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Incomplete dominanceFor some
traits, neither allele is dominant
Blending occurs
Ex. Snapdragons◦Cross a red
flower with a white flower and get pink flowered offspring
ProblemA true breeding red
snapdragon is crossbred with a true breeding white snapdragon. Red and white colors are considered to be incompletely dominant. What will the phenotypic ratio of their offspring be?
Codominance Both alleles are expressed in the
same organismEx. Chicken feathers
◦Black feathered rooster + a white feathered hen = offspring with some white feathers and some black feathers
ProblemIn a certain type of
rabbit, black fur is codominant with brown fur resulting in an agouti color. If a true breeding black fur rabbit and an agouti rabbit are bred, what will the phenotypic ratio of the offspring be?
Multple AllelesEach organism
has only 2 possible alleles for a trait
Multiple alleles for the trait may exist in the entire population
Ex. Human Blood types◦A, B, AB, O
Blood TypesA and B are CODOMINANTA and B are DOMINANT over OWritten as IA, IB, and I
◦IA IA produce A blood type◦IA i produce A blood type◦IB IB produce B blood type◦IB i produce B blood type◦ii proudces O blood type
What type of parents can produce offspring with any blood type?
ProblemA man with type O
blood marries a woman with type A blood. They have a son with type O blood. Several years later they get a divorce and the wife is suing the husband for child support. Is it possible that he should have to pay?
ProblemA man with type O
blood marries a woman with type A blood. They have a son with type B blood. Several years later they get a divorce and the wife is suing the husband for child support. Is it possible that he should have to pay?
Polygenetic TraitsSome traits are controlled by
more than one gene◦Ex. In humans
Height Eye color Skin color
Intermediate Patterns of Inheritance
Pattern of Inheritance
Description Example
Incomplete Dominance
One allele is not completely dominant over another
Snapdragons: Red flowered plant and white flowered plant crossed make pink flowered plants
Codominance Both alleles contribute to the phenotype
Chicken variety: black feathered chicken X with a white feathered chicken = white and black feathered chicken
Multiple alleles A gene has more than 2 alleles possible
Human blood type: IA and IB (codominant) and i
Polygenetic Traits
Traits are controlled by more than 1 gene
Human skin color: wide range of colors due to several different genes
Chromosomes2 types Autosomes are any
type of chromosome other than a sex chromosome
Sex chromosomes are X and Y
Females XX – always donate an X
Males XY – determines gender of offspring
Sex – Linked TraitsDetermined by alleles
located on the sex chromosomes (X and Y chromosomes)
Males have 1 X chromosome meaning they are more likely to show a sex linked recessive trait
Ex. Albinism, hemophilia, blindness, muscular dystrophy are sex linked
KaryotypesVisual display of all the
chromosomes in an organismIndicate chromosomal
abnormalities◦Broken, missing, or extra◦Down’s syndrome
Turner’s syndromeOnly 1 X chromosome and no YAlways FemaleUsually develop normally but
may result in shorter stature