Need help preparing for the Biology section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Dominance, Leakage, Penetrance, and Expressivity, Hybrid Viability and Gene Pool of Mendelian Concepts. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Mendelian genetics is the study of biological inheritance first proposed by a Christian monk named Gregor Mendel in the nineteenth century. At first, Mendel’s theories were controversial, but soon, with more study, they became accepted into the scientific community. Several concepts derived from Mendelian genetics help explain the inheritance patterns of reproduction. These concepts include dominance, leakage, penetrance, expressivity, hybrid viability, and gene pools. All of these concepts are important to understand for the MCAT exam.
Dominance
There are three different types of dominance that are important to understand for the MCAT exam - complete dominance, incomplete dominance, and codominance. Complete dominance is when the dominant allele completely masks the effects of the recessive allele in heterozygotes. The homozygous dominant plant with RR alleles has red flowers. The homozygous recessive plant with rr alleles has white flowers. Lastly, the heterozygote with Rr alleles once again has red flowers. In complete dominance, the heterozygote has a phenotype entirely determined by the dominant allele, and the recessive allele is masked.
However, in incomplete dominance, neither allele is dominant. In this way, the phenotype of the heterozygote is an intermediate of the two homozygous phenotypes. The plant with the RR alleles has red flowers, the plant with the rr alleles has white flowers. The heterozygote, Rr, has pink flowers. Pink is not the phenotype of either homozygous plants but is instead an intermediary of them.
The last type of dominance is codominance, in which both alleles are expressed. The typical example of codominance is human blood types. In terms of blood types, there are three different levels: the IA allele, the IB allele, and the i allele. Individuals who have the genotype ‘IA IA’ or ‘IA I’, have the A antigen on their red blood cells and their blood type is A. Individuals with the genotype ‘IB IB’ or ‘IB I’, have the B antigen on their red blood cells and their blood type is B. Those who have the genotype ‘IA IB’, express both the A and B antigens on their red blood cells and their blood type is AB. The blood type of these individuals is an example of codominance because they express both alleles. Lastly, those with the genotype ‘i i’, do not express any antigens on their red blood cells, and their blood type is O.
Leakage
Leakages refers to the transfer of genes from one organism to another. This concept is often discussed with regards to genetically modified organisms, or GMOs. For example, a particular crop could be genetically modified to have resistance to pesticides. In this way, farmers can grow these genetically modified crops and apply pesticides that will kill the weeds but spare the crops. If the pesticide resistance gene is transferred from crops to weeds, this represents leakage. Transference of this gene is a concern in the field of GMOs because if the weeds gain the pesticide resistance gene, they will be hard to get rid of, which poses a significant problem for farmers.
Penetrance & Expressivity
Penetrance is another concept important in Mendelian genetics. It refers to the percentage of organisms with a particular allele that have the associated phenotype. Organisms with a particular allele do not always have to express a particular phenotype. For example, 70% of mice with a mutation and the COL1A1 gene have bone fractures, which is the phenotype associated with this allele. In this way, since only 70% of mice with this allele have this phenotype, the COL1A1 mutation has a 70% penetrance.
Expressivity refers to the degree to which a trait is expressed in individuals. These are often traits that are not necessarily black and white, but rather a shade of colors. Penetrance is visualized in the top row. Individuals with the allele either have the phenotype, or they do not. Expressivity is visualized in the second row. In this row, all individuals have the phenotype, but the degree to which they have the phenotype varies. In this way, the phenotype is stronger in some individuals and weaker and in others.
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