Genetics Archive

X-linked recessive disorders

Let’s look at the pedigrees of disorders caused by rare recessive alleles of genes located on the X chromosome. Such pedigrees typically show the following features: 1. Many more males than females show the rare phenotype under study. The reason …

Autosomal inheritance in humans

Human matings, like those of experimental organisms, show many examples of the inheritance patterns described above. Because controlled experimental crosses cannot be made with humans, geneticists must resort to scrutinizing records in the hope that informative matings have been made …

Penetrance and expressivity

Many mutations show incomplete penetrance: not every individual with the genotype expresses the corresponding phenotype. Thus penetrance is defined as the percentage of individuals with a given allele who exhibit the phenotype associated wit many mutations show incomplete penetrance: not …

Epistasis

In trying to find evidence of gene interaction, one approach is to look for cases of a type of gene interaction called epistasis. This word means “stand upon,” referring to the ability of a mutation at one locus to override …

Recessive Lethal Alleles

Many mutant alleles are capable of causing the death of an organism; such alleles are called lethal alleles. The human disease alleles provide examples. A gene whose mutations may be lethal is clearly an essential gene.The ability to determine whether …

Chromosomal theory of Inheritance

Historical development  of the chromosome theory The theory and practice of genetics took a major step forward in the early part of the twentieth century with the development of the notion that the genes, as identified by Mendel, are parts …

Mendel’s law segregation

Mendel’s model for the pea-color example, translated into modern terms, was as follows 1. A hereditary factor called a gene is necessary for producing pea color. 2. Each plant has a pair of this type of gene. 3. The gene comes …