Each strand of the helix is a linear arrangement of repeating units called nucleotides that consist of one sugar, one phosphate, and a nitrogen-containing molecule called a base. The basic form of a DNA molecule is that of a twisted ladder or double helix. It is made up of tightly wound strands of deoxyribonucleic acid (DNA) organized, in humans, into 23 pairs of chromosomes: 22 autosome pairs (numbered 1–22) and one sex chromosome pair (XX in females and XY in males). Our genome is the blueprint for all cellular structures and activities and is stored in the nucleus of every cell. The Molecular Basis of Genetic Inheritance Exceptions to Mendel's laws of inheritance are described later in this chapter. 1, 2Īs advances in genetics have confirmed and illuminated the mechanisms underlying Mendel's observations, we have also discovered the need to adapt and modify his principles.
If the offspring have an intermediate phenotype, such as moderate height in an individual born to a very tall and a very short parent, the alleles are considered semidominant. When neither allele exerts a stronger effect, both are considered codominant, and the offspring may show the phenotypic features of both alleles, as is the case in individuals with type AB blood, who have features of blood groups AA and BB. The effects of allele B in this case are apparent only in the homozygous state (BB). For instance, if the phenotypes produced by the combinations AA and AB are the same, then A is dominant to B (or conversely, B is recessive to A). Different alleles can exert different phenotypic effects broadly speaking, most genes are either dominant or recessive. Each unit of inheritance can have alternate states (alleles) that segregate at meiosis, with each gamete receiving only one allele ( the principle of segregation, Mendel's first law) different alleles assort independently in the gametes ( the principle of independent assortment, Mendel's second law).
Gregor Mendel crossed various pure lines of garden peas and, by following their hybrid progeny, observed that traits are inherited as alternate states of independent units of inheritance or genes (which Mendel called “factors”), and that these units come in pairs.
MENDELS PRINCIPLE OF DOMINANCE SERIES
Our basic laws of inheritance were derived from a simple series of experiments with garden peas more than a century ago. Mendel's Laws and Principles of Inheritance
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