19. Multiple alleles & other extensions to Mendel Flashcards
Ratios of monohybrid cross with complete dominance - 1 gene
genotype ratio: 1:2:1
phenotype ratio: 1:3
Incomplete dominance
A form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele
Phenotypic ratio of incomplete dominance
1:2:1 (same for genotypic ratio)
What is co-dominance
The full effect of both alleles can be seen in the phenotype of the heterozygote
Monohybrid ratio with codominance
For both genotype and phenotype - 1:2:1
Example of co-dominance in humans
sickle cell anemia
• HbA - normal beta-globin
•HbS - abnormal beta-globin
HbA and HbA - normal
HbA and HbS - normal and sickled
HbS and HbS - sickled
Define pleiotropy
Pleiotropy occurs when one gene will code and control the phenotype or expression of several different and unrelated traits
Pleiotropy example - sickle cell anemia
•genotype: HbS/HbS
•abnormal haemoglobin - sickling of RBCs
• clumping of cells causing interference with blood circulation
•local failures in blood supply can lead to:
- heart failure
- pneumonia
- brain; paralysis etc.
Example of pleiotropy in cats
Allele that decides fur colour also influences the eye colour and hearing
- most cats with blue eyes and white fur are deaf
Pleiotropy example - Marfan syndrome
- Marfan syndrome (autosomal dominant) - mutation in fibrillin gene important for connective tissue
- skeleton: loose joints, curved spine
- problems with heart
- eyes: short sighted
Define phenocopy
- a copy of an inherited condition but results from an environmental factor
- occurs when the environment modifies the phenotype to mimic the effect of a specific genotype
- a phenocopy is NOT inherited
Example of phenocopy - meromelia
- meromelia - mutation that affects limb development
- limb deformity causes by taking thalidomide during pregnancy for morning sickness (this is the phenocopy because it is not inherited - due to consumption of thalidomide)
Vitamin D phenocopy example
- inherited trait: vitamin D-resistant rickets
* phenocopy: low Vitamin D in diet and lifestyle
Define expressivity
The degree or range in which a particular phenotype is expressed by individuals
e.g. mild, moderate, severe
Example of expressivity in dogs
- beagle shows variable expression fur colour and pattern
* same genotype but different distributions of black and white fur
What is penetrance?
The proportion of genotypes that show the expected phenotype
What is reduced penetrance?
When a portion of those with the genotype do not show the trait
ie. Polydactyly - everyone with genotype Pp may not have extra digits
Example of completely penetrant trait
Huntington’s disease - all of those with the mutation will develop the disease
Example of condition that demonstrates both variable expression and reduced penetrance
Polydactyly - extra digits
variable - both Pp genotype, but someone can get 1 extra finger while someone has 2 extra
Reduced penetrance - both Pp but one shows trait and other doesn’t
What factor can contribute to penetrance and expressivity?
Phenotype also influenced by environmental factor
•ie. temperature (Flower patterns in Zinnias, siamese cats)
What is lethality
An allele, genotype or mutation which results in death
When are ‘recessive lethals’ and ‘dominant lethals’ lethal?
Recessive lethal
• only lethal as a homozygote (ie. aa, or BB)
Dominant lethal
• Both HH and Hh are lethal
•late onset so allele (H) is passed to next generation
