Mendelian Genetics Flashcards
Autosomal Dominant
- Affected will pass to 1/2 offpspring
- Males and females are affected equally
- Male to male transmission
- No skipped generations
Autosomal Recessive
- 2 Carriers: 1/4 offspring affected
- Males and females equally affected
- Not typically seen in multiple generations
- Affected + unaffected: all children are carriers
- Affected + carrier: 1/2 carrier + 1/2 affected
*When looking at siblings, if individual is NOT affected, but sibling is…then there is a 2/3 chance that person is a carrier
X-linked
Recessive
- Affected males: all daughters are obligatory carriers
- Carrier females: 1/2 sons affected, 1/2 daughters carriers
- No male to male transmission
- Skips generations
- Heterozygous females are usually unaffected, but some may express condition w/ variable sverity
Dominant
- Affected females: 1/2 children at risk to be affected
- Typically lethal in males; all daughters would be affected
- Tend to see only females affected
Compound heterozygous
&
Double heterozygous
- Compound: different mutant alleles at a given locus
ex: cystic fibrosis - Double: one mutatnt allele at each of 2 different loci
*person is carrier for two conditions
Mendel’s laws (3)
- Unit inheritance: genes come in pairs, one from each parent. Individual genes have different alleles (dom/rec)
- Segregation: Alleles always segregate during gametogenesis
- Independent assortment: Different gene pairs assort independently of one another; random recombination
Mendelian inheritance
- Pattern of transmission of single gene disorders
2.
Lyon Hypothesis
- One X chromosome ine ach somatic cell is **inactivated **
- Occurs during early embryonic development; random
- once an X is inactivated, same one remains inactive in all descendents
*3 exceptions:
- Can have skewed X inactivation; not 50-50
- abnormal X
- Turner’s syndrome (XO)
Y-linked
- Male to male transmission only
- No females
New Mutation
*Factor that can complicate risk assessment*
- Change in a gene that occurs during gametogenesis; mutation not in parent
- person with new mutation will pass mutation to 1/2 offspring
Example: achondroplasia (>80% caused by mutation)
- Association btwn increasing paternal age and increased risk for new mutations to occur
- Typically in autosomal dominant conditions b/c clearest seen
New mutation in X-linked recessive conditions (4)
- 1/3 of patients result of a new mutation, 2/3 have carrier mother
- woman with affected father is an obligate carrier
- women with affected brother and son is an obligate carrier
- single male member affected: 1/3 chance caused by new mutation
Age of onset
*Factor that complicates risk assessment*
- Neonatal, childhood, or adulthood onset
- Assessment can be misleading if later age of onset is not taken into accound
Ex: Huntington disease
- mean age of onset: 35-44
- ~100% show symptoms at age 70
Expressivity
*Factor that complicates risk assessment*
EXTENT to which genetic defect is shown
(mild to severe)
Ex: Neurofibromastosis I
- Cafe-au-lait spots
- fibromas
- Lisch nodules in iris
- risk for malignant tumors
1/2 cases are result of new mutation
Penentrance
*Facrot that complicates risk assessment*
Probability that a gene will have a phenotypic expression
(all or nothing)
*Percentage of people with a particular genotype who are actually affected
- Capable of having affected children
Example: split-hand deformity (autosomal dominant)
- Shows “skipping” of generations
- 70% penetrant
