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all pictures and artwork on this web site are the exclusive property of Joe Pociask Pythons 2002, 2003, 2004

 

Simple Mendelian Inheritance of a Recessive Trait

     This page was created to help explain how certain traits, such as albinism (the lack of production of the dark pigment melanin), are inherited in ball pythons.  In order to develop a good understanding of this subject, one must begin at the beginning.
     Many physical  (and even behavioral) characteristics in living things are controlled by genes.   In plants and animals, genes are located on the chromosomes.  Chromosomes are long, but compact, coiled strands of DNA (deoxyribonucleic acid).  In animals, the chromosomes exist in pairs.  We get one half of the pair from our fathers, and one half of the pair from our mothers.
     Oftentimes, genes can exist in two or more different forms, called alleles.  Many of these alleles originate from mutations (mistakes in the DNA).  For instance, in ball pythons, there are two alleles for the gene that controls the production of melanin:  One that allows melanin to be produced normally, and one that does not.  Later, when we address how albinism is inherited in ball pythons, we will use the capital letter "A" to represent the allele for normal melanin production and the lowercase letter "a" to represent the albino allele.
     Remember that alleles for a given trait exist in pairs, one on the chromosome
inherited from the father and one on the chromosome inherited from the mother.  In the case of albinism, and many other traits in ball pythons, one allele is "stronger" than the other.  This stronger allele masks the weaker one.  We call this stronger allele the dominant allele, and the weaker allele the recessive allele.  Because the dominant allele masks the recessive one, only one copy of that allele need be present for that trait to be shown.  In ball python albinism, the dominant allele is the one responsible for normal coloration.  The allele for albinism is recessive.
     An animal's combination of alleles is known as its genotype.  An animal's physical appearance, brought about by its genotype, is known as its phenotype.  With regards to albinism, there are three possible genotypes and two possible phenotypes:

 
genotypes phenotypes
2 copies of the dominant allele (AA) Normal coloration
2 copies of the recessive allele (aa) Albino coloration
1 copy of each allele (Aa) Normal coloration  (because normal is dominant)

     An organism that has two copies of the same allele is said to be homozygous (either homozygous dominant or homozygous recessive).  An organism that has one copy of each different allele is said to be heterozygous.  Heterozygous individuals are commonly called "hets".  When sex cells, or gametes, are formed, the parent can pass on either copy of its pair of alleles.  This process is completely random.  Therefore, a homozygous dominant individual (AA) can pass on either copy of the normal allele.  Likewise, a homozygous recessive individual can pass on either copy of the albino allele.  A heterozygous individual, or "het" (Aa), can pass either its normal or albino allele to its offspring.  Geneticists use a simple diagram called a Punnett square to illustrate what the potential offspring of a given mating might be.  Punnett squares can only tell us the probability of any one offspring possessing a specific genotype or phenotype.  Below you will find Punnett squares that can be used to predict the outcome of given matings:

 

The Punnett square below shows the mating of an albino (aa) to a normal (AA) ball python.
 
   Mom:  Albino (aa)          

possible allele
passed to offspring:
        a                     or                              a
 

Dad:  Normal (AA)


 

                    possible allele
             passed to offspring:
 


 


        A     

 
 or


A


Aa

Normal coloration, het. for albinism


Aa

Normal coloration, het. for albinism


Aa

Normal coloration, het. for albinism


Aa

Normal coloration, het. for albinism

*In this case, each offspring has a 4 in 4 (100%) chance of being heterozygous for albinism

 



The Punnett square below shows the mating of an albino (aa) to a heterozygous for albino ball python (Aa).
 
   Mom:  Albino (aa)          

possible allele
passed to offspring:
        a                     or                              a
 

Dad:
 Het Albino (Aa)


 

                    possible allele
             passed to offspring:
 


 


        A     

 
 or


a


Aa

Normal coloration, het. for albinism


Aa

Normal coloration, het. for albinism


aa

Albino


aa

Albino

*In this case, each offspring has a 2 in 4 (50%) chance of being an albino, and a 2 in 4 (50%) chance of being heterozygous for albinism.  Note:  It is not appropriate to assume that 50% of the offspring from this cross will be albinos!  We can only say that each egg will have a 50% chance of containing an albino.  Each egg is an individual "flip of the coin".  If you flip a coin ten times, and it lands on heads ten times, the eleventh toss still has a 50% chance of landing on heads!

 



The Punnett square below shows the mating of two heterozygous for albino (Aa) ball pythons.
 
   Mom:  Het Albino (Aa)          

possible allele
passed to offspring:
        A                     or                              a
 

Dad:
 Het Albino (Aa)


 

                    possible allele
             passed to offspring:
 


 


        A     

 
 or


a


AA

Normal coloration, Normal genotype


Aa

Normal coloration, het. for albinism


Aa


Normal coloration, het. for albinism


aa

Albino

*In this case, each offspring has a 1 in 4 (25%) chance of being an albino, a 1 in 4 (25%) chance of having a normal appearance and genotype, and a 2 in 4 (50%) chance of being het for albino.  Note:  You can not tell the difference between the completely normal offspring and the het albinos.  Together, the normal looking offspring from this cross are sometimes called "66% hets".  When you purchase these 66% hets, you may be buying a het albino or you may be buying a normal snake.

 

 

The Punnett square below shows the mating of a heterozygous for albino (Aa) ball python and a normal ball python (AA).

   Mom:  Normal (AA)          

possible allele
passed to offspring:
        A                     or                              A
 

Dad:
 Het Albino (Aa)


 

                    possible allele
             passed to offspring:
 


 


        A     

 
 or


a


AA

Normal coloration, Normal genotype


AA

Normal coloration, Normal genotype


Aa

Normal coloration, het. for albinism


Aa

Normal coloration, het. for albinism

*In this case, each offspring has a 2 in 4 (50%) chance of being het for albino, and a 2 in 4 (50%) chance of having a normal appearance and genotype. Note:  You can not tell the difference between the completely normal offspring and the het albinos.  Together, the normal looking offspring from this cross are sometimes called "50% hets".  When you purchase these 50% hets, you may be buying a het albino or you may be buying a normal snake.

 

While the simple recessive inheritance pattern regulates albinism, axanthism, piebaldism, and many other ball python appearances or "morphs", some traits are controlled by one of several other types of inheritance.  Click to learn more about the inheritance of co-dominant and dominant traits in ball pythons.


 

 


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