117 Key Terms
- allele
- gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes
- autosomes
- any of the non-sex chromosomes
- blending theory of inheritance
- hypothetical inheritance pattern in which parental traits are blended together in the offspring to produce an intermediate physical appearance
- carriers
- heterozygous individual or organism that carries a recessive allele that is not contributing to the phenotype. Relevant in medicine for recessive alleles that cause genetic disease(s) in homozygous offspring
- chromosomal basic of heredity
- also known as Chromosomal Theory of Inheritance. A theory proposing that chromosomes are the genes’ vehicles and that their behavior during meiosis is the physical basis of the inheritance patterns that Mendel observed
- codominance
- in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic
- continuous variation
- inheritance pattern in which a character shows a range of trait values with small gradations rather than large gaps between them
- dihybrid
- result of a cross between two true-breeding parents that express different traits for two characteristics
- discontinuous variation
- inheritance pattern in which traits are distinct and are transmitted independently of one another
- dominant
- trait which confers the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait
- dominant lethal
- inheritance pattern in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age
- epistasis
- antagonistic interaction between genes such that one gene masks or interferes with the expression of another
- F1
- first filial generation in a cross; the offspring of the parental generation
- F2
- second filial generation produced when F1 individuals are self-crossed or fertilized with each other
- genetics
- the study of heredity
- genotype
- underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism
- hemizygous
- presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant
- heterozygous
- having two different alleles for a given gene on the homologous chromosome
- homozygous
- having two identical alleles for a given gene on the homologous chromosome
- hybridization
- process of mating two individuals that differ with the goal of achieving a certain characteristic in their offspring
- incomplete dominance
- in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype
- law of dominance
- in a heterozygote, one trait will conceal the presence of another trait for the same characteristic
- law of independent assortment
- genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur
- law of segregation
- paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors
- linkage
- phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together
- Mendelian genetics
- patterns of inheritance discovered by Gregor Mendel that include law of segregation, law of independent assortment, genotypes and phenotypes
- model system
- species or biological system used to study a specific biological phenomenon to be applied to other different species
- monohybrid
- result of a cross between two true-breeding parents that express different traits for only one characteristic
- P0
- parental generation in a cross
- phenotype
- observable traits expressed by an organism
- product rule
- probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone
- Punnett square
- visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid
- recessive
- trait that appears “latent” or non-expressed when the individual also carries a dominant trait for that same characteristic; when present as two identical copies, the recessive trait is expressed
- recessive lethal
- inheritance pattern in which an allele is only lethal in the homozygous form; the heterozygote may be normal or have some altered, nonlethal phenotype
- reciprocal cross
- paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross
- sex-linked
- any gene on a sex chromosome
- sum rule
- probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities
- test cross
- cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait
- trait
- variation in the physical appearance of a heritable characteristic
- true breeding
- parental organisms that always produce offspring that look like the parent (because they are homozygous for that trait)
- wild type
- term for most common phenotype or genotype among wild animals of a gene
- X-linked
- gene present on the X, but not the Y chromosome