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MM15 - Medelian Genetics 1 (ii) (Autosomal Recessive (things that…
MM15 - Medelian Genetics 1 (ii)
Autosomal Recessive
phenotype only in homozygotes (aa)
no normal allele
Aa heterozygotes (1 mutated allele) = carriers
no phenotype
not @ a selective disadvantage
hence mutation can become widespread
hence recessive condition more common than dominant
consanguinity increases frequency of inheritance by 2-4%
parents have same carrier mutations
horizontal inheritance pattern
2 carrier parents...
50% chance of carrier offspring
25% chance of affected offspring
25% chance of unaffected offspring
1 affected + 1 normal parent...
100% chance of carrier offspring
1 affected + 1 carrier parent...
50% carrier offspring
50% chance affected offspring
identifying AR disease - newborn screening for diseases with high incidence in the country
e.g. in Ire: cf, PKU, galactosemia, maple syrup urine disease
things that complicate AR disease identification
small family size
case can appear as single + sporadic
uncertain parentage
random gene segregation
disease with decreased fertility/ increased childhood mortality
de novo mutations
if a child has an AR disease, the parents are obligate carriers/heterozygotes
test for a 50% decrease in enzyme activity
genetic heterogeneity
different genotypes result in similar phenotypes
allelic heterogeneity
similar phenotype is produced by different alleles within the same gene
locus heterogeneity
similar phenotype is produced by mutations at different loci
Ethnic variations in allele frequencies
CF (1/25 Irish are carriers)
PKU (high incidence in Ire)
Sickle-cell anaemia (high in Africa as it protects against malaria)
thalassaemias (high in pacific isles)