Genetic Inheritance
INTRODUCTION
henever one wants to eat a fruit like a mango, one goes to a mango tree to pluck some. This is because such fruit is expected to be found on mango trees, not on orange trees. When a goat gives birth, one does not expect the cry of a human being as a newborn. The possibility of like giving rise to like is because each generation passes components across to the next generation.
Have you wondered why a goat has never given birth to a horse, a horse never gave birth to a duck nor a mango produce an apple? It is because of material that is passed across generations from parents to offspring. Although organisms of the same species share significant traits in common, there are still obvious differences that can help us tell individuals apart. This variation could be in eye colour, body size and body shape, hair patterning, disease or behaviour. When this change is observed among individuals of the same species, the organisms are called variants (differing individuals). An example is seen in humans, whereby among two individuals, there is certainly a difference.
It was this principle that Mendel studied before he proposed that something was being stably passed down without alteration, from parent to offspring through their gametes, across successive generations. In his attempt to give it nomenclature, he called those things as ‘factors’. across successive years, scientists named those factors genes, which are presently described as the units of inheritance. Genes contain every information that is required to express a particular trait in an organism. However, it does not give the final phenotype of most traits of an organism because the environment could influence its impact.
DEFINITION
Therefore, Inheritance is the process by which characters are passed on from parent to progeny. Such characteristics could include height, weight, colour, shape, and even intelligence quotient. The traits are encoded in the gene. So, genes that code for a pair of contrasting traits are known as alleles, i.e., slightly different forms of the same gene
As more insight was revealed on Mendel’s finding, scientists decided to reduce confusion by adopting the use of alphabetical symbols to denote every gene coding for a trait. To achieve that, in the case of the character of round, R is used for the round trait and r for the wrinkle, and R and r are alleles of each other. Therefore, in organisms, the pair of alleles for the round would be RR, Rr, or rr. Traits with gene representation of RR or rr are described as being homozygous whereas traits that have Rr are heterozygous, this is due to the contrasting traits that they express. The gene inscription of RR, Rr, and rr are called the genotype of the organism while the descriptive terms round and wrinkle are the organism’s phenotypes.
MONOHYBRID INHERITANCE
This is simply defined as a cross between parents that involves only one gene. It can also be explained as an inheritance pattern of a characteristic, governed by one gene or an allele’s pair. An example is seen in a cross between a true-breeding tall pea plant and a true-breeding dwarf pea plant (Figure 21.1a). If at the end of their hybridization, their seeds are collected and grown to generate plants of the first hybrid generation (fillial1 progeny or F1). The probability of the offspring’s genotype can conveniently be predicted using Punnett square.
a
b
Figure 21.1 Punnett square. a. All plants are tall and are heterozygote. b. Phenotype ratio based on tall: dwarf is 3 : 1, while the genotype ratio of TT : Tt : tt is 1 : 2 : 1.
DIHYBRID INHERITANCE
This inheritance pattern involves a cross between two pairs of contrasting characters. An example can be drawn from Mendel’s experiment where he crossed pea plants that differed in two characters. One of the plants had seeds with yellow colour and round shape while the other had green coloured and wrinkle-shaped seeds. If the dominant and recessive alleles are known, Punnett square can be used to know the possible genotypes and their ratio that are available for offspring to possess.
Figure 21.2 Dihybrid cross producing 16 offspring.
round yellow : round green : wrinkle yellow : wrinkled green
9 : 3 : 3 : 1
MENDELIAN TRAIT
These are traits being controlled by one gene, hence, called monogenic traits. Those were the traits studied by Mendel during his experiment with garden peas. Due to the number of genes that control it, any mutation on the gene will be expressed if the mutant is the dominant allele or the gene is homozygous recessive. Such traits follow the Mendelian inheritance pattern. Mendelian inheritance is the basis of the classical genetics. Such a pattern of inheritance is simple because the environment does not influence them. An example of such traits is albinism. Mendelian traits have discrete futures without character ranges in between.
NON-MENDELIAN TRAIT
These are traits that are controlled by more than one gene, hence, called polygenic traits. Due to the number of genes involved, mutation on one gene may not be very significant as all the genes act in concert in adding or detracting from the phenotype. Such traits have two extreme features, with numerous ranges of traits between the two extreme traits. Due to the number of genes, controlling such traits (diseases) is difficult. An example of such traits is height.
CHARACTERISTICS OF MENDELIAN INHERITANCE
i) Equal contribution of materials to offspring by both male and female parents.
ii) Segregation produces the phenotypic ratio of 3:1 and genotypic ratio of 1:2:1 in the F2 generation of a monohybrid cross.
EXERCISE
1) Cross between parents for trait which is governed by only one gene is called…….
a) Hybridization b) cross fertilization c) monohybrid inheritance d) crossing
2) Process by which traits pass down from parent to progeny is called…………
a) Pollination b) transfer c) heritage d) inheritance
3) What is the phenotype of offspring produced from cross between homozygously dominant and homozygously recessive individuals for trait height?
a) ½ tall b) 1/4 tall c) 3 tall d) 4 tall
4) From number 3 question above, what is the offspring’s genotype?
a) ½ heterozygous b) ¼ heterozygous c) 3 heterozygous d) 4 heterozygous
5) Inheritance pattern whereby there is exchange between two pairs of contrasting traits is called………
a) Dihybrid inheritance b) monohybrid inheritance c) hybrid crossing d) cross
REFERENCES
Griffiths, J. F., Wessler, R. S., Lewontin, C. R., Gelbart, M. W., Suzuki, T. D. & Miller, H. J. (2005). Introduction to Genetic Analysis. New York: W. H. Freeman and Company.
Snustad, D. P. & Simmons, M. J. (2012). Principles of Genetics (6th ed.). United State of America: John Wiley & Sons, Inc.
Watson, J. D., Baker, T. A., Bell, S. P., Gann, A., Levine, M., Losick R. & Harrison, S. C. (2013). Molecular Biology of the Gene (7th ed.). New York: Cold Spring Harbor Laboratory Press.
Zimmerman, D. (2013). Genotype and phenotype definition. eHow. Retrieved from http///:recent%20project%20downloads/Environmental%20Factors_files/Genotype%2 0&%0Phenotype%20Definition%20_%20eHow.com.htm
