Modifications of Mendelian Ratios
Dominance is not always complete
Incomplete dominance : The F1 hybrid resembles neither purebred parent (often an intermediate phenotype)
o Genotype and phenotype ratio is 1:2:1
o Phenotype ratio is equal to the genotype ratio
Among snapdragons there is a gene which produces flower pigment (a protein).
The allele Ar produces the wild type red pigment and the allele Aw produces white (lack of red
When red (ArAr) plants are crossed with white (AwAw) plants the resulting F1 are pink (ArAw).
only one “dose” of red pigment is produced (from Ar ), resulting in pink offspring that do
not resemble either of their parents.
Note* it is proper to designate incompletely dominant or co-dominant alleles as both uppercase
Codominance : Alternative traits are both visible in the F1 hybrid
o Shows same genotype and phenotype ratio as incomplete dominance: 1:2:1
o Neither allele is dominate or recessive to the other
o Multiple alleles of a single gene
o Codominance: ABO Blood Groups
The gene controlling the cell surface sugars responsible for the ‘ABO’ blood grouping system is a
multiple allele system in which IA and IB are codominant to each other but they are both
dominant to the i allele.
Allele IA attaches a sugar different to that of allele IB and allele I does not attach the sugar at all.
Heterozygotes IAIB have both types of sugars on their cell surface.
Phenotype (blood type) Possible genotype
O Codominance is described by phenotype
IA = makes enzyme A to add A sugar
IB = makes enzyme B to add B sugar
i = no enzymes - no sugars added
IAIB = both enzymes to add both A + B sugars
single gene (I) with altered alleles (IA + IB + i )
IA + IB are codominant to each other and dominant to i
Polymorphic gene as the 3 alleles are commonly found
Variations on complete dominance are consistent with Mendel’s law of segregation
o The type of dominance relationship exhibited by different alleles does not affect the way they are
transmitted. Rather, it is a reflection of the way in which the proteins they encode act in the cell.
A gene may have more than two alleles
Multiple alleles exist for most genes
o e.g., blood type : A (IA), B (IB), O (i)
o e.g., trait of lentil seed coat pattern: spotted, dotted, clear (no pattern), marble-1 or marble-2
o e.g., drosophila eye colours white, red, or eosin
o e.g., mouse coat colour black, black and tan, or agouti
o e.g., rabbit coat colour agouti, albino, chinchilla, Himalayan
Reciprocal crosses can be conducted between purebreeding lines representing all phenotypes, to establish the
dominance relationships between all possible pairs of alleles.
This reveals a dominance series, in which alleles are listed in order from dominant to recessive.
o A dominance series: Four alleles for coat pattern o Crosses using homozygotes to establish dominance series
o Many mutant alleles found for coat colour
c+_ Wild type
c ch Himalayan
albino killed by predation in wild
in nature c+ has a frequency of > 99%
c+ is a monomorphic allele gene
o Do not assume the wild-type allele is always dominant!
o New alleles arise through mutation
Chance alterations in the genetic material arise spontaneously in nature
The percentage of the total number of copies of a gene in a population represented by a particular allele
o Wild-type allele : greater than 1 %
o Mutant allele : less than 1 %
o Monomorphic : gene with only one wild-type allele
o Polymorphic : gene with more than one wildtype allele
One gene may contribute to several visible characteristics
o Pleiotropy – Multiple phenotypic effects caused by a single gene
Sickle cell syndrome: an example of pleiotropy
o A variation on pleiotropy: Some alleles may cause lethality
Some alleles may result not only in a visible phenotype, but also affect viability
The assignment of dominant and recessive pertains only to the phenotype being regarded
Recessive lethal alleles - yellow coat
o Allele for yellow coat is dominant for coat colour, but recessive for lethality
(recessive lethal allele)
o Two heterozygotes for A+ and yellow (AY) produce only yellow and agouti
offspring in 2:1 ratio
Manx cats - spinal development
o Homozygous fetus dies in utero
o All Manx cats are heterozygous for the Manx allele o Manx allele is dominant for taillessness and recessive for lethality (recessive
Both are examples of 1 allele affecting 2 different phenotypes (pleiotropy)
just because its dominant for one phenotype, does not mean it has to