Glossary of Genetic Chapter Two: Fundamentals of Gene Expression

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Unlinked genes
Two genes having loci on different chromosomes
Independent assortment
Sorting of alleles during meiosis after which the gametes can end up with any combination of the maternal and paternal chromosomes; classes of gametes produced in equal frequency
Linked genes
Genes having loci on the same chromosome; physically "linked" to each other
Dependent assortment
Sorting of alleles during meiosis after which the gametes can end up with only certain combinations of maternal and paternal chromosomes because of linked genes
Parental gametes
Gametes produced from dependent assortment without recombination
Individual in which the maternal and paternal alleles of a gene are different
X chromosome
Sex chromosome of which females have two and males have one
Y chromosome
Sex chromosomes of which males have one along with one X chromosome
Heteromorphic pair
Chromosomes such as the X and Y in males that are not homologous and usually differ in size
Having one copy of each X- and Y-linked gene (males)
Producing two types of gametes depending on sex chromosome content (male mammals)
Producing one type of gamete only containing X chromosomes (female mammals)
Y-linked genes
Present only in males; passed to all sons
Inheritance confined to males caused by Y-linked genes
Mendelian phenotypes
Phentoypes controlled by a single gene
Polygenic phenotypes
Phenotypes controlled by multiple different genes acting together
Individuals with identical copies of each autosomal gene
Dominant allele
Determines the phenotypes in the heterozygote
Recessive allele
Has no effect on the phenotype in the heterozygote
Null alleles
Nonfunctional recessive alleles; the phenotype they cause is the result of no gene function
Incomplete dominance
A heterozygote's phenotype is intermediate between the phenotypes of the homozygotes
A heterozygote shows the phenotypes of both alleles
Gene or allele that affects multiple phenotypes
Temperature sensitive
Codominant allele of a phenotype that is defined biochemically
Null alleles of genes that are essential for survival; usually recessive
Deleterious alleles (sublethals, semilethals)
Cause a non-lethal disadvantageous condition; can be dominant, incompletely dominant, or recessive
Incomplete penentrance
Genotype that does not always cause the expected phenotype; can be the result of genetic background
Variable expressivity
Phenotype is present, but not constant (causes a characteristic that is variable)
Genetic background
All of the genes of the organism other than those obviously involved with one phenotype; affects the internal environment of an organism
Dosage compensation
Mechanism that ensures that males and females have the same amount of X chromosome activity, regardless of the number of X chromosomes
Barr body
An inactive X chromosome that appears as a darkly stained structure in the nucleus of the cell
X inactivation
Inactivation of all but one X chromosome in mammals
Genetic mosaic
"Patchwork" of phenotypes controlled by two chromosomes, such as X chromosomes in females
Cell non-autonomous
Phenotype controlled by an X-linked gene in one cell that affects all of the cells in the body
Cell autonomous
An X-linked gene that controls a phenotype restricted to the cell in which it was made; causes a "genetic mosaic"
Occurence in male meiosis during which both the X and Y chromosomes will go to the same pole or no poles at all, producing gametes with two sex chromosomes or none at all
Form of polygenic inheritance in which the genotype at one locus overrides or modifies the phenotype caused by another locus
Additive inheritance
Form of polygenic inheritance in which multiple genes control a phenotype, and each allele of each gene has an incremental affect on phenotype
Phenotype that looks like a genetically-caused phenotype, but is really caused by the environment
Diseases that are genetically determined
Idiopathic, sporadic
Diseases that are environmentally induced
Norm of reaction
Range of the phenotype that can be produced from a single genotype, given the cariation in the environment
The proportion of the phenotype that is caused by the genotype

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