Wednesday, April 20, 2011

Mendel's postulates explained


Source

This is a continuation "Introduction to Mendelian Genetics"
After reading this article, you can go to Rattus Genetics Quiz #2.






Gregor Mendel experimented on the hybridization of garden peas (Pisum sativum), and after he saw consistent results from the tests, he came up with four postulates regarding heredity.



Postulate #1: Unit factors are formed in pairs

“Genetic factors are controlled by unit factors that exist in pairs.”


These unit factors are called ‘genes’ which are formed by a pair of two alternative alleles. The gene for an agouti colored rat is ‘AA’ or ‘Aa’, an albino rat’s gene is ‘cc’.





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.”


Dominant traits are always written in uppercase (such as AA or Aa for agouti). Recessive traits on the other hand are always written in lowercase (e.g. aa for non-agouti or black).

Dominant traits always suppress the recessive from appearing. Example, agouti (AA) is dominant over non-agouti or black (aa). When the parents produce a heterozygous offspring which has a genotype of ‘Aa’, the dominant agouti will be the offspring’s phenotype.

There are some genes, like the hooded locus (H) which appears to be partially dominant.





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.”


Example: (See table below)

Rat A has a genotype of AA (agouti).
Rat B has a genotype of aa (black).

One allele (one pair of the gene) from each parent will separate and combine with the other.

So, AA from Rat A will split into ‘A’ and ‘A’;
‘aa’ from Rat B will split into ‘a’ and ‘a’.

Each of the allele from Rat A will combine with the allele from Rat B.

All four combinations result in ‘Aa’.


a
a
A
Aa
Aa
A
Aa
Aa




Postulate #4: Independent assortment

“During gamete formation, segregating pairs of unit factors assort independently of each.”



Example: (See table below)

Rat A has standard ears ‘DuDu’ (dominant).
Rat B has dumbo ears ‘dudu’ (recessive), and pink eyes 'pp'.

All of the offspring will have standard ears but will carry dumbo. The probability of an offspring having standard or dumbo ear will not be affected by the presence of a pink eye gene (pp).


du
du
Du
Dudu
Dudu
Du
Dudu
Dudu









If you're ready, please take the short quiz below

Bibliography
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 http://www.hobart.k12.in.us/jkousen/Biology/inccodom.htm

Ratz. Rat health care & information: Rat genetics. Retrieved April 15, 2011 from http://www.ratz.co.uk/genetics.html

Wikipedia. Dominance (genetics). Retrieved April 15, 2011 from http://en.wikipedia.org/wiki/Dominance_(genetics) 

Wikipedia. Mendelian inheritance. Retrieved April 15, 2011 from http://en.wikipedia.org/wiki/Mendelian_inheritance 

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