What is Homozygous?

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Image of Homozygous and Heterozygous chromosomes

In order to know about Homozygous, it is necessary to know about the Zygosity. Zygosity refers to the similarity of genes for a trait (inherited characteristic) in an organism. If both the genes are the same, the organism is ‘Homozygous’ for the trait. If both genes are different, the organism is ‘Heterzygous’. If one gene is missing, it is ‘Hemizygous’. If both genes are missing, it is “Nullizygous’. Diploid organisms are organisms having 2 matching sets of chromosomes which have the same genes on each of their 2 sets of chromosomes; except that the sequence of theses genes may differ between the 2 chromosomes in a matching pair and that a few chromosomes may be mismatched as a part of a sex-determining system. The DNA sequence of a gene usually varies from one individual to another. Those variations are called “alleles”. In diploid organisms, one allele is inherited from the male parent and one form the female parent. Zygosity is a description of whether those 2 alleles have identical or different DNA sequences.


What are the different types of Zygose?

The words homozygous, heterozygous and hemizygous are used to describe the genotype of a diploid organism. Diploid organisms reproduce sexually as humans. They are described as below.

  • Homozygous: A cell is known to be homozygous for a particular gene when identical alleles are present on both homologous chromosomes. Homologous chromosomes are similar in shape and content. The cell or organism in question is called “Homozygote”. True breeding organisms are always homozygous for the traits that are to be held constant. An individual that is homozygous dominant for a particular trait carries 2 copies of the allele that codes for the dominant trait. This allele, often called the “dominant allele”, is normally represented by a capital letter (such as “P” for the dominant allele producing purple flowers in pea plants). Dominance in genetics is a relationship between 2 variant forms (alleles) of a single gene, in which one allele masks the expression f the other influencing some trait. When an organism is homozygous dominant for a particular trait, the genotype is represented by a doubling of the symbol for that trait, such as “PP”. Someone who is homozygous dominant for a trait such as brown eyes will have brown-eyed children because only one copy of a dominant trait is needed for the trait to be expressed. 
  • An individual that is homozygous recessive for a particular trait carries 2 copies of the allele that codes for the recessive trait. This allele, often know as the “recessive allele”, is usually represented by lower case form of the letter such as, with the reference to the example above, “p” for the recessive allele producing white flowers in pea plants. The genotype of an organism that is homozygous recessive trait is denoted by a doubling of the letter, such as, “pp”. People with blue eyes are an example of a homozygous recessive. A person with blue eyes will pass the gene down, but his or her child will not necessarily have blue eyes unless the child inherits the blue-eyed gene from the other parent as well.
  • Heterozygous: A diploid organism is heterozygous when its cells contain 2 different alleles of a gene. Heterozygous genotype are represented by an upper case letter (representing the dominant allele) and a lower case letter (representing the recessive allele), such as “Rr” or “Ss”. The capital letter is usually written first.
  • Hemizygous: A chromosome in a diploid is hemizygous when only one copy is present. The cell or organism is known as “hemizygote”. This is observed when one copy of a gene is deleted. Male honey bees are completely hemizygous organisms. They develop from unfertilized eggs and their entire genome is haploid, unlike female honey bees which are diploid.
  • Nullizygous: A nullizygous organism carries 2 mutant alleles for the same gene. The mutant alleles are complete loss-of-function or ‘null’ alleles, so homozygous null and nullizygous are synonyms. The mutant cell or organism is called a nullizygote.


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