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How can a recessive allele become represented in a significant number of individuals in a population

How can a recessive allele become represented in a significant number of individuals in a population


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Darwin believed that individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve.

-individuals in a species show a wide range of variation this variation is because of differences in their genes

-individuals with characteristics most suited to the environment are more likely to survive and reproduce. The genes that allow these individuals to be successful are passed to their offspring.

In genetic terms, evolution is any change in the allele frequency of a population.

A recessive allele can become represented in a significant number of individuals in a population, how?


A recessive allele is one whose phenotype can be masked by a dominant allele. A recessive allele is not necessarily deleterious, and the dominant allele does not necessarily have higher fitness than the recessive allele. Fitness and dominance are often confused, but they are not the same thing.

In some scenarios, the recessive allele indeed confers a fitness benefit, and when it does, it is predicted to increase in frequency rapidly because homozygous recessive individuals have higher fitness than both homozygous dominant genotypes and heterozygotes.

A recessive allele can also persist in a population if it is deleterious in homozygotes but confers a heterozygote advantage. The most famous example of this phenomenon is in Africa, where individuals heterozygous at the sickle cell locus have a fitness advantage because they are resistant to malaria. A higher prevalence of sickle cell phenotypes occur because selection for heterozygotes maintains a high frequency of recessive alleles in the population. For more, see Templeton (1982, http://www.indiana.edu/~curtweb/L567/readings/Templeton.pdf) and Allison (1954, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2093356/).


Dominance

Dominance explains the relationship between alleles in determining the phenotype. Consider two allelesAanda. IfAis dominant, then individualsAAand individualsAapresent the same dominant phenotype while individualaapresent the recessive phenotype.

Selection and dominance

When you ask

A recessive allele can become represented in a significant number of individuals in a population, how?

I am not sure why it seems unlikely to you. No one said that the recessive phenotype was less fit than the dominant genotype.

If the allelesaandAare present at the same frequency in the population, then most individuals will carry the dominant phenotype. But the allelesAandamay well not be at the same frequency. Selection (as well as other evolutionary processes that I won't talk about here) will affect the frequencies of these alleles. If the dominant phenotype has a higher fitness than the recessive phenotype, then the alleleAwill increase in frequency. If, on the other hand, the recessive phenotype has higher fitness than the dominant phenotype, then the alleleawill increase in frequency.

From allele frequency to genotype frequency

Let $p$ and $q$ be equal to the frequencies of the alleleaandA, respectively, such that $p = 1-q$ is necessarily true. Under a few assumptions (see here), the frequencies of the genotypesAA,Aaandaaare $p^2$, $2pq$ and $q^2$, respectively. For more information on why this is true, you can have a look at Solving Hardy Weinberg problems.

Let's imagine a scenario where, the recessive phenotype has a higher fitness. The alleleawill increase in frequency in the population until eventually reaching a frequency of $p=0.95$. At such allele frequency, $0.95^2 = 0.902$ (or 90.2%) of the population will carry the recessive phenotype.

Note that at very low frequency of the recessive allele, the recessive phenotype is very rare (as it's frequency is the square of the frequency of the allele) and there is almost no selection acting on the locus of interest. But all of that is a story for another time.


My answer below demonstrates how recessive alleles can fixate and persist in potentially large numbers of the population. /a/61176/9771


Recessive diseases are insidious because most of the time the dominant (healthy) allele masks them. Then every once in a while, someone in the village gets a wide range of chronic, unexplainable symptoms that the local shaman explains as a random curse.

While these are a no-brainer in the developed world, they are a daunting problem for primitive cultures and third-world nations where superstition is much more commonly practiced than science and no one knows what a "genome" is.

With even distribution, a quarter of the population has the disease and half of the population have 1 allele for the disease.