Ch. 5 Extensions and Modifications of Basic Principles

Question 1

complete dominance

Answer 1

type of dominance in which the same phenotype is expressed in homozygous (AA) and in heterozygotes (Aa); only the dominant allele is expressed in a heterozygote

Question 2

incomplete dominance

Answer 2

type of dominance in which the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes

Question 3

codominance

Answer 3

type of allelic interaction in which the heterozygote simultaneously expresses traits of both homozygotes

Question 4

incomplete penetrance

Answer 4

a case in which some individuals possess the genotype for a trait but do not express the expected phenotype

Question 5

penetrance

Answer 5

percentage of individuals with a particular genotype that express the phenotype expected of that genotype

Question 6

expressivity

Answer 6

degree to which a trait is expressed

Question 7

lethal allele

Answer 7

allele that causes the death of an individual organism, often early in development, so that the organism does not appear in the progeny of a genetic cross. a recessive lethal allele kills individuals that are homozygous for the allele; a dominant lethal allele kills both heterozygotes and homozygotes

Question 8

multiple alleles

Answer 8

presence of more than two alleles at a locus in a group of diploid individuals; however, each individual member of the group has only two of the possible alleles

Question 9

compound heterozygote

Answer 9

an individual with two different recessive alleles at a locus that results in a recessive phenotype

Question 10

gene interaction

Answer 10

interaction between genes at different loci that affect the same characteristic

Question 11

epistasis

Answer 11

type of gene interaction in which a gene at one locus masks or suppresses the effects of a gene at a different locus

Question 12

epistatic gene

Answer 12

gene that masks or suppresses the effect of a gene at a different locus

Question 13

hypostatic gene

Answer 13

gene that is masked or suppressed by the action of a gene at a different locus

Question 14

complementation test

Answer 14

test designed to determine whether two different mutations are at the same locus (are allelic) or at different loci (are nonallelic). two individuals that are homozygous for two independently derived mutations are crossed, producing F1 progeny that are heterozygous for the mutations. if the mutations are at the same locus, the F1 will have a mutant phenotype. if the mutations are at different loci, the F1 will have a wild-type phenotype

Question 15

complementation

Answer 15

manifestation of two different mutations in the heterozygous condition as the wild-type phenotype; indicates that the mutations are at different loci

Question 16

sex-influenced characteristic

Answer 16

characteristic encoded by autosomal genes that are more readily expressed in one sex. for example, an autosomal dominant gene may have higher penetrance in males than in females, or an autosomal gene may be dominant in males but recessive in females

Question 17

sex-limited characteristic

Answer 17

characteristic encoded by autosomal genes and expressed in only one sex. both males and females carry genes for sex-limited characteristics, but the characteristics appear in only one of the sexes

Question 18

cytoplasmic inheritance

Answer 18

inheritance of characteristics encoded by genes located in the cytoplasm. because the cytoplasm is usually contributed entirely by one parent, most cytoplasmically inherited characteristics are inherited from only one parent

Question 19

genetic maternal effect

Answer 19

determination of the phenotype of an offspring not by its own genotype, but by the nuclear genotype of its mother

Question 20

genomic imprinting

Answer 20

differential expression of a gene that depends on the sex of the parent that transmitted the gene

Question 21

epigenetics

Answer 21

phenomena due to alterations in DNA that do not include changes in the base sequence; often affect the way in which DNA sequences are expressed. such alterations are often stable and heritable in the sense that they are passed to descendant cells or individuals

Question 22

anticipation

Answer 22

increasing severity or earlier age of onset of a genetic trait in succeeding generations. for example, symptoms of a genetic disease may become more severe as the trait is passed from generation to generation

Question 23

temperature-sensitive allele

Answer 23

allele that is expressed only at certain temperatures

Question 24

phenocopy

Answer 24

phenotype produced by environmental effects that is the same as the phenotype produced by a genotype

Question 25

discontinuous characteristic

Answer 25

characteristic that exhibits only a few, easily distinguished phenotypes. an example is seed shape in which seeds are either round or wrinkled

Question 26

continuous characteristic

Answer 26

characteristic that displays a large number of possible phenotypes that are not easily distinguished, such as human height

Question 27

quantitative characteristic

Answer 27

continuous characteristic; displays a large number of possible phenotypes or is encoded by multiple genetic factors

Question 28

polygenic characteristic

Answer 28

characteristic encoded by genes at many loci

Question 29

pleiotropy

Answer 29

ability of a single gene to influence multiple phenotypes

Question 30

multifactorial characteristic

Answer 30

characteristic determined by multiple genes and environmental factors