Determining which gene is more dominant is a complex topic in genetics. There are several factors that influence whether one gene will be expressed over another gene when both are present in an organism. In this article, we will explore the basics of genetic dominance, look at some examples of dominant and recessive genes, discuss what makes one gene more dominant over another, and explain how complete dominance, incomplete dominance, and co-dominance work.
What is Genetic Dominance?
Genetic dominance refers to the relationship between two versions (alleles) of a gene, in which one allele masks or overrides the expression of the other. The dominant allele is expressed phenotypically (in the organism’s traits) while the recessive allele is masked and not expressed. This occurs because the dominant allele codes for a functioning protein while the recessive allele may have a mutation that leads to a non-functional protein.
In diploid organisms like humans that have two copies of each gene, one inherited from each parent, the expression of traits is determined by the two alleles. If the two alleles are the same, the organism is homozygous for that gene. If the alleles are different, the organism is heterozygous. When an organism is heterozygous for a gene, the dominant allele will be expressed while the recessive allele will be masked.
Examples of Dominant and Recessive Genes
Here are some common examples of dominant and recessive genes:
- Height: The allele for tall stature (T) is dominant over the allele for short stature (t). Someone who is TT or Tt will be tall while someone tt will be short.
- Widow’s peak: The allele for widow’s peak hairline (W) is dominant over no widow’s peak (w). WW or Ww individuals will have a widow’s peak while ww will not.
- Dimples: The allele for dimples (D) is dominant over no dimples (d). DD or Dd people will have dimples while dd people will not.
- Blood type: The IA and IB alleles that confer blood types A and B are co-dominant over the i allele that confers blood type O. IAIA and IAi individuals have type A, IBIB and IBi have type B, and ii have type O.
- Sickle cell anemia: The allele for normal hemoglobin (HbA) is dominant over the allele for sickle hemoglobin (HbS). People with the genotype HbAHbA or HbAHbS will not have the disease while people HbSHbS will.
As you can see, in each case the dominant trait is expressed when the dominant allele is present, even if only one copy is present. The recessive trait is only seen when two copies of the recessive allele are present.
What Makes a Gene Dominant?
There are a few key factors that determine whether one allele is dominant over another:
- Protein function: If one allele codes for a functional protein and the other codes for a nonfunctional or less efficient protein, the functional allele is almost always dominant. This is because one functional copy of the gene is enough to produce sufficient protein for the trait to be expressed.
- Transcription factors: Dominant alleles often code for transcription factors that can activate gene expression on their own. Recessive alleles may lack this ability.
- Toxin genes: Some dominant alleles code for products that are outright toxic and dangerous if overexpressed. Having two copies leads to serious diseases.
- Haplosufficiency: For some genes, having one functional copy is not enough to produce enough protein for normal function. In these cases, the allele that leads to sufficient protein production tends to be dominant.
Essentially, dominance comes down to the allele that leads to a functional, advantageous trait being expressed. This helps preserve useful traits in a population despite the presence of nonfunctional recessive alleles.
Types of Genetic Dominance
There are three main patterns of genetic dominance that can occur between two alleles:
Complete Dominance
In complete dominance, one allele is fully dominant and the other is fully recessive. The dominant trait is observed any time the dominant allele is present. An example is the dominance of the allele for brown eyes (B) over the allele for blue eyes (b).
Genotype | Phenotype |
---|---|
BB | Brown eyes |
Bb | Brown eyes |
bb | Blue eyes |
As seen, anyone with a B allele will have brown eyes. Both alleles must be recessive for the blue eye trait to show.
Incomplete Dominance
With incomplete dominance, the heterozygous genotype leads to a phenotype that is a blend of the two homozygous phenotypes. Neither allele is completely dominant over the other. For example, in snapdragons the allele for red flowers (R) and white flowers (W) show incomplete dominance.
Genotype | Phenotype |
---|---|
RR | Red flowers |
RW | Pink flowers |
WW | White flowers |
The heterozygote RW has a blended phenotype of pink flowers. Neither red nor white is completely dominant.
Co-dominance
In co-dominance, both alleles are equally expressed in the heterozygote. A good example is the ABO blood types in humans mentioned earlier. Someone who is genotype IAIB will have type AB blood that expresses both the A and B alleles equally.
Genotype | Phenotype |
---|---|
IAIA or IAi | Type A blood |
IBIB or IBi | Type B blood |
IAIB | Type AB blood |
ii | Type O blood |
Neither the A or B allele is dominant over the other. Instead, they are expressed together when both are present.
Conclusion
Determining genetic dominance is complex and depends on the specific alleles and genes involved. In general, alleles that code for functional proteins, transcription factors, and other beneficial products tend to be genetically dominant. Complete dominance, incomplete dominance, and co-dominance describe the potential relationships between two alleles. Understanding how alleles interact provides insight into how genetic traits are expressed and inherited.