117 Key Terms

gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes
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
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
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
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
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
antagonistic interaction between genes such that one gene masks or interferes with the expression of another
first filial generation in a cross; the offspring of the parental generation
second filial generation produced when F1 individuals are self-crossed or fertilized with each other
the study of heredity
underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism
presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant
having two different alleles for a given gene on the homologous chromosome
having two identical alleles for a given gene on the homologous chromosome
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
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
result of a cross between two true-breeding parents that express different traits for only one characteristic
parental generation in a cross
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
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
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
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
gene present on the X, but not the Y chromosome


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