traits Expressed as Simple Ratios • Monohybrid Crosses A monohybrid cross is a cross that involves one pair of contrasting traits. • Mendel’s Results Mendel observed that contrasting traits appear in offspring according to simple ratios. In Mendel’s experiments, only one of the two contrasting forms of a trait was expressed in the F1 generation. The other form reappeared in the F2 generation in a 3:1 ratio.

5. Section 2 Mendel’s Theory Objectives: • Describe the four major hypotheses Mendel developed. • Define the terms homozygous, heterozygous, genotype, and phenotype. • Compare Mendel's two laws of heredity.

6. Section 2 Mendel’s Theory A Theory of Heredity Mendel’s Hypotheses Different versions of a gene are called alleles. An individual usually has two alleles for a gene, each inherited from a different parent. Mendel’s Findings in Modern Times Individuals with the same two alleles for a gene are homozygous; those with two different alleles for a gene are heterozygous. Genotype and Phenotype The set of alleles that an individual has is called its genotype. The physical appearance of a trait or outward expression of the genotype is called the phenotype.

7. Section 2 Mendel’s Theory The Laws of Heredity • The Law of Segregation The law of segregation states that the two alleles for a trait separate when gametes are formed. • The Law of Independent Assortment The law of independent assortment states that two or more pairs of alleles separate independently of one another during gamete formation.

8. Section 3 Studying Heredity Objectives: • Predict the results of monohybrid genetic crosses by using Punnett squares. • Apply a test cross to determine the genotype of an organism with a dominant phenotype. • Predict the results of monohybrid genetic crosses by using probabilities. • Analyze a simple pedigree.

9. Section 3 Studying Heredity Punnett Squares • Function of Punnett Squares The results of genetic crosses can be predicted with the use of Punnett squares. • One Pair of Contrasting Traits Punnett squares can be used to predict the outcome of a monohybrid cross with one pair of contrasting traits. • Determining Unknown Genotypes A test cross can be used to determine whether an individual expressing a dominant trait is heterozygous or homozygous.

10. Section 3 Studying Heredity Outcomes of Crosses • Probability of a Specific Allele in a Gamete The probability of a specific allele in a gamete can be predicted with the use of probabilities. For a gene with two alleles, the chance of contributing one allele or the other to the gamete is 1/2. • Probability of the Outcome of a Cross The results of a genetic cross can be predicted with the use of probabilities. To find the probability that a combination of two independent events will occur, multiply the separate probabilities of the two events.

11. Section 3 Studying Heredity Inheritance of Traits • Pedigrees A trait’s pattern of inheritance within a family can be determined by analyzing a pedigree. • Patterns of Inheritance Scientists use pedigrees to determine whether a trait is autosomal or sex-linked, dominant or recessive, and heterozygous or homozygous.

12. Section 4 Complex Patterns of Heredity Objectives: • Identify five factors that influence patterns of heredity. • Describe how mutations can cause genetic disorders. • List two genetic disorders, and describe their causes and symptoms. • Evaluate the benefits of genetic counseling.

13. Section 4 Complex Patterns of Heredity Complex Control of Traits • Traits Influenced by Several Genes Many traits—weight, hair color, and skin color—are polygenic traits that involve several genes influencing the trait. • Intermediate Traits A trait that is intermediate between the two parental types is a condition known as incomplete dominance. • Traits Controlled by Genes with Three or More Alleles Some traits, such as the ABO blood type alleles, are controlled by three or more alleles.

14. Section 4 Complex Patterns of Heredity Complex Control of Traits continued • Traits with Two Forms Displayed at the Same Time Codominance occurs if two alleles are dominant and thus both forms of the trait are expressed at the same time. • Traits Influenced by the Environment A phenotype often depends on conditions in the environment.

15. Section 4 Complex Patterns of Heredity Genetic Disorders • Sickle Cell Anemia Sickle cell anemia is a recessive genetic disorder that causes an abnormal form of hemoglobin protein. • Cystic Fibrosis Cystic fibrosis is a fatal recessive trait that causes a defective chloride-ion transport protein. • Hemophilia Hemophilia is a recessive genetic disorder that leads to a defective blood-clotting factor. • Huntington’s Disease Huntington’s disease is a dominant genetic disorder that leads to the production of an inhibitor of brain-cell metabolism.

16. Section 4 Complex Patterns of Heredity Treating Genetic Disorders • Genetic Counseling Genetic counseling can help patients concerned about a genetic disorder. • Gene Therapy A promising new gene technology called gene therapy will allow scientists to replace defective genes with copies of healthy genes.