Biology / 유전자와 유전형

유전자(Gene)

유전의 기능단위(functional unit)

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유전자 자리(locus)

각 유전자는 염색체의 특정한 위치에 존재, 그 위치를 locus라 부른다.

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대립유전자(allele)

하나의 유전자는 DNA 변이(variation)에 의해 갖는 두 개 이상의 형태, 조합에 따라 3가지 유전형을 가진다.

AA, AB, BB

- homozygote(동형접합체): 2개의 동일한 대립유전자형의 조합(AA, BB)

- heterozygote(이형접합체): 2개의 다른 대립유전자형의 조합(AB)

- major allele/minor allele: 형질의 기능보다도 출현빈도를 근거로 판단, 어느 집단의 유전자 자리(locus)에 2~3개의 allele가 존재한다고 하면 그 빈도가 A1가 80%, A2가 17%, A3가 2%라면 A1은 major allele가 되고, 나머지는 minor allele가 된다.

- allele frequency: 한 집단 내에서 같은 유전자를 구성하는 서로 다른 대립유전자 사이의 구성 비율

Calculation of allele frequencies from genotype frequencies

If f(AA), f(Aa), and f(aa) are the frequencies of the three genotypes at a locus with two alleles, then the frequency p of the A-allele and the frequency q of the a-allele are obtained by counting alleles. Because each homozygote AA consists only of A-alleles, and because half of the alleles of each heterozygote Aa are A-alleles, the total frequency p of A-alleles in the population is calculated as

Similarly, the frequency q of the a allele is given by

It would be expected that p and q sum to 1, since they are the frequencies of the only two alleles present. Indeed they do:

and from this we get:

q = 1 − p and p = 1 − q

If there are more than two different allelic forms, the frequency for each allele is simply the frequency of its homozygote plus half the sum of the frequencies for all the heterozygotes in which it appears. Allele frequency can always be calculated from genotype frequency, whereas the reverse requires that the Hardy–Weinberg conditions of random mating apply. This is partly due to the three genotype frequencies and the two allele frequencies. It is easier to reduce from three to two.

An example population

Consider a population of ten individuals and a given locus with two possible alleles, A and a. Suppose that the genotypes of the individuals are as follows:

AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA

Then the allele frequencies of allele A and allele a are:

so if an individual is chosen at random there is a 70% chance it will carry the A allele, and a 30% chance it will have the a allele 

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이배체(diploid)

하나는 아버지로부터 다른 하나는 어머니로부터 받음.