January 20, 2012
Multifactorial trait determine by two or more genes or by the interaction of genes with
If a hybrid gets the trait under consideration identical to that of one parent, the allele
carried by that parent is deemed dominant to the allele of the parent whose trait is not
expressed by the hybrid. This is complete dominance giving rise to a 3:1 ratio of
Incomplete Dominance the hybrid does not resemble either purebreeding parent; their
phenotype is often an intermediate between the parents’.
Codominanceboth traits show up equallt in heterozygote’s phenotype (both spotted and
Both incomplete dominance and codominance give rise to a 1:2:1 F rati2
Cells carry two copies of each genea pair of either similar or dissimilar allelesthat
segregate during gamete formation. Variations in dominance relations thus do not detract
from Mendel’s laws of segregation.
I gives rise to blood type A by specifying an enzyme that adds sugar A, I results in
blood type B by specifying an enzyme that adds sugar B, i does not induce a functional
sugar adding enzyme and is recessive to the two I alleles. Blood type O is therefore a
result of homozygosity for allele i. This is an example of multiple alleles. A persons whose cells carry only A molecules produce antiB antibodies; B people have
antiA antibodies; AB individuals make neither antibody; and O individuals produce both
antibodies. These antibodies cause coagulation of cells displaying the foreign molecule.
O is the universal donor
AB is the universal recipient
Dominance Series alleles are listed in order from most dominant to most recessive
Histocompatibility Antigens play a critical role in facilitating proper immune response
that destroys intruders while leaving the bodies tissues intact. Has between 20100 alleles
so no two people, excluding identical twins, carry the same array of cell surface
Mutations alterations of genetic material that arise spontaneously in nature. Once they
occur in gamete producing cells they are inherited. They make it possible to follow gene
transmission as long as there are two alleles.
Allele Frequency the percentage of the total number of gene copies that an allele of a
gene accounts for
Wildtype Allele most common allele in a population; often designated by the subscript
Mutant Allele a rare allele in the same population
Monomorphic a gene with only one common wildtype allele
Polymorphic genes with more than one common allele
Common Variants high frequency alleles of a polymorphic gene
Pleiotropy a single gene determining a number of distinct and seemingly unrelated
Mutations in almost any gene may have pleiotropic effects because more proteins act in a
variety of tissues.
A variation of pleiotropy occurs in alleles that not only produce a phenotype but also
Lucien Cuenot’s Odd Yellow Mice
Mating of inbred agouti mice to yellow
mice always results in a 1:1 ratio of
yeylow to agouti.
A is dominant to A for coat colour. Mating of yellow to yellow mice always results in a 2:1 ratio of yellow to agouti because
the A allele is a recessive lethal.
Recessive Lethal Allele an allele such as A that negatively affects the survival of a
homozygote. They usually remain silent until rare homozygocity occurs
Some mutations cause the homozygote to survive beyond birth but to die later from the
deleterious consequences of the genetic defect
Heterozygous carriers can only pass on recessive alleles causing prenatal or early
childhood lethality to subsequent generations because affected homozygotes die before
they can mate. However, for late onset lethal alleles, homozygous patients can pass on the
lethal allele before they become debilitated.
Early Onset gene is necessary for cellular function, death at embryogenesis or early on
Late Onset gene is essential for survival but not until the individual has matured
Semi Lethal kills some mutant individuals in the population, but not all
Conditional the mutation is only lethal under certain environmental conditions (e.g.
Lethal alleles cause deviations from Mendial ratios
The ChiSquare Test
Are the deviations from the expected 3:1 phenotypic ratio due to
Null Hypothesis: the observed deviation from the expected
phenotypic ratio is due to sampling error
Phenotype Observed Expected (Observed−Expected)
Red Eyes 3470 3189 24.8
White Eyes 782 1063 74.3
4252 4252 99.1
The P value is the probability that the deviation is due to sampling error.
Our statistic is 99.1 which is way off of the chart, meaning it is much lower than 0.05, so
the deviation much be caused by some sort of biological mechanismlike the white eyed
are dying off.
What Mendel Extension Extension’s Effect Extension’s Effect
Descried on Het. Pheno. on Ratios of F1
Complete Incomplete Unlike either Phenos coincide
dominance dominance, homozygote with geno in a ratio
Codominance of 1:2:1
Two alleles Multiple alleles Multiplicity of A series of 3:1 ratios
All alleles equally Recessive lethal No effect 2:1 instead of 3:1
One gene Pleiotropy: one gene Several traits Different ratios,
determines one trait influences several affected in diff ways depending on