Download Human Genetics

Complex Inheritance
Simple Recessive Heredity
• Most genetic disorders are caused by
recessive alleles; the person must be
homozygous recessive in order to
express the disorder
– Cystic Fibrosis
– Tay-Sachs
– PKU
Simple Dominant Heredity
• Only one dominate allele must be present
for the person to express the phenotype
• Some examples of traits that follow this
pattern are:
–
–
–
–
Hitchhiker’s Thumb
Tongue rolling
Free earlobes
Mid-digital hair
Incomplete Dominance
• Phenotype of the heterozygote is an
intermediate between the two
homozygous parents
• Neither allele is dominant…the alleles
blend together if offspring is
heterozygous
Incomplete Dominance
• In snapdragons, neither red or white is
completely dominant
• Red is represented by R and white by
R’ (R prime)
• RR=red flowers
• R’R’= white flowers
• RR’=pink flowers
Incomplete Dominance
in Humans
• Curly hair is incompletely dominant
• HH = straight hair
• H’H’ = curly hair
• HH’ = wavy hair
Codominance
• Phenotypes of both homozygous parents
are expressed in the heterozygous
offspring in equal amounts
• BOTH alleles are dominant AND BOTH
alleles will be seen if genotype is
heterozygous
Codominance
• Chickens with black
feathers are homozygous
for the B allele (BB)
• Chickens with white
feathers are homozygous
for the W allele (WW)
• A heterozygous chicken
(BW) would have black and
white feathers
Codominance in Humans
• Sickle Cell Anemia
– Common in AfricanAmerican populations
– Hemoglobin crystallizes,
changing the shape of
RBC
– Sickle shaped cells slow
blood flow and block small
vessels
– Sickle cells have short
life span causing low blood
cell count and anemia
Multiple Alleles
• The presence of 3 or more alleles for a
genetic trait
• 3 or more genotypes are possible for one
trait…so you get 3 or more phenotypes are
possible for one trait
• Ex: Pigeons
– There are three alleles that code for feather
color, but each individual can have only two.
– BA=ash red > B=grey > b = brown
Multiple Alleles
• BABA or BAB = Ash red feathers
• BB or Bb = Grey feathers
• bb = Brown feathers
Multiple Alleles in Humans
• In humans, there are 3 alleles that code for
blood type
– 3 alleles (IA, IB, and i) determine 4 blood types (A,
AB, B, and O).
– IAIA or IAi result in type A blood
– IBIB or IBi result in type B blood
– IAIB results in type AB blood
– ii results in type O blood
• If a person is given the wrong blood during a
transfusion, incompatible blood cells can clump
together and cause death.
Sex-Linked Inheritance
• Traits are controlled by genes located on sex
chromosomes
• Sex chromosomes determine sex
– XX = female
– XY = male
• Y chromosome doesn’t have same genes as X
• Disorders affect males more than females
Sex-linked Inheritance
• Ex: Fruit flies
– XR=Red Eyes
– Xr=White Eyes
• Females
– XRXR or XRXr = Red Eyes
– XrXr = White Eyes
• Males
– XRY = Red Eyes
– XrY = White Eyes
Sex-Linked Inheritance
in Humans
• Red-green color blindness
– Person cannot tell the
difference between red
and green.
– Recessive allele affects
the red and green
receptors in the eye
– Inherited on the X
chromosome
Sex-Linked Inheritance
in Humans
• Hemophilia
– Missing clotting factor so
cannot stop bleeding
– Treatments include
clotting factor injections
and blood transfusions
– Inherited on the X
chromosome
Polygenic Inheritance
• Inheritance pattern of a trait
controlled by two or more genes…genes
may be on the same chromosome or
different chromosomes
• 2 or more genes control the trait
• Genes can be on different chromosomes
Polygenic Inheritance
in Humans
• Involves 3-4 different
genes
• Offspring look like an
intermediate or
combination of parents’
skin color
• Eye and Hair Color are
also polygenic
Environmental Influences
• Temperature
– Siamese Cats: body temp determines fur color
• Warm areas—white fur
• Cooler areas—black/brown fur
• Light
– Sun exposure increases melanin production,
resulting in tan skin
• Chemicals
– Pesticide use can result in mutations and resistance
Environmental Influences
• Hormones
– Feathers of peahens versus peacocks
• Females are dull and males have bright, attractive plumage
– Sex changing fish
• Age
– Cells cannot divide indefinitely, eventually cannot
repair themselves