What difference homozygous and heterozygous?

Homozygosity and heterozygosity are two terms used in genetics to describe the genotype of a diploid organism at a particular locus or gene. As it refers to a specific locus, an organism can be homozygous for some genes and heterozygous for others.

In diploid organisms, such as humans, each cell has two homologous chromosomal copies, and therefore two copies of each gene. Each copy of the same gene is called an allele and appears in the same position on its respective chromosome, this position is known as a locus. For example, human cells have 23 pairs of chromosomes, a total of 46.

Alleles can exist in multiple forms and a diploid organism, having two copies, may have the two identical copies or not, that is, each allele of the same genetic locus may present a different form. For example, an individual may have both the allele for blue eyes and the allele for brown eyes, one on each chromosome, or it may have both alleles for blue eyes.

When the degree of similarity between the copies or alleles of a gene is classified, the possible combinations give rise to several categories:

  • homozygous: the two alleles are the same.
  • heterozygous: each allele is different.
  • hemizygous: there is only one copy of the gene, that is, a single allele. This can often be seen in genes linked to sex chromosomes. For example, almost all genes linked to the X chromosome are hemizygous.
  • Nullizygote: Both alleles are nonfunctional mutations.

Homozygous

Homozygosity describes a locus of a diploid genotype in which the two alleles are the same or identical, one on one chromosome and one on the homologous chromosome. Homozygous alleles can be either dominant or recessive:

1.- Homozygous dominant: The allele combination contains two dominant alleles and therefore expresses the dominant phenotype. Dominant alleles are usually represented by capital letters, for example the letter "P" for the dominant allele that gives rise to the phenotype with purple flowers in pea plants. The genotype of a homozygous dominant would be represented as “PP”, since it has two P alleles.

2.- Homozygous recessive: contains two recessive alleles and expresses the recessive phenotype. Recessive alleles are not expressed if there are dominant alleles, so for an individual to express the phenotype associated with recessive alleles, the genotype must be with the two alleles equal and recessive, that is, homozygous recessive. Recessive alleles are represented by lowercase letters. For example, the recessive 'p' allele codes for white flowers in the pea plant, and plants with the 'pp' genotype will show the phenotype with white flowers.

heterozygosity

In the two previous cases, the homozygotes express a phenotype, since they have the same two alleles. If the two alleles are different, the phenotype expressed depends on the exact combination of alleles that is present, with dominant alleles being expressed over recessive alleles.

Continuing with the example of pea plants, a heterozygous “Pp” genotype will give rise to plants with purple flowers, since the P allele is dominant and carries the genetic information to produce purple flowers.

Generally, there are more than two forms of the same allele, which makes the possible combinations of alleles in heterozygous individuals enormous. While homozygotes always display characteristics of a particular phenotype, heterozygotes can display a wide variety of phenotypes.

In addition, the dominance of some alleles over others can occur to different degrees:

  • complete dominance: the phenotype of the heterozygous individual corresponds to the expression of the dominant allele that completely masks the expression of the recessive allele.
  • incomplete dominance: the phenotype is a mixture of the expression of the dominant allele and the recessive allele, with predominance of the characteristic that gives the dominant allele.
  • co-dominance: the phenotype is a mixture of the expression of two alleles without a predominance of one over the other being observed.

Homozygous and heterozygous mutations

In the case of mutations, if the mutation is identical and occurs in the two alleles of the same locus, that is, it affects the same allele in the two homologous chromosomes, it is said to be a homozygous or homozygous mutation.

If a mutation affects only one of the alleles, it is said to be a heterozygous mutation. In heterozygous mutations, the mutation is expressed as a function of the dominance or recessivity of the affected allele.

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