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Genotype

genotype definition

genotype
n., ˈdʒiːn.oʊ.taɪp
The genetic makeup of a cell, an organism, or an individual that contributes to its trait or phenotype

The characteristics or traits of an organism may be elucidated using the terms genotype and phenotype. What does genotype mean? A genotype pertains to the entire set of genes of an organism (or a taxon). Genotype and phenotype are related in a way that the genotype refers to all the genes of an organism and when they are expressed they will determine its phenotype. In simpler words, the genotype is the genetic contributor to the phenotype of that organism. However, the genotype is not the only factor that will determine its phenotype. Other factors, such as environmental factors and random variation, have a role as well. The genotype interacts with these factors to define the distinctive phenotype of an organism.

Genotype Definition

What is a genotype? In biology and genetics, the term genotype refers to the entire genetic makeup of a cell, an organism, or an individual. Etymologically, it came from the Greek “genos”, meaning “race” and Latin “typus”, meaning “type”. The word “genotypic” pertains to, relates to, or characterizes genotype. For example, a genotypic ratio is a ratio that describes the number of times a genotype would appear in the offspring after a test cross. For the actual genotypic ratio, see the section: Genotype Examples.

Compare: phenotype.
Sentence example:
Your physical attributes and behavior are a result of “nature”, which is your genotype, and “nurture”, which pertains to your environment and life experiences.

Genotype Types and Characteristics

In humans, the gene for a particular character or trait may exist in pairs called alleles. The paired alleles will occupy the same spot on a chromosome (called a locus). They will also control the same trait. They are paired because one of them comes from the mother (via the ovum) and the other, from the father (via the sperm cell that fertilized the ovum). The ovum and the sperm cell are haploid cells. Hence, their union (fertilization) shall produce a diploid cell (called a zygote) that will eventually grow and develop as a new human being. The offspring shall subsequently inherit the genes of the parents. Thus, the genes that will determine certain traits of the offspring may occur in allelic pairs.

If the trait follows a Mendelian pattern of inheritance, then one of the alleles will be expressed while the other will not. The allele that is expressed is said to be the dominant allele whereas the allele that is not is described as recessive. In instances wherein the dominant allele is absent, the expression of the recessive allele will occur.

If the dominant allele is labeled as “A” and the recessive allele, “a”, three different genotypes are possible: “AA”, “aa”, and “Aa”. The term “homozygous” is used to describe the pairs “AA” and “aa” because the alleles in the pair are the same, i.e. both dominant and both recessive, respectively. In contrast, the term “heterozygous” is used to describe the allelic pair, “Aa”. Because the dominant allele constitutes the pairs, “AA” and “Aa”, the dominant trait will manifest in the organism’s phenotype. As for the allelic constitution of aa, the dominant allele is absent. This means that the recessive trait will be expressed.

Not all traits will follow a Mendelian pattern of inheritance. Codominance, incomplete dominance, and polygenic inheritance are examples of a non-Mendelian type of inheritance. In humans, many of the observable traits are non-Mendelian. The height and skin color, for example, are brought about by the interactions of not just a pair of alleles but of various alleles.

Genotype vs Genome

Both genotype and genome pertain to the genetic makeup of an organism. Genome, however, is a more inclusive concept. It is defined as the total genetic composition of an organism. This definition seems applicable to genotype as well (refer to the definition above). Nevertheless, the difference between genome and genotype lies in the scope and the intent of use. As for the scope, “genome” is used to pertain to all the genetic material and that means it includes not just the coding nuclear DNA (called genes) but also the non-coding nuclear DNA and the extranuclear genetic material (e.g. mtDNA and cpDNA). Comparative genomics, in particular, is a science field concerned chiefly on determining, analyzing, and comparing genomes of different species. (Ref. 1) The scientific data on the genomes already sequenced can be viewed in the National Human Genome Research Institute (NHGRI) website, genome.gov. They are able to sequence the complete genomes of hundreds of animals and plants, including the human genome, which they completed sequencing in 2003. (Ref. 1)

Studies on genotype are concerned primarily with determining the interactions and the expression of the genes so as to explain the genetic attribute of a trait or the phenotype of an organism. Thus, it is sometimes used in a broad sense where it entails the entire genes responsible for the phenotype. At other times, it is used in a narrow sense where it focuses on the specific gene(s) responsible for a particular trait. For instance, the genotype for a specific trait (e.g. flower color) can be represented by just one pair of alleles.

Genotype vs Allele

As pointed out earlier, alleles are the genetic variants for a particular trait. They may occur in pairs or constitute a set of multiple alleles. The interaction between or among alleles will genetically determine the trait. Each allele carries a specific DNA sequence that when expressed will manifest as an observable characteristic. The alleles may be the same (as in the case of homozygous) or different (as in the case of heterozygous). In essence, the alleles represent the genotype of a specific gene. (Ref. 2)

Genotype Examples

Genotype in the broad sense

What is the genotype of the man? According to the NHGRI’s Human Genome Project, humans are estimated to have about 20,000 to 25,000 genes. (Ref. 3) The genotype can be inherited and passed down to the next generation. Accordingly, the genes of humans do not vary much. Only less than 1% of the total genes vary between them. The slight genetic difference accounts for the unique physical features of a person. (Ref. 3)

Genotype in the narrow sense

As mentioned above, the genotype in a narrow sense pertains to the pair or the set of alleles that determine a particular trait. For example, the genotype for the gene that determines the color of the pea flower is represented by two alleles, “B” and “b”. “B” is the dominant allele whereas “b” is the recessive allele. The dominant allele codes for the purple flower trait whereas the recessive allele codes for the white flower trait. See the chart picture below.

This diagram is an example of a Punnett square. It shows the relation between the genotype and the phenotype. The flower images on the left side and at the top represent the flower-color traits of the female plant parent and the male plant parent, respectively. Both male and female parents have a heterozygous dominant genotype, “Bb”, and therefore produce purple flowers. However, a test cross between them shows that not all their offspring will produce similar purple flowers. The chart shows that out of four, one of them will bear white flowers. This example shows that manifesting a trait that deviates from the parents and the siblings is a normal occurrence. This means that the progeny that produces white flowers inherited a pair of alleles that are both recessive.

The Punnett square is an essential tool in genetics to predict inheritance patterns and ratios. Based on the above example, of the four offspring, three possess the purple-flower trait (“BB” and “Bb” genotypes) and one possesses the white-flower trait (“bb” genotype). As for the genes, the possible genotypes from the cross are as follows: one “BB”, two “Bb”, and one “bb”. Given this, the genotypic ratio is 1:2:1 whereas the phenotypic ratio is 3:1. The genotypic ratio refers to the number of times a genotype would appear in the offspring after a test cross whereas the phenotypic ratio refers to the number of the offspring expressing the traits based on the genotypes. For more info and description on the phenotypic ratio, read here.

See Also

References

    1. Comparative Genomics Fact Sheet. (2019). Genome.Gov. https://www.genome.gov/about-genomics/fact-sheets/Comparative-Genomics-Fact-Sheet
    2. genotype | Learn Science at Scitable. (2014). Nature.Com. https://www.nature.com/scitable/definition/genotype-234/
    3. Genetics Home Reference. (2020). What is a gene? Genetics Home Reference. https://ghr.nlm.nih.gov/primer/basics/gene

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