A heterozygous in diploid organisms is an organism that has the condition of possessing two different variants of a gene, in contrast to homozygous, that possess two copies of the same gene. Each of these variants of the gene is known as an allele.
A significant number of organisms are diploid; that is, they have two sets of chromosomes in their cells (except for gametes, ovules and sperm, which are haploid, so they only have one set of chromosomes). Humans have 23 pairs of chromosomes, 46 in total: half inherited from the mother and half from the father.
If in each pair of chromosomes both alleles are different for a given gene, the individual is said to be heterozygous for that characteristic.
What is a gene?
Before delving into the concepts and examples of heterozygotes, it is necessary to define what a gene is. A gene is a DNA sequence that codes for some phenotype.
Generally, in eukaryotes, the sequence of genes is interrupted by non-coding sequences called introns.
There are genes that code for messenger RNAs, which in turn produce a sequence of amino acids; that is, proteins.
However, we cannot restrict the concept of gene to sequences that code for proteins, since there are a significant number of genes whose function is regulatory. In fact, the definition of gene is interpreted differentially depending on the area in which it is studied.
Complete dominance
The physical appearance and observable characteristics of an individual are the phenotype of this, while the genetic makeup is the genotype. Logically, the expression of the phenotype is determined by the genotype and by the environmental conditions that exist during development.
If the two genes are the same, the phenotype is the exact reflection of the genotype. For example, suppose that eye color is determined by a single gene. Surely this character is influenced by a greater number of genes, but we will simplify the matter for didactic purposes.
The "A" gene is dominant and is related to brown eyes, while the "a" gene is recessive and is related to blue eyes.
If the genotype of a certain individual is "AA", the expressed phenotype will be brown eyes. Similarly, the "aa" genotype will express a blue-eyed phenotype. Since we are talking about diploid organisms, there are two copies of the gene related to eye color.
However, when studying the heterozygote, things get complicated. The genotype of the heterozygote is "Aa", the "A" may come from the mother and the "a" from the father or vice versa.
If dominance is complete, the dominant "A" allele will completely mask the recessive "a" allele and the heterozygous individual will have brown eyes, just like the dominant homozygous. In other words, the presence of the recessive allele is irrelevant.
Incomplete dominance
In this case, the presence of the recessive allele in the heterozygote does play a role in the expression of the phenotype.
When dominance is incomplete, the phenotype of the individual is intermediate between the homozygous dominant and the homozygous recessive.
If we go back to our hypothetical example of eye color and assume that the dominance of these genes is incomplete, the individual with the "Aa" genotype would have eyes between blue and brown.
A real example from nature is that of the flowers of the Antirrhinum genus plant. They are known as snapdragon or dragon head. These plants produce white ("BB") and red ("bb") flowers.
If we carry out an experimental cross between white flowers and red flowers, we will obtain pink flowers ("Bb"). The crossing of these pink flowers results in a variety of red, pink and white flowers.
This phenomenon occurs because the "A" allele produces red pigment and the "b" allele produces white pigment. In the heterozygous individual, half of the flower pigment is produced by the "A" allele and the other half by the "a" allele, generating a pink flower phenotype. Note that the phenotype is intermediate.
It is necessary to mention that, although the phenotype is intermediate, it does not mean that the genes "mix". Genes are discrete particles that do not mix and are passed intact from one generation to the next. What is mixed is the product of the gene, the pigment; for this reason the flowers are pink.
Codominance
In the phenomenon of codominance, both alleles are expressed equally in the phenotype. Therefore, the heterozygous is no longer an intermediate between the dominant and recessive homozygous, as we saw in the previous case, but includes the phenotypes of the homozygous.
The most studied example is that of blood groups, denoted by the letters MN. This gene codes for proteins (antigens) expressed on red blood cells.
Unlike the famous ABO or Rh antigens, which produce important immune reactions if the body detects a non-self, MN antigens do not generate these reactions.
The L M allele codes for the M antigen and the L N codes for the N antigen. An individual with the L M L M genotype produces exclusively M antigens on his erythrocytes and belongs to the M blood group.
Similarly, organisms with the L N L N genotype only have the N antigen and that is their blood group. In the case of the heterozygote, L M L N expresses the two antigens equally and the blood group is MN.
Barr corpuscles
In the case of sex chromosomes, females have XX sex chromosomes and males XY.
In mammals, one of the X chromosomes is inactivated in development and condenses into a structure known as a body or Barr's corpuscle. The genes in this body are not expressed.
The choice of the X chromosome that is inactive is totally random. Therefore, female heterozygous mammals are said to be a mosaic, in which some cells express an X chromosome from the father and in other cells the X chromosome from the mother is silenced and active.
References
- Campbell, NA, & Reece, JB (2007). Biology. Panamerican Medical Ed.
- Curtis, H., & Schnek, A. (2006). Invitation to Biology. Panamerican Medical Ed.
- Lewin, B. (1993). Genes Volume 1. Reverte.
- Pierce, BA (2009). Genetics: A conceptual approach. Panamerican Medical Ed.
- Starr, C., Taggart, R., Evers, C., & Starr, L. (2015). Biology: The unity and diversity of life. Nelson Education.