Inheritance Patterns Flashcards
What are the 3 principles of genetics?
Segregation
Dominance
Independent assortment
What is meant by independent assortment?
Every gene has 2 alleles that code for a trait.
In heterozygotes, one allele is dominant meaning it will always show, one is recessive and is masked by the dominant allele
What is meant by segregation?
Allele pairs separate/segregate randomly from each other during meiosis each cell has a single allele for each trait
What is meant by independent assortment?
Traits are transmitted to offspring independently of one another
What is Mendelian inheritance?
Autosomal and sex-linked
Dominant and recessive
What is non-mendelian inheritance?
Imprinting
Mitochondrial inheritance
Multifactorial
(Mosaicism)
What are the categories of genetic disorders?
Chromosome abnormalities
Single gene disorders
Multi-factorial and polygenic disorders e.g. spina bifida, cleft lip and palate
What is a autosome?
Any chromosome, other than the sex chromosomes (X or Y), that occurs in pairs in diploid cells
What is meant by recessive?
Manifest only in homozygotes
What is an allele?
One or more alternative forms of a gene at a given location (locus)
What is meant by homozygous?
Presence of identical alleles at a given locus
Homozygotes are affected
What is meant by heterozygous?
Presence of two different alleles at a given locus
Heterozygotes are unaffected and are usually referred to as carriers
What is meant by allelic heterogeneity?
The situation where different mutations within the same gene result in the same clinical condition e.g. cystic fibrosis. Thus an individual with an autosomal recessive condition may be a compound heterozygote for two different mutations
What are features of autosomal recessive inheritance?
Typical features:
•Male and females affected in equal proportions
•Affected individuals only in a single generation
•Parents can be related, i.e. consanguineous
What is cysticfibrosis?
Chronic condition affecting mainly lungs and gut, variable presentation, can affect other systems
What is meant by consanguinity?
Reproductive union between two relatives
What is meant by autozygosity?
Homozygosity by descent, i.e. inheritance of the same altered allele through two branches of the same family.
What are features of autosomal dominant inheritance?
Typical features:
•Male and females affected in equal proportions
•Affected individuals in multiple generations
•Transmission by individuals of both sexes, to both sexes
•But don’t forget penetrance & variability!
What is meant by peretrance?
The percentage of individuals with a specific genotype showing the expected phenotype
What is meant by expression?
Refers to the range of phenotypes expressed by a specific genotype
What is meant by recurrence rich?
50% for transmission of mutation
•BUT will the person be affected?
•Depends on penetrance and expression
What is meant by anticipation?
Whereby genetic disorder affects successive generations earlier or more severely, usually due to expansion of unstable triplet repeat sequences
Example – Myotonic Dystrophy
What is somatic mosaicism?
Genetic fault present in only some tissues in body.
What is gonadal mosaicism?
Genetic fault present in gonadal tissue.
What is late-onset?
Condition not manifest at birth (congenital), classically adult-onset
•Example – Hypertrophic Cardiomyopathy
What is meant by sex limited?
Condition inherited in AD pattern that seems to affect one sex more than another
•Example – BRCA1/2
What is meant by predictive testing?
Testing for a condition in a pre-symptomatic individual to predict their chance of developing condition
What are features of x-linked inheritance?
Typical features:
•Usually only males affected
•Transmitted (usually) through unaffected females
•No male-to-male transmission
What is lyonization?
Generally only one of two X chromosomes active in each female cell. Can be skewed
What is genomic imprinting?
Genomic imprinting is an epigenetic (non-genetic influence on gene expression) phenomenon that causes genes to be expressed in a parent-of-origin-specific manner
What is PWS?
Deletion of paternal genes absence of active paternal genes – maternal UPD (uniparental disomy)
What is Angelman syndrome?
Loss of function of maternal UBE3A due to point mutation or deletion paternal UPD
What is homoplasmy?
A eukaryotic cell whose copies of mitochondrial DNA are all identical (identically normal or have identical mutations)
What is heteroplasmy?
•there are multiple copies of mtDNA in each cell
•the name given to denote mutations which affect only a proportion of the molecules in a cell
•the level of heteroplasmy can vary between cells in the same tissue or organ, from organ to organ within the same person, and between individuals in the same family
What is mitochondrial genetic disease?
Group of disorders caused by dysfunctional mitochondria
Caused by mutations in mitochondrial DNA
Caused by mutations in nuclear genes, whose gene products are imported into the mitochondria
Acquired conditions caused by e.g. drugs
Example of multifactoral inheritance?
Cleft lip and palate
- In many families thought due to combination of both genetic and environmental factors
- Increased risk of recurrence above general population risk in future child
What shape is used for males in pedigree drawing?
Square
What shape is used for females in pedigree drawing?
Circle
