Download Genetics Genetics, a discipline of biology, is the science of genes

Genetics
Genetics, a discipline of biology, is the science of genes, heredity, and variation in living
organisms. Genetics concerns the process of trait inheritance from parents to offspring, including
the molecular structure and function of genes, gene behavior in the context of a cell or organism
(e.g. dominance and epigenetics), gene distribution, and variation and change in populations (such
as through Genome-Wide Association Studies). Given that genes are universal to living organisms,
genetics can be applied to the study of all living systems; including bacteria, plants, animals, and
humans. The modern science of genetics, seeking to understand this process, began with the work
of Gregor Mendel in the mid-19th century.
Mendel observed that organisms inherit traits by way of discrete 'units of inheritance.' This term,
still used today, is a somewhat ambiguous definition of a gene. A more modern working definition
of a gene is a portion (or sequence) of DNA that codes for a known cellular function. This portion
of DNA is variable, it may be small or large, have a few subregions or many subregions. The word
'Gene' refers to portions of DNA that are required for a single cellular process or single function,
more than the word refers to a single tangible item. The sequence of nucleotides in a gene is read
and translated by a cell to produce a chain of amino acids which in turn spontaneously fold into
proteins. The order of amino acids in a protein corresponds to the order of nucleotides in the gene.
This relationship between nucleotide sequence and amino acid sequence is known as the genetic
code. The amino acids in a protein determine how it folds into its unique three-dimensional shape; a
structure that is ultimately responsible for the proteins function. Proteins carry out many of the
functions needed for cells to live. A change to the DNA in a gene can change a protein's amino acid
sequence, thereby changing its shape and function, rendering the protein ineffective or even
malignant. When a gene change occurs, it is referred to as a mutation.
Features of inheritance
Discrete inheritance and Mendel's laws
At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units,
called genes, from parents to progeny. This property was first observed by Gregor Mendel, who
studied the segregation of heritable traits in pea plants. In his experiments studying the trait for
flower color, Mendel observed that the flowers of each pea plant were either purple or white—but
never an intermediate between the two colors. These different, discrete versions of the same gene
are called alleles.
In the case of pea, which is a diploid species, each individual plant has two copies of each gene, one
copy inherited from each parent. Many species, including humans, have this pattern of inheritance.
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.
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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.
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.
Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous
features (e.g. human height and skin color). These complex traits are products of many genes. 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
heritability.
Molecular basis for inheritance
DNA and Chromosomes
The molecular basis for genes is deoxyribonucleic acid (DNA). DNA is a molecule that encodes the
genetic instructions used in the development and functioning of all known living organisms and
many viruses. DNA and RNA are nucleic acids; alongside proteins, they compose the three major
macromolecules essential for all known forms of life. Most DNA molecules are double-stranded
helices (the double helix structure of DNA was first discovered by James Watson and Francis
Crick), consisting of two helical chains each coiled round the same axis or, in other words, two long
biopolymers made of simpler units called nucleotides—each nucleotide is composed of a
nucleobase (guanine, adenine, thymine, and cytosine), recorded using the letters G, A, T, and C, as
well as a backbone made of alternating sugars (deoxyribose) and phosphate groups (related to
phosphoric acid), with the nucleobases (G, A, T, C) attached to the sugars. DNA is well-suited for
biological information storage, since the DNA backbone is resistant to cleavage and the doublestranded structure provides the molecule with a built-in duplicate of the encoded information. The
two strands of DNA run in opposite directions to each other and are therefore anti-parallel. Attached
to each sugar is one of four types of molecules called nucleobases (informally, bases). It is the
sequence of these four nucleobases along the backbone that encodes genetic information. This
information is read using the genetic code, which specifies the sequence of the amino acids within
proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA in a
process called transcription.
Within cells, DNA is organized into long structures called chromosomes. During cell division these
chromosomes are duplicated in the process of DNA replication, providing each cell its own
complete set of chromosomes. Eukaryotic organisms (animals, plants, fungi, and protists) store
most of their DNA inside the cell nucleus and some of their DNA in organelles, such as
mitochondria or chloroplasts. In contrast, prokaryotes (bacteria and archaea) store their DNA only
in the cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and
organize DNA. These compact structures guide the interactions between DNA and other proteins,
helping control which parts of the DNA are transcribed.
Biological functions
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DNA usually occurs as linear chromosomes in eukaryotes, and circular chromosomes in
prokaryotes. The set of chromosomes in a cell makes up its genome; the human genome has
approximately 3 billion base pairs of DNA arranged into 46 chromosomes. 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. The information carried by
DNA is held in the sequence of pieces of DNA called genes. A gene is a sequence of DNA that
contains genetic information and can influence the phenotype of an organism. Transmission of
genetic information in genes is achieved via complementary base pairing. For example, in
transcription, when a cell uses the information in a gene, the DNA sequence is copied into a
complementary RNA sequence through the attraction between the DNA and the correct RNA
nucleotides. Usually, this RNA copy is then used to make a matching protein sequence in a process
called translation, which depends on the same interaction between RNA nucleotides. In alternative
fashion, a cell may simply copy its genetic information in a process called DNA replication. Cell
division is essential for an organism to grow, but, when a cell divides, it must replicate the DNA in
its genome so that the two daughter cells have the same genetic information as their parent. The
double-stranded structure of DNA provides a simple mechanism for DNA replication. Here, the two
strands are separated and then each strand's complementary DNA sequence is recreated by an
enzyme called DNA polymerase. This enzyme makes the complementary strand by finding the
correct base through complementary base pairing, and bonding it onto the original strand.
Genes and genomes
Genomic DNA is tightly and orderly packed in the process called DNA condensation to fit the small
available volumes of the cell. In eukaryotes, DNA is located in the cell nucleus, as well as small
amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an irregularly
shaped body in the cytoplasm called the nucleoid. The genetic information in a genome is held
within genes, and the complete set of this information in an organism is called its genotype. A gene
is a unit of heredity and is a region of DNA that influences a particular characteristic in an
organism.
Reproduction
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).
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.
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