The roots of genetics can be traced back to the 1800s when an Austrian monk named Gregor Mendel first began studying inheritance patterns using garden-variety pea plants. The results of his studies provided important groundwork for modern genetic science and are much of the reason why the science of genetics has prospered to date.
The work of Gregor Mendel established fundamental rules for patterns of inheritance. The principles of genotypes and phenotypes must first be understood before mastering Mendel's rules. Although unfamiliar, these terms have a simple explanation.
A phenotype is nothing more than the manner in which a gene is expressed. For example, a dimpled chin is a widely recognized example of a genetic trait. Whether or not you have a dimpled chin determines your phenotype. In other words, if you have a dimple in your chin, the trait is expressed. This also means the trait is dominant. A trait that is hidden, no dimpled chin for example, is what is known as a recessive trait.
Conversely, a genotype is the full genetic information of an individual. So this means a genotype represents both of copies of the gene for a particular trait. Remember, everyone inherits two copies of a gene -- one is from each parent.
The rules of dominance in inheritance patterns are relatively clear-cut. An individual who inherits two recessive alleles will always mask or hide the trait. This individual would also be considered homozygous recessive.
Recessive alleles are typically indicated with lowercase letters. So, if we say the trait for dimpled chin is represented by a lowercase letter d, then an individual who inherits two recessive alleles, one that came from each parent, will have the dd genotype. Again, someone with this genotype is said to be a recessive homozygote.
Consequently, if an individual inherits two dominant alleles, the trait will always be expressed. This is also known as homozygous dominant. Dominant alleles are written with, you guessed it, capital letters; therefore, someone with two dominant alleles, DD, for the dimpled chin trait will surely have a dimpled chin.
Then what if a person inherits one dominant allele and one recessive allele -- the genotype of Dd? This would mean they are heterozygous; therefore, they will always express the trait. The reasoning behind this is that dominant alleles are just that -- dominant. Even if inherited alongside a recessive allele, a dominant trait will inevitably be expressed.
Another fascinating feature of recessive traits is that they hold more significance than merely being hidden. In certain cases, inheriting two copies of a recessive allele may indicate that a person will have a particular genetic disease. A few common recessive inherited disorders are Sickle Cell Anemia, Tay Sachs, and Cystic Fibrosis.
These are examples of autosomal disorders, which are diseases that are linked to the 22 pairs of chromosomes that do not determine our gender. Interestingly enough, there are also dominant inherited disorders which are quite lethal and sex-linked disorders that have to do with mutations in the genes located on our X or Y chromosomes.
Blood type is a special type of inherited trait that is governed by multiple alleles, or gene variants. Most often referred to as the ABO blood group, these three alleles, A, B, and O, can produce four phenotypes from six genotypes. Of the three possible alleles, A and B are always dominant to O.
Likewise, A and B are codominant which means that if inherited together, both A and B will be expressed. As with any other inherited trait, only one gene copy will be donated from a parent. Possible mixtures are AO, AA, AB, OO, BO, and BB. The resultant phenotypes are type A blood for the AO and AA genotypes, type AB blood for the AB genotype, type O blood for the OO genotype, and type B blood for the BO and BB genotypes. - 15254
The work of Gregor Mendel established fundamental rules for patterns of inheritance. The principles of genotypes and phenotypes must first be understood before mastering Mendel's rules. Although unfamiliar, these terms have a simple explanation.
A phenotype is nothing more than the manner in which a gene is expressed. For example, a dimpled chin is a widely recognized example of a genetic trait. Whether or not you have a dimpled chin determines your phenotype. In other words, if you have a dimple in your chin, the trait is expressed. This also means the trait is dominant. A trait that is hidden, no dimpled chin for example, is what is known as a recessive trait.
Conversely, a genotype is the full genetic information of an individual. So this means a genotype represents both of copies of the gene for a particular trait. Remember, everyone inherits two copies of a gene -- one is from each parent.
The rules of dominance in inheritance patterns are relatively clear-cut. An individual who inherits two recessive alleles will always mask or hide the trait. This individual would also be considered homozygous recessive.
Recessive alleles are typically indicated with lowercase letters. So, if we say the trait for dimpled chin is represented by a lowercase letter d, then an individual who inherits two recessive alleles, one that came from each parent, will have the dd genotype. Again, someone with this genotype is said to be a recessive homozygote.
Consequently, if an individual inherits two dominant alleles, the trait will always be expressed. This is also known as homozygous dominant. Dominant alleles are written with, you guessed it, capital letters; therefore, someone with two dominant alleles, DD, for the dimpled chin trait will surely have a dimpled chin.
Then what if a person inherits one dominant allele and one recessive allele -- the genotype of Dd? This would mean they are heterozygous; therefore, they will always express the trait. The reasoning behind this is that dominant alleles are just that -- dominant. Even if inherited alongside a recessive allele, a dominant trait will inevitably be expressed.
Another fascinating feature of recessive traits is that they hold more significance than merely being hidden. In certain cases, inheriting two copies of a recessive allele may indicate that a person will have a particular genetic disease. A few common recessive inherited disorders are Sickle Cell Anemia, Tay Sachs, and Cystic Fibrosis.
These are examples of autosomal disorders, which are diseases that are linked to the 22 pairs of chromosomes that do not determine our gender. Interestingly enough, there are also dominant inherited disorders which are quite lethal and sex-linked disorders that have to do with mutations in the genes located on our X or Y chromosomes.
Blood type is a special type of inherited trait that is governed by multiple alleles, or gene variants. Most often referred to as the ABO blood group, these three alleles, A, B, and O, can produce four phenotypes from six genotypes. Of the three possible alleles, A and B are always dominant to O.
Likewise, A and B are codominant which means that if inherited together, both A and B will be expressed. As with any other inherited trait, only one gene copy will be donated from a parent. Possible mixtures are AO, AA, AB, OO, BO, and BB. The resultant phenotypes are type A blood for the AO and AA genotypes, type AB blood for the AB genotype, type O blood for the OO genotype, and type B blood for the BO and BB genotypes. - 15254
About the Author:
RJ Sullivan manages several profitable home-based businesses and is an accomplished editor and writer. Ms. Sullivan also writes for In the Gno, a blog dedicated to demystifying the science of genetics in an interesting and easy-to-understand manner. Visit her blog to learn more about mendelian genetics or blood types.
