Genetics

This article is about the general scientific term. For the scientific journal, see http://www.book4doc.com/17383.
For a generally accessible and less technical introduction to the topic, see http://www.book4doc.com/14375.
Genetics (from http://www.book4doc.com/73131 γενετικός genetikos, genitive and that from γένεσις genesis, originhttp://www.book4doc.com/45659http://www.book4doc.com/87338http://www.book4doc.com/65458), a discipline of http://www.book4doc.com/29928, is the http://www.book4doc.com/58051 of http://www.book4doc.com/78003 and http://www.book4doc.com/64396 in living http://www.book4doc.com/88861.http://www.book4doc.com/65793http://www.book4doc.com/75272 The fact that living things inherit traits from their parents has been used since http://www.book4doc.com/38139 times to improve crop plants and animals through http://www.book4doc.com/86289. However, the modern science of genetics, which seeks to understand the process of inheritance, only began with the work of http://www.book4doc.com/79714 in the mid-nineteenth century.http://www.book4doc.com/18061 Although he did not know the physical basis for heredity, Mendel observed that organisms inherit traits in a http://www.book4doc.com/41778 mannerthese basic units of inheritance are now called http://www.book4doc.com/85947.
http://www.book4doc.com/53888http://www.book4doc.com/53888
http://www.book4doc.com/11005, the molecular basis for inheritance. Each strand of DNA is a chain of http://www.book4doc.com/27517, matching each other in the center to form what look like rungs on a twisted ladder.

Genes correspond to regions within http://www.book4doc.com/11005, a molecule composed of a chain of four different types of http://www.book4doc.com/27517the sequence of these nucleotides is the genetic information organisms inherit. DNA naturally occurs in a double stranded form, with nucleotides on each strand complementary to each other. Each strand can act as a template for http://www.book4doc.com/17772 a new partner strandthis is the physical method for making copies of genes that can be inherited.
The sequence of nucleotides in a gene is translated by http://www.book4doc.com/36207 to produce a chain of http://www.book4doc.com/68082, creating http://www.book4doc.com/55291the order of amino acids in a protein corresponds to the order of nucleotides in the gene. This is known as the http://www.book4doc.com/78483. The amino acids in a protein determine how it folds into a three-dimensional shape; this structure is, in turn, responsible for the protein’s function. Proteins carry out almost all the functions needed for cells to live. A change to the DNA in a gene can change a protein’s amino acids, changing its shape and function: this can have a dramatic effect in the cell and on the organism as a whole.
Although genetics plays a large role in the appearance and behavior of organisms, it is the combination of genetics with what an organism experiences that determines the ultimate outcome. For example, while genes play a role in determining a person’s http://www.book4doc.com/26312, the http://www.book4doc.com/34263 and http://www.book4doc.com/51866 that person experiences in childhood also have a large effect.

Main article: http://www.book4doc.com/20851
http://www.book4doc.com/45237 http://www.book4doc.com/45237
Morgan’s observation of -linked inheritance of a mutation causing white eyes in http://www.book4doc.com/43754 led him to the hypothesis that genes are located upon chromosomes.

Although the science of genetics began with the applied and theoretical work of http://www.book4doc.com/79714 in the mid-1800s, other theories of inheritance preceded Mendel. A popular theory during Mendel’s time was the concept of http://www.book4doc.com/69634: the idea that individuals inherit a smooth blend of traits from their parents. Mendel’s work disproved this, showing that traits are composed of combinations of distinct genes rather than a continuous blend. Another theory that had some support at that time was the http://www.book4doc.com/52699: the belief that individuals inherit traits strengthened by their parents. This theory (commonly associated with http://www.book4doc.com/85593) is now known to be wrongthe experiences of individuals do not affect the genes they pass to their children.http://www.book4doc.com/62626 Other theories included the http://www.book4doc.com/86285 of http://www.book4doc.com/71307 (which had both acquired and inherited aspects) and http://www.book4doc.com/38847‘s reformulation of pangenesis as both particulate and inherited.http://www.book4doc.com/66184

The modern science of genetics traces its roots to http://www.book4doc.com/15008, a German-Czech Augustinian http://www.book4doc.com/24754 and scientist who studied the nature of inheritance in plants. In his paper “Versuche ber Pflanzenhybriden” (“http://www.book4doc.com/38435“), presented in 1865 to the Naturforschender Verein (Society for Research in Nature) in http://www.book4doc.com/11328, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically.http://www.book4doc.com/58045 Although this pattern of inheritance could only be observed for a few traits, Mendel’s work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios.
The importance of Mendel’s work did not gain wide understanding until the 1890s, after his death, when other scientists working on similar problems http://www.book4doc.com/10469 his research. http://www.book4doc.com/55387, a proponent of Mendel’s work, coined the word genetics in 1905.http://www.book4doc.com/47387http://www.book4doc.com/75419 (The adjective genetic, derived from the http://www.book4doc.com/42673 word genesis – γένεσις, “origin” and that from the word genno – γεννώ, “to give birth”, predates the noun and was first used in a biological sense in 1860.)http://www.book4doc.com/25483 Bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in 1906.http://www.book4doc.com/58547
After the rediscovery of Mendel’s work, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1910, http://www.book4doc.com/21155 argued that genes are on http://www.book4doc.com/51228, based on observations of a -linked white eye mutation in fruit flies.http://www.book4doc.com/70733 In 1913, his student http://www.book4doc.com/35282 used the phenomenon of http://www.book4doc.com/49612 to show that genes are arranged linearly on the chromosome.http://www.book4doc.com/52745

http://www.book4doc.com/34531 http://www.book4doc.com/34531
http://www.book4doc.com/34141 (pictured) and http://www.book4doc.com/88419 determined the structure of DNA in 1953.

Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNAscientists did not know which of these was responsible for inheritance. In 1928, http://www.book4doc.com/44021 discovered the phenomenon of http://www.book4doc.com/12091 (see http://www.book4doc.com/54112): dead bacteria could transfer genetic material to “transform” other still-living bacteria. Sixteen years later, in 1944, http://www.book4doc.com/19455, http://www.book4doc.com/17143 and http://www.book4doc.com/46358 identified the molecule responsible for transformation as http://www.book4doc.com/11005.http://www.book4doc.com/68504 The http://www.book4doc.com/18162 in 1952 also showed that DNA (rather than protein) was the genetic material of the viruses that infect bacteria, providing further evidence that DNA was the molecule responsible for inheritance.http://www.book4doc.com/73340
http://www.book4doc.com/34141 and http://www.book4doc.com/88419 determined the structure of DNA in 1953, using the http://www.book4doc.com/14708 work of http://www.book4doc.com/29537 that indicated DNA had a http://www.book4doc.com/65812 structure (i.e., shaped like a corkscrew).http://www.book4doc.com/70831http://www.book4doc.com/87308 Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what looks like rungs on a twisted ladder.http://www.book4doc.com/33719 This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for duplication: if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand.
Although the structure of DNA showed how inheritance worked, it was still not known how DNA influenced the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of http://www.book4doc.com/55291 production. It was discovered that the cell uses DNA as a template to create matching http://www.book4doc.com/45622 (a molecule with nucleotides, very similar to DNA). The nucleotide sequence of a messenger RNA is used to create an http://www.book4doc.com/68082 sequence in protein; this translation between nucleotide and amino acid sequences is known as the http://www.book4doc.com/78483.
With this molecular understanding of inheritance, an explosion of research became possible. One important development was chain-termination http://www.book4doc.com/16630 in 1977 by http://www.book4doc.com/46483: this technology allows scientists to read the nucleotide sequence of a DNA molecule.http://www.book4doc.com/81343 In 1983, http://www.book4doc.com/33760 developed the http://www.book4doc.com/47021, providing a quick way to isolate and amplify a specific section of a DNA from a mixture.http://www.book4doc.com/86576 Through the pooled efforts of the http://www.book4doc.com/71047 and the parallel private effort by http://www.book4doc.com/34298, these and other techniques culminated in the sequencing of the human http://www.book4doc.com/51732 in 2003.http://www.book4doc.com/41912

Discrete inheritance and Mendel’s laws
Main article: http://www.book4doc.com/74365
http://www.book4doc.com/61064 http://www.book4doc.com/61064
A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms

At its most fundamental level, inheritance in organisms occurs by means of discrete traits, called http://www.book4doc.com/85947.http://www.book4doc.com/62291 This property was first observed by http://www.book4doc.com/79714, who studied the segregation of heritable traits in http://www.book4doc.com/54974.http://www.book4doc.com/58045http://www.book4doc.com/48396 In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white – and never an intermediate between the two colors. These different, discrete versions of the same gene are called http://www.book4doc.com/53236.
In the case of pea plants, each organism has two alleles of each gene, and the plants inherit one allele from each parent.http://www.book4doc.com/64307 Many organisms, including humans, have this pattern of inheritance. Organisms with two copies of the same allele are called http://www.book4doc.com/76953, while organisms with two different alleles are http://www.book4doc.com/13248.
The set of alleles for a given organism is called its http://www.book4doc.com/60208, while the observable trait the organism has is called its http://www.book4doc.com/75170. When organisms are heterozygous, often one allele is called http://www.book4doc.com/28610 as its qualities dominate the phenotype of the organism, while the other allele is called http://www.book4doc.com/74507 as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have http://www.book4doc.com/21812 by expressing an intermediate phenotype, or http://www.book4doc.com/62441 by expressing both alleles at once.http://www.book4doc.com/89734
When a pair of organisms http://www.book4doc.com/73496, 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 http://www.book4doc.com/55851 or the Law of Segregation.

http://www.book4doc.com/20247 http://www.book4doc.com/20247
Genetic pedigree charts help track the inheritance patterns of traits.

Geneticists use diagrams and symbols to describe inheritance. A gene is represented by a letter (or letters) – the capitalized letter represents the dominant allele and the recessive is represented by lowercase.http://www.book4doc.com/76718 Often a “+” symbol is used to mark the usual, non-mutant allele for a gene.
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 http://www.book4doc.com/62118.
When studying human genetic diseases, geneticists often use http://www.book4doc.com/85299 to represent the inheritance of traits.http://www.book4doc.com/15647 These charts map the inheritance of a trait in a family tree.

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Human height is a complex genetic trait. http://www.book4doc.com/38847‘s data from 1889 shows the relationship between offspring height as a function of mean parent height. While correlated, remaining variation in offspring heights indicates environment is also an important factor in this trait.

Organisms have thousands of genes, and in ually reproducing organisms assortment of these genes are generally independent 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 “http://www.book4doc.com/78025” 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 http://www.book4doc.com/49612, a topic discussed later in this article.)
Often different genes can interact in a way that influences the same trait. In the http://www.book4doc.com/70669 (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: color or 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 http://www.book4doc.com/50175, with the second gene epistatic to the first.http://www.book4doc.com/24475
Many traits are not discrete features (eg. purple or white flowers) but are instead continuous features (eg. human height and skin color). These http://www.book4doc.com/80893 are the product of many genes.http://www.book4doc.com/64866 The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism’s genes contribute to a complex trait is called http://www.book4doc.com/74120.http://www.book4doc.com/81623 Measurement of the heritability of a trait is relative – in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a complex trait with a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62%.http://www.book4doc.com/79119

Molecular basis for inheritance
DNA and chromosomes
Main articles: http://www.book4doc.com/11005 and http://www.book4doc.com/14325
http://www.book4doc.com/43645 http://www.book4doc.com/43645
The http://www.book4doc.com/85490 of DNA. Bases pair through the arrangement of http://www.book4doc.com/16375 between the strands.

The http://www.book4doc.com/49214 basis for genes is http://www.book4doc.com/41084 (DNA). DNA is composed of a chain of http://www.book4doc.com/27517, of which there are four types: http://www.book4doc.com/49358 (A), http://www.book4doc.com/21669 (C), http://www.book4doc.com/58849 (G), and http://www.book4doc.com/51671 (T). Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.http://www.book4doc.com/56228 http://www.book4doc.com/10896 are the only exception to this rulesometimes viruses use the very similar molecule http://www.book4doc.com/81159 instead of http://www.book4doc.com/11005 as their genetic material.http://www.book4doc.com/76134
DNA normally exists as a double-stranded molecule, coiled into the shape of a http://www.book4doc.com/23038. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. 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: http://www.book4doc.com/17772 duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.http://www.book4doc.com/78839
Genes are arranged linearly along long chains of DNA sequence, called http://www.book4doc.com/51228. In http://www.book4doc.com/85565, each cell has a single circular chromosome, while http://www.book4doc.com/77193 organisms (which includes 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 247 million http://www.book4doc.com/72055 in length.http://www.book4doc.com/60939 The DNA of a chromosome is associated with structural proteins that organize, compact, and control access to the DNA, forming a material called http://www.book4doc.com/49466; in eukaryotes, http://www.book4doc.com/49466 is usually composed of http://www.book4doc.com/74047, repeating units of DNA wound around a core of http://www.book4doc.com/74588 proteins.http://www.book4doc.com/32595 The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the http://www.book4doc.com/51732.
While http://www.book4doc.com/55487 organisms have only one copy of each chromosome, most animals and many plants are http://www.book4doc.com/48218, containing two of each chromosome and thus two copies of every gene.http://www.book4doc.com/78747 The two alleles for a gene are located on identical http://www.book4doc.com/64684 of sister http://www.book4doc.com/66299, each allele inherited from a different parent.
http://www.book4doc.com/32918 http://www.book4doc.com/32918
http://www.book4doc.com/65793‘s 1882 diagram of eukaryotic cell division. Chromosomes are copied, condensed, and organized. Then, as the cell divides, chromosome copies separate into the daughter cells.

An exception exists in the http://www.book4doc.com/78482, specialized chromosomes many animals have evolved that play a role in determining the of an organism.http://www.book4doc.com/31990 In humans and other mammals, the Y chromosome has very few genes and triggers the development of male ual characteristics, while the X chromosome is similar to the other chromosomes and contains many genes unrelated to determination. Females have two copies of the X chromosome, but males have one Y and only one X chromosome – this difference in X chromosome copy numbers leads to the unusual inheritance patterns of http://www.book4doc.com/15454 disorders.

Main articles: http://www.book4doc.com/48181 and http://www.book4doc.com/73496
When cells divide, their full genome is copied and each daughter cell inherits one copy. This process, called http://www.book4doc.com/29890, is the simplest form of reproduction and is the basis for http://www.book4doc.com/48181. Aual 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 http://www.book4doc.com/79536.
http://www.book4doc.com/20252 organisms often use http://www.book4doc.com/73496 to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of ual reproduction alternates between forms that contain single copies of the genome (http://www.book4doc.com/55487) and double copies (http://www.book4doc.com/48218).http://www.book4doc.com/78747 Haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. 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 http://www.book4doc.com/27710.
Although they do not use the haploid/diploid method of ual reproduction, http://www.book4doc.com/85565 have many methods of acquiring new genetic information. Some bacteria can undergo http://www.book4doc.com/89985, transferring a small circular piece of DNA to another bacterium.http://www.book4doc.com/88703 Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genome, a phenomenon known as http://www.book4doc.com/12091.http://www.book4doc.com/61666 This processes result in http://www.book4doc.com/84839, transmitting fragments of genetic information between organisms that would be otherwise unrelated.