22. Genes and alleles. Dominance and recessivity - mechanisms, medical examples. Penetrance and expressivity.
Covers the basic vocabulary of genes, alleles, genotype and phenotype, then dominance and recessivity with the test cross and Mendel's pea examples. It goes on to three medical mechanisms (gain-of-function, dominant-negative and loss-of-function) with named disorders, followed by incomplete dominance, codominance, multiple alleles and X-linked inheritance including Barr bodies. It closes with penetrance and expressivity, illustrated by osteogenesis imperfecta, Marfan syndrome and feline polydactyly, and includes four mnemonics.
Topic 23 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 8 sections, about 1,312 words (9 min of reading), with 21 flashcards.
In this note
Genes and Alleles. Dominance and recessivity - Mechanisms, medical examples. Penetrance and expressivity.
Dominance and Recessivity: Mechanisms and Medical Examples
Medical and Genetic Mechanisms Behind Dominance
Variations in Dominance
Multiple Alleles
Sex-Linked Inheritance
Penetrance and Expressivity
MNEMONICS
Genes and Alleles. Dominance and recessivity - Mechanisms, medical examples. Penetrance and expressivity.
The study of genetics began in the 1860s with Gregor Mendel’s experiments using pea plants. His work revealed that traits are inherited in a discrete, predictable manner.
A gene is a unit of heredity that determines a specific trait in an organism.
Variations or alternative versions of the same gene are known as alleles. For example, there are alleles for purple and white flower petals in pea plants.
The complete set of genes in an organism is called its genotype. The physical traits that result from these genes make up the phenotype.
Diploid organisms, such as humans and most animals, carry two copies of each autosomal gene—one from each parent.
If both copies of a gene are the same, the organism is said to be homozygous for that gene.
If the two copies are different, the organism is heterozygous for that gene.
A dominant allele is usually represented by a capital letter, and it masks the effect of a recessive allele, which is shown with a lowercase letter.
When studying the inheritance of a single gene pair, the process is called a monohybrid cross. If two gene pairs are studied together, it is called a dihybrid cross. Studying three pairs is called a trihybrid cross.
Genetic crosses can be used to determine the genotype of individuals by examining the traits of their offspring.
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