Q.How is Nicotiana exhibit self-incompatibility? Explain its mechanism.

ABO blood grouping is the classic example of multiple allelism: three alleles (IA, IB, i) of one gene determine four possible phenotypes.

Multiple alleles occur when a single gene locus has more than two allelic forms present in a population (though any individual carries only two). For the ABO blood group gene, the three alleles IA, IB and i produce six possible genotypes and four phenotypes (A, B, AB and O), with IA and IB showing co-dominance (both expressed together in AB blood) and both dominant over the recessive i allele. This is not sex-linked (the …

ABO blood grouping is controlled by a single gene with three alleles (IA, IB, i) -- the textbook example of multiple allelism in humans.

The ABO blood group gene (I gene) has three alleles: IA (produces antigen A), IB (produces antigen B), and i (produces no antigen). Since more than two alleles exist for this single locus in the population (any individual carries only two of the three, but the population pool has three), it is called multiple allelism; IA and IB are codominant to each other and both dominant over i, giving the four phenotypes A, B, AB and O. It is not sex-linked (a) -- the I gene is autosomal, located on chromosome 9, not on the X or Y chromosome. It …

(a) I^A I^B — the codominant genotype that produces the AB blood group.

Blood group AB arises from the codominant combination of the I^A and I^B alleles.

The ABO blood-group locus has three alleles — I^A, I^B and i — where I^A and I^B are codominant with each other and both are dominant over i. A person with genotype I^A I^B expresses both A and B antigens on the red-cell surface simultaneously (since neither allele masks the other), …

Correct answer: (b) I^B i

Blood group B arises from either I^B I^B or I^B i; among the given options, only I^B i is listed.

Human ABO blood groups are controlled by a single gene with three alleles: I^A and I^B (co-dominant to each other) and i (recessive to both). Blood group B results from the genotype I^B I^B (homozygous) or I^B i (heterozygous) — in either case the I^B allele is expressed since i is recessive. Among the given options, I^A i and I^A I^A both …