The Theory Of Heredity Essay

Question:

Explain The Theory of Heredity.

Answer:

In 1866, Gregor Mendel, an European monk, tells us about the theory of heredity, but unfortunately it was recognized long after he died in 1900. For his experiments, Mendel used common pea plants which are also named as Pisum sativum. He found that some traits appear in progeny with no mixing of characteristics of the parents. For example, when he cross pollinated the progeny of the pea plant, he found that its flowers appeared either purple or white in colour, but not as the intermediate colours. He studied seven traits of the pea plant with discrete alternative forms such as colour of a plant's flowers, position of the flower on the plant, seeds colour and shape , pea pods colour and shape, and the length of plant stems. (Mendelian laws of inheritance, 2016)

According to Mendel, the hereditary determinants, which are also called genes, are of a specific nature. While studying the each trait, he found that a pair of genes contains two alleles and each parent in their each cell has two genes. After crossing the two pure line breeds, he observed that the F1 generation progeny has one allele for the recessive phenotype and one for the dominant phenotype. He observed that gene pair of one member segregates into a gamete. He analyzed his pea plant number and invented three laws of inheritance, i.e., Law of Dominance, Law of Segregation and Law of Independent Assortment.

As indicated by his law of dominance, in a pair of traits, one gene dominates the other in inheritance except in the case when both the genes in the pair are recessive.

Likewise, law of segregation says that amid the gametes production, the pair of each inherited gene segregate and from each parent, progeny gets one gene.

Similarly law of independent assortment expresses that the laws of chance represent which specific qualities of the parental pairs will arise in an individual progeny. (Mendel's Genetics, 2013)

In a cross, let one of the parent who is homozygous non tongue roller (recessive) is denoted as rr and he / she will be unable to roll his / her tongue due to the absence of dominant gene. (Science Brainwaves, 2011)But the other parent who is heterozygous roller is denoted as Rr and he / she will be able to roll his tongue due to the presence of dominant gene.

So, a cross between Rr and rr produces a genotypic ratio of 2 Rr : 2 rr or 1 Rr: 1 rr.

R

r

r

Rr

rr

r

Rr

rr

So, it is found that the genotype of the children is Rr and rr, that is, 50% of the children will be heterozygous (Rr) and 50% will be homozygous recessive (rr).

The persons who can taste the chemical named as Phenylthiocarbamide (PTC) are associated with the taster gene. So, both Homozygous PP and heterozygous Pp persons show the "P" phenotype, but only a homozygous pp person show the "t" phenotype. As the phenotype of the Pp person look like that of the PP persons, so, the P allele is dominant to the p allele. (Primer of Mendelian Genetics, 2012)

So, from Punnett Square diagram, it is found that a cross between two heterozygous parents, (Pp x Pp) produces a genotypic ratio of 1 PP: 2 Pp: 1 pp.

P

p

P

PP

Pp

p

Pp

pp

It is found that 25% of the progeny will be homozygous dominant (PP), 50% of the progeny will be heterozygous (Pp) and other 25% of the progeny will be homozygous recessive (pp).

References:

Mendelian laws of inheritance, 2016. [online]. Available at: [Accessed 27 May 2016].

Mendel's Genetics, 2013. [online]. Available at: [Accessed 27 May 2016].

Primer of Mendelian Genetics, 2012. [online]. Available at: [Accessed 27 May 2016].

Science Brainwaves, 2011. Dominant and Recessive Genes In Humans. [online]. Available at: [Accessed 27 May 2016].

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