Genetic Chapter Two: Fundamentals of Gene Expression
Terms
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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
- Heterozygous
- 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
- Hemizygous
- Having one copy of each X- and Y-linked gene (males)
- Heterogametic
- Producing two types of gametes depending on sex chromosome content (male mammals)
- Homogametic
- Producing one type of gamete only containing X chromosomes (female mammals)
- Y-linked genes
- Present only in males; passed to all sons
- Holandric
- 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
- Homozygous
- 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
- Codominance
- A heterozygote shows the phenotypes of both alleles
- Pleiotropic
- Gene or allele that affects multiple phenotypes
- Temperature sensitive
- Codominant allele of a phenotype that is defined biochemically
- Lethals
- 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"
- Non-disjunction
- 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
- Epistasis
- 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
- Phenocopy
- Phenotype that looks like a genetically-caused phenotype, but is really caused by the environment
- Familial
- 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
- Heritability
- The proportion of the phenotype that is caused by the genotype