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Chapter 4. Heredity and Evolution. Selective Breeding. Genetic Principles Discovered by Mendel. Gregor Mendel (1822-1884) laid down the basic principles of heredity. Plant hybrids. Traits Mendel Studied: Peas. Results: One Trait at a Time. Principle of Segregation.
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Chapter 4 Heredity and Evolution
Genetic PrinciplesDiscovered by Mendel • Gregor Mendel (1822-1884) laid down the basic principles of heredity. • Plant hybrids
Dominance and Recessiveness • Homozygous vs. heterozygous • Recessive traits are not expressed in heterozygotes. • Dominant traits are governed by an allele that can be expressed in the presence of another, allele.
Punnett square • 1⁄4 of the plants can be expected to be homozygous tall (TT). • 1/2 can be expected to be tall but will be heterozygous (Tt). • The remaining 1⁄4 are homozygous for the recessive “short” allele (tt).
Principle of Independent Assortment • The distribution of one pair of alleles into gametes does not influence the distribution of another pair. • The genes controlling different traits are inherited independently of one another.
Phenotype • The observable or detectable physical characteristics of an organism. • The detectable expressions of genotypes, frequently influenced by environmental factors. • Phenotypic ratio - The proportion of one phenotype to other phenotypes in a group of organisms.
Mendelian Traits • Characteristics that are influenced by alleles at only one genetic locus. • Examples include many blood types, such as ABO. • Many genetic disorders such as sickle-cell anemia and Tay-Sachs disease are also Mendelian traits.
Mendelian Inheritance in Humans • Over 4,500 human trains are known to be inherited according to Mendelian principles.
Results of Crosses When Traits Are Considered Simultaneously
Antigens • Large molecules found on the surface of cells.
Codominance • The expression of two alleles in heterozygotes. • In this situation, neither allele is dominant or recessive so they both influence the phenotype.
Autosomal Dominant Trait • Inheritance of an autosomal dominant trait: a human pedigree for brachydactyly. How can individuals 5, 11, 14, 15, and 17 be unaffected?
Pattern of Inheritance of Autosomal Dominant Traits • (a) Diagram of possible gametes produced by one parent with brachydactyly and another with normal hands and fingers. • (b) Punnett square depicting possible genotypes in the offspring of one parent with brachydactyly (Bb) and one with normal hands and fingers (bb).
Partial Pedigree for Albinism • Individuals 6 and 7, children of unaffected parents, are affected. Four individuals are definitely unaffected carriers. Which ones are they?
An African Albino • An African albino. This young man has a greatly increased likelihood of developing skin cancer.
Phenotypically Normal Parents, Both Carriers of the Albinism Allele • Offspring: Homozygous dominants, Heterozygotes, (carriers), or Homozygous recessives
Polygenic Inheritance • Polygenic traits are continuous traits governed by alleles at more than one genetic locus.
Discontinuous Distribution of Mendelian Traits • Shows the discontinuous distribution of ABO blood type in a hypothetical population. • The expression of the trait is described in terms of frequencies.
Continuous Expressionof a Polygenic Trait • Represents the continuous expression of height in a large group of people.
Pleiotropy • Pleiotropy is a situation where a single gene influences more than one phenotypic expression. • Example: The autosomal recessive disorder phenylketonuria (PKU). • Individuals who are homozygous for the PKU allele don’t produce phenylketonurase, the enzyme involved in the conversion of the amino acid phenylalanine to the amino acid, tyrosine.
Mitochondrial Inheritance • Animals of both sexes inherit their mtDNA, and all mitochondrial traits, from their mothers. • All the variation in mtDNA is caused by mutation.
The Modern Synthesis Evolution is a two-stage process: • The production and redistribution of variation. • Natural selection acting on this variation. • A current definition of evolution - a change in allele frequency from one generation to the next.
Mutation • Mutation is a molecular alteration in genetic material.
Gene Flow • Gene flow is the exchange of genes between populations.
Genetic Drift • Genetic drift occurs when some individuals contribute a disproportionate share of genes to succeeding generations.
Founder Effect • Genetic drift in which allele frequencies are altered in small populations that are taken from, or are remnants of, larger populations.
Recombination • In sexually reproducing species both parents contribute genes to offspring and the genetic information is reshuffled every generation.
Natural Selection • Natural selection provides directional change in allele frequency relative to specific environmental factors. • If the environment changes, selection pressures also change. • If there are long-term environmental changes in a consistent direction, then allele frequencies should also shift gradually each generation.
Genetic Variation • Cheetahs, like many other species, have passed through a genetic bottleneck. • As a species, they have little genetic variation.
