Chimera (genetics)

In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different varieties, subspecies, species or genera through sexual reproduction. Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from a different organism is called a chimera. Hybrids are not always intermediates between their parents such as in blending inheritance (a now discredited theory in modern genetics by particulate inheritance), but can show hybrid vigor, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.

Species are reproductively isolated by strong barriers to hybridization, which include genetic and morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization and others after it. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes. A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat, where the number of chromosomes has been doubled.

A human–animal hybrid (or animal–human hybrid) is a hypothetical organism that incorporates elements from both humans and non-human animals. In a technical sense, a human–animal hybrid would be defined as an organism in which each cell contains both human and non-human genetic material. This contrasts with a non-human chimera in which some cells are human and the other are derived from a non-human organism (a human chimera, by contrast, consists entirely of human cells from different zygotes.)

Examples of human–animal hybrids mainly include humanized mice that have been genetically modified by xenotransplantation of human genes. Humanized mice are commonly used as small animal models in biological and medical research for developing human therapeutics.

Gene targeting is a biotechnological tool used to change the DNA sequence of an organism (hence it is a form of genome editing). It is based on the natural DNA-repair mechanism of homology directed repair (HDR), including homologous recombination. Gene targeting can be used to make a range of sizes of DNA edits, from larger DNA edits such as inserting entire new genes into an organism, through to much smaller changes to the existing DNA such as a single base-pair change. Gene targeting relies on the presence of a repair template to introduce the user-defined edits to the DNA. The user (usually a scientist) will design the repair template to contain the desired edit, flanked by DNA sequence corresponding (homologous) to the region of DNA that the user wants to edit; hence the edit is targeted to a particular genomic region. In this way Gene Targeting is distinct from natural homology-directed repair, during which the 'natural' DNA repair template of the sister chromatid is used to repair broken DNA (the sister chromatid is the second copy of the gene). The alteration of DNA sequence in an organism can be useful in both a research context – for example to understand the biological role of a gene – and in biotechnology, for example to alter the traits of an organism (e.g. to improve crop plants).

Mosaicism or genetic mosaicism is a condition in which a multicellular organism possesses more than one genetic line as the result of genetic mutation. This means that various genetic lines resulted from a single fertilized egg. Mosaicism is one of several possible causes of chimerism, wherein a single organism is composed of cells with more than one distinct genotype.

Genetic mosaicism can result from many different mechanisms including chromosome nondisjunction, anaphase lag, and endoreplication. Anaphase lagging is the most common way by which mosaicism arises in the preimplantation embryo. Mosaicism can also result from a mutation in one cell during development, in which case the mutation will be passed on only to its daughter cells (and will be present only in certain adult cells). Somatic mosaicism is not generally inheritable as it does not generally affect germ cells.

As an adjective, obligate means "by necessity" (antonym facultative) and is used mainly in biology in phrases such as:

• Obligate aerobe, an organism that cannot survive without oxygen
• Obligate anaerobe, an organism that cannot survive in the presence of oxygen
• Obligate air-breather, a term used in fish physiology to describe those that respire entirely from the atmosphere
• Obligate biped, Bipedalism designed to walk on two legs
• Obligate carnivore, an organism dependent for survival on a diet of animal flesh.
• Obligate chimerism, a kind of organism with two distinct sets of DNA, always
• Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.
• Obligate intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cell
• Obligate parasite, a parasite that cannot reproduce without exploiting a suitable host
• Obligate photoperiodic plant, a plant that requires sufficiently long or short nights before it initiates flowering, germination or similarly functions
• Obligate symbionts, organisms that can only live together in a symbiosis