genotype the genetic constitution of an organism phenotype an observable characteristic or set of characteristics of an organism produced by the interaction between its genotype and the environment true-breeding when a trait under investigation does not change from parent to offspring for many generations monohybrid crosses crosses between true-breeding strains that differ in a single trait reciprocal crosses matings done both ways: female A + male B, female B + male A what Mendel called genes particulate factors locus a particular spot on a chromosome where a gene is located Mendel's First Law Principle of Segregation: each allele separates from each other during the formation of gametes Mendel's Second Law Principle of Independent Assortment: factors for different traits assort independently of each other testcross a cross of an individual with a homozygous recessive genotype with another individual with an unknown genotype, which can reveal the unknown's genotype. allele One of two or more alternative forms of a single gene proband The person at which a pedigree analysis reveals was the starting point for the trait propositus a male proband proposita a female proband They proposed the chromosome theory of inheritance Sutton & Boveri autosomes non-sex chromosomes hemizygous the gene is only present on a single chromosome (not paired alleles in two autosomes) crisscross inheritance example: male parent --> female offspring --> male grandchild non-disjunction when homologous chromosomes fail to segregate correctly during meiosis, resulting in a gamete with both chromosomes and another gamete with none. aneuploidy One or more chromosomes missing, or present in more than usual number of copies In this system, sex is determined by the ratio of X chromosomes to sets of autosomes X chromosome-autosome balance system Barr body These are highly condensed masses of chromatin, containing a disabled X chromosome The process of disabling an X chromosome lyonization Saccharomyces cerevisiae has this type of system for sex determination A genic system -- sexes are determined by simple allelic differences at a small number of gene loci. The "sexes" are "A" and alpha and are called mating types. Turtles have this type of sex determination Environmental sex determination (by temperature) Hemophilia A is an example of this X-linked recessive trait A trait resulting from a mutant gene carried on the Y chromosome holandric, or Y-linked Incomplete dominance Plumage color in chickens, palomino horses are examples codominance When a heterozygote shows the phenotypes of both homozygotes epistasis When one gene masks the phenotypic expression of another gene penetrance The frequency at which a dominant trait manifests itself in individuals (% of group that show the trait at all) expressivity The degree at which a trait is expressed (% of individual trait) a mutation which is not passed on to the next generation of offspring somatic mutation a mutation which is passed on to the next generation of offspring germ-line mutation base-pair substitution mutation involves a change in the DNA from one base-pair to another transition mutation a mutation from one purine-pyr to the other purine-pyr pair (AT -> GC, GC -> AT, TA -> CG, CG -> TA). transversion mutation a mutation from a purine-pyr to a pyr-purine pair (AT -> TA, AT -> CG, etc.). missense mutation a mutation in which a base pair change in DNA results in a different mRNA codon nonsense mutation a mutation in which a base pair change in DNA results in a stop mRNA codon silent mutation a mutation in which a base pair change in DNA results in the same mRNA codon neutral mutation a mutation in which a base pair change in DNA results in a similar protein is encoded frameshift mutation a mutation where an addition or deletion scrambles the protein encoding forward mutation change from wild-type to mutant back mutation change from mutant back to wild-type. AKA reversions, reverse mutations. true reversion changes the mRNA message back to the exact wild-type message partial reversion changes the mRNA message back to a protein that is compatible with the wild-type message suppressor mutation a mutation at a different site which diminishes effects of original mutation When a stop codon is missing or not sensed, this could result in an overly long protein known as a read-through protein intercalating agents insert themselves beteen adjacent bases in one or both strands of DNA This is the standard test for possible carcinogens The Ames Test This is added during the Ames Test to see if the liver could cause a carcinogen to form S9 extract In E. coli, these two genes are key to controlling the SOS system lexA, recA In E. coli, a significant chunk of DNA can be resequenced by methylation of the A in this sequence; it is called GATC; methyl-directed mismatch repair This can fix thymine dimers photoreactivation/light repair The genes uvrA, B, C, D form the basis of dark repair, excision repair system; AKA nucleotide excision repair (NER) Pseudodominance Unexpected expression of a recessive trait caused by the absence of a dominant allele Pericentric Includes the centromere Paracentric Excludes the centromere translocation mutation a chromosomal mutation where portions of chromosomes are swapped