Haploid cell
A cell containing only one set of chromosomes.
Hardy-Weinberg equilibrium
The steady-state relationship between relative frequencies of two or more alleles in an
idealized population; both the allele frequencies and the genotype frequencies will remain
constant from generation to generation in a population breeding at random in the absence
of evolutionary forces.
Hardy-Weinberg theorem
An axiom maintaining that the sexual shuffling of genes alone cannot alter the overall
genetic makeup of a population.
Haustorium
In parasitic fungi, a nutrient-absorbing hyphal tip that penetrates the tissues of the host but
remains outside the host cell membranes.
Haversian system
One of many structural units of vertebrate bone, consisting of concentric layers of
mineralized bone matrix surrounding lacunae, which contain osteocytes, and a central
canal, which contains blood vessels and nerves.
Heat
The total amount of kinetic energy due to molecular motion in a body of matter. Heat is
energy in its most random form.
Heat of vaporization
The amount of heat required to change a given amount of a liquid into a gas; 540 calories
are required to change 1 gram of liquid water into vapor.
Heat-shock protein
A protein that helps protect other proteins during heat stress, found in plants, animals, and
microorganisms.
Helper T cell (TH)
A type of T cell that is required by some B cells to help them make antibodies or that helps
other T cells respond to antigens or secrete lymphokines or interleukins.
Heme
The iron-containing group of heme proteins such as hemoglobin and the cytochromes.
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