What are alleles?
1. Alleles indicate the overall size of the genome.
2. They are restricted only to allosomes.
3. They are two or more versions of a chromosome.
4. They are two or more versions of a gene.
Answer: 4 (They are two or more versions of a gene).
Definition: These are variant forms of the same gene that occupy the same locus on homologous chromosomes, and govern variations in the production of the same gene product. This means, at the functional level, alleles of the same gene will interact in several ways, which leads to variations in the type of dominance and the occurrence of different phenotypic effects with different allelic combinations.
Examples: The ABO blood groups of humans are represented by three alleles of the same gene that show different types of interactions with one another and produce four blood types in the ABO group system. These allelic series include three major alleles, i, I A, and I B, but a single person contains only two of the three alleles or two copies of one of them.
Hence, overall, six different genotypes are possible: three homozygous and three different types of heterozygous.
Read more about alleles, genes, and genotypes here.
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