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5.3 Mendelian Genetics





⏱️  2 min read

written by

Caroline Koffke

caroline koffke

May 30, 2020

The Father of Modern Genetics

Gregor Mendel, the father of modern genetics, came up with some really important laws, including the law of independent assortment, that allows for scientists to determine how genes are inherited from generation to generation.

Essential Vocabulary

Phenotype - the physical appearance of an organism, or the actual depiction of a trait (think: phenotype, PHYSICAL). Ex. red, purple, white, sparkly, spiky.

Genotype - the alleles that make up an individual trait (think: genotype, GENES). Ex. AA, Aa, aa OR homozygous dominant, heterozygous, homozygous recessive.

Allele - a version of a gene. Usually an allele can be dominant or recessive. For Mendelian genetics, all genes have two alleles. Homozygous Recessive - an organism that has two recessive alleles. The organism will have the recessive phenotype.

Dominant - a trait that produces enough protein or product in order to overtake another trait.

Recessive - a trait that does not produce enough protein or product and is overpowered by dominant traits.

Homozygous Dominant - an organism that has two dominant alleles. The organism will have the dominant phenotype.

Heterozygous - an organism that has one dominant and one recessive allele. The organism will have the dominant phenotype.

Punnett Squares

Note that only the homozygous recessive genotype leads to the recessive phenotype. All of the vocabulary above is used frequently and should be memorized and thoroughly understood. 

Because of the rules that Mendel created, the frequency of inheritance can be determined when two individuals are crossed. This can be shown with a Punnett Square.


Image courtesy of WikiMedia Commons.

As shown in the Punnett square above, when a heterozygous (Yy) and homozygous recessive (yy) individual is crossed, there is a 50% chance that the offspring will show the dominant (yellow) phenotype and a 50% chance that the offspring will show the recessive (green) phenotype. This can be done for any trait that has a simple inheritance pattern. By knowing the genotype of the parents, the various possible offspring can be calculated with their frequencies.

Unfortunately, most traits do not have a simple dominant/recessive inheritance pattern and, therefore, do not fit Mendel’s rules. These traits, referred to as Non-Mendelian traits, are explained next.

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