Definition: Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype.
Example:
In cross-pollination experiments between red and white snapdragon plants, the resulting offspring are pink. The dominant allele that produces the red color is not completely expressed over the recessive allele that produces the white color.
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When scientists first started to classify life, everything was designated as either an animal or a plant. But as new forms of life were discovered and our knowledge of life on Earth grew, new categories, called "Kingdoms," were added. There eventually came to be five Kingdoms in all - Animalia, Plantae, Fungi, Protista, and Bacteria.
The five Kingdoms were generally grouped into two categories called Eukarya and Prokarya. Eukaryotes represent four of the five Kingdoms (animals, plants, fungi and protists). Eukaryotes are organisms whose cells have a nucleus -- a sort of sack that holds the cell's DNA. Animals, plants, protists and fungi are all eukaryotes because they all have a DNA-holding nuclear membrane within their cells.
The cells of prokaryotes, on the other hand, lack this nuclear membrane. Instead, the DNA is part of a protein-nucleic acid structure called the nucleoid. Bacteria are all prokaryotes.
However, new insight into molecular biology changed this view of life. A type of prokaryotic organism that had long been categorized as bacteria turned out to have DNA that is very different from bacterial DNA. This difference led microbiologist Carl Woese of the University of Illinois to propose reorganizing the Tree of Life into three separate Domains: Eukarya, Eubacteria (true bacteria), and Archaea.
Archaea look like bacteria - that's why they were classified as bacteria in the first place: the unicellular organisms have the same sort of rod, spiral, and marble-like shapes as bacteria. Archaea and bacteria also share certain genes, so they function similarly in some ways. But archaeans also share genes with eukaryotes, as well as having many genes that are completely unique.
Archaea are so named because they are believed to be the least evolved forms of life on Earth ('archae' meaning 'ancient'). The ability of some archaea to live in environmental conditions similar to the early Earth gives an indication of the ancient heritage of the domain.
The cells of prokaryotes, on the other hand, lack this nuclear membrane. Instead, the DNA is part of a protein-nucleic acid structure called the nucleoid. Bacteria are all prokaryotes.
However, new insight into molecular biology changed this view of life. A type of prokaryotic organism that had long been categorized as bacteria turned out to have DNA that is very different from bacterial DNA. This difference led microbiologist Carl Woese of the University of Illinois to propose reorganizing the Tree of Life into three separate Domains: Eukarya, Eubacteria (true bacteria), and Archaea.
Archaea look like bacteria - that's why they were classified as bacteria in the first place: the unicellular organisms have the same sort of rod, spiral, and marble-like shapes as bacteria. Archaea and bacteria also share certain genes, so they function similarly in some ways. But archaeans also share genes with eukaryotes, as well as having many genes that are completely unique.
Archaea are so named because they are believed to be the least evolved forms of life on Earth ('archae' meaning 'ancient'). The ability of some archaea to live in environmental conditions similar to the early Earth gives an indication of the ancient heritage of the domain.
The early Earth was hot, with a lot of extremely active volcanoes and an atmosphere composed mostly of nitrogen, methane, ammonia, carbon dioxide, and water. There was little if any oxygen in the atmosphere. Archaea and some bacteria evolved in these conditions, and are able to live in similar harsh conditions today. Many scientists now suspect that those two groups diverged from a common ancestor relatively soon after life began.
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