Friday, April 15, 2011

Introduction to Mendelian Genetics

Crazy about peas!

[Please right-click on highlighted words for Wikipedia definition, and don't forget to post a comment below or share this article with your friends!]

Before we begin with some essentials on rat genetics, let me give you a brief introduction to Mendelian Genetics. As we all know, an offspring inherits traits from its parents; but not all traits from both parents will always appear. There are some laws governing genes and how they affect the outward appearance (phenotype) of a rat. This introduction is necessary in order to make the proceeding articles intelligible and easily understood.


Mendel was born to a peasant family in Heinzendorf (Czech Republic) in 1822. He lived his life in Austria as a monk and scientist. In 1856, he performed his first hybridization test on garden peas (Pisum sativum). He postulated laws on inheritance of traits based on his findings, which became the foundation of modern genetics. His scientific research continued until 1868, when he became abbot and had to commit all of his time and energy to administrative responsibilities. After his death in 1884 (kidney failure), his work was rediscovered in 1900.


Mendel began with his monohybrid test on garden peas (crossing for one trait only i.e. for color only, or for height only). Based on the consistent patterns of the tests, he was able to derive his postulates on inheritance. (Details of each law will be elaborated in a separate article, right-click here to read the article.)

Green or yellow? Tall or short? Fill-in the blanks!

Postulate #1: Unit factors are formed in pairs
“Genetic factors are controlled by unit factors that exist in pairs.”

Postulate #2: Dominance and recessiveness
“When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, which is said to be recessive.”

Postulate #3: Segregation
During the formation of gametes, the paired unit factors separate or segregate randomly so that each gamete receives one or the other with equal likelihood.”

Postulate #4: Independent assortment
“During gamete formation, segregating pairs of unit factors assort independently of each.”


Please right-click on highlighted words to check out their definition:

Basic unit factors in genetics
Allele: One alternative of a pair or group of genes that could occupy a specific position on a chromosome. (e.g. 'A' is the allele of an 'Aa' gene)

Chromosome: A linear strand of DNA harboring many genes.

Gene: A unit of genetic information that occupies a specific position on a chromosome and comes in multiple versions called alleles (e.g. the gene responsible for agouti coloring in rats is 'AA' or 'Aa')

Locus: the specific location of a gene (or DNA sequence) in a chromosome (e.g. the P-locus, which determines whether a rat will have black or pink eyes)

DNA (Deoxyribonucleic acid): nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms

Heterozygous vs. Homozygous
Heterozygous: Having a genotype with two different and distinct alleles for the same trait (e.g. 'Aa')

Homozygous: Having a genotype with two of the same alleles for a trait (e.g. 'AA' or 'aa')

Dominant vs. Recessive
Dominant: an allele that causes a trait to become visible; it masks (or hides) the recessive trait from appearing in its phenotype, and is always written in uppercase letters (e.g. 'AA' or 'Aa')

Recessive: an allele that causes a trait to become visible only in a homozygous genotype, and is always written in lowercase letter (e.g. 'aa', 'bb')

Carrier: an organism that inherited a trait, but does not display the characteristic outwardly (e.g. in the 'Aa' gene, the rat will appear to have agouti color, but also carries the gene for black)

Modifier genes (epistasis): the presence of one gene masks or suppresses the other gene(s) (e.g. any rat that has 'cc' will always be albino, regardless having any other color or marking such as 'AA' or 'aa')

Lethal gene: some genes are said to be lethal when homozygously paired (e.g. the high-white marking on rats is associated with Megacolon; another example is the homozygote Pearl gene 'PePe')

Incomplete (partial dominance) vs. Co-dominance
Incomplete dominance: the phenotype of a rat's heterozygous genotype is an intermediate of the phenotypes of the homozygous genotypes; contribution of both alleles do not overpower each other, but rather, produces a blended or intermediate effect (e.g. the hooded (H) gene in rats)

The difference between these two is that in:

  • Incomplete dominance - we get a blending of the dominant and recessive traits (e.g. white flower x red flower = pink flower)
  • Co-dominance - the dominant and recessive traits appear simultaneously in the hybrid's phenotype (e.g. white flower x red flower = we get a flower that has both red and white petals)

Genotype vs. Phenotype
Genotype: the genetic constitution of an organism (e.g. an agouti rat's genotype is written as 'AA')

Phenotype: the outward or physical appearance of an organism (e.g. "agouti or black colored coat")

Punnett square: a simple diagram used to predict the outcome of a cross of traits


If you're done reading this article, please test your understanding by clicking here.

Klug, W. & Cummings, M. (2002) Essentials of Genetics (4th Edition). Chapter 3: Mendelian Genetics. Prentice-Hall, USA

Incomplete and co-dominance. Retrieved April 15, 2011 from

Ratz. Rat health care & information: Rat genetics. Retrieved April 15, 2011 from

Torigoe, Asano et al. (2010). Genetic analysis of modifiers for the hooded phenotype in the rat. Graduate School of Veterinary Medicine, Hokkaido University, Sapporo Japan. Retrieved April 15, 2011 from

Wikipedia. Dominance (genetics). Retrieved April 15, 2011 from

Wikipedia. Mendelian inheritance. Retrieved April 15, 2011 from
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