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