Genes are segments of DNA located on chromosomes that contain the instructions for protein production. Scientists estimate that humans have as many as 25,000 genes. Genes exist in more than one form. These alternative forms are called alleles and there are typically two alleles for a given trait. Alleles determine distinct traits that can be passed on from parents to offspring. The process by which genes are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.

Gene Transcription

Genes contain the genetic codes, or sequences of nucleotide bases in nucleic acids, for the production of specific proteins. The information contained within DNA is not directly converted to proteins, but must first be transcribed in a process called DNA transcription. This process takes place within the nucleus of our cells. Actual protein production takes place in the cytoplasm of our cells through a process called translation.

Transcription factors are special proteins that determined whether or not a gene gets turned on or off. These proteins bind to DNA and either aid in the transcription process or inhibit the process. Transcription factors are important for cell differentiation as they determine which genes in a cell are expressed. The genes expressed in a red blood cell, for example, differ from those expressed in a sex cell.

An Individual's Genotype

In diploid organisms, alleles come in pairs. One allele is inherited from the father and the other from the mother. Alleles determine an individual's genotype or gene composition. The allele combination of the genotype determines the traits that are expressed or the phenotype. A genotype producing the phenotype of a straight hairline, for example, differs from the genotype resulting in a V-shaped hairline.

Inherited Through Both Asexual and Sexual Reproduction.

Genes are inherited through both asexual reproduction and sexual reproduction. In asexual reproduction, resulting organisms are genetically identical to a single parent. Examples of this type of reproduction include budding, regeneration, and parthenogenesis.

Gametes Fuse to Form a Distinct Individual

Sexual reproduction involves the contribution of genes from both male and female gametes that fuse to form a distinct individual. The traits exhibited in these offspring are transmitted independently of one another and may result from several types of inheritance.

In complete dominance inheritance, one allele for a particular gene is dominant and completely masks the other allele for the gene.

In incomplete dominance, neither allele is completely dominant over the other resulting in a phenotype that is a mixture of both parent phenotypes.

In co-dominance, both alleles for a trait are fully expressed.

Some Traits Determined by More Than One Gene

Not all traits are determined by a single gene. Some traits are determined by more than one gene and are therefore known as polygenic traits. Some genes are located on sex chromosomes and are called sex-linked genes. There are a number of disorders that are caused by abnormal sex-linked genes including hemophilia and color blindness.

Variation Helps Adapting to Changing Situations

Genetic variation is a change in the genes that occur in organisms in a population. This variation typically occurs through DNA mutation, gene flow (movement of genes from one population to another) and sexual reproduction. In unstable environments, populations with genetic variation are typically able to adapt to changing situations better than those that do not contain genetic variation.

Mutations Are From Errors and the Environment

A gene mutation is an alteration in the sequence of nucleotides in DNA. This change can affect a single nucleotide pair or larger segments of a chromosome. Changing gene segment sequences most often results in non-functioning proteins.

Some mutations can result in disease, while others may have no negative impact on or may even benefit an individual. Still, other mutations may result in unique traits such as dimples, freckles, and multicolored eyes. Gene mutations are most commonly the result of environmental factors (chemicals, radiation, ultraviolet light) or errors that occur during cell division (mitosis and meiosis).