Neurogenetics is a broad and important topic for the neurology board, RITE, and shelf exams. Genetics of various disorders will be covered in other topic chapters in more depth. In this chapter, we will discuss the fundamentals of genetics, and touch upon the genetic inheritance patterns and disorders that are the most frequently tested. It is important to understand genetics concepts and inheritance patterns, as well as memorize some key chromosomes and genes, which we have included both in this chapter and in the chapter flashcards.
Author: Steven Gangloff, MD
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Table of Contents
Table of Contents
Definitions
Genotype
- The markup of a set of two alleles for one gene. For example “B” and “b” where each letter represents a given allele for a gene. A capital letter indicates a dominant allele while a lowercase represents a recessive allele.
Phenotype
- The physical presentation of the genotype. For example, for eye color, one may write the dominant allele for brown eyes as “B” and the recessive allele for blue eyes as “b”. In this scenario, the genotype for a heterozygote would be written as “Bb,” while the phenotype would be “brown”.
Homozygous
- Both alleles are the same. Example: BB
Heterozygous
- Each allele is different. Example: Bb
Mendelian Patterns of Inheritance
- Humans have 23 pairs of chromosomes (46 total).
- Each gene has two copies, one on each chromosome.
- This second copy can be identical or different. These are called alleles.
- Genes are segments of DNA that are transcribed into RNA and then translated into protein.
- Whether a genotypic allele is expressed (i.e. seen “phenotypically”) depends on the protein that is transcribed by the gene.
- Each chromosomal copy is obtained from the male and female haploid cells (sperm and egg, respectively), which combine by a somewhat random shuffling process known as recombination.
- Because diploid organisms like humans have 2 copies of each allele, every haploid (sperm and egg) has a 1 in 2 chance of holding any given allele.
- When the sperm fertilizes the egg, a 46-chromosome diploid embryo is made. Usually, one of the two alleles will be seen phenotypically (they can also be expressed together, but we will discuss this later). This is the foundation of mendelian genetics.
Autosomal dominant
- An autosomal dominant allele will “overpower” a recessive allele for expression as the phenotype.
- In autosomal dominant diseases, therefore, only one mutated copy of an allele is required to express the mutated phenotype.
- These are usually “gain of function” mutations.
- The risk of inheritance in an offspring is 50% if one parent is affected.
- The inheritance pattern is described as “vertical” on the pedigree chart (no generations are skipped).
- Some of the most commonly tested autosomal dominant disorders include…
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