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.
Mendel's work provided basic laws establishing the rules for inheritance patterns. Understanding these patterns requires grasping the concepts of genotypes and phenotypes. These terms might sound unfamiliar, but there is a simple explanation for each.
A phenotype is the physical manifestation of a gene. A dimpled or cleft chin is an example of an inherited trait. Your phenotype is established by whether you have a dimple in your chin. In other words, you express the trait if you have a dimpled chin. In addition, this means the trait is dominant. A hidden trait, for example no dimpled chin, is known as recessive.
On the other hand, a genotype is an individual's actual genetic information, whether hidden or expressed. In other words, a genotype indicates both allelic copies of the trait. Don't forget, everyone gets two copies of a gene -- one comes 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 normally written out in lowercase letters. For example, if we indicate the trait for dimpled chin with a lowercase letter d, then a person who inherits two copies of the recessive allele, one from each parent, will have the genotype of dd. Again, this individual would be considered homozygous recessive.
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.
So, what happens if someone inherits one dominant and one recessive allele, effectively Dd? This means that they are heterozygous for the trait; however, they will always express it. The reason for this is because dominant alleles are exactly that -- dominant. Even in the presence of a recessive allele, a dominant trait will always 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 examples are known as autosomal disorders, or diseases linked to the 22 pairs of chromosomes that do not determine gender. Also interesting is that there are dominant inherited disorders which are highly lethal and X- or Y-linked disorders related to mutations in genes located on our sex-determining chromosomes.
Blood type is a unique type of genetic trait which is governed by multiple alleles. Often spoken of as the ABO blood group, these three alleles, A, B, and O, can yield a possible four phenotypes and six different genotypes. In the ABO blood group, A and B are permanently 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. - 16759
Mendel's work provided basic laws establishing the rules for inheritance patterns. Understanding these patterns requires grasping the concepts of genotypes and phenotypes. These terms might sound unfamiliar, but there is a simple explanation for each.
A phenotype is the physical manifestation of a gene. A dimpled or cleft chin is an example of an inherited trait. Your phenotype is established by whether you have a dimple in your chin. In other words, you express the trait if you have a dimpled chin. In addition, this means the trait is dominant. A hidden trait, for example no dimpled chin, is known as recessive.
On the other hand, a genotype is an individual's actual genetic information, whether hidden or expressed. In other words, a genotype indicates both allelic copies of the trait. Don't forget, everyone gets two copies of a gene -- one comes 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 normally written out in lowercase letters. For example, if we indicate the trait for dimpled chin with a lowercase letter d, then a person who inherits two copies of the recessive allele, one from each parent, will have the genotype of dd. Again, this individual would be considered homozygous recessive.
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.
So, what happens if someone inherits one dominant and one recessive allele, effectively Dd? This means that they are heterozygous for the trait; however, they will always express it. The reason for this is because dominant alleles are exactly that -- dominant. Even in the presence of a recessive allele, a dominant trait will always 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 examples are known as autosomal disorders, or diseases linked to the 22 pairs of chromosomes that do not determine gender. Also interesting is that there are dominant inherited disorders which are highly lethal and X- or Y-linked disorders related to mutations in genes located on our sex-determining chromosomes.
Blood type is a unique type of genetic trait which is governed by multiple alleles. Often spoken of as the ABO blood group, these three alleles, A, B, and O, can yield a possible four phenotypes and six different genotypes. In the ABO blood group, A and B are permanently 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. - 16759
About the Author:
RJ Sullivan manages several profitable home-based businesses and is a distinguished editor and writer. Ms. Sullivan also writes for In the Gno, a blog dedicated to presenting the science of genetics in an interesting and easy-to-understand manner. Visit her blog to learn more about mendelian genetics or blood types.
