Dominance for Autosomal Genes

 

As previously stated,  dominance relationships can only be inferred from heterozygous individuals.  In genetic laboratories such investigations start out with homozygous parental lines.

Experimental Note: In many settings it is extremely difficult to obtain homozygous lines.  In such cases, an experienced geneticist will carry out a number of different crosses to determine the dominance patterns.

 

Example: Consider two alleles of a gene (the alleles are A1 and A2). 

You have two homozygous parental lines with the genotypes  A1 A1 and A2 A2 .

The homozygote A1 A1 has a unique recognizable phenotype we will call ONE.  Similarly the homozygote A2 A2 has a unique recognizable phenotype we will call TWO.  

At this point, we know nothing about dominance relationships.  For purpose of this example, we will assume that this is an autosomal gene.

By the appropriate F1 cross we "construct"  a heterozygote with genotype A1 A2 . (Note: Since this gene is known to be autosomal we do not need to do a reciprocal F1 cross as well).

 The phenotypes of the heterozygote should be uniform (Mendel's Principle of Uniformity

There are five possible outcomes of this F1 cross

1.  The phenotype of the heterozygote is ONE. Complete dominance
     
2. The phenotype of the heterozygote is TWO. Complete dominance
     
3. The phenotype of the heterozygote is unique and appears to be somewhere between the phenotypes of ONE and TWO (e.g., it has phenotype of ONE AND THREE-QUARTERS. Incomplete dominance
     
4. The phenotype of the heterozygote is unique and appears to be more extreme than either ONE or TWO  (e.g., it has phenotype of FOUR) Overdominance
     
5. The heterozygote has the phenotypes of ONE and TWO simultaneously. Codominance
 

Complete Dominance: 

In situation 1, the A1 allele would be completely dominant to the A2 allele.   In situation 2, the A2 allele would be completely dominant to the A1 allele. 

With complete dominance,  the phenotype of the heterozygote is indistinguishable from the phenotype of one the parental lines.

Complete dominance refers to the parental phenotype that occurs in the F1 generation (that is, in heterozygotes). The term recessive phenotype is essentially the parental phenotype that was not dominant.  More formally, the recessive phenotype or trait is the phenotype that disappears in the F1 generation and then reappears in the F2 generation.

Return to 5 outcomes

Incomplete dominance, partial dominance and semidominance (sometimes even called  blending inheritance).

These three terms all refer to the same thing.  With incomplete dominance, the phenotype of the heterozygote is intermediate in appearance to the phenotypes of the two homozygous parental lines.  That is, the parental homozygotes  have the "extreme" phenotypes and the heterozygote has the intermediate phenotype.

Mendel knew about incomplete dominance although he did not study it in this original paper.

"With some of the more striking characters, those, for instance, which relate to the form and size of the leaves, the pubescence of the several parts, etc., the intermediate, indeed, is nearly always to be seen..." (Mendel 1865)

Example   There are  two homozygous lines of Four O'clock flowers:  red and white.  A heterozygous Four    O'clock has an intermediate pink phenotype (a blend of red and white).
Return to 5 outcomes

 OverdominanceThe heterozygote has a more extreme phenotype than either of the homozygous parents. Overdominance might refer to a simple phenotype (such as number of wing spots) but very often refers to survival or reproductive fitness.

Mendel described a case of overdominance in his original paper with the trait of stem length.  Mendel, however, called stem length completely dominant and attributed the more extreme form of the heterozygous phenotype to "hybrid vigor".

"With regard to this last character it must be stated that the longer of the two parental stems is usually exceeded by the hybrid, a fact which is possibly only attributable to the greater luxuriance which appears in all parts of plants when stems of very different lengths are crossed. Thus, for instance, in repeated experiments, stems of 1 ft. and 6 ft. in length yielded without exception hybrids which varied in length between 6 ft. and 7½ ft"

Example Warfarin is a rat poison that prevent coagulation.  Rats eat Warfarin and ultimately die of internal bleeding.  Warfarin prevents Vitamin K from interacting with clotting proteins.  This thins the capillaries and promote hemorrhaging.

Some rats have a single gene mutation that makes them resistant to Warfarin that is inherited as autosomal trait.  

The resistance allele prevents Warfarin from interacting with Vitamin K so that rats do not hemorrhage after eating Warfarin.  However, rats homozygous for the Warfarin resistance allele have great difficulty  absorbing Vitamin K from their food. These rats tend to die of Vitamin K deficiency.

Heterozygotes for the Warfarin resistance allele are still resistant to Warfarin but only need 2 or 3 times the normal dietary Vitamin K to stay healthy.  These heterozygotes are overdominant for survival.

Summary:

At this locus there is a resistance allele (A1) and a wildtype non-resistance allele (A2 )

  • A2A2 homozygotes are killed by Warfarin.

  • A1A1 homozygotes are resistant by Warfarin but die due to Vitamin K deficiency. 

  • A1 A2 heterozygotes  are resistant to Warfarin poisoning and generally get enough additional vitamin K to stay healthy. 

 
Return to 5 outcomes

Codominance.

Codominance is perhaps the hardest dominance concept to understand.  With codominance, the heterozygote has the phenotype of both of the homozygous parental lines simultaneously.

At the most basic biochemical level, all structural genes are codominant.  Each gene codes for a protein.  Different alleles of that gene code for slightly different proteins.  A heterozygous individual would make both types of proteins. That is, the heterozygote has both parental phenotypes.

Roan color is cattle is often described as a codominant traits. It is possible to get homozygous red cattle and it is possible to get homozygous white cattle.  When these cattle are crossed to make a heterozygote, you get a cow that has both red and white hair patches. That is, both red and white hair (the parental phenotypes) are present.  This is unlike the Four O'Clock flowers where the color is exactly intermediate to the parents (half the amount of red pigment).

Return to 5 outcomes
Test your Understanding of Dominance for Autosomal Traits


 

Basic Dominance Issues

1 Naming of Dominance Relationships
2 Dominance for Autosomal Genes
3 Fixing a New Dominant Mutation
4 Some Dominance Practice Problems.
5. Review the terms homozygous, heterozyogus and hemizygous
  Home