Genetics and Related Topics

Gene

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Basic unit of heredity and a sequence of nucleotides in DNA that encodes the synthesis of a gene product, either RNA or protein.

Fluorescent microscopy image of a human female karyotype, showing 23 pairs of chromosomes. The DNA is stained red, with regions rich in housekeeping genes further stained in green. The largest chromosomes are around 10 times the size of the smallest.

Schematic of a single-stranded RNA molecule illustrating a series of three-base codons. Each three-nucleotide codon corresponds to an amino acid when translated to protein

Protein coding genes are transcribed to an mRNA intermediate, then translated to a functional protein. RNA-coding genes are transcribed to a functional non-coding RNA.

Inheritance of a gene that has two different alleles (blue and white). The gene is located on an autosomal chromosome. The white allele is recessive to the blue allele. The probability of each outcome in the children's generation is one quarter, or 25 percent.

A sequence alignment, produced by ClustalO, of mammalian histone proteins

Depiction of numbers of genes for representative plants (green), vertebrates (blue), invertebrates (orange), fungi (yellow), bacteria (purple), and viruses (grey). An inset on the right shows the smaller genomes expanded 100-fold area-wise.

Gene functions in the minimal genome of the synthetic organism, Syn 3.

Comparison of conventional plant breeding with transgenic and cisgenic genetic modification.

Twenty years later, in 1909, Wilhelm Johannsen introduced the term 'gene' and in 1906, William Bateson, that of 'genetics' while Eduard Strasburger, amongst others, still used the term 'pangene' for the fundamental physical and functional unit of heredity.

The structure of the DNA double helix. The atoms in the structure are colour-coded by element and the detailed structures of two base pairs are shown in the bottom right.

DNA

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Chemical structure of DNA; hydrogen bonds shown as dotted lines. Each end of the double helix has an exposed 5' phosphate on one strand and an exposed 3' hydroxyl group (—OH) on the other.

A section of DNA. The bases lie horizontally between the two spiraling strands ([[:File:DNA orbit animated.gif|animated version]]).

DNA major and minor grooves. The latter is a binding site for the Hoechst stain dye 33258.

From left to right, the structures of A, B and Z DNA

DNA quadruplex formed by telomere repeats. The looped conformation of the DNA backbone is very different from the typical DNA helix. The green spheres in the center represent potassium ions.

A covalent adduct between a metabolically activated form of benzo[a]pyrene, the major mutagen in tobacco smoke, and DNA

Location of eukaryote nuclear DNA within the chromosomes

T7 RNA polymerase (blue) producing an mRNA (green) from a DNA template (orange)

DNA replication: The double helix is unwound by a helicase and topoisomerase. Next, one DNA polymerase produces the leading strand copy. Another DNA polymerase binds to the lagging strand. This enzyme makes discontinuous segments (called Okazaki fragments) before DNA ligase joins them together.

Interaction of DNA (in orange) with histones (in blue). These proteins' basic amino acids bind to the acidic phosphate groups on DNA.

The lambda repressor helix-turn-helix transcription factor bound to its DNA target

The restriction enzyme EcoRV (green) in a complex with its substrate DNA

Recombination involves the breaking and rejoining of two chromosomes (M and F) to produce two rearranged chromosomes (C1 and C2).

The DNA structure at left (schematic shown) will self-assemble into the structure visualized by atomic force microscopy at right. DNA nanotechnology is the field that seeks to design nanoscale structures using the molecular recognition properties of DNA molecules.

Maclyn McCarty (left) shakes hands with Francis Crick and James Watson, co-originators of the double-helix model.

Pencil sketch of the DNA double helix by Francis Crick in 1953

A blue plaque outside The Eagle pub commemorating Crick and Watson

Deoxyribonucleic acid (DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.

Heredity

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Passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic information of their parents.

DNA structure. Bases are in the centre, surrounded by phosphate–sugar chains in a double helix.

Aristotle's model of inheritance. The heat/cold part is largely symmetrical, though influenced on the father's side by other factors; but the form part is not.

Table showing how the genes exchange according to segregation or independent assortment during meiosis and how this translates into Mendel's laws

An example pedigree chart of an autosomal dominant disorder.

An example pedigree chart of an autosomal recessive disorder.

An example pedigree chart of a sex-linked disorder (the gene is on the X chromosome)

Hereditary defects in enzymes are generally inherited in an autosomal fashion because there are more non-X chromosomes than X-chromosomes, and a recessive fashion because the enzymes from the unaffected genes are generally sufficient to prevent symptoms in carriers.

On the other hand, hereditary defects in structural proteins (such as osteogenesis imperfecta, Marfan's syndrome and many Ehlers–Danlos syndromes) are generally autosomal dominant, because it is enough that some components are defective to make the whole structure dysfunctional. This is a dominant-negative process, wherein a mutated gene product adversely affects the non-mutated gene product within the same cell.

The study of heredity in biology is genetics.

Gregor Mendel

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Biologist, meteorologist, mathematician, Augustinian friar and abbot of St. Thomas' Abbey in Brünn (Brno), Margraviate of Moravia.

Dominant and recessive phenotypes. (1) Parental generation. (2) F1 generation. (3) F2 generation.

Mendel was born in a German-speaking family in the Silesian part of the Austrian Empire (today's Czech Republic) and gained posthumous recognition as the founder of the modern science of genetics.

Dominance (genetics)

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Inheritance of dwarfing in maize. Demonstrating the heights of plants from the two parent variations and their F1 heterozygous hybrid (centre)

This Punnett square illustrates incomplete dominance. In this example, the red petal trait associated with the R allele recombines with the white petal trait of the r allele. The plant incompletely expresses the dominant trait (R) causing plants with the Rr genotype to express flowers with less red pigment resulting in pink flowers. The colors are not blended together, the dominant trait is just expressed less strongly.

A and B blood types in humans show co-dominance, but the O type is recessive to A and B.

This Punnett square shows co-dominance. In this example a white bull (WW) mates with a red cow (RR), and their offspring exhibit co-dominance expressing both white and red hairs.

On the other hand, hereditary defects in structural proteins (such as osteogenesis imperfecta, Marfan's syndrome and Ehlers–Danlos syndromes) are generally autosomal dominant, because it is enough that some components are defective to make the whole structure dysfunctional. This is a dominant-negative process, wherein a mutated gene product adversely affects the non-mutated gene product within the same cell.

In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome.

Gregor Mendel, the Moravian Augustinian monk who founded the modern science of genetics

Mendelian inheritance

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Type of biological inheritance that follows the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and popularized by William Bateson.

Characteristics Mendel used in his experiments

P-Generation and F1-Generation: The dominant allele for purple-red flower hides the phenotypic effect of the recessive allele for white flowers. F2-Generation: The recessive trait from the P-Generation phenotypically reappears in the individuals that are homozygous with the recessive genetic trait.

Myosotis: Colour and distribution of colours are inherited independently.

F1 generation: All individuals have the same genotype and same phenotype expressing the dominant trait ( red ).
F2 generation: The phenotypes in the second generation show a 3 : 1 ratio.
In the genotype 25 % are homozygous with the dominant trait, 50 % are heterozygous genetic carriers of the recessive trait, 25 % are homozygous with the recessive genetic trait and expressing the recessive character.

In Mirabilis jalapa and Antirrhinum majus are examples for intermediate inheritance. As seen in the F1-generation, heterozygous plants have " light pink " flowers—a mix of " red " and "white". The F2-generation shows a 1:2:1 ratio of red : light pink : white

A Punnett square for one of Mendel's pea plant experiments – self-fertilization of the F1 generation

Segregation and independent assortment are consistent with the chromosome theory of inheritance.

When the parents are homozygous for two different genetic traits (llSS and LL sP sP), their children in the F1 generation are heterozygous at both loci and only show the dominant phenotypes (Ll S sP). P-Generation: Each parent possesses one dominant and one recessive trait purebred (homozygous). In this example, solid coat color is indicated by S (dominant), Piebald spotting by sP (recessive), while fur length is indicated by L (short, dominant) or l (long, recessive). All individuals are equal in genotype and phenotype. In the F2 generation all combinations of coat color and fur length occur: 9 are short haired with solid colour, 3 are short haired with spotting, 3 are long haired with solid colour and 1 is long haired with spotting. The traits are inherited independently, so that new combinations can occur. Average number ratio of phenotypes 9:3:3:1

For example 3 pairs of homologous chromosomes allow 8 possible combinations, all equally likely to move into the gamete during meiosis. This is the main reason for independent assortment. The equation to determine the number of possible combinations given the number of homologous pairs = 2x (x = number of homologous pairs)

Its most vigorous promoter in Europe was William Bateson, who coined the terms "genetics" and "allele" to describe many of its tenets.

The logarithm of fitness as a function of the number of deleterious mutations. Synergistic epistasis is represented by the red line - each subsequent deleterious mutation has a larger proportionate effect on the organism's fitness. Antagonistic epistasis is in blue. The black line shows the non-epistatic case, where fitness is the product of the contributions from each of its loci.

Population genetics

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Gene flow is the transfer of alleles from one population to another population through immigration of individuals. In this example, one of the birds from population A immigrates to population B, which has fewer of the dominant alleles, and through mating incorporates its alleles into the other population.

The Great Wall of China is an obstacle to gene flow of some terrestrial species.

Current tree of life showing vertical and horizontal gene transfers.

Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology.