What is Genotype?
Genotype is a word that is used commonly, but not easily understood because of its accompanying vocabularies when defining it. Truly, the work of Gregory Mendel with the pea plant began to open people’s understanding of genes. He experimented on Pisum sativum (pea plant), in order to determine or know the characteristics that they pass down to the next generation. On a whole, it can be said that all organisms, be it a member of the plant or animal kingdom, is a by-product of their genetic makeup and environment. Therefore, a whole new way or dimension will be taken in dissecting what is genotype. In addition, examples will be used to illustrate what, and how it works in real life. Well before a definition is given, identification and definition of keywords have to be made.
Definition of Allele and other Keywords in Genotype
Allele; These are variant forms of genes that occur at a position on the chromosomes
Trait; This is a distinguished genetic characteristic of an organism
Dominant Allele; This is a variant of a gene that surpasses another allele in the production of phenotypic characteristics.
Recessive Allele; This is a variant of a gene that is overshadowed by another allele in the expression of phenotypic characteristics.
Locus; This is the location or position on a chromosome that an allele can be found.
Homozygous Alleles; These are the same copies of alleles, either dominant or recessive. For example, (HH or rr)
Heterozygous Alleles; These are different copies of alleles, with one being dominant, and the other recessive. For example, (Hr)
Now that some keywords pertaining to genotype has been defined, we can now move forward with understanding genotype and how it relates to us.
A genotype is an array, assemblage, or combination of genes that are responsible for the many genetic traits of an organism. When genotype is mentioned, it is referring to the organism’s gene, not traits. That is the organism’s raw information in the DNA.
Furthermore, it is an expression of an organism’s genetic information that has been passed down from its parent. Hence, it can be said that genotypes are inherited. Also, it can be the information carried by alleles in an organism’s DNA. Therefore, genetic information or genotype is determined by the makeup of the alleles. In addition, an allele can be homozygous, that is, having a pair of dominant or recessive genes, or heterozygous, that is having both dominant and recessive genes in its paring.
On the whole, the genotype is the unit of hereditary information that cannot be seen by simple physical observation, except through biological testing.
It is difficult to give a specific example pertaining to genotype. This is simply because of the many genes that are found in an organism that cannot be quantified. Therefore an example will be given with respect to the human eye color gene. To illustrate this, a representation of the alleles will be, capital B for the dominant brown eye color allele and small letter b for recessive blue eye color allele. as shown below;
The result is BB, Bb, bB, and bb. Meaning that the probability of the offspring inheriting the genes for blue eyes is 1 out of 4. That is when both parents are heterozygous.
The result is Bb, Bb, bb, and bb. This means that the probability of the offspring inheriting the genes for blue eyes is 2 out of 4 when one parent is heterozygous and the other is homozygous recessive.
The result is Bb, Bb, Bb, and Bb. This shows that the probability of an offspring inheriting the genes for blue eyes is 0 out of 4 when one parent is homozygous dominant and the other is homozygous recessive. But the offspring will be heterozygous.
The result is bb, bb, bb, and bb. The is showing the probability of the offspring inheriting the genes for a blue eye is 4 out of 4 when both parents are homozygous recessive.
The result is BB, BB, BB, and BB. This result shows that the offspring has a 0 probability of inheriting the genes responsible for blue eyes from its parents when both of them are homozygous dominant.
From the above illustration, it can be seen that there are three possible genotypes namely; homozygous dominant BB (co-dominance) and recessive bb and the heterozygous Bb or bB (incomplete dominance) which is carrying the recessive gene.
Genotype cannot be fully understood, alone without explaining phenotype. They both work together because one is a product of the other. Hence, the word “pheno” which means observe, gives us an idea that phenotypes are the observable characters of an organism. Also, these traits can easily be seen and quantified. For example, the height of an organism, hair color, wing length, presence or absence of disease, and flower color. Furthermore, it is fair to say that phenotypes are greatly influenced or affected by the environment of the organism where it develops. Thus an organisms’ phenotype is determined by its genotype and environment.
In addition, phenotype covers a living things’ morphology or physical form and structure. Meaning its biochemical, physiological, developmental, and behavioral processes. To illustrate it mathematically;
Phenotype = genotype (G) + environment (E)
a more advanced equation will be
Phenotype = genotype (G) + environment (E) + genotype and environment (GE)
Furthermore, there are two types of phenotypes, the first is called extreme phenotype which happens when both alleles of the parent produce a hybrid characteristic that differs and surpasses that of the parents. The act of this formation is called transgressive traits. Secondly, there is a recombinant phenotype, and this arises when an individual possesses a phenotype that is different from its parents.
Example of Phenotype
An example of two birds with heterozygous long wing genotypes(Ww) to determine the phenotypes of their offspring.
The above representation shows that the offspring is likely to inherit long wings as there is a 75% chance and a 25% chance to inherit short wings.
Also, there is a 25% chance that the offspring will be extremely phenotypic if it inherits (WW) genotype and another 25% chance that the offspring will be recombinant if it inherits the (ww) genotype.
Differences between Phenotype and Genotype
|Basis for comparison||Genotype||Phenotype|
|consisting of||hereditary characteristics which may be expressed or shown in the next generation or not.||consisting of physical and structural form which are made of developmental, behavioral, biological, and physiological|
|Inherited||They are inherited from parents to offspring||They are not inherited|
|Determined by||They are determined by a biological test such as RFLP and polymerase reaction (PCR)||they are easily observed and determined with the human eye|
|Changes||They remain the same throughout the organisms life||They might change from time to time, like height and hair color|
|Relation||same genotype responsible for the same phenotype, except during heritable mutations||The same phenotype may be expressed by a different genotype or of the same genotype.|
|Environmental factors||They are not affected by the environment||They are affected by the environment|
|Affected by||they are affected by genetic composition, heritable mutations, and sexual reproduction||Affected by only genotype and various environmental conditions|
|Observable||They are present in the chromosome of the organism, and thus cannot be seen with the naked eye||They can be observed by merely looking at an organism because the characteristics are found on the body of the organism|
|Definition||Genetic composition of an individual consisting of genes that can be inherited||Observable characteristics of an individual that results from the interaction of genotype and the environment the individual belongs to|
This is a tool used to graphically represent possible genotypes of offspring from a particular cross-breeding event. Therefore a punnet square is created from the knowledge of the genetic composition of the parent. Furthermore, the combination is in a tabular format. Also, the punnet square was created in the twentieth century years after the experiment of Mendel. Again, it contains possible combinations of the parents’ gamete, with each box representing a fertilization event.
Functions of Punnett square
Firstly, it is used as a tool for marriage and couples counseling. Especially those that have the desire of having kids or children. For instance, a case study of both parents with a carrier for an autosomal recessive disease such as sickle cell anemia. These couples have a 25% chance of transferring both recessive alleles to their child, causing the child to suffer from the illness, and a 50% chance that the offspring be carrying the recessive allele. However, if one of the parents has the disease and the other neither possesses the carrier allele nor suffers from the disease, it will lead to the offspring not developing any sickle cell anemia since it will carry only one abnormal gene or allele.
Secondly, in an experiment of a larger nature, like that conducted by Mendel, using a Punnett square can give an accurate prediction of the genotypic and phenotypic ratios. For example, a true breed short pea plant is cross-fertilized with pollen from a true breed tall pea plant, this will lead to a prediction of 100% tall pea plants. But this generation will be having a heterozygous allele. If a further cross-breeding of the heterozygous alleles is done, it can be predicted that 75% of the new generation will be tall and 25% will be short pea plants. Further analysis will show that one-third of this generation will remain true breeders (TT or tt) and two-third of the breeds will be heterozygous (Tt).
Hence this tool (Punnett square) is therefore used by animal and plant breeders to get a desired trait by cross-breeding the appropriate specimen.
Types of Punnett square
There are basically two types that are commonly used in crossbreeding. The first is called a monohybrid cross square, as it involves single trait crossing. Furthermore, in this type, the boxes are 4 with each box representing an event between the parent gamete. Likewise, the second one is used for analyzing two traits or characteristics and is called the dihybrid cross square. In this type, the boxes are 16 in number since each parent can produce four types of gamete.
Examples of monohybrid and dihybrid cross
In a punnet square, an allele is represented by the dominant allele in this case capital letter “G” for green and then followed by the recessive allele small letter “g” for yellow. The alleles are allowed to independently segregate into gametes are represented in the table below.
In the test cross above, there is a 50-50 prediction that the offspring might be green in color or yellow. that is gG, gG, gg, and gg. It also indicates that the possible genotype of the offspring will either be heterozygous green (gG) or homozygous yellow (gg).
Seed color and plant Height
In this test cross, two traits or characteristics will be analyzed namely, seed color and plant height. Therefore, the alleles that are represented in the table below and subsequently crossed are as follows; GT(green and tall), gT(yellow and tall), Gt(green and short), and gt(yellow and short)
From the above tabular representation, it can be seen that the majority of the offspring will be tall and have green seed color. Also, there is a 1% chance that a new allele combination will appear that is yellow seed color and short plant (ggtt).
Limitation of Punnet square
- It can not be used to understand complex genetic inheritance such as distribution of genotype in the presence of genetic linkage
- It can not accurately predict the distribution of phenotype in humans because of the large number of genes affecting a single phenotype.
- It can not show the result from genes inherited completely from one parent such as the Y- chromosomes.
Genotypic Ratio and Phenotypic Ratio
The genotypic ratio describes the number of times a genotype from a parent will be seen in an offspring after a test cross. On the other hand, The phenotypic ratio describes the relative number of offspring manifesting a singular or particular character or combination of traits after doing a test cross that is based on the genotype of the offspring.
Examples of Genotypic and Phenotypic Ratio
When two purple flowers heterozygous plants(Pp) are cross-bred, the following representation using Punnett square will be as follows;
PP, Pp, Pp, and pp will be the resulting probability of the genotypes the offspring will likely inherit. Thus the genotypic ratio is 1:2:1 meaning 1 homozygous dominant allele(PP), 2 heterozygous alleles(Pp, Pp), and 1 homozygous recessive allele(pp). On the other hand, the phenotypic ratio will be 3:1, meaning 3 out of 4 will be purple in color(PP, Pp, and Pp) and only one will be white in color(pp).
The ratio can also be represented mathematical into percentages. For instance, the genotypic ratio of 1:2:1 can be seen as a 25% chance of Homozygous dominant, 50% chance of heterozygous, and 25% chance of homozygous recessive. Likewise in the case of phenotypic ratio. There is a 75% chance of the flowers being purple in color and a 25% chance of it being white in color.
Genotype in blood group and Haemoglobin
Blood grouping is a phenotypic representation of the genotypes that are involved. Therefore there are over 25 blood groups, but the common and widely recognized grouping systems are that of the ABO and the Rhesus D (Rh D). Thus, in the ABO system, there are 4 possible genotypes namely; A, B, AB, and O. And on the other side of the Rhesus D system there are only 2 namely; Rhesus D negative and Rhesus D positive.
The tabular representation below will show how the paring or crossing brought about the four blood types.
Blood types alleles; IA co-dominant, IB co-dominant, and Io recessive.
|IA||IA IA||IA IB||IA Io|
|IB||IA IB||IB IB||IBIo|
|Io||IA Io||IB Io||Io Io|
|Type A||IAIA IAIo|
|Type B||IBIB IBIo|
The Rhesus D system will fall under any of the four blood types as either positive or negative. It is important to know your blood type in case of an emergency that will require a blood transfusion. Again you can also predict your child’s blood type when both parents know their blood types.
Furthermore, in the aspect of the genotype of the blood haemoglobin, which has been sounded over the years for intending couples to know theirs in other to prevent medical exhaustion when they give birth to a child with an inherited disease. Haemoglobin is the protein that is found in the red blood cells and there are 5 types, AA, AS, AC, SS, and SC. With AC and SC being rare. In addition, the AS and AC haemoglobin are carriers of the sickle cell anemia disease trait, and the SC and SS Haemoglobin are said to have the sickle cell anemia disease. It is therefore advisable that intending couples should know their blood haemoglobin genotypes in other to prevent the passing of the sickle cell allele to their offspring. Below is a chart for the cross-matching of intending couples.
|Partner X||Partner Y||Possible Combination||Remark|
|AA||AA||AA, AA, AA, AA||can marry|
|AA||AS||AA, AS, AA, AS||can marry|
|AS||AS||AA, AS, AS, SS||not to marry|
|SS||AA||AS, AS, AS, AS||can marry|
|SS||SS||SS, SS, SS, SS||not to marry|
|AS||SC||SS, AS, AC, SC||not to marry|
|AS||CC||AC, AC, SC, SC||not to marry|
|AA||SC||AS, AC, AS, AC||can marry|
|AA||CC||AC, AC, AC, AC||can marry|
The above medical genotype chart table for intending couples is a guideline and a tool to predict the haemoglobin your child or children stands to inherit from you as the parents. It is advisable to know that this will help you in planning for the future of your child or children. Note that the major haemoglobin are AA (normal), AS (carrier of the sickle cell trait), and SS (sickler). It is also important to know your genotype before marriage.
To draw the curtain on this, the topic “genotype” is very important in understanding biology. It is what differentiates us as living things. Because it is said that no two living things have the same genetic makeup, even identical twins have different genetic compositions. Hence, groundbreaking studies in this field have helped scientists to understand plants (flora), and animals (fauna) down to their genetic composition that is even to their DNA helix. Furthermore, cross breeders in botany and zoology have produced better and stronger plant breeds to withstand pests and harsh environmental factors. Whereas animal breeders have produced new breeds of dogs and farm animals that can withstand and thrive in a challenging environment. In addition, the combination of different genotypes to form a new one has led to an increase in biodiversity. As nee species are being discovered due to adaptation and a slight change in genotype and phenotype.
- what is the strongest or best genotype? Ans: The best genotype is AA as it is normal. And anyone who has such can marry anyone with the other genotypes.
- Can Genotype Change? Ans: Genotype can not change throughout the lifetime of an organism, Except in the cases of some sorts of mutations that rarely occur.
- Can you predict genotypes? Ans: Genotypes can be predicted when a test cross is done using the Punnett square as a tool for such test cross.