Genetics Terms Starting With H

Haploid is the condition of a cell or organism that contains a single complete set of chromosomes, equal to half the number found in the diploid somatic cells of the same species.

Haploid cells are produced by meiosis and in most animals exist only transiently as gametes. Fertilization of two haploid gametes restores the diploid chromosome number in the zygote. Some organisms, including many fungi, algae, and male honeybees (Apis mellifera), exist as haploids throughout most or all of their life cycle.

Bread mold (Neurospora crassa) spends the majority of its life cycle as a haploid, which made it especially useful to George Beadle and Edward Tatum in their 1941 experiments linking genes to enzymes.

Hardy-Weinberg Equilibrium is the theoretical state in which allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary forces.

Equilibrium requires a large randomly mating population with no mutation, no natural selection, no genetic drift, and no gene flow. The principle is useful not because real populations meet all conditions but because deviations from expected genotype frequencies reveal which evolutionary forces are acting. For two alleles with frequencies p and q, the expected genotype frequencies are p-squared for homozygous dominant, 2pq for heterozygous, and q-squared for homozygous recessive.

Godfrey Hardy and Wilhelm Weinberg independently published this relationship in 1908, Hardy in a brief letter to the journal Science and Weinberg in a German medical journal.

Hemizygous is a genetic state in which an individual has only one copy of a gene rather than the usual two, usually because the gene is located on a sex chromosome present in only one copy.

In humans, males are hemizygous for genes on the X chromosome because they carry only one X and one Y. A hemizygous individual expresses whatever allele is present, whether dominant or recessive, because no second allele exists to mask it. This explains why X-linked recessive disorders such as hemophilia and color blindness are far more common in males than females.

Roughly 8 percent of males have some form of red-green color blindness, compared to less than 0.5 percent of females, a difference that directly reflects hemizygosity at the relevant X-linked loci.

Hemophilia is a group of X-linked recessive inherited bleeding disorders caused by deficiencies in specific blood clotting factors, most commonly factor VIII in hemophilia A or factor IX in hemophilia B.

Affected individuals cannot form stable blood clots, leading to prolonged bleeding after injury and spontaneous hemorrhages into joints and muscles. Hemophilia A is caused by mutations in the F8 gene and hemophilia B by mutations in the F9 gene, both located on the X chromosome. Because the genes are X-linked, hemophilia predominantly affects males, while females with one mutant copy are usually carriers with normal or mildly reduced clotting.

Severe hemophilia A, defined as less than 1 percent of normal factor VIII activity, affects approximately 1 in 5,000 male births worldwide.

Heredity is the biological process by which traits and genetic information are transmitted from parents to offspring through reproduction.

Heredity operates through the faithful replication and transmission of DNA from one generation to the next. Gregor Mendel first described the patterns governing individual traits in the 1860s using garden pea plants (Pisum sativum), establishing that traits segregate and assort in predictable ratios. Both genetic sequence and epigenetic information can pass from parents to offspring, contributing to inherited variation in traits.

Environmental factors interact with inherited information to shape the final phenotype, which is why identical twins raised apart can differ measurably in traits such as body weight or disease risk.

Heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation, expressed as a value between zero and one.

Broad-sense heritability includes all genetic contributions to phenotypic variance, while narrow-sense heritability includes only additive genetic effects and predicts the response to selection. Heritability is a population-specific statistic that depends on both the genetic and environmental variation present in a particular group at a particular time. A high heritability does not mean a trait cannot be changed by environment; it means only that genetic differences account for much of the variation seen within that specific population.

Estimates can shift substantially when the same trait is measured in a population with different environmental conditions, such as comparing height heritability in food-secure versus food-insecure communities.

Heterozygous is a genetic state in which an individual carries two different alleles at a given genetic locus, one on each homologous chromosome.

Heterozygosity at a locus means the two alleles may produce different molecular products, and the phenotypic outcome depends on the dominance relationship between them. For autosomal recessive conditions, heterozygous individuals are usually unaffected carriers who can transmit the recessive allele to their offspring. Heterozygosity is a standard measure of genetic diversity within populations, with higher values indicating more genetic variation.

Conservation geneticists use genome-wide heterozygosity estimates to assess the health of small or endangered populations, such as the Amur leopard (Panthera pardus orientalis), which shows critically low heterozygosity due to its small census size.

Histone is one of a family of positively charged proteins around which DNA is wound to form the nucleosome, the fundamental packaging unit of eukaryotic chromatin.

Five major histone types exist: H1, H2A, H2B, H3, and H4. Core histones H2A, H2B, H3, and H4 assemble into an octamer around which approximately 147 base pairs of DNA are wrapped to create each nucleosome. Chemical modifications to histone tails, including acetylation and methylation, regulate chromatin compaction and gene expression without altering the DNA sequence itself.

These modifications are written and erased by dedicated enzyme complexes, giving cells a dynamic mechanism to switch gene activity in response to developmental signals or environmental cues.

Homozygous is a genetic state in which an individual carries two identical alleles at a given genetic locus, one on each homologous chromosome.

Homozygous individuals are described as homozygous dominant if both alleles are the dominant form, or homozygous recessive if both alleles are the recessive form. True-breeding lines used in genetic crosses are homozygous at the loci of interest, ensuring consistent phenotypic expression across generations. Homozygosity for recessive disease alleles results in expression of the disease phenotype, while homozygosity for beneficial alleles fixes those alleles in a lineage.

Prolonged inbreeding or strong directional selection can drive entire populations toward homozygosity at specific loci, as seen in domesticated crop varieties selected for uniform grain color or disease resistance.

Hybrid is the offspring of two genetically distinct parents that differ in one or more heritable characteristics, usually produced by crossing different species, varieties, or inbred lines.

Hybrids produced by crossing two different pure-breeding lines often display hybrid vigor, also called heterosis, in which the hybrid outperforms both parents in traits such as growth, yield, and stress resistance. Interspecific hybrids, produced between different species, are frequently sterile because of incompatibilities in chromosome pairing during meiosis. Plant and animal breeding programs rely heavily on hybrid production to achieve superior crop varieties and livestock.

The first commercially successful hybrid maize varieties were introduced in the United States during the 1930s and contributed to dramatic yield increases across the Corn Belt.