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

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.

Section 1

The Origins of GeneticsMendel’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.

Section 1

The Origins of GeneticsTraits 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.

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.

Section 2

Mendel’s TheoryA Theory of HeredityMendel’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.

Section 2

Mendel’s TheoryThe 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.

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.

Section 3

Studying HeredityPunnett 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.

Section 3

Studying HeredityOutcomes 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.

Section 3

Studying HeredityInheritance 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.

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.

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.

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.

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.

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.