Carbon copies of chromosomes and DNA hardly serve the purpose of accounting for genetic and trait variations within a species. Offspring are not identical copies of their parents, rather they are the combination of both their parents genetic information. How does evolution and genetics account for genetic variability in a species? There are a number of ways such as the crossing over of chromosomes, dominant and recessive traits, and independent assortment.
In the case of meiotic replication, the shared genetic information conveyed and expressed by an organism is governed by certain genetic principles. Mendel's laws were as follows:
These genetic principles have become characteristically known as the law of independent assortment (Mendel's second principle), and Dominants and Recessives (Mendel's third principle). The second principle is simply as it says, that a trait's expression is not dependent on another traits expression, i.e. a trait for blue eyes is not dependent on the trait for height within an individual organism.
A genotype is the entire genetic make up of an organism. The phenotype is the actual expressed traits or characteristics found within an organism. The distinction between genotype and phenotype can be made based on dominant and recessive genes. A dominant gene is an expressed characteristic trait within an organism, where as a recessive trait is not. This is best illustrated in the example of human blood types.
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Blood Types in Humans |
A | B | O |
| A |
AA (AA) |
AB (AB) |
AO (A) |
| B |
AB (AB) | BB (B) | BO (B) |
| O | AO (A) | BO (B) | OO (O) |
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Human blood type is determined by 3 alleles, or genes, A, B, and O. Both A and B blood types are co-dominant, meaning that if a human inherits the AB blood type, both types will be expressed as AB. Alternately if a human inherits the A allele and the O allele, the blood type of the offspring will be A. In parenthesis above are the actual phenotypic expression of the traits listed above. |
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The example of human blood types is however unique from Mendel's laws, in that Mendel stated that there are only 2 traits from each parent which determine the dominance or recessiveness of a species. Within the example of blood type there are 3 unique alleles, two of which are co-dominant. An example of Mendel's laws with two traits can be illustrated in the human trait of tongue rolling.
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Tongue Rolling Trait |
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| TT | tt | ||
| TT | TT | Tt | |
| tt | Tt | tt | |
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T= Dominant trait for tongue rolling. t = Recessive trait for tongue rolling. |
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The above illustrations display each possible trait for tongue rolling a parent may have. The phenotypic expression of a gene is determined by dominance. For example, with tongue rolling if a parent had even one T in the pair, the T trait will be expressed- the parent can roll their tongue. Only in the case of entirely recessive inheritance, where both parents give their offspring the tt gene combination, will the tongue rolling trait not be expressed. The above illustration has a ratio of 3:1, meaning that there is a three in four chance that the dominant T gene will be expressed in the offspring, leaving a one in four chance that the offspring will not be able to roll their tongue.
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