Chapter 14: Mendel and the Gene Idea PDF

# Chapter 14: Mendel and the Gene Idea ## Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance - **Law of segregation** - Character variants -> Traits - Two alleles segregate during meiosis and end up in different gametes. - **Independent assortment** -...

# Chapter 14: Mendel and the Gene Idea ## Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance - **Law of segregation** - Character variants -> Traits - Two alleles segregate during meiosis and end up in different gametes. - **Independent assortment** - Two or more genes assort independently. ### Mendel's Experiments - He also used varieties that were **true-breeding** (plants that produce offspring of the same variety when they self-pollinate). - In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called **hybridization**. - The true-breeding parents are the **P generation**. - The hybrid offspring of the P generation are called the **F₁ generation**. - When F₁ individuals self-pollinate or cross-pollinate with other F₁ hybrids, the **F₂ generation** is produced. ## The Law of Segregation - Mendel derived the law of segregation by following a single character. - The F₁ offspring produced in this cross were **monohybrids**, heterozygous for one character. - A cross between such heterozygotes is called a **monohybrid cross**. **Advantages of Pea Plant as a Model System:** - Short generation time - Single flower producer: Sperm & egg. - Large number of offspring - Mating can be controlled (Self-pollinate or cross-pollinate) ### Principle of Segregation - **Phenotypic Ratio**: 3:1 (monohybrid cross, F₂ generation) - **Genotypic Ratio**: 1:2:1 (monohybrid cross, F₂ generation) ## The Law of Independent Assortment - Mendel identified his second law of inheritance by following two characters at the same time. - Crossing two true-breeding parents differing in two characters produces **dihybrids** in the F₁ generation, heterozygous for both characters. - A **dihybrid cross**, a cross between F₁ dihybrids, can determine whether two characters are transmitted to offspring as a package or independently. ### Principle of Independent Assortment <start_of_image> Schematic: | **Predicted offspring of F₂ generation** | **Hypothesis of dependent assortment** | **Hypothesis of independent assortment** | |---|---|---| | | **Sperm** 1/2 YR 1/2 yr | **Sperm** 1/4 YR 1/4 Yr 1/4 yR 1/4 yr | | **Eggs** 1/2 YR | YYRR YyRr | YYRR YYRr YyRR YyRr | | **Eggs** 1/2 yr | YyRr yyrr | YyRr YYrr YyRr Yyrr | | | | YyRr yyRR yyRr yyrr | | **Phenotypic ratio**: 3:1 | | **Phenotypic ratio**: 9:3:3:1 | - **Results**: Phenotypic ratio approximately 9:3:3:1 ## Concept 14.2: Probability laws govern Mendelian inheritance - Mendel's laws of segregation and independent assortment reflect the rules of probability. - When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss. - In the same way, the alleles of one gene segregate into gametes independently of another gene's alleles. ## The Multiplication and Addition Rules Applied to Monohybrid Crosses <start_of_image> Illustration with coin toss: *R (dominant allele) and r (recessive allele)*. - **The multiplication rule**: The probability that two or more independent events will occur together is the product of their individual probabilities. ### The Addition Rule - **The addition rule**: States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities. ## Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics - The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied. - Many heritable characters are not determined by only one gene with two alleles. - However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance. ## Extending Mendelian Genetics for a Single Gene - Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: - When alleles are not completely dominant or recessive. - When a gene has more than two alleles. - When a gene produces multiple phenotypes. ### Degrees of Dominance - **Complete dominance**: Occurs when phenotypes of the heterozygote and dominant homozygote are identical. - **Incomplete dominance**: The phenotype of F₁ hybrids is somewhere between the phenotypes of the two parental varieties. - **Codominance**: Two dominant alleles affect the phenotype in separate, distinguishable ways. ### Pleiotropy - Most genes have multiple phenotypic effects, a property called **pleiotropy**. - For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease. ## Extending Mendelian Genetics for Two or More Genes - Some traits may be determined by two or more genes. - **Epistasis**: One gene affects the phenotype of another due to interaction of their gene products. - **Polygenic inheritance**: Multiple genes independently affect a single trait. ### Epistasis - **Epistasis**: Expression of a gene at one locus alters the phenotypic expression of a gene at a second locus. - E/e is epistasis to B/b ### Polygenic Inheritance - **Quantitative characters**: Are those that vary in the population along a continuum. - **Polygenic inheritance**: Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype. - **Height**: Over 180 genes affect height. - **Skin color in humans**: Is also controlled by many separately inherited genes. ## Nature and Nurture: The Environmental Impact on Phenotype - **Environmental impact on phenotype**: Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype. - **Phenotypic range**: The phenotypic range is broadest for polygenic characters. - **Multifactorial traits**: Traits that depend on multiple genes combined with environmental influences are called multifactorial. ## Concept 14.4: Many human traits follow Mendelian patterns of inheritance: Pedigree Analysis - A **pedigree** is a family tree that describes the interrelationships of parents and children across generations. - Inheritance patterns of particular traits can be traced and described using pedigrees. ### Recessively Inherited Disorders - Recessively inherited disorders show up only in individuals homozygous for the allele. - **Carriers**: are heterozygous individuals who carry the recessive allele but are phenotypically normal. ### Dominantly Inherited Disorders - Dominant inherited disorders show up in individuals heterozygous and homozygotes for the allele. - Dominant trait shows in every generation. ### Multifactorial Disorders - **Simple Mendelian disorders**: Are rare hereditary diseases caused by mutations in a single gene. - **Multifactorial diseases**: Have both genetic and environmental components. - **Lifestyle choices**: Can significantly reduce the risk of developing multifactorial diseases such as exercise, a healthy diet, not smoking, and stress management. ## Genetic Testing and Counseling - **Genetic counseling and testing**: Can assess the risk of genetic disorders before conception or during early pregnancy. - **Mendelian genetics and probability rules**: Can determine the risk of passing on recessive genetic disorders. - **Carrier screening**: Can identify carriers of recessive alleles, helping individuals make informed decisions about having children. - **Prenatal testing**: Can detect genetic disorders in the fetus. - **Genetic Information Nondiscrimination Act**: Protects against discrimination based on genetic test results. ## Chapter 14 Review **Mendel's laws of inheritance:** - **Law of segregation:** Genes have alternative forms (alleles) that segregate during meiosis and gamete formation - **Law of independent assortment:** Alleles for different genes segregate into gametes independently. **Probability laws in Mendelian inheritance:** - **Multiplication rule:** The probability of two or more events occurring together is the product of their individual probabilities. - **Addition rule:** The probability of an event occurring in two or more mutually exclusive ways is the sum of their individual probabilities. **Complex inheritance patterns:** - **Environmental impact**: Genotype expression can be influenced by environmental factors - **Polygenic traits**: Polygenic characters are impacted by genes and the environment. **Analysis of family pedigrees:** - Can be used to deduce possible genotypes and predict genetic disorders in offspring. **Genetic disorders:** - Many genetic disorders are inherited as simple recessive traits. - Lethal dominant alleles are often eliminated if affected individuals die before reproducing. - Some genetic disorders have both genetic and environmental components and do not follow simple Mendelian inheritance patterns. **Test Your Understanding** - **Punnett squares**: Used to predict genotypic and phenotypic ratios in offspring, allowing for the determination of inheritance patterns and the probability of certain traits appearing in future generations. - **Analysis of offspring:** Can be used to determine the genotypes of individuals - **Genetic counseling**: Can provide information about the probability of certain genetic disorders appearing in offspring based on family history and known inheritance patterns.