What is Homozygous? - Definition, Traits & Example

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  • 0:02 Chromosomes, Genes and…
  • 1:00 Homozygous and Heterozygous
  • 1:30 Dominant and Recessive
  • 2:35 Homozygous and…
  • 3:55 Sex-Linked Traits
  • 4:25 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

Since diploid organisms have two copies of each gene, they can be homozygous or heterozygous at any place on the chromosome. Find out what it means to be homozygous, and get some examples of phenotypes that are caused by being homozygous.

Chromosomes, Genes, and Alleles

As you may know, a diploid organism has two copies of each chromosome, one from its father and one from its mother. Humans have 23 chromosomes: one sex chromosome (X or Y) and 22 other chromosomes (called autosomes). Since we are diploid, these chromosomes all come in pairs, bringing our grand total to 46 chromosomes in each cell's nucleus.

Since we have two copies of each chromosome, we have two copies of each gene. Our genes give rise to traits, or observable characteristics, like height, eye color, freckles, dimples, normal hearing versus deafness, normal vision versus color blindness, and so forth.

In a population of organisms, there are variations in the sequences of genes. Different forms of the same gene are called alleles, and they can lead to different traits. Alleles are commonly represented by letters: for example, for a gene related to the albinism trait, the alleles could be called A and a.

Homozygous and Heterozygous

If an organism has two copies of the same allele, for example AA or aa, it is homozygous for that trait. If the organism has one copy of two different alleles, for example Aa, it is heterozygous.

Keep in mind that an organism can't simply be 'homozygous,' period. For each of the organism's thousands of genes, it is either homozygous or heterozygous at that genetic locus.

Dominant and Recessive

Alleles can be dominant or recessive. A dominant allele takes precedence over a recessive allele. For example, the allele that gives rise to albinism is recessive, and the allele for normal pigment production is dominant. That means that a heterozygous individual with the genotype Aa, who has one copy of the normal allele and one copy of the albinism allele, would not be an albino. This is because since A is dominant, one copy of A is enough to give the normal phenotype. Make sense?

So, in order for an organism to show a recessive trait, it must be homozygous for the recessive allele. In that case, there's no dominant allele there to throw its weight around in terms of phenotype. So, what's the genotype of an albino in our example? It has to be aa, homozygous recessive.

In addition to albinism, some human phenotypes caused by being homozygous recessive for a certain gene include straight hair, attached earlobes, thin lips, congenital deafness, nearsightedness, and grey, green, blue, or hazel eyes.

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