Medical Genetics

INTRODUCTION

High-Yield Terms

Autosome

Any one of the 22 non-sex determining chromosomes

Pedigree

An orderly diagram of a family’s genetic features, relative to a particular disease in the family, extending over several generations

Punnett square

A Punnett square is a graphical representation of the possible genotypes of offspring arising from a particular mating

Polymorphism

Refers to any one of two or more variants of a particular gene or DNA sequence. The most common are single nucleotide polymorphisms, SNP (“snip”)

Incomplete penetration

The lack of phenotype in an individual harboring a dominant mutant allele

Obligate carrier

Specifically refers to females carrying a mutant allele for an X-linked gene. Can often be seen used when describing the parents of a child manifesting an autosomal recessive disease

Concordance/Discordance

Concordance refers to the probability that a pair of individuals will both have a certain characteristic such as the presence of the same trait in both members of a pair of twins. Discordance refers to the lack of the same trait in both members of a pair of twins

Karyotype

Refers to a picture of the number and types of chromosomes in an individual

Linkage disequilibrium

When an observed haplotype frequency does equal the product of the frequencies of the alleles constituting that haplotype

GENETIC BASIS OF INHERITANCE

Medical genetics can be defined as the diagnosis and management of inherited disorders. Specifically medical genetics is the application of genetics to the practice of medical care. Genetics requires an understanding of the nature of the hereditary material (Table 35–1) and the mechanisms for how this information is transmitted from cell to cell. The hereditary material is, of course, the DNA in the nucleus that constitutes the human genome.

TABLE 35–1Basic genetic terminologies.

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TABLE 35–1 Basic genetic terminologies.

Term

Description

Locus

Refers to the particular location, or position, of a segment of DNA; can refer to the position of a single gene or to the position of a segment of DNA containing many genes such as the β-globin locus on chromosome 11 which contains five functional genes and one pseudogene

Gene

The term that defines a unit of heredity; there are both coding (protein-coding genes) and noncoding genes

Allele

All humans inherit a copy of their DNA from each parent and each parental set of chromosomes contains different versions (approximately 3–5 million different bases) termed alleles; for any given gene there are two alleles: the maternal and the paternal versions; for any given gene there is a common allele referred to as the wild-type allele

Variant (Mutant)

Refers to the sequence differences in a gene relative to that of the wild-type gene due to the presence of a mutation; can refer to mutation causing disease but does not necessarily imply difference in function

Polymorphism

Denotes variant ...

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Chapter 35: Medical Genetics

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INTRODUCTION

GENETIC BASIS OF INHERITANCE

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