Haldane’s rule
Common observation that among the offspring of crosses between two species, one sex is sterile, absent, or rare. Often, male hybrids are sterile and female hybrids are fertile. Haploid (n) a cell or an individual with one copy of each nuclear chromosome.
Haplo-sufficient
Describing a gene that can support the normal wild-type phenotype when present in only one copy (heterozygous condition) in a diploid cell. A haplo-sufficient gene exhibits complete dominance in genetic crosses.
Haplotype
A set of specific SNP alleles at particular SNP loci that are close together in one small region of a chromosome.
Haplotype map (HapMap)
A complete description of all of the haplotypes known in all human populations tested, as well as the chromosomal location of each of these haplotypes.
Hardy–weinberg law
An extension of mendel’s laws of inheritance that describes the expected relationship between gene frequencies in natural populations and the frequencies of individuals of various genotypes in the same populations.
Hemizygous
Possessing only one copy (allele) of a gene in a diploid cell. Usually applied to genes on the x chromosome in males with the xy genotype.
Hereditary trait
A characteristic that results from gene action and is transmitted from one generation to another. Also called character.
Heritability
The proportion of phenotypic variation in a population attributable to genetic factors.
Hermaphroditic
Referring to animal species in which each individual has both testes and ovaries and to plant species in which both stamens and pistils are on the same flower.
Heterochromatin
Chromatin that remains condensed throughout the cell cycle and is usually not transcribed.
Heterodimer
A dimer containing one copy each of two different polypeptides.
Heteroduplex DNA
A region of double-stranded DNA with different sequence information on the two strands.
Heterogametic sex
In a species, the sex that has two types of sex chromosomes (e.g., x and y) and therefore produces two kinds of gametes with respect to the sex chromosomes. In mammals, the male is the heterogametic sex.
Heterogeneous nuclear RNA (hnRNA)
A group of RNA molecules of various sizes that exist in a large population in the nucleus and include precursor mRNAs (pre-mrnas).
Heteroplasmon
Cell of individuals with diseases caused by mtDNA defects in which there is a mixture of normal and mutant mitochondria. Also called cytohet.
Heterosis
The superiority of heterozygous genotypes regarding one or more characters compared with the corresponding homozygous genotypes based on growth, survival, phenotypic expression, and fertility. Also called heterozygote superiority or overdominance.
Heterozygosity (h
A measure of genetic variation; with respect to a particular locus, the proportion of individuals within a population that are heterozygous at that locus.
Heterozygote superiority
The superiority of heterozygous genotypes regarding one or more characters compared with the corresponding homozygous genotypes based on growth, survival, phenotypic expression, and fertility. Also called heterozygote superiority or overdominance.
Heterozygous
Describing a diploid organism having different alleles of one or more genes and therefore producing gametes of different genotypes.
Hfr (high-frequency recombination)
Designation for an e. Coli cell that has an f factor integrated into the bacterial chromosome. When an hfr cell conjugates with a recipient (f–) cell, bacterial genes are transferred to the recipient with high frequency.
Highly repetitive DNA
A class of DNA sequences, each of which is present in 105 to 107 copies in the haploid chromosome set.
Histone
One of a class of basic proteins that are complexed with DNA in chromatin and play a major role in determining the structure of eukaryotic nuclear chromosomes.
Holandric trait
A characteristic controlled by a gene carried on the y chromosome for which there is no corresponding gene locus on the x chromosome. Also called holandric or “wholly male” trait.
Homeobox
A 180-bp consensus sequence found in many genes that regulate development.
Homeodomain
The 60-amino acid part of proteins that corresponds to the homeobox sequence in genes. All homeodomain-containing proteins can bind to DNA and function in regulating transcription.
Homeotic genes
Group of genes in drosophila that specify the body parts (appendages) that will develop in each segment, thus determining the identity of the segments.
Homeotic mutation
Any mutation that alters the identity of a particular body segment, transforming it into a copy of a different segment.
Homodimer
A dimer containing two copies of the same polypeptide monomer.
Homogametic sex
In a species, the sex that has one type of sex chromosome (e.g., x) and therefore produces only one kind of gamete with respect to the sex chromosomes. In mammals, the female is the homogametic sex.
Homolog
Each individual member of a pair of homologous chromosomes.
Homologous
Referring to genes that have arisen from a common ancestral gene over evolutionary time; also used in reference to proteins encoded by homologous genes. Homologous chromosomes chromosomes that have the same arrangement of genetic loci, are identical in their visible structure, and pair during meiosis.
Homologous recombination
Recombination between identical or highly similar DNA sequences; it is most common during meiosis.
Homozygous
Describing a diploid organism having the same alleles at one or more genetic loci and therefore producing gametes of identical genotypes.
Homozygous dominant
A diploid organism that has the same dominant allele for a given gene locus on both members of a homologous pair of chromosomes.
Homozygous recessive
A diploid organism that has the same recessive allele for a given gene locus on both members of a homologous pair of chromosomes.
Human genome project (hgp)
A project to determine the sequence of the complete 3 billion (3!109) nucleotide pairs of the human genome and to map all of the genes along each chromosome.
Hybridization
In experiments, the complementary base pairing a single-stranded DNA or RNA probe to a single stranded DNA or RNA target molecule. One of the probe and target molecules is labeled, which one depending on the experiment.
Hypersensitive sites
Regions of DNA around transcriptionally active genes that are highly sensitive to digestion by DNase 1. Also called hypersensitive regions.
Hypothetico-deductive method of investigation
Research method involving making observations, forming hypotheses to explain the observations, making experimental predictions based on the hypotheses, and, finally, testing the predictions. The last step produces new observations, so a cycle is set up leading to a refinement of the hypotheses and perhaps eventually to the establishment of a law or an accepted principle.