Cytogenetics in the context of Karyotype

Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.

Genes are the common factor of the qualities of most human-inherited traits. Study of human genetics can answer questions about human nature, can help understand diseases and the development of effective treatment and help us to understand the genetics of human life. This article describes only basic features of human genetics; for the genetics of disorders please see: medical genetics. For information on the genetics of DNA repair defects related to accelerated aging and/or increased risk of cancer please see: DNA repair-deficiency disorder.

Giemsa stain (/ˈɡiːmzə/), named after German chemist and bacteriologist Gustav Giemsa, is a nucleic acid stain used in cytogenetics and for the histopathological diagnosis of malaria and other parasites.

Barbara McClintock (June 16, 1902 – September 2, 1992) was an American scientist and cytogeneticist who was awarded the 1983 Nobel Prize in Physiology or Medicine. McClintock received her PhD in botany from Cornell University in 1927. There she started her career as the leader of the development of maize cytogenetics, the focus of her research for the rest of her life. From the late 1920s, McClintock studied chromosomes and how they change during reproduction in maize. She developed the technique for visualizing maize chromosomes and used microscopic analysis to demonstrate many fundamental genetic ideas. One of those ideas was the notion of genetic recombination by crossing-over during meiosis—a mechanism by which chromosomes exchange information. She is often erroneously credited with producing the first genetic map for maize, linking regions of the chromosome to physical traits. She demonstrated the role of the telomere and centromere, regions of the chromosome that are important in the conservation of genetic information. She was recognized as among the best in the field, awarded prestigious fellowships, and elected a member of the National Academy of Sciences in 1944.

During the 1940s and 1950s, McClintock discovered transposons and used it to demonstrate that genes are responsible for turning physical characteristics on and off. She developed theories to explain the suppression and expression of genetic information from one generation of maize plants to the next. Due to skepticism of her research and its implications, she stopped publishing her data in 1953.

Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or through biochemical analysis to measure specific protein output. In a medical setting, genetic testing can be used to diagnose or rule out suspected genetic disorders, predict risks for specific conditions, or gain information that can be used to customize medical treatments based on an individual's genetic makeup. Genetic testing can also be used to determine biological relatives, such as a child's biological parentage (genetic mother and father) through DNA paternity testing, or be used to broadly predict an individual's ancestry. Genetic testing of plants and animals can be used for similar reasons as in humans (e.g. to assess relatedness/ancestry or predict/diagnose genetic disorders), to gain information used for selective breeding, or for efforts to boost genetic diversity in endangered populations.

The variety of genetic tests has expanded throughout the years. Early forms of genetic testing which began in the 1950s involved counting the number of chromosomes per cell. Deviations from the expected number of chromosomes (46 in humans) could lead to a diagnosis of certain genetic conditions such as trisomy 21 (Down syndrome) or monosomy X (Turner syndrome). In the 1970s, a method to stain specific regions of chromosomes, called chromosome banding, was developed that allowed more detailed analysis of chromosome structure and diagnosis of genetic disorders that involved large structural rearrangements. In addition to analyzing whole chromosomes (cytogenetics), genetic testing has expanded to include the fields of molecular genetics and genomics which can identify changes at the level of individual genes, parts of genes, or even single nucleotide "letters" of DNA sequence. According to the National Institutes of Health, there are tests available for more than 2,000 genetic conditions, and one study estimated that as of 2018 there were more than 68,000 genetic tests on the market.

Panmixia (or panmixis) means uniform random fertilization, which means individuals do not select a mate based on physical traits. A panmictic population is one where all potential parents may contribute equally to the gamete pool, and that these gametes are uniformly distributed within the gamete population (gamodeme). This assumes that there are no hybridising restrictions within the parental population: neither genetics, cytogenetics nor behavioural; and neither spatial nor temporal (see also Quantitative genetics for further discussion). True panmixia is rarely, if ever, observed in natural populations. It is a theoretical model used as a null hypothesis in population genetics. It serves as a point of comparison to understand how deviations from random mating affect allele and genotype frequencies.[1] Therefore, all gamete recombination (fertilization) is uniformly possible. Both the Wahlund effect and the Hardy Weinberg equilibrium assume that the overall population is panmictic.

In genetics and heredity, random mating usually implies the hybridising (mating) of individuals regardless of any spatial, physical, genetical, temporal or social preference. That is, the mating between two organisms is not influenced by any environmental, nor hereditary interaction. There is no tendency for similar individuals (positive assortative mating) or dissimilar individuals (negative assortative mating) to mate.[2] Hence, potential mates have an equal chance of being contributors to the fertilizing gamete pool. If there is no random sub-sampling of gametes involved in the fertilization cohort, panmixia has occurred. This scenario is considered rare as it is very idealized. In real life, there are many different factors that can influence mate choice.Such uniform random mating is distinct from lack of natural selection: in viability selection for instance, selection occurs before mating.

Fluorescence in situ hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes that bind to specific parts of a nucleic acid sequence with a high degree of sequence complementarity. It was developed by biomedical researchers in the early 1980s to detect and localize the presence or absence of specific DNA sequences on chromosomes. Fluorescence microscopy can be used to determine where the fluorescent probe is bound to the chromosomes. FISH is often used to find specific features in DNA for genetic counseling, medicine, and species identification.

FISH can also be used to detect and localize specific RNA targets (mRNA, lncRNA, and miRNA) in cells, circulating tumor cells, and tissue samples. In this context, it helps define the spatial and temporal patterns of gene expression within cells and tissues.