Genotypes and phenotypes are characteristics of individual organisms. The word ‘genotype’ is usually used when talking about the genetics of certain traits (such as eye color). The phenotype is determined by genetic and environmental influences, for example, height, weight and skin color.
Genes are a basic functional unit of heredity which is a focal point in modern genetic science. In all branches of genetic science, genes are a common thread that unites diversity in the conduct of experiments.
Geneticists have great attention to the transmission of genes from generation to generation, the physical structure of genes, variations in genes, and how genes reduce the characteristics of a species. In this chapter, we will trace how the concept of genes emerged.
We can see that genetics is an abstract science which generally starts from a series of hypotheses in the minds of geneticists and then is identified in physical form. The concept of genes (not the word ‘gen’) was first introduced in 1865 by Gregor Mendel. After that, not much progress has been made in understanding heredity.
The idea that was prevailing at that time was that sperm and egg cells contained a sampling / digestive essence from various parts of the mother’s body; so that in the process of fertilization, this essence is mixed somehow to form the new individual traits that are produced.
This idea, called ” blending inheritance”, was arranged to explain the fact that offspring usually exhibit some of the same characteristics as the two parents. However, some problems result from this idea, one of which is that the offspring is not always a mixture of the characteristics of the two parents.
Efforts to develop and improve this theory do not lead to a better understanding of heredity. Then as a result of his research on the pea plant, Mendel proposed an alternative theory of ” particulate inheritance”.
According to Mendel’s theory, characters are determined by units that have their characteristics that are inherited intact to the next generation.
Understanding Genotype and Phenotype Factors
Genotype is the arrangement of genes from an individual that is not visible from the outside (Pane, 1986 in Ferdy 2010). Phenotype is a characteristic (both structural, biochemical, physiological, and behavioral) that can be observed from an organism that is governed by genotype and environment and their interactions. The definition of phenotype includes various levels in the expression of genes of an organism.
At the organism level, a phenotype is something that can be seen/observed/ measured, something trait or character. At this level, examples of phenotypes such as eye color, body weight, or resistance to a particular disease.
At the biochemical level, phenotypes can be in the form of certain chemical substances in the body. For example, blood sugar levels or protein content in rice. At the molecular level, the phenotype can be either the amount of RNA produced or the detection of DNA or RNA bands on electrophoresis (Anonymous, 2011).
Inheritance of traits or crosses
In natural inheritance or crosses, there are principles that we must remember, namely:
- Genes that play a role in the regulation and determination of properties are given letter symbols.
- Dominant genes are expressed in capital letters, for example, genes that determine the nature of a tall bar are written with the letter “T” (derived from the high word).
Recessive genes are expressed in lowercase letters, for example, genes that determine the nature of the short stem are written with the letter “t”. So, it can be interpreted that the tall stem is dominant to the short stem, and conversely, the short stem is recessive to the high stem.
In humans and vertebrate animals, the union of sperm and ovum, each of which is haploid (n), will form a zygote. Zygotes grow and develop into diploid individuals (2n), so individuals who have these traits are expressed by two letters Example :
- TT: Symbol for tall trunked plants, gametes formed by T and T.
- tt: Symbol for short-stemmed plants, gametes formed by t and t.
- MM: Symbol for red flowering plants, gametes formed by M and M.
- mm: Symbol for white flowering plants, gametes formed m and m.
- Mm: Symbol for plants with pink flowers, gametes formed by M and m.
The composition of genes that determine the nature of an individual is called genotype (cannot be seen with the eye). The genotype of an individual is given a symbol with double letters because the individual is generally diploid, for example, MM, Mm and mm. Genotype has a pair of genes.
These genes are located in the corresponding loci of homologous chromosomes. A pair of genes located in the same position on a pair of chromosomes is called an allele.
Thus, the allele is a member of a pair of genes such as M = the gene for red flower color, m = gene for white flower color, T = gene for tall plants, and t = genes for low plants. M and m are alleles but M and t are not alleles.
The nature of an individual whose genotype that has the same genes from each type of gene is called homozygous, for example, RR, rr, TT, AABB, aabb and so on.
Homozygous dominant occurs when the individual genetically RR, AA, TT; whereas homozygous recessive if the individual genetically rr, aa, tt and so on.
The nature of an individual whose genotype consists of different genes from each type of gene is called heterozygous, for example, Rr , Aa, Tt, AaBb and so on. Observable characters or physical characteristics (shape, color, blood type, etc.) are called phenotypes. The phenotype is determined by genes and the environment.
The phenotype is not given a symbol but is written according to the appearance such as sweet fruit taste, straight hair, blue flower color and so on. Plants that have round seeds are phenotypically written round seeds and genotypes are written BB or Bb if B is dominant against b.
Two individuals who have the same phenotypic traits may have different genotype traits, for example, two individual plants that have the same phenotype as round seeds, have the possibility of genotype is BB or Bb.
Gen B is dominant so that the B gene overcomes or covers the recessive gene b. Therefore plants with BB or Bb have a round seeded phenotype.
Difference between Genotype and Phenotype
The genotype of certain organisms or traits refers specifically to genetic information that describes the visible nature. An apparent characteristic is that, like eye color or hair color, it cannot be described as a genotype. While the phenotype is specifically referred to as the properties that can be described from an observation.
Genotypes are factors that cause phenotypes that are specific to exist. Whereas phenotype is a “visible characteristic” in an individual and can be observed with the five senses, for example, the color of red flowers, curly hair, large body, sweet fruit, and so on.
Phenotype is a combination of genotype and environmental factors. So an individual with the same phenotype may not necessarily have the same genotype.
Crosses of Two Individuals with One Difference (Monohybrid)
Crossing two individuals with one different trait will decrease the dominant trait if the offspring is the same as one of the parents. Example: Mendel takes pollen from flower plants whose seeds are curled and pollinated to the pistil from flower plants whose seeds are round. All F1 breeds take the form of rounded seeds. Then the F1 plants were allowed to self-pollinate to get F2 offspring showing a phenotype ratio of 3 round seeds: 1 frozen seed.
Sat intermediates are inherited traits that are owned by both parents. An example is a flowering plant at four (Mirabilis jalapa ) pure red strains (MM) crossed with pure white strains (mm). From these crosses obtained results F1 all of which have pink flowers. If F1 is pollinated with each other, then F2 produces red, pink and white flowering plants in the ratio of 1: 2: 1.
Crosses of Two Individuals with Two Different Traits (Dihybrid)
Dihybrid is a cross between two individuals with two or more different traits that produce offspring with a comparison of certain phenotypes and genotypes. In his experiments, Mendel crossed pure strain peas which had yellow round seeds with pure strains that had green wrinkled seeds.
F1 seeds are then replanted and pollinated among themselves to get F2. The crossing is a cross between two individuals with two different characteristics, namely the shape of the seeds and the color of the seeds. The descendants of F2 are as follows:
B: round, dominant to wrinkles
K: yellow, dominant towards green
Crosses of Two Individuals with Three Different Traits (Trihibrid)
Trihibrid is a cross between two individuals with three or more different traits that produce offspring by comparison of certain phenotypes and genotypes. In his experiments, Mendel crossed pea with three different properties, namely tall stems, round seeds, and yellow seeds with short-stemmed peas, wrinkled seeds, and green seeds.
Tall, round, and yellow is dominant to short, wrinkled, and green, so the whole F1 is pea with high trunk, round seeds, and yellow. F1 descent can be seen in the trihibrid crossing chart
The F1 seeds are then replanted and pollinated among themselves to get F2. The crossing is a cross between two individuals with three different characteristics, namely the size of the stem, the shape of the seeds and the color of the seeds. The descendants of F2 are as follows:
T: tall, dominant to short
B: round, dominant to wrinkles
K: yellow, dominant towards green
Conclusion – Genotype vs Phenotype
In a cross, there are genotypes and phenotypes. Genotype is the genetic makeup of an individual (something that cannot be observed). The genotypic nature of an individual is given a symbol with double letters. For example, genotypes for high trunk plants = TT, genotypes for low trunk plants = tt. The letters T and t are called gametes.
Genes are divided into two types, some are homozygous genes and some are heterozygous genes. Homozygotes are traits of an individual whose genotype consists of the same genes from each type of gene, for example, AA and aa.
Meanwhile, heterozygotes are traits of an individual whose genotype consists of different genes from each type of gene, for example, Aa and Bb.
Outward form or observable traits are called phenotypes. Phenotype is strongly influenced by genes and the environment. If the genes of a plant have high properties, but if the environment is not supportive, then these plants are not going to achieve a high they should be.
The phenotype is not given a symbol but is written according to the appearance. For example, red flower color, sweet fruit taste, tall or short stems. A flower is red, the phenotype is called red, and the genotype is written MM or Mm.
Two individuals who have the same phenotype do not necessarily have the same genotype. The genotype can be homozygous or heterozygous. For example, two tall trunked trees can have the TT or Tt genotype with the same phenotype.
In crossing, there are several kinds of crosses, namely: monohybrid (one different character ), dihybrid (two different characters) and trihibrid (three different characters).