Mendelian Genetics and Chromosomal Inheritance: every key term you need (+ practice quiz)
25 flashcard terms for General Biology I Topic 8, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 15-question quiz — free, no account needed.
The experimental choices that made the pattern visible: true-breeding lines, sharply contrasting characters, controlled pollination, and counting large numbers of offspring across successive generations.
Particulate inheritance
The conclusion that heredity is carried by discrete factors that remain intact across generations, replacing the older idea that parental traits blend irreversibly in the offspring.
Allele
One of the alternative versions of a gene found at a given chromosomal position. A diploid individual carries two, which may be the same or different, one inherited from each parent.
Genotype and phenotype
The genetic constitution of an individual versus its observable characteristics. The same phenotype can arise from more than one genotype, which is why test crosses are needed.
Dominant and recessive
A description of how alleles interact in a heterozygote, not of how common or how beneficial an allele is. Many recessive alleles simply produce a nonfunctional or reduced-activity product.
Law of segregation
The rule that the two alleles of a gene separate during gamete formation so each gamete carries one. Its physical basis is the separation of homologs during the first meiotic division.
Law of independent assortment
The rule that alleles of different genes are distributed to gametes independently. It holds for genes on different chromosomes or far apart on one, but not for closely linked genes.
Monohybrid cross
A cross tracking one character, which yields a three to one phenotypic ratio in the second generation from heterozygous parents and an underlying one to two to one genotypic ratio.
Dihybrid cross
A cross tracking two independently assorting characters, giving the nine to three to three to one second generation ratio that first revealed independent assortment.
Test cross
A cross of an individual showing the dominant phenotype to a homozygous recessive partner. Any recessive offspring reveals the unknown parent as a heterozygote.
Punnett square
A grid that combines the gamete types of two parents to enumerate offspring genotypes and their expected proportions, useful for one or two genes before combinatorics becomes unwieldy.
Incomplete dominance
A pattern in which the heterozygote shows an intermediate phenotype because one functional allele yields insufficient product. The second generation ratio becomes one to two to one for phenotype as well.
Codominance
A pattern in which both alleles are fully and separately expressed in the heterozygote, as when two distinct surface molecules appear together rather than blending into an intermediate.
Multiple alleles
The situation in which a population contains more than two versions of a gene even though any individual carries only two, as with the three common alleles of the classic blood group locus.
Pleiotropy
One gene influencing several apparently unrelated traits, usually because its product functions in more than one tissue or pathway. Many hereditary disorders show characteristic clusters of symptoms.
Epistasis
An interaction in which one gene masks or modifies the expression of another, as when a pigment deposition gene overrides whichever color a separate gene specifies, distorting the expected ratios.
Polygenic inheritance
Additive contributions from many genes producing a continuous, roughly bell-shaped distribution of a trait such as height or skin tone, rather than a small number of discrete classes.
Penetrance and expressivity
Two ways genotype and phenotype loosen: penetrance is the fraction of carriers showing the trait at all, and expressivity is how strongly it appears in those who do.
Chromosome theory of inheritance
The proposal by Sutton and Boveri that genes reside on chromosomes, argued from the exact parallel between the behavior of chromosomes in meiosis and the behavior of Mendel's factors.
Morgan white eyed fly
The finding that a mutant eye color appeared almost only in males and followed the pattern of a specific chromosome, tying a particular gene to a particular chromosome for the first time.
Sex linkage
Inheritance of genes on the sex chromosomes, which produces unequal patterns between the sexes because a male carrying only one copy expresses a recessive allele with no possible masking partner.
X inactivation
The developmental silencing of one X chromosome in each cell of a female mammal, chosen at random and inherited by that cell's descendants, producing patchy mosaic expression such as tortoiseshell coat color.
Genetic linkage
The tendency of genes near each other on a chromosome to be inherited together, showing up as parental combinations in excess of the frequencies independent assortment would predict.
Recombination frequency mapping
The use of the percentage of recombinant offspring as a distance measure, since crossovers are more likely between genes that are farther apart, with one percent defined as one map unit.
Aneuploidy
An abnormal chromosome number arising from nondisjunction, such as a trisomy in which three copies of one chromosome are present. Dosage imbalance rather than a mutated gene causes the effects.