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Genetics is a branch of biology concerned with the study of genes , genetic variation , and heredity in organisms. Though heredity had been observed for millennia, Gregor Mendel , Moravian scientist and Augustinian friar working in the 19th century in Brno , was the first to study genetics scientifically.
Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring. He observed that organisms pea plants inherit traits by way of discrete "units of inheritance".
This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene. Trait inheritance and molecular inheritance mechanisms of genes are still primary principles of genetics in the 21st century, but modern genetics has expanded beyond inheritance to studying the function and behavior of genes.
Gene structure and function, variation, and distribution are studied within the context of the cell , the organism e. Genetics has given rise to a number of subfields, including molecular genetics , epigenetics and population genetics. Organisms studied within the broad field span the domains of life archaea , bacteria , and eukarya. Genetic processes work in combination with an organism's environment and experiences to influence development and behavior , often referred to as nature versus nurture.
The intracellular or extracellular environment of a living cell or organism may switch gene transcription on or off. A classic example is two seeds of genetically identical corn, one placed in a temperate climate and one in an arid climate lacking sufficient waterfall or rain. While the average height of the two corn stalks may be genetically determined to be equal, the one in the arid climate only grows to half the height of the one in the temperate climate due to lack of water and nutrients in its environment.
The observation that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. His second law is the same as what Mendel published. In his third law, he developed the basic principles of mutation he can be considered a forerunner of Hugo de Vries. Other theories of inheritance preceded Mendel's work. A popular theory during the 19th century, and implied by Charles Darwin 's On the Origin of Species , was blending inheritance : the idea that individuals inherit a smooth blend of traits from their parents.
Blending of traits in the progeny is now explained by the action of multiple genes with quantitative effects. Another theory that had some support at that time was the inheritance of acquired characteristics : the belief that individuals inherit traits strengthened by their parents. This theory commonly associated with Jean-Baptiste Lamarck is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children,  although evidence in the field of epigenetics has revived some aspects of Lamarck's theory.
Modern genetics started with Mendel's studies of the nature of inheritance in plants. The importance of Mendel's work did not gain wide understanding until , after his death, when Hugo de Vries and other scientists rediscovered his research. Bateson both acted as a mentor and was aided significantly by the work of other scientists from Newnham College at Cambridge, specifically the work of Becky Saunders , Nora Darwin Barlow , and Muriel Wheldale Onslow.
After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. In , Nettie Stevens began studying the mealworm. Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNA, and scientists did not know which of the two is responsible for inheritance.
In , Frederick Griffith discovered the phenomenon of transformation see Griffith's experiment : dead bacteria could transfer genetic material to "transform" other still-living bacteria.
The structure also suggested a simple method for replication : if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. This property is what gives DNA its semi-conservative nature where one strand of new DNA is from an original parent strand.
Although the structure of DNA showed how inheritance works, it was still not known how DNA influences the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of protein production. The nucleotide sequence of a messenger RNA is used to create an amino acid sequence in protein; this translation between nucleotide sequences and amino acid sequences is known as the genetic code. With the newfound molecular understanding of inheritance came an explosion of research.
In this theory, Ohta stressed the importance of natural selection and the environment to the rate at which genetic evolution occurs. This technology allows scientists to read the nucleotide sequence of a DNA molecule. At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called genes , from parents to offspring.
These different, discrete versions of the same gene are called alleles. In the case of the pea, which is a diploid species, each individual plant has two copies of each gene, one copy inherited from each parent. Diploid organisms with two copies of the same allele of a given gene are called homozygous at that gene locus , while organisms with two different alleles of a given gene are called heterozygous. The set of alleles for a given organism is called its genotype , while the observable traits of the organism are called its phenotype.
When organisms are heterozygous at a gene, often one allele is called dominant as its qualities dominate the phenotype of the organism, while the other allele is called recessive as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have incomplete dominance by expressing an intermediate phenotype, or codominance by expressing both alleles at once.
When a pair of organisms reproduce sexually , their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as Mendel's first law or the Law of Segregation. Geneticists use diagrams and symbols to describe inheritance. A gene is represented by one or a few letters. In fertilization and breeding experiments and especially when discussing Mendel's laws the parents are referred to as the "P" generation and the offspring as the "F1" first filial generation.
When the F1 offspring mate with each other, the offspring are called the "F2" second filial generation. One of the common diagrams used to predict the result of cross-breeding is the Punnett square. When studying human genetic diseases, geneticists often use pedigree charts to represent the inheritance of traits. Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other.
This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as " Mendel's second law " or the "law of independent assortment," means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. Some genes do not assort independently, demonstrating genetic linkage , a topic discussed later in this article.
Often different genes can interact in a way that influences the same trait. In the Blue-eyed Mary Omphalodes verna , for example, there exists a gene with alleles that determine the color of flowers: blue or magenta.
Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its flowers are white—regardless of whether the first gene has blue or magenta alleles.
This interaction between genes is called epistasis , with the second gene epistatic to the first. Many traits are not discrete features e. These complex traits are products of many genes. The degree to which an organism's genes contribute to a complex trait is called heritability. For example, human height is a trait with complex causes. The molecular basis for genes is deoxyribonucleic acid DNA. DNA is composed of a chain of nucleotides , of which there are four types: adenine A , cytosine C , guanine G , and thymine T.
Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain. DNA normally exists as a double-stranded molecule, coiled into the shape of a double helix. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.
Genes are arranged linearly along long chains of DNA base-pair sequences. In bacteria , each cell usually contains a single circular genophore , while eukaryotic organisms such as plants and animals have their DNA arranged in multiple linear chromosomes. These DNA strands are often extremely long; the largest human chromosome, for example, is about million base pairs in length. DNA is most often found in the nucleus of cells, but Ruth Sager helped in the discovery of nonchromosomal genes found outside of the nucleus.
While haploid organisms have only one copy of each chromosome, most animals and many plants are diploid , containing two of each chromosome and thus two copies of every gene. Many species have so-called sex chromosomes that determine the gender of each organism.
In evolution, this chromosome has lost most of its content and also most of its genes, while the X chromosome is similar to the other chromosomes and contains many genes. This being said, Mary Frances Lyon discovered that there is X-chromosome inactivation during reproduction to avoid passing on twice as many genes to the offspring. When cells divide, their full genome is copied and each daughter cell inherits one copy. This process, called mitosis , is the simplest form of reproduction and is the basis for asexual reproduction.
Asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Offspring that are genetically identical to their parents are called clones. Eukaryotic organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of sexual reproduction alternates between forms that contain single copies of the genome haploid and double copies diploid.
Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell gametes such as sperm or eggs. Some bacteria can undergo conjugation , transferring a small circular piece of DNA to another bacterium. Natural bacterial transformation occurs in many bacterial species, and can be regarded as a sexual process for transferring DNA from one cell to another cell usually of the same species.
The diploid nature of chromosomes allows for genes on different chromosomes to assort independently or be separated from their homologous pair during sexual reproduction wherein haploid gametes are formed.
In this way new combinations of genes can occur in the offspring of a mating pair. Genes on the same chromosome would theoretically never recombine. However, they do, via the cellular process of chromosomal crossover. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. Meiotic recombination , particularly in microbial eukaryotes , appears to serve the adaptive function of repair of DNA damages.
The first cytological demonstration of crossing over was performed by Harriet Creighton and Barbara McClintock in Their research and experiments on corn provided cytological evidence for the genetic theory that linked genes on paired chromosomes do in fact exchange places from one homolog to the other. The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between the points.
For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated. The amounts of linkage between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome.
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Genetics is a branch of biology concerned with the study of genes , genetic variation , and heredity in organisms. Though heredity had been observed for millennia, Gregor Mendel , Moravian scientist and Augustinian friar working in the 19th century in Brno , was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring. He observed that organisms pea plants inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.