Genetics Midterm Textbook Notes
Mendelian Genetics: pg. 40-48
 The Principle of Dominance- in a heterozygote, one allele may conceal the presence of another
 The Principle of Segregation- in a heterozygote, two different alleles segregate from each other during the formation of gametes
 The Principle of Independent Assortment- The alleles of different genes segregate, or as we sometimes say, assort, independently of each other
 Mendel studied the inheritance of 7 different traits in garden peas, each trait being controlled by a different gene
Modifications of Mendelian Ratios: pg. 62-82
 Incomplete dominance- expression of two alleles in a heterozygote that allows the heterozygote to be distinguished from either of its two parents
 Codominant- alleles that produce independent effects when heterozygous
 Polymorphic- the existence of two or more variants in a population of individuals, with at least two of the variants having frequencies greater than 1 percent  Amorphic- mutant allele that completely abolishes gene expression
 Hypomorphic- mutant allele that has less expression than a wild-type allele but does not completely abolish expression
 Genes often have multiple alleles
 Mutant alleles may be dominant, recessive, incompletely dominant, or codominant  If a hybrid that inherited a recessive mutation from each of its parents has a mutant phenotype, then the recessive mutations are alleles of the same gene; if the hybrid has a wild phenotype, then the recessive mutations are alleles of different genes
 Most genes are encoded polypeptides
 In homozygous condition, recessive mutations often abolish or diminish polypeptide activity
 Some dominant mutations produce a polypeptide that interferes with the activity of the polypeptide encoded by the wild-type allele of a gene
 Epistasis- interactions between products of nonallelic genes. Genes suppressed are said to be hypostatic. Dominance is associated with members of allelic pairs, whereas epistasis results from interactions of the products of nonalleles
 Pleiotropic- condition in which a single gene influences more than one trait
 Gene action is affected by biological and physical factors in the environment
 Two or more genes may influence a trait
 A mutant allele is epistatic to a mutant allele of another gene if it has an overriding effect on the phenotype
 A gene is pleiotropic if it influences many different phenotypes
 Inbreeding increases the frequency of homozygotes and decreases the frequency of heterozygotes

Pedigree Analysis: pg. 53-56
 Pedigree- a table, chart, or diagram representing the ancestry of an individual
 Albinism- absence of pigment (melanin) in skin, hair, and eyes of an animal.
Absence of chlorophyll in plants
 Pedigrees are used to identify dominant and recessive traits in human families
 The analysis of pedigrees allows genetic counselors to asses the risk that an individual will inherit a particular trait
Chromosomes: pg. 89-94, 100-103, 113, 119-123
 Chromatin- the complex of DNA and proteins in eukaryotic chromosomes
 Euchromatin- genetic material that is not stained so intensely by certain dyes during interphase and that comprises of many different kinds of genes
 Heterochromatin- chromatin staining darkly even during interphase, often containing repetitive DNA with few genes
 Sex chromosomes- chromosomes that are connected with the determination of sex  Autosomes- any chromosome that is not a sex chromosome
 Individual chromosomes becomes visible during cell division; between divisions they form a diffuse network of fibers called chromatin
 Diploid somatic cells have twice as many chromosomes as haploid gametes
 Sex chromosomes are different between the two sexes, whereas autosomes are the same
 Hemizygote- an individual that carries one copy of a chromosome or gene, as in sex linkage or as a result of deletion
 Nondisjunction- failure of