Mendelian Genetics

Introduction

The invention of better microscopes in the 1890s enabled biologists to ascertain various fundamental details about sexual reproduction and cell division.   The center of the study of genetics then moved to the understanding of what takes place during the transmission of inheritance traits from parents to their children. Researches came up with various hypotheses to explain the concept of heredity, but Gregor Mendel, an Australian monk, is the only one who got it more or less precise and his works became famous in 1900 long after his death (Khanna, 2010).

Mendel’s research dealt with plants, which are the essential principle of heredity but the ideas can also relate to animals and people. This is because the system of heredity is the same for all organisms.  His principles became the contemporary discipline of genetics. He reasoned out that for any organism to be relevant in genetic experiments it should posses certain characteristics. The organism should have a number of varied traits that one can study, and the progeny of the self-fertilizing plants must be perfectly fertile. The other trait is that the plant used should be self-fertilizing, and the structure of its flower should be that, which confines accidental contacts.  Mendel made two contributions to the science of genetics. He developed pure lines and counted the results he got along with keeping statistical notes. A pure line was a significant innovation since any segregating generation that is non-pure did not confound the genetic experiment results. It was about a population whose breeding is true for a specified trait (Khanna, 2010).

Mendel used an ordinary garden pea as the experimental organism. The pea has several advantages for genetics. The plant is available in different varieties with unique features that are heritable with diverse traits. Their flowers are prone to self-pollination, and each pea has both sexual organs. The male organs called anthers produce pollen that has the male gametes. The female part includes the stigma, ovary, and the style. The ovary makes the female gamete. The transfer of pollen grains to the stigma from the anther takes place when the flower of the pea opens. The pollen grain grows a pollen tube that enables the sperm to pass through the stigma and style to the ovary. The wall of the ovary (pea pod) becomes the fruit when it ripens (Walker & Wood 2008).

Mendel tested the 34 varieties of peas that he got through seed dealers,  where each character he studied had two different forms such as smooth or wrinkled, tall or short and so on. In a crossbreeding experiment, he would hybridize true different reproductive pea variety and allow the F1 hybrids to self-pollinate and make an F2 generation.  The true reproductive parents are the p generation and their hybrid offspring are the F1 generation.  Below are the results from Mendel’s experiment (O’Neil, 2012).

Parental cross F1 phenotype F2 phenotype ratio F2 ratio Round x wrinkled

Seed

Round 5474 round:1850 wrinkled 2.96.1 Yellow x  green seeds Yellow 6022 yellow:2001 green 3.01.1 Red x white seeds Red 705 red:224 white 3.15.1 Tall x dwarf plants Tall 1787 tall:227 dwarf 2.84.1

Through careful crossbreeding of common pea plants over several generations, Mendel found out that there are certain traits that show up in the offspring without any combination of parent distinctiveness. Example the pea plant is either white or purple since intermediate colors fail to appear in the offspring of the cross-pollinated pea plants (O’Neil, 2012).  Mendel discovered several traits that are recognizable and happen in only one or two forms.  They include the color of the flower is purple or white, the shape of the seed is round or wrinkled, the stem is long or short, and the pod color is either yellow or green (O’Neil, 2012).

The other traits are the position of the flower is either terminal or axil, seed color is yellow or green, and the shape of the pod is either constricted or inflated. The study that these traits fail to happen in intermediate form was crucial. This is because the principal theory in biology during that time was that the inherited attributes mingle from one generation to the other. Either when Mendel cross-pollinated plants that produce yellow or green seeds, he found out that the generation of the offspring always produced yellow seeds unlike the following generation that consistently had a yellow to green ratio of 3:1 (O’Neil, 2012).

The ratio occurs in the other generations too, and Mendel found out that this was the means to understanding the fundamental system of inheritance.

From the above experiment, Mendel arrived at three fundamental conclusions, which includes, genes determine the inheritance of each characteristic, a character acquires one such factor from each parent for every feature and a trait can pass on to the next generation but fail to appear in an individual.  It is crucial to note that, in this experiment, the preliminary parent plants were homozygous for seed color that is two yellow and two green. All the plants in the first generation were heterozygous that is each of them had acquired two dissimilar alleles one from each parent plant. This is evident when one looks at the actual genetic makeup instead of only the physical characteristics that are observable (O’Neil, 2012).

Each First generation plants above acquired a Y allele from specified parent and from the other a G allele.  When the breeding of the first generation plants takes place, each has the same possibility on each offspring either the Y or G allele. From all the features that Mendel observed, one trait prevails than the others. For example, the genotype for pee seed color is YG, the phenotype is Y but the prevailing yellow allele does not affect the recessive green in any way. The alleles can both pass on to the following generation unaffected. From Mendel’s observations, two principles came up that is, the principle of segregation and the independent assortment theory (O’Neil, 2012).

Segregation principle

In this principle, he first studied the height of the seed. He crossed a true- breeding that was tall with an assortment that always created dwarf seeds. The resulting seeds were tall. In the subsequent year, he planted the seeds from the first generation and left them to self-fertilize.  From this experiment, he recovered 2014 seeds, where 1787 were tall and 227 dwarf.  He marked and numbered these generations to make record keeping easier. P1 denotes the parental generation while the F1 stands for the offspring of the P1 generation. F2 stands for the F1 self-fertilizing generation.

These data was accessible on the Punnett square with the gametes from the F1 generation. Although the ratio of the phenotype was 3:1, the resultant genotype had a ratio of 1:2:1.  He performed a backcross to confirm this principle of segregation. The first cross was between two pure line parents and it produced an F1 heterozygote (Hoffee, 2004).

Instead of Mendel self-crossing the F1 generation at this point, he crossed it with a pure yellow homozygote plant.

Backcross: Dd x dd

Male
Gametes D Female
Gametes D DD
(Tall) d dd
(Dwarf)

Backcross One or (BC1) Phenotypes: 1 Tall: 1 dwarf

BC1 Genotypes: 1 Dd: 1 dd

Theory of independent assortment

According to this experiment, different pairs of alleles pass on to offspring independent of each other. The result is the new combination of genes, which are not there in any of the parent. Example, the inheritance ability of the pea plant to produce purple flowers in place of white flowers does not give a possibility that it will produce yellow pea seeds instead of white. This principle also explains why the inheritance of an eye color in humans does not amplify or lessen the possibility of having six fingers in on each hand. This is because the genes for independent assortment traits exist on different chromosomes. These two principles and understanding of inheritance and dominance units are the beginning of modern science of genetics (Cummings, 2011).

Mendel concluded that heritable determinants are of a specified nature. These determinants are genes. Every parent has a gene pair in each cell for each studied unit. The F1 from a cross of two pure lines encloses one allele for the prevailing type and one for the recessive type. These two alleles include the gene pair. One constituent of the generic material pair separates into a gamete; therefore, each gamete only holds one member of the gene pair. Gametes join haphazardly and separate from the other gene pairs (Cummings, 2011).

Mendelian Genetic Definitions

Allele refers to one alternative pair of the existing allelic pair. Example green and yellow are the alleles for the color of a pea plant. Any specified gene can contain more than one-two alleles, but only one of them is within an individual.

Homozygote is the amalgamation of two alleles that contain the gene pair.

Heterozygote refers to an individual that encloses one of every member of the gene pair such as the Dd heterozygote.

Genotype entails the precise allelic combination for a certain set of genes. Using symbols one can represent the cross of tall and dwarf pea plants using the manner as shown

Backcross refers the cross of an F1 hybrid to one of the homozygous parents; for pea plant shape, the cross would be Rr x RR or Rr x rr; most of the time, a backcross is a cross that occurs to an entirely recessive parent (Cummings, 2011).

Testcross is the cross of an individual to a recessive parent that is homozygous and it determines whether the individual is homozygous or heterozygous dorminant(Cummings, 2011).

Monohybrid cross refers to a cross between parents that are different based on the single gene pair (usually AA x aa) (O’Neil, 2012).

Monohybrid refers to the offspring of two homozygous parents for the exchange of alleles of a gene pair. Monohybrids are essential for determining the association between alleles. A homozygous allele shows the phenotype of the heterozygote that allows one to verify the relationship of the alleles (Cummings, 2011).

Dominance is the capability of an allele to show its phenotype at the cost of a varying allele. The main form of interface between alleles usually the dominant allele makes a gene product that the recessive type fails to produce. The dominant allele will express itself when it is present (O’Neil, 2012).

Conclusion

The system of heredity is the modern science of genetics. For an organism to be useful in genetic experimentation, it should posses certain features and a number of different traits that one can study. The offspring of the self-fertilizing plants must be perfectly fertile. There are certain traits that appear in the offspring without any combination of parent uniqueness. Genes determine the inheritance of each trait because of this the genes for independent assortment characters exist on different chromosomes. The principle of independent assortment, segregation and the understanding of the components of inheritance with dominance are the beginning of modern science of genetics (Hoffee, 2004).

References

Cummings, M. R. (2011). Human heredity: Principles & issues. Belmont, CA: Brooks/Cole, Cengage Learning.

Hoffee, P. A. (2004). Quick look genetics. S.I.: Hayes Barton Press.

Khanna, P. (2010). Essentials of genetics. New Delhi, India: IK International.

O’Neil, D. (2012). Mendel’s Genetics. Retrieved from             http://anthro.palomar.edu/mendel/mendel_1.htm

Walker, P., & Wood, E. (2008). Hands-on general science activities with real-life applications:     Ready-to-use labs, projects, & activities for grades 5-12. San Francisco: Jossey-Bass.