Patterns of Inheritance

Patterns of inheritance are how genes interact with each other.  An individual gets a set of genes (chromosome) from each parent and the alleles for a gene can be different.  Alleles can either be dominant, recessive, codominant, or incomplete dominant.  One of the key points to understanding interactions of genes is the understanding of the terms homozygous and heterozygous.  Homozygous refers to having two identical alleles at the same loci of a pair of chromosomes.  If an allele is denoted by “A,” then a homozygous individual would be “AA,” meaning that both alleles at that loci on both chromosomes are “A.”  If an organism is heterozygous for a trait, then it has two different alleles at the specific loci on its chromosome pair, making its trait look something like “Aa.”

Now that homozygousity and heterozygocity has been learned, it is easier to understand the characteristics of pattern of inheritance.  For the previously mentioned dominant allele, it is expressed regardless of whether it is present either with another of its kind or with a recessive allele.  In simple genetics, dominate alleles are usually denoted as capital letters.  Recessive alleles are those that can be present in a heterozygous trait, but not expressed unless they are homozygous for the trait.  Usually recessive alleles are denoted in lower case letters, like the little “a.”  For the recessive allele to be expressed the organism’s alleles would have to be homozygous recessive, or “aa.”  Therefore, a recessive allele can be present in an organism’s genotype without being observed in the phenotype if the organism is heterozygous for that trait.  An example of dominate and recessive traits seen in humans is the unattached or attached earlobe.  Unattached earlobes (image A) are the dominate trait for earlobes.  An individual like the one in image A could be homozygous for unattached earlobes, “AA,” or heterozygous for unattached earlobes, “Aa.”  The attached earlobes in image B are the recessive form of the trait and an individual with attached earlobes must be homozygous for the triat, or “aa.”

Codominance is much like the name suggests, with the alleles for the trait both being expressed as they would without the other’s being there.  An example of codominance in humans is the ABO blood type.  Individuals with type A blood have the “A” antigen present on the outside of the red blood cell.  An individual with type A blood may be “AA” or “AO,” because the characteristic of type O blood is that it does not have any antigens present on the red blood cell.  For a person with type B blood, the same principle applies.  A person with type AB blood has inherited an allele from one parent that is “A” and one parent that is “B” and both alleles are expressed normally, so the red blood cell has both A and B antigens present.  A person with type O blood must be homozygous for “O.”


Incomplete dominance is a situation in which both of the traits are passed on to the next generation, but they are incompletely expressed of themselves when in the presence of the other allele.  The snapdragon’s in the image below are “AA” for bright red and “aa” for white petals.  When bred together, the resulting offspring is a blending of the colors, making it pink.  However, the alleles for red and white are not diminished in the first generation, they are just incompletely expressed when the flower is heterozygous for the trait, “Aa.”  The fact that the mating did not just have a blending effect can be seen in the second generation when two of the first generation “Aa” pink flowers are bred together and the bright red “AA” trait, the pink “Aa” trait, and the white “aa” trait can all be found in the resulting offspring.



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