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What is Allele

It is an established fact that a specific Gene controls a specific phenotypic trait. This finding is not always true. Studies on ‘gene expression’ have revealed that a gene often influences more than one phenotypic trait. This phenomenon of multiple effects of a single gene is called pleiotropism. In such a genic influence more conspicuous expression of a phenotypic trait by a gene is called its major effect. If the gene causes other less conspicuous phenotypic changes, it is known as secondary effect. Such genes responsible for multiple effects are called pleiotropic genes. Vestigeal wings in Drosophila are caused by a recessive gene in homozygous condition. A keen observation has shown that this gene affects other traits as well. They are
(i) the small halters or balancers behind the wing
(ii) structure of reproductive organs
(iii) egg production
(iv) life duration and
(v) bristles on the body.

Hardy-Weinberg Law   During the 20th century, scientists showed great interest in applying the theories of Mendel to human traits. They were able to identify a dominant trait namely brachydactyly that agreed with the Mendelian theories and showed Mendelian ratio in the human populations. Brachydactyly is characterised by abnormal shortening of the phalanges while the rest of the portion of the arms remain normal. An objection to this example was raised. If the trait brachydactyly is a dominant character, three-fourth of the human populations should possess the abnormally shortened phalanges. But in reality brachydactyly is seen rarely in human population. This occurrence appeared to contradict Mendelian theories. However, until 1908 it remained unexplained among the scientists. Two scientists G.E. Hardy and Wilhelm Weinberg (worked independently) put forth the theory Hardy-Weinberg Law. This theory explains why some characters though dominant (like brachydactyly) appear rarely in a population.

This law states that under specified conditions the genotypic frequency in a population remains constant. It simply means, that the common traits remain common and the rare traits remain rare. It further explains that the Mendelian terms dominant and recessive refer to the phenotypic appearance of the heterozygotes and not to the abundance of any trait in the population. The relative abundance of a genotypye in a population is referred to as its genotypic frequency.
The genotypic frequency of any genotype in a population could be expressed mathematically between 0 to 1 (or 100%) Let us consider that aparticular trait has two alleles ‘A’, ‘a’. ‘A’ remains dominant to ‘a’. Let us assume that the number of individuals in a new population (with AA and aa) is 100 or 1.Then the number of individuals (or frequency) with AA is ‘p’ and the number of individuals with ‘aa’ (or frequency of aa) is ‘q’. With that we can draw the equation p+q =1 (or 100). As both the characters are considered as homozygotes the individuals with AA will produce all gametes with ‘A’. Like wise the ‘aa’ individuals will produce all gametes with ‘a’. If all the gametes are viable, combine at random, all have the chance of survival and no new gametes are introduced in to the population, we can calculate the proportion of next generation with genotype AA using the usual Punnet Square method.

Substitute these values for the symbols p and q and find out the percentage of heterozygotes found in the population when it reaches the equilibrium.

Reminders: Convert the percentage values into decimal fractions before

Where the percentage of population having Hetero zygotes is 48.
It is clear that this law is applicable only to populations with following features
a) Infinitely large populations
b) No mutation
c) No selection
d) Random mating
e) Isolated from any other population of the same species.

This law could be mathematically proved not only to a new population but also to the populations already existing with equilibrium. Such populations that has reached equilibrium remain unchanged from generation after generation. This means that they remain constant and have no chance for evolution

1.If a population of fruit flies has a gene frequency of 20% for the recessive allele causing vestigial wings, what proportion shows this trait phenotypic ally? What proportion of the population is heterozygous?

2.Assume that brown eyes are dominant to blue eyes. On a certain island about 9% of the population is blue eyed. What proportion of the population is heterozygous.

This post first appeared on Namaste Kadapa, please read the originial post: here

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What is Allele


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