Inheritance Patterns
• Not all genes migrate and follow
strict Medelian patterns.
Because of this a spectrum of
dominance was created.
• Complete Dominance
– Mendelian pattern of the F1
(heterozygote) being
indistinguishable from the
dominant (homozygote)
phenotype
• Codominance
– both effect the phenotype in
distinguishable ways
– blood types are an example (A &
B are dominant, O is recessive)
Inheritance Patterns
• Incomplete Dominance
– the expression of the dominant
phenotype is lessened by the existence
of the recessive gene
– Red & White true breeding snapdragons
form pink F1 offspring
– the degree to which the dominant gene
is expressed is called penetrance
• Tay Sachs is a disease that causes fat
accumulation in the neural tissues of
children leading to death in the
homozygous individual. The disease is due
to the loss of function of a needed
enzyme.
– Although the allele for the disease is
dominant the heterozygote does not exhibit
disease symptoms. 1/2 of the enzyme
production is sufficient to eliminate
symptoms.
– The disease has very low penetrance on the
heterozygous state.
Aberrations of Mendelian genetics
• Frequency of recessive alleles
– in many instances the recessive
allele is much more common
than the dominant allele
– polydactally (6 or more digits)
is a dominant trait but only
expressed in ~1/400 births
because the majority of the
population is homozygous for
the recessive gene
• Multiple alleles
– most than 2 form of alleles
exist for most human traits
(unlike Mendel's 2 allele peas)
– bood type IA, IB, & i (O)
• Pleiotropy
– most genes exert more than
one phenotypic effect (pick any
genetic disease)
Aberrations of Mendelian genetics
• Epistasis
– a gene at 1 locus alters the effect of a gene at
another
– hair color is effected by a gene at a different
locus that is responsible for deposition of the
pigment in the follicles
– if the animal is homozygous for the color gene
the animal is white even though it may be
homozygous for the pigment (black)
• Polygenic Inheritance (quantitative
inheritance)
– describes traits that are effected by inheritance
of genes on different chromosomes
– the effect of the traits may be additive or
pleiotrophic
– skin color is additive with at least 3 genes
inherited on 3 different locus and different
genes
– gives us a distribution of skin color instead of
dark brown, tan, or white
• Nature vs Nurture
– the environment in which an organism lives can
alter its phenotypic expression
Pedigree Chart
• Recessive disorders
–
–
–
–
homozygous individuals are called carriers
cystic fibrosis
sickle-cell
mating of close relatives increases the
penetrance of recessive disorders
• Dominant disorders
– achondroplasia
– Huntington's
• Multifactoral (Nature + Nurture)
– diseases that have a genetic basis but there
penetrance is tied to environmental issues
– heart disease
– diabetes (type II)
Testing
• Fetal testing
– amniocentesis - looking at cells
from the amniotic fluid
• in the 14th to 16th week
– chorionic villi sampling (CVS)
• investigation of the cells from the
placenta
• newborn screening
– PKU
– Tay Sachs
– Hemophilia
• Genetic counseling
– offered to individuals related to
or who have genetic diseases