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an organism's genotype is its specific combination of alleles for a given gene

What is meant by this? Is it talking about heterozygous and homozygous (like GG or Gg) or is it talking about the nuclear bases?

Thanks

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  • $\begingroup$ I'm voting to close this question as off-topic because there is no source given for the fragment quoted. $\endgroup$ – David Apr 12 '18 at 17:49
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From wikipedia

An Allele is a variant form of a given gene.

Let's consider a simplified example. There is a gene for eye color and there are different alleles at this gene. The allele for blue eye, the allele for brown eye etc...

In a diploid (see ploidy) individual (humans are diploid), we have two copies of each gene (exception of sex chromosomes). Let's imagine, at a givne gene, there are 3 possible alleles called A, B and C. The possible combinations are

AA
AB
AC
BB
BC
CC

The genotype of an individual for a given gene (I should say locus for the sake of generality) is called the genotype of an individual.

Is it talking about heterozygous and homozygous[?]

The terms "homozygous" or "heterozygous" are adjectives to define whether, in a diploid genotype, the same allele is found on both haplotypes or not. In the above examples, 3 genotypes are homozygous (AA, BB and CC) and 3 genotypes are heterozygous (AB,AC and BC).

[I]s it talking about the nuclear bases?

For two alleles to be different, they must differ in their sequence of nuclear bases of course.

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Alleles are different versions of the same gene; they have different nucleotide sequences. Which versions of each gene an organism gets defines its genotype.

Zygosity (heterozygous/homozygous) is a related idea. When creatures have more than one copy of a chromosome (e.g. one from each parent) the alleles of each gene on each chromosome may be the same (homozygous) or different (heterozygous).

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