complex inheritance. simple recessive heredity most genetic disorders are caused by recessive...
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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 African-
American 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 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