19. Multiple alleles and extensions to Mendel Flashcards
What is the phenotypic ratio of incomplete dominance when crossing 2 heterozygotes?
1 red : 2 pink : 1 white
What is codominance?
The full effect of both alleles can be seen in the phenotype of the heterozygote
How can codominance be seen in sickle cell anaemia alleles?
- HbA and HbA give normal
- HbA and HBS give normal and sickled cells
- HbS and HbS give sickled
What are the 6 extensions to Mendel?
- Multiple alleles
- Pleiotropy
- Phenocopy
- variable expression
- Reduced penetrance
- Lethality
What is pleiotropy?
A single gene may affect many features of the phenotype.
What is a phenocopy?
A phenocopy is a copy of an inherited condition but results from an environmental factor. Occurs when environment modifies the phenotype to mimic the effect of a specific genotype - not inherited
What is variable expression?
The degree or range in which a particular phenotype is expressed by individuals (mild to severe). Example is polydactyly
What is penetrance?
Referes to the proportion of genotypes that actually show the expected phenotype, some can escape it
What two extensions to Mendel does polydactyly demonstrate?
Variable expression and reduced penetrance
What is lethality?
An allele, genotype or mutation which results in death
What is a recessive lethal trait?
Lethal only as a homozygote either aa or BB. Recessive lethal could be the dominant phenotype in the heterozygote (need 2 copies of lethal gene to die)
What is a dominant lethal?
Lethal as either HH or Hh, only need one copy of the lethal gene to die
