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Chapter 14 Mendel and the Gene Mendel’s Discoveries Character - a heritable feature Trait - each variant for a character True-breeding - when the plants self-pollinate all their offspring are of the same variety Hybridization - the mating or crossing of two varieties (Fig 14.2) Monohybrid cross - a cross that tracks the inheritance of a single trait (Fig 14.1) P generation - parental, true breeding parents F1 generation - first filial, hybrid offspring F2 generation - second filial, hybrid offspring of the F1 generation Mendel’s Principals Law of Segregation Law of Independent Assortment Law of Segregation (Fig 14.4) Alternate versions of genes account for variations in inherited characters Alleles - alternate versions of genes (Fig 14.3) For each character, an organism inherits two alleles, one from each parent If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance The two alleles for each character segregate during gamete production Genetic Vocabulary Homozygous - has a pair of identical alleles for a character Heterozygous - has two different alleles for a character (Fig 14.6) Phenotype - an organism’s appearance (Fig 14.5) Genotype - an organism’s genetic makeup Law of Independent Assortment Dihybrid cross - the mating of parental varieties differing in two characters (Fig 14.7) Types of Dominance Complete Dominance - the phenotypes of the heterozygotes and the dominant homozygote are indistinguishable Incomplete Dominance - the F1 hybrids have an appearance somewhere in between the phenotypes of the two parental varieties (Fig 14.9) Codominance - alleles separately manifest in the phenotype Dominance/Recessiveness Relationships They range from complete dominance, though various degrees of incomplete dominance, to codominance They reflect the mechanism by which specific alleles are expressed in phenotype and do not involve the ability of a one allele to subdue another at the level of the DNA They do not determine the relative abundance of alleles in the population Multiple alleles (Fig 14.10) Pleiotropy - the ability of a gene to affect an organism in many ways Epistasis - a gene at one locus alters the phenotypic expression of a gene at a second locus (Fig 14.11) Polygenic inheritance - an additive effect of two or more genes on a single phenotypic character (Fig 14.12) Quantitative characters - characters that vary in the population along a continuum Nature vs Nurture Norm of Reaction - a range of phenotypic possibilities over which there may be variation due to environmental influence Multifactorial characters - many factors, both genetic and environmental, collectively influence phenotype Mendelian Inheritance in Humans Pedigree - a family history for a particular trait assembled into a family tree describing the interrelationships of parents and children across the generations (Fig 14.14) Carriers - heterozygotes who are phenotypically normal but can transmit the recessive allele to their offspring Recessively Inherited Disorders Cystic Fibrosis Tay-Sachs disease Sickle-cell disease (Fig 14.15) Dominantly Inherited Disorders Achondroplasia Huntington’s disease (Fig 14.16) Multifactorial Disorders Heart disease Diabetes Alcoholism Schizophrenia Alzheimer’s disease Genetic Testing Carrier recognition Fetus Testing (Fig 14.17) Amniocentesis Chorionic villi sampling (CVS) Newborn screening