Exceptions to Mendel’s Laws
 Mendelian traits tend to be the exception in the study of genetics.
 Phenotypes are often controlled by more than one gene that express themselves in some other form than
dominant or recessive
Allele- alternate form of same gene
Loci- location of a particular gene
Chromosome- carries genes
Chapter 5 (pp 95-113)
Mendelian Ratios Wrong?
a. Mendel’s Ratios
i. Monohybrid - 3:1
ii. Dihybrid – 9:3:3:1
b. Lethal allele combinations (in humans many lead to “miscarriages”)
i. any genotype that causes death
ii. in a population sense it is more specifically any genotype that prevents passage of genes
to the next generation
EX. Mexican Hairless Dogs- Hairless is a dominant trait, Homozygous dominant is lethal
c. Multiple alleles- (several hundred in humans)
i. Humans have 2 alleles for all autosomal traits however the gene can exist in more than 2
forms
ii. Leads to variation in phenotypes
EX. ABO blood group
d. Incomplete Dominancei. the heterozygous phenotype is intermediate to the both homozygotes
I.
EX
SnapdragonsRR – Red
rr – White
Rr - Pink
Hair CurlinessCC- Curly
cc – Strait
Cc - Wavy
e. Codominanti. both alleles are expressed in the heterozygote
EX. Coat Color in Horses
Red and White both homozygous
Roan (both red and white hairs) is heterozygous
ii. ABO blood group
1. 4 blood types- A, AB, B, O
2. 3 alleles- IA, IB, i (IO) (I = isoagglutingen = antigen)
Genotype
IA i
IA IA
IA IB
IB i
IB IB
ii
Antigen Present
A
A
A and B
B
B
none
Phenotype
A
A
AB
B
B
O
3. A and B are dominant to O, but Codominant to each other
4. Important for compatible blood transfusion
f. Epistasisi. One gene effects the expression of another gene
EX – Bombay Factor- rare recessive mutation
Genotype - IA IB at one loci and
hh at another loci
 A and B not expressed no antigens Phenotype - O
g. Penetrance and Expressivity – explain the degree of gene expression
i. completely penetrant - everyone who inherits the combination of alleles has some
symptoms
ii. incompletely penetrant - some individuals do not express the phenotype or have no
symptoms
1. Use percentages to describe Penetrance numerically
EX. Polydactyly- (extra fingers or toes)
iii. variably expressive – intensity varies in different people
1. some people may have an extra digit on every extremity or some may just have a
partial digit on one extremity
h. Pleiotropy - one gene (protein) controls several functions or has more than one effect
i. Phenocopy – an environmentally caused trait that appears to be inherited
EX. an injury to the pancreas can mimic diabetes
j. Genetic Heterogeneity – different genes can produce the same phenotype
EX. 132 forms of deafness
II.
Mitochondrial Genes
a. Mitochondrial genes only come from mother
i. mothers usually pass all disorders to all offspring
b. Only DNA fingerprinting that can be done with hard remains (bone / teeth) Trade Center 9/1/01!
III.
Linkage – transmission of genes on the same chromosome
a. do not follow Mendelian rations because genes do not independently assort
b. Recombinant – a series of alleles on a chromosome that differs from the series of either parent
i. a result of crossing over