Heredity is thought to have been discovered by Gregor Mendel in the 17th century. He is famous for cross breeding various types of pea plants based on the traits they exhibited.
He used "true breeding" plants for each trait (short, tall, yellow pod, green pod, wrinkled, smooth, etc). After crossing a tall plant with a short plant for example, the resultant generation (the F1 generation) would be 100% tall. This is because the tall phenotype is dominant to the short, or dwarf, phenotype. The F1 generation plants were allowed to self fertilize and creat the F2 generation. This is where the variation was seen and recorded. Mendel observed a 3:1 ratio of tall to dwarf respectively.
In humans, eye color is an example of an inherited characteristic: an individual might inherit the "brown-eye trait" from one of the parents. Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype.
The complete set of observable traits (phenotype) arise from the interaction of its genotype with the environment. As a result, many aspects of an organism's phenotype are not inherited. For example, suntanned skin comes from the interaction between a person's phenotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in their genotype: a striking example is people with the inherited trait of albinism, who do not tan at all and are very sensitive to sunburn.
Heritable traits are known to be passed from one generation to the next via DNA. The sequence of bases along a particular DNA molecule specifies the genetic information: this is comparable to a sequence of letters spelling out a passage of text.
If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.
While this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by a quantitative trait locus (multiple interacting genes) within and among organisms. Developmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlie some of the mechanics in developmental plasticity and canalization.
Recent findings have confirmed important examples of heritable changes that cannot be explained by direct agency of the DNA molecule.
DNA methylation marking chromatin, self-sustaining metabolic loops, gene silencing by RNA interference, and the three dimensional conformation of proteins (such as prions) are areas where epigenetic inheritance systems have been discovered at the organismic level.
Heritability may also occur at even larger scales such as ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effect that modifies and feeds back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors. Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits, group heritability, and symbiogenesis. These examples of heritability that operate above the gene are covered broadly under the title of multilevel or hierarchical selection, which has been a subject of intense debate in the history of evolutionary science.
It was not until later that punnett squares were created so Mendel had no model to explain this ratio. He repeated this experiment with several other traits always finding a 3:1 ratio. He concluded that each trait was controlled by a functional unit and was passed down from parent to offspring in an exact quantity.
Mendel further realized that some traits must be dominant to others and only the dominant traits would be shown in the phenotype.
Mendel cross-bred hundred of plants creating thousands of trials and he hand counted each phenotypic ratio.
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