Introduction to Human Genetics
Overview
gene
: fundamental unit of heredity
genome
: complete DNA sequence; contains the complete genetic info for the organism
size
Þ
3.6 x 10
9
basepairs
number of genes is more important than size (~100K in humans);
Ý
gene #,
Ý
complexity
98% of human genome is non-coding (non-coding region is not ‘junk’ - still has certain functions)
DNA variation
: differences between DNA sequences of haploid genomes within a species
humans are diploid (two sets of homologous chromosomes)
all variation is the result of mutation
humans differ an average of 1 bp per 1000
rate of mutation per gene per generation = 1x10
6
Þ
1/5 children will inherit a mutation
genetic lethal: mutation that causes death before reproduction
Þ
mutation is not passed on
genotype + environment + chance
Þ
phenotype
alleles
: different forms of DNA sequences at a specific site in the genome
sequence variations due to:
single nucleotide substitution
small or large deletions
small or large insertions
different alleles may result in different phenotypes; or may result in the same phenotype
i.e. genetic variation does not equal phenotypic variation
wild-type allele: predominant allele in a population (the ‘normal’ allele)
mutant type allele: variation that causes a different phenotype, not necessarily disease
organisms with diploid genomes may be either homozygous or heterozygous at a given locus
exceptions: sex chromosomes in males; anuploidy
genetic variation can lead to resistance to certain diseases (i.e. HIV)
Classifications of Genetic Diseases
chromosomal
: addition or deletion of parts of a chromosome
most are fatal before birth
single gene
: disease is due to single gene variation (Mendelian)
most phenotypes appear at time of birth
polygenic
: disease due to variation in more than one gene
multifactorial
: disease due to one or more gene variations coupled with environmental factors or chance
most appear in adulthood
mortality from genetic diseases is increasing relative to strictly environmental diseases
because we are better at treating environmental diseases