The father of genetics, Gregor Mendel, first began studying patterns of inheritance using garden-variety pea plants back in the 1800s. Today, his findings have provided the groundwork necessary for the science of genetics to prosper.
Mendel's discoveries established basic rules for genetic principles and inheritance patterns. Understanding the concepts of genotypes and phenotypes is necessary to master Mendelian rules. Though these terms may be unfamiliar, each has 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.
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.
Dominance rules of inheritance patterns are mostly unambiguous. If a person receives two recessive alleles, they will always hide or mask the trait. This person would also be known as a recessive homozygote.
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.
Accordingly, if a person receives two copies of a dominant allele, the trait will always be expressed. The expression of two dominant alleles is termed homozygous dominant. Dominant alleles are typically indicated by, you guessed it, capital letters; thus, an individual who has two dominant alleles, DD, for the dimpled chin trait will absolutely have a dimpled chin.
So, if an individual has the genotype of Dd, this means they inherited one dominant and one recessive allele. This person would also be what is known as heterozygous for the trait and they will always express it. The rationale is that dominant alleles are precisely that -- dominant. Even inherited with a recessive allele, a dominant trait will without question be expressed.
Another interesting point on recessive alleles and traits is that they are more significant than merely being hidden. In some cases, two copies of a recessive allele can mean that a person will inherit a genetic disease. Some commonly known recessive inherited diseases are Cystic Fibrosis, Sickle Cell Anemia, and Tay Sachs.
These recessive diseases are examples of autosomal disorders, which are disorders linked to the 22 chromosome pairs which do not determine gender. Also curious is that there are dominant inherited disorders which are very lethal and sex-linked disorders that are related to genetic mutations on the X and 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.
Mendel's discoveries established basic rules for genetic principles and inheritance patterns. Understanding the concepts of genotypes and phenotypes is necessary to master Mendelian rules. Though these terms may be unfamiliar, each has 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.
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.
Dominance rules of inheritance patterns are mostly unambiguous. If a person receives two recessive alleles, they will always hide or mask the trait. This person would also be known as a recessive homozygote.
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.
Accordingly, if a person receives two copies of a dominant allele, the trait will always be expressed. The expression of two dominant alleles is termed homozygous dominant. Dominant alleles are typically indicated by, you guessed it, capital letters; thus, an individual who has two dominant alleles, DD, for the dimpled chin trait will absolutely have a dimpled chin.
So, if an individual has the genotype of Dd, this means they inherited one dominant and one recessive allele. This person would also be what is known as heterozygous for the trait and they will always express it. The rationale is that dominant alleles are precisely that -- dominant. Even inherited with a recessive allele, a dominant trait will without question be expressed.
Another interesting point on recessive alleles and traits is that they are more significant than merely being hidden. In some cases, two copies of a recessive allele can mean that a person will inherit a genetic disease. Some commonly known recessive inherited diseases are Cystic Fibrosis, Sickle Cell Anemia, and Tay Sachs.
These recessive diseases are examples of autosomal disorders, which are disorders linked to the 22 chromosome pairs which do not determine gender. Also curious is that there are dominant inherited disorders which are very lethal and sex-linked disorders that are related to genetic mutations on the X and 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.
About the Author:
RJ Sullivan manages several successful home-based businesses and is an accomplished editor and writer. Ms. Sullivan also writes for In the Gno, a blog devoted 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.
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