Genetic Inheritance Of Disease Flashcards
Define genetic diversity
genetic diversity refers to the differences in genetic sequences and chromosomal structures between different human beings
this is driven by genetic mutations e.g. different physical traits represent genetic diversity in a population
What is genetic diversity driven by
Genetic diversity is driven by genetic mutations
What is the human genome composed of
23 pairs of homologous chromosomes
22 pairs of autosomes
1 pair of sex chromosomes (XX vs XY)
How many base pairs approx in human genomes and what percentage is non-coding
3 billion base pairs, 98% non-coding
Other 2% is made up of around 21,000 genes
Most genetic mutations occur in areas where they have ______ effect on life
Most genetic mutations occur in areas where they have little/no effect on life
Define allele
1 of 2 alternative forms of a gene present in a chromosome
or
Alternative versions of the same gene
(Aka allelomorph)
Factors that affect allele frequencies in a population include
Population size
Mutation rate
Genetic drift (random- affecting small isolated populations)
Natural selection
Environment
Migration
Non-random mating
define variants
Variants: DNA changes that occur in areas that do not usually affect human health (but can do)
define polymorphism
occurrence of a chromosome/genetic character in more than 1 form, resulting in coexistence of more than 1 morphological type in same population
Variations in population caused by differing alleles (e.g. redheads within a thousand ppl)
define mendelian/monogenic inheritance
A disease or trait determined by a single gene (e.g. CFTR - cystic fibrosis is a monogenic recessive disorder)
define multifactorial inheritance
A disease or trait determined by a small number of genes (called oligogenic trait), or a large number of genes with individually small effects (polygenic traits). May be ‘triggered’ by environmental factors (ex. Hirschprung disease affects ability to pass faeces).
define complex inheritance
A disease or trait determined by a large number of genes and their effects is also altered by a number of environmental factors (such as diet, stress, smoking, etc) (ex. diabetes, cancer, coronary heart disease)
Describe homozygous alleles
If both alleles at a specific position on a gene are identical on the chromosomes (one from the mother and one from the father).
Describe heterozygous alleles
If the two alleles at a specific position on a gene are not identical
define dominant allele
Dominant allele – a gene is in the heterozygous state and one allele is preferentially expressed over the other allele.
It will determine the phenotype both in the homozygous state (2 dominant alleles) and the heterozygous state.
define recessive allele
Recessive allele – the allele which does not affect the phenotype in the heterozygous state but will only influence the phenotype in the homozygous state (called homozygous recessive when 2 recessive alleles only are present)
what are autosomal chromosomes
Chromosomes that are not directly involved in determining sex -
chromosomes 1-22
modes of inheritance in mendelian traits depend on:
Whether the gene is on an autosomal or sex chromosomes
Whether the trait is determined by a dominant or recessive genotype
what are the 5 main types of modes of inheritance
Autosomal dominant
Autosomal recessive
X-linked dominant
X-linked recessive
Y-linked
what is a dominant allele in simple terms
one copy is enough to bring about disease
If the father is affected then male children are spared; if mother is affected then she can pass trait to both boys and girls
Penetrance
Penetrance is
proportion of individuals carry a disease-causing allele
Expressivity
Expressivity - extent to which a genotype exhibits its phenotypic expression
variable penetrance
Variable penetrance: genetic background, environment; an affected individual MAY be asymptomatic
Variable expressivity
Variable expressivity: variation in severity of symptoms
i.e. 1 genotype produces a range of phenotypes
Anticipation in autosomal dominant inheritance meaning
Anticipation: disease manifests at a younger age in successive generations (e.g. Huntington’s disease, triplet repeat expansion)
i.e. disease gets worse as generations go on it mutates more
explain what is meant by complete penetrance
complete penetrance: if all individuals who have the disease-causing mutation have clinical symptoms of the disease
explain what is meant by incomplete penetrance
incomplete penetrance= when we know genotype is present but the phenotype is not observable
Define autosomal dominant
autosomal dominant= gene is on a non-sex chromosome, only need 1 copy of mutated allele to cause disorder/characteristic
Define autosomal recessive
autosomal recessive= gene is on a non-sex chromosome, need 2 copies of mutated allele to cause disorder/characteristic
define x-inactivation in simple terms
early in embryonic development
1 of 2 X chromosomes is randomly + permanently inactivated in cells other than egg cells
define X-linked dominant mode of inheritance
trait is on X chromosome
if father is affected, male children r spared
if mother affected, she passes trait to boys + girls
define X-linked recessive mode of inheritance
expressed in females if there’s 2 copies of mutated allele.
expressed in males if there’s 1 copy of mutated X-linked recessive allele (as man has 1 X)
what is Y-linked inheritance what do we call an affected individual
y-linked inheritance is only father to son transmission
y chromosomes contain few genes (only 200 + most code for male sexual characteristics) so are VERY RARE
affected individuals are called HEMIZYGOTES
define hemizygote
hemizygote= only 1 allele is required for disease
what is a consanguineous marriage
consanguineous marriage - a union between two individuals who are related as second cousins or closer
What is a SNP
Single nucleotide polymorphism= is a variation at a single position (A,C,G,T) in a DNA sequence among individuals
