Q. Which of the following statements concerning alleles is true for diploid organisms?
I. at most only two alleles occur at a given locus in all organisms genome
duh - this is genetics 101. Because we are talking about a DIPLOID organism, that means one set of chromosomes came from mom & the other from dad. To explain this further, think of a gene as something expressible, example of a gene is eye color. There are different variations of eye color, each variation is called an allele (for example, blue allele, green allele, brown allele, etc).
II. alleles occupy an identical locus in homologous chromosomes
Yes. Since we are talking about DAT genetics which is a very basic form of genetics, all alleles of a specific gene are occupied on the same location (same locus) of the two chromosomes (one chromo from mom & other from dad)... This figure might help you understand this better
III alleles of a given gene usually occur on homologous or non homologous chromosomes
The problem with this statment is the use of the word "usually" and "or non homologous chromosomes". You see, it is possible to have alleles be spread out on different chromosomes (non-homologous), but its not the "usual" form of how we study genetics. Basic Genetics (like the one you need to know for the DAT) is usually taught on the basis that the different forms of alleles for a given gene are all located on homologous chromosomes (like the figure I drew above)
IV a single chromosomes usually carries two alleles of each gene
A gene "USUALLY" occupies one locus on a chromosome and that locus will express the allele it carries. "USUALLY" the gene locus has 1 allele attached to it. So for example, the chromosome from mom has the allele "blue eye color" and the dad's chromosome has the allele "brown eye color"... It is unusual to see one locus having both alleles (both brown and blue color)
correct answer is I and II
would you explain me in detail why other choices III and IV are incorrect statement?
