Genetics study guide

Deoxyribonucleic acid, or DNA, is a fundamental molecule of life, serving as the carrier of genetic information in most organisms. It is a long, complex polymer. Its structure is famously known as a double helix, resembling a twisted ladder. This double helix is formed by two strands that coil around each other. These strands are not directly joined but are linked by a series of complementary base pairs. These base pairs are held together by weak hydrogen bonds. Each individual strand of DNA is itself a polymer made up of repeating units called nucleotides. Every nucleotide consists of three main components: a sugar molecule (deoxyribose), a phosphate group, and one of four distinct nitrogenous bases. The bases are adenine (A), guanine (G), cytosine (C), and thymine (T). The sugar and phosphate groups form the backbone of each DNA strand, with the bases attached to the sugar component and extending inwards, forming the 'rungs' of the ladder. The specific pairing of A with T, and G with C, is crucial for DNA's function in replication and heredity.

The inheritance of the ABO blood groups is a classic example of both multiple alleles and codominance. Unlike traits determined by two alleles, the ABO system involves three different alleles for a single gene: Iᔧ, Iᔥ, and i. The Iᔧ allele codes for the A antigen on red blood cells, while the Iᔥ allele codes for the B antigen. The allele i codes for no antigen. Alleles Iᔧ and Iᔥ are codominant with each other, meaning that if both are present in an individual (genotype IᔧIᔥ), both A and B antigens will be expressed, resulting in blood type AB. However, both Iᔧ and Iᔥ are dominant over the i allele. This means a person with genotype Iᔧi will have blood type A, and a person with genotype Iᔥi will have blood type B. Individuals with genotype ii will have blood type O. This genetic system explains the four primary blood types: A, B, AB, and O, and their specific inheritance patterns.