HMB265H1 Chapter Notes -Null Allele, Zygosity, Wild Type
6.1 Interactions between the Alleles of a single gene: Variations on Dominance
multiple alleles/allelic series: the known mutant alleles of a gene and its wild-type
Complete Dominance and recessiveness
Full/complete dominance: allele that expresses itself the same in single copy
(heterozygote) as in double copy (homozygote).
Null mutation: a mutation that results in complete absence of function for the gene.
Dominant Negative mutation: mutant allele that in a single dose (heterozygote) wipes
out gene function by a spoiler effect on the protein.
For most genes, a single copy is adequate for full expression (haplosufficient), and null
mutations are fully recessive. Harmful mutations of haplosufficient genes are often
dominant. Mutations in genes that encode units in homo- or heterodimers ca behave as
dominant negatives, acting through “spoiler” proteins.
Incomplete Dominance: heterozygote shows a phenotype quantitatively (but nt exactly)
intermediate between the corresponding homozygote phenotypes. 1:2:1 ratio: inheritance
pattern is based on two alleles of a single gene.
Codominance: expression of both alleles of a heterozygote.
The type of dominance is determined by the molecular functions of the alleles of a gene
and by the investigative level of analysis.
Recessive Lethal Alleles: An allele that is capable of causing death of an organisms. 2:1
ratio can be explained by lethal genes. One allele can be lethal when homozygous.
Pleiotropic allele: an allele that affects several different properties of an organism.
Temperature-sensitive (ts) mutations: a conditional mutation that produces the mutant
phenotype in restrictive temperature range and the wild-type phenotype in the permissive
To see if a gene is essential, a null allele is tested for lethality.