Covalently closed circle
A double-stranded DNA molecule with no free ends. The two strands are interlinked and will remain together even after denaturation. In its native form, a CCC will adopt a supercoiled configuration.

Co-variance
A measure of the statistical association between variable; the extent to which two variables vary together.

Critical breed
In AnGR: A breed where the total number of breeding females is less than 100 or the total number of breeding males is less than or equal to five; or the overall population size is close to, but slightly above 100 and decreasing, and the percentage of pure-bred females is below 80%. (Source: FAO, 1999)

Critical-maintained breed; endangered-maintained breed
In AnGR: Categories where critical or endangered breeds are being maintained by an active public conservation programme or within a commercial or research facility. (Source: FAO, 1999)

Cross hybridization
The hydrogen bonding of a single-stranded DNA sequence that is partially but not entirely complementary to a single-­stranded substrate. Often, this involves hybridizing a DNA probe for a specific DNA sequence to the homologous sequences of different species.

Crossing over
A process in which homologous chromosomes exchange material through the breakage and reunion of two chromatids. A single crossover represents one reciprocal breakage and reunion event. A double crossover requires two simultaneous reciprocal breakage and reunion events. a.k.a, recombination; recombination event.