geneticists use diagrams and symbols to describe inheritance. a gene is represented by one or a few letters. 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 punnett square. when studying human genetic diseases, geneticists often use pedigree charts to represent the inheritance of traits. these charts map the inheritance of a trait in a family tree. multiple gene interactions human height is a trait with complex genetic causes. francis galton's data from 1889 shows the relationship between offspring height as a function of mean parent height. 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. different genes often interact to influence 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.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. 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 trait with complex causes. it has 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%. molecular basis for inheritance dna and chromosomes the molecular structure of dna. bases pair through the arrangement of hydrogen bonding between the strands. dna sequence the molecular basis for genes is deoxyribonucleic acid (dna). dna is composed of deoxyribose (sugar molecule), a phosphate group, and a base (amine group). there are four types of bases: adenine (a), cytosine (c), guanine (g), and thymine (t). the phosphates make hydrogen bonds with the sugars to make long phosphate-sugar backbones. bases specifically pair together (t&a, c&g) between two backbones and make like rungs on a ladder. the bases, phosphates, and sugars together make a nucleotide that connects to make long chains of dna. genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the dna chain. these chains coil into a double a-helix structure and wrap around proteins called histones which provide the structural support. dna wrapped around these histones are called chromosomes. viruses sometimes use the similar molecule rna instead of dna as their genetic material. dna normally exists as a double-stranded molecule, coiled into the shape of a double helix. 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: dna replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand. schematic karyogram of a human, showing 22 homologous chromosome pairs, both the female (xx) and male (xy) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (at bottom left). further information: karyotype 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 247 million base pairs in length. the dna of a chromosome is associated with structural proteins that organize, compact, and control access to the dna, forming a material called chromatin; in eukaryotes, chromatin is usually composed of nucleosomes, segments of dna wound around cores of histone proteins. the full set of hereditary material in an organism (usually the combined dna sequences of all chromosomes) is called the genome. 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. in plants, these are often found in the chloroplasts and in other organisms, in the mitochondria. these nonchromosomal genes can still be passed on by either partner in sexual reproduction and they control a variety of hereditary characteristics that replicate and remain active throughout generations. 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 two alleles for a gene are located on identical loci of the two homologous chromosomes, each allele inherited from a different parent. walther flemming'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. many species have so-called sex chromosomes that determine the sex of each organism. in humans and many other animals, the y chromosome contains the gene that triggers the development of the specifically male characteristics. 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. lyon's discovery led to the discovery of x-linked diseases. reproduction main articles: asexual reproduction and sexual 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. 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. although they do not use the haploid/diploid method of sexual reproduction, bacteria have many methods of acquiring new genetic information. some bacteria can undergo conjugation, transferring a small circular piece of dna to another bacterium. bacteria can also take up raw dna fragments found in the environment and integrate them into their genomes, a phenomenon known as transformation. these processes result in horizontal gene transfer, transmitting fragments of genetic information between organisms that would be otherwise unrelated. 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). transformation requires the action of numerous bacterial gene products, and its primary adaptive function appears to be repair of dna damages in the recipient cell. recombination and genetic linkage main articles: chromosomal crossover and genetic linkage thomas hunt morgan's 1916 illustration of a double crossover between chromosomes 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. this process of chromosomal crossover generally occurs during meiosis, a series of cell divisions that creates haploid cells. 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 1931. 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. for genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage; alleles for the two genes tend to be inherited together. 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. gene expression genetic code the genetic code: using a triplet code, dna, through a messenger rna intermediary, specifies a protein. genes generally express their functional effect through the production of proteins, molecules responsible for most functions in the cell. proteins are made up of one or more polypeptide chains, each composed of a sequence of amino acids. the dna sequence of a gene is used to produce a specific amino acid sequence. this process begins with the production of an rna molecule with a sequence matching the gene's dna sequence, a process called transcription. this messenger rna molecule then serves to produce a corresponding amino acid sequence through a process called translation. each group of three nucleotides in the sequence, called a codon, corresponds either to one of the twenty possible amino acids in a protein or an instruction to end the amino acid sequence; this correspondence is called the genetic code. the flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but it never transfers from protein back into the sequence of dna—a phenomenon francis crick called the central dogma of molecular biology. the specific sequence of amino acids results in a unique three-dimensional structure for that protein, and the three-dimensional structures of proteins are related to their functions. some are simple structural molecules, like the fibers formed by the protein collagen. proteins can bind to other proteins and simple molecules, sometimes acting as enzymes by facilitating chemical reactions within the bound molecules (without changing the structure of the protein itself). protein structure is dynamic; the protein hemoglobin bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood. a single nucleotide difference within dna can cause a change in the amino acid sequence of a protein. because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. for example, sickle-cell anemia