Mendelian Concepts

Genes are DNA sequences that tells a cell which traits from the parents should be passed down and these genes shape the physical and biochemical features of an organism. In molecular biology, we see that genes make up the coding for the protein structure, which acts to bring about a particular trait. Genotype of an individual refers to the genetic combination possessed by that individual, and the manifestation of a given genotype as a trait that can be observed is known as the phenotype.1 Height and eye color are examples of the phenotype of a person. Structures called chromosomes organize all of this to ensure that genetic material is passed easily to daughter cells during the processes of mitosis and meiosis. Each gene may have alternative forms called alleles. Each human being possesses two copies of each chromosome, called homologues, except for male sex chromosomes. Each gene is located on a specific area of a chromosome and that area is known as a locus.1 An allele is a variant of a gene and a gene may have a number of alleles. Alleles of the same gene would naturally exist at the same position on the chromosome or at the same locus. A cell holds one maternal allele and one paternal allele. When a gene has only 2 alleles, then that’s called single allele. When a gene has more than 2 alleles, then that’s called multiple alleles. For example, blood type is governed 3 alleles: IA IB and i. Because a cell can only hold 2 of these alleles, the different combinations an individual can have are:

Genotype Blood type (phenotype)
ii O


Homozygous alleles are when the two alleles that an individual carries are the same. For example, AA or aa. Heterozygous alleles are when the two alleles that an individual carries are different. For example, Aa. In the case of the X chromosome in males, only one allele is present. This kind of genotype is referred to as a hemizygous genotype. The normal phenotype or allele expressed for an organism is known as the wild type. The wild-type is usually the most dominating allele, although it doesn’t hold true in some cases. Recessiveness refers to the weak allele. The recessive allele is only expressed if both copies are present. Only a single copy is needed for the dominant allele. Lower case letters usually indicate recessive alleles while upper case letters are used to represent the dominant allele. One instance of this is for blond hair. Blond hair alleles are recessive and the two recessive alleles need to be present, otherwise the hair will be dark. When only one dominant and one recessive allele exist for a given gene, there is said to be complete dominance.1 In this case, the presence of one dominant allele will mask the recessive allele, if present. When more than one dominant allele exists for a given gene, there is codominance. For example, a person with one allele for the A blood antigen and one allele for the B blood antigen will express both antigens simultaneously. Finally, incomplete dominance occurs when a heterozygote expresses a phenotype that is intermediate between the two homozygous genotypes. An example of incomplete dominance is the color of chickens. Black chickens and white chickens will have bluish grey offsprings due to incomplete dominance. Genetic leakage is a flow of genes between species. There are cases of organisms from different (but closely related) species can mate to produce a hybrid offspring. Many hybrid offspring, such as the mule (hybrid of a male horse and a female donkey), are not able to reproduce because they have odd numbers of chromosomes. Mules are hybrids but they cannot undergo normal homologous pairing in meiosis resulting in the inability to form gametes. This is because horses have 64 chromosomes while donkeys have 62. The mules would then have 63 chromosomes which cannot go through meiosis. In some cases, however, a hybrid can reproduce with members of one species or the other, such as the beefalo (a cross between cattle and American bison). The hybrid is a carrier of the genes from both parent species, and this results in an overall flow of genetic material from one species to the other.

Penetrance is a population measure defined as the section or amount of individuals in a particular population that carry the allele that will actually be express phenotypically. In other words, it is the probability that, given a particular genotype, a person will express the phenotype. Alleles can be grouped by their level of penetrance and Huntington’s disease is an example of this phenomena. Full penetrance is when a 100% of persons with a particular allele show will manifest the phenotype. An example is Huntington’s disease as once individuals have over 40 sequence they will manifest the disease. Individuals that do not have as high of a sequence repeat will manifest high penetrance and most, but not all of those with the allele will demonstrate symptoms of the disease. Even fewer sequence repeats and the gene will exhibit reduced penetrancelow penetrance, or even non-penetrance. Expressivity is to what degree a penetrant gene is expressed. If expressivity is constant, then all individuals with a given genotype express the same phenotype. However, if expressivity is variable, then individuals with the same genotype may have different phenotypes. Hybridization can refer to the process of two complementary, single-stranded DNA or RNA combining together, producing a double-stranded molecule through base pairing. This technique is used for interbreeding between individuals of genetically distinct populations. Gene pool is the sum of all genes/alleles in a population at a given time. Gene pools with high ratings have more genetic diversity and thus more fitness.




1) Churchill, F. (1974). William Johannsen and the genotype concept. Journal of the History of Biology, 5 -30.

2) Freeman, S. (2011.). Biological Science (6th ed.). Hoboken, NY: Pearson.

3) Hartl DL, C. A. (2007). Principles of population genetics. Sunderland, MA: Sinauer.

4) Baker, J. M. (2005). Adaptive speciation: The role of natural selection in mechanisms of geographic and non-geographic speciation. Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences, 303 – 326.

5) S. Blair Hedges, S. K. (2003, April). Genomic clocks and evolutionary timescales. Retrieved from Hedges Lab:


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