Biology
KEY CONCEPTS:
The O (or H) antigen is generated in all individuals, and consists of a particular carbohydrate group that is added to proteins. The ABO locus codes for a galactosyltransferase enzyme that adds a further sugar group to the O antigen. The specificity of this enzyme determines the blood group. The A allele produces an enzyme that uses the cofactor UDP-N-acetylgalactose, creating the A antigen. The B allele produces an enzyme that uses the cofactor UDP-galactose, creating the B antigen. The A and B versions of the transferase protein differ in 4 amino acids that presumably affect its recognition of the type of cofactor. The O allele has a mutation (a small deletion) that eliminates activity, so no modification of the O antigen occurs.
- Mendelian Genetics Part No. 1 - Defining Key Vocabulary
Solving Mendelian genetics problems can seem somewhat daunting from the onset, especially with the vocabulary that is associated with them. Here we will define key terms that are used in genetics problems, but also, walk you through the proper steps needed...
- Genetic Terms
Genotype is the genetic composition of an organism. Phenotype is the set of characteristic manifested by an organism. An individual has a heterozygous condition if he has 2 different alleles of the same gene An individual has a homozygous condition if...
- Individual Genomes Show Extensive Variation
KEY TERMS:Polymorphism (more fully genetic polymorphism) refers to the simultaneous occurrence in the population of genomes showing variations at a given position. The original definition applied to alleles producing different phenotypes. Now it is also...
- Mutations In The Same Gene Cannot Complement
KEY TERMS:A complementation test determines whether two mutations are alleles of the same gene. It is accomplished by crossing two different recessive mutations that have the same phenotype and determining whether the wild-type phenotype can be produced....
- A Gene Codes For A Single Polypeptide
KEY TERMS:A homomultimer is a protein composed of identical subunits. A heteromultimer is a protein that is composed of nonidentical subunits (coded by different genes). KEY CONCEPTS: The one gene : one enzyme hypothesis summarizes the basis of modern...
Biology
A locus may have more than one wild-type allele
KEY TERMS:
- Polymorphism (more fully genetic polymorphism) refers to the simultaneous occurrence in the population of genomes showing variations at a given position. The original definition applied to alleles producing different phenotypes. Now it is also used to describe changes in DNA affecting the restriction pattern or even the sequence. For practical purposes, to be considered as an example of a polymorphism, an allele should be found at a frequency > 1% in the population.
- A locus may have a polymorphic distribution of alleles, with no individual allele that can be considered to be the sole wild-type.
There is not necessarily a unique wild-type allele at any particular locus. Control of the human blood group system provides an example. Lack of function is represented by the null type, O group. But the functional alleles A and B provide activities that are codominant with one another and dominant over O group. The basis for this relationship is illustrated in Figure 1.30.
The O (or H) antigen is generated in all individuals, and consists of a particular carbohydrate group that is added to proteins. The ABO locus codes for a galactosyltransferase enzyme that adds a further sugar group to the O antigen. The specificity of this enzyme determines the blood group. The A allele produces an enzyme that uses the cofactor UDP-N-acetylgalactose, creating the A antigen. The B allele produces an enzyme that uses the cofactor UDP-galactose, creating the B antigen. The A and B versions of the transferase protein differ in 4 amino acids that presumably affect its recognition of the type of cofactor. The O allele has a mutation (a small deletion) that eliminates activity, so no modification of the O antigen occurs.This explains why A and B alleles are dominant in the AO and BO heterozygotes: the corresponding transferase activity creates the A or B antigen. The A and B alleles are codominant in AB heterozygotes, because both transferase activities are expressed. The OO homozygote is a null that has neither activity, and therefore lacks both antigens.
Neither A nor B can be regarded as uniquely wild type, since they represent alternative activities rather than loss or gain of function. A situation such as this, in which there are multiple functional alleles in a population, is described as a polymorphism.
- Mendelian Genetics Part No. 1 - Defining Key Vocabulary
Solving Mendelian genetics problems can seem somewhat daunting from the onset, especially with the vocabulary that is associated with them. Here we will define key terms that are used in genetics problems, but also, walk you through the proper steps needed...
- Genetic Terms
Genotype is the genetic composition of an organism. Phenotype is the set of characteristic manifested by an organism. An individual has a heterozygous condition if he has 2 different alleles of the same gene An individual has a homozygous condition if...
- Individual Genomes Show Extensive Variation
KEY TERMS:Polymorphism (more fully genetic polymorphism) refers to the simultaneous occurrence in the population of genomes showing variations at a given position. The original definition applied to alleles producing different phenotypes. Now it is also...
- Mutations In The Same Gene Cannot Complement
KEY TERMS:A complementation test determines whether two mutations are alleles of the same gene. It is accomplished by crossing two different recessive mutations that have the same phenotype and determining whether the wild-type phenotype can be produced....
- A Gene Codes For A Single Polypeptide
KEY TERMS:A homomultimer is a protein composed of identical subunits. A heteromultimer is a protein that is composed of nonidentical subunits (coded by different genes). KEY CONCEPTS: The one gene : one enzyme hypothesis summarizes the basis of modern...