Mendelian Transmission and Extensions to Simple Inheritance: every key term you need (+ practice quiz)
25 flashcard terms for Genetics Topic 1, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
One of the alternative forms of a gene at a given locus. A diploid individual carries two, which may be identical or different.
Genotype versus phenotype
Genotype is the allele combination carried; phenotype is the observable trait. The same phenotype can arise from more than one genotype, which is why test crosses exist.
Law of segregation
The two alleles of a gene separate into different gametes. This follows directly from homologous chromosomes parting at the first meiotic division.
Law of independent assortment
Alleles of different genes assort independently, because homologous pairs orient at random on the spindle. It fails for genes on the same chromosome that are close together.
Test cross
A cross to a fully recessive individual, which reveals the unknown genotype directly because every offspring phenotype reports the allele contributed by the tested parent.
Monohybrid ratio
The three to one phenotypic ratio among offspring of two heterozygotes, the signature of a single gene with complete dominance.
Dihybrid ratio
The nine to three to three to one ratio expected when two independently assorting genes each show complete dominance.
Incomplete dominance
The heterozygote shows an intermediate phenotype, so the phenotypic and genotypic ratios become identical and no allele is masked.
Codominance
Both alleles are expressed fully and separately in the heterozygote, as in the blood group system where two antigens appear together.
Multiple alleles
A gene with more than two variants in the population, though any individual still carries only two of them.
Lethal allele
An allele that kills some genotypic class before scoring, which distorts observed ratios, most visibly turning three to one into two to one.
Epistasis
One gene masks the effect of another, modifying the standard two-gene ratio into patterns such as nine to seven or twelve to three to one.
Complementation test
Crosses two recessive mutants; a normal offspring means the mutations lie in different genes, while a mutant offspring means they lie in the same gene.
Penetrance
The fraction of individuals with a genotype who show the associated phenotype at all. Incomplete penetrance makes traits appear to skip generations.
Expressivity
How strongly a phenotype is expressed among those who show it, which can vary widely even within one family.
Pleiotropy
One gene affecting several apparently unrelated traits, usually because its product functions in more than one tissue or pathway.
Sex-linked inheritance
Inheritance of genes on the sex chromosomes, producing unequal phenotype frequencies between sexes and reciprocal crosses that give different results.
Hemizygosity
Carrying only one copy of a gene, as males do for most genes on the X chromosome, so a single recessive allele is expressed directly.
One X chromosome is randomly silenced in each cell of a female embryo, producing patches of tissue expressing different alleles.
Sex-limited trait
A trait expressed in only one sex although the genes are carried by both, driven by hormonal or developmental context rather than chromosome location.
Sex-influenced trait
A trait whose dominance relationship differs between the sexes, so the same heterozygous genotype gives different phenotypes.
Pedigree analysis
Infers the mode of inheritance from a family diagram, using unaffected parents with affected children and the pattern across sexes as the key evidence.
Chi-square test in genetics
Compares observed offspring counts with those expected under a proposed model, and a large value means the model should be rejected rather than the data doubted.
Product and sum rules
Independent events multiply and mutually exclusive outcomes add. Applied together they replace large Punnett squares with a short calculation.
Carrier
A heterozygote for a recessive condition who is unaffected but transmits the allele, and whose frequency far exceeds that of affected individuals for rare traits.