Inheritance Patterns - Phenotype And Genotype, Alleles, Dominance Relations, Molecular Meaning Of Dominance And Recessiveness, Autosomal Dominant Inheritance - Mitochondrial Inheritance

Inheritance Patterns - Phenotype And Genotype

An organism's observable characteristics, such as height, hair texture, skin color, or ear shape, are known as the phenotype of that organism. The phenotype is determined partly by the environment and partly by the set of genes that the organism inherited from its parents. Adult height, for instance, is due partly to nutrition (an environmental influence), and partly to a set of genes gover…

Inheritance Patterns - Alleles

Inheritance Patterns - Dominance Relations

When both alleles for a particular trait are identical, the organism is said to be homozygous for that trait. When the alleles differ, the organism is heterozygous. The presence of two different alleles raises the question of whether one or the other, or both, will determine the phenotype of the organism. For his experiments on peas, Mendel chose traits for which one allele of each pair had a deci…

Inheritance Patterns - Molecular Meaning Of Dominance And Recessiveness

The terms "dominant" and "recessive" imply some competitive interaction between alleles over which one will control the phenotype. This is not the case, however. Alleles do not interact in the nucleus. Instead, both alleles are expressed (in most cases), and the phenotype reflects the result. How, then, can one allele determine the phenotype to the exclusion of another?…

Inheritance Patterns - Autosomal Dominant Inheritance

Autosomal dominant inheritance is due to a dominant allele carried on one of the autosomes. Autosomal dominant alleles need only be inherited from one parent, either the mother or the father, in order to be expressed in the phenotype. Because of this, any child has a 50 percent chance of inheriting the allele and expressing the trait if one parent has it. Many normal human traits are due to autoso…

Inheritance Patterns - Autosomal Recessive Inheritance

Autosomal recessive inheritance is due to recessive alleles carried on autosomes. An individual possessing only one recessive allele is known as a carrier. An individual must inherit two recessive alleles, one from each parent, in order to express the recessive trait. When two carrier parents have off-spring, each offspring has a 25 percent chance of inheriting two alleles and expressing the trai…

Inheritance Patterns - Sex-linked Inheritance

The two sex chromosomes differ in the genes they carry. The Y chromosome is very small, and appears to carry very few genes other than the SRY gene that determines male sex. Many genes are carried on the X chromosome, however, and these are as essential for males as they are for females. Genes carried on the X chromosome are said to be X-linked. X-linked dominant alleles affect both males and fema…

Inheritance Patterns - Mosaicism

The reason females are less often affected by recessive alleles is not as simple as it is for autosomes. Since females have twice the number of X chromosomes as males, the question arises as to whether they make twice the amount of each X-encoded protein as males do. In fact they do not, and are prevented from doing so by the random inactivation of one X chromosome in each cell. Therefore, about h…

Inheritance Patterns - Penetrance, Expressivity, And Anticipation

The presence of a dominant allele, or two recessive alleles, is not always a guarantee that the trait will be displayed in the phenotype, a phenomenon called incomplete penetrance. Variable expressivity also occurs, with some individuals more affected than others of the same genotype. In most cases the reasons for these differences are unknown, but they are assumed to be due at least in part to ot…

Inheritance Patterns - Imprinting

Some cases of incomplete penetrance appear to be due to imprinting. In this phenomenon, expression of an allele is governed by whether it is derived from the mother or the father. Imprinted alleles are located on autosomes, but are "stamped" with the sex of the parent that contributed it. The chemical basis of the imprint is the addition of methyl (-CH3) groups to the allele's…

Inheritance Patterns - Polygenic, Multifactorial, And Complex Traits

Proteins, which are the products of genes, interact with one another in complex ways to determine the phenotype. Almost every trait we observe, such as height, normal metabolic level, or intelligence, is really the product of many genes. Many traits, however, also reflect the influence of the environment. Such traits are called complex traits, to distinguish them from simple traits that are govern…

Inheritance Patterns - Pleiotropy And Epistasis

Most single genes affect more than one observable trait, a phenomenon know as pleiotropy. For example, the alleles for melanin pigment affect skin color, eye color, and hair color. The ion channel gene affected in cystic fibrosis acts in the lungs, the pancreas, and other passageways, and defects cause symptoms in both these organs, as well as elsewhere in the body. Proteins are also involved in h…