2. Review of Mendel’s Principles
Genes are passed parents offspring;
get one allele from each parent
During Meiosis, the alleles for a gene
segregate from each other.
During Meiosis, genes independently
assort with each other.
3. Exceptions to Mendel’s principles
Sometimes, there is no dominant or
recessive gene, or the trait is controlled
by many alleles or genes.
4. In Incomplete Dominance, every genotype has its own
phenotype. (One allele not completely dominant over the
other.) Third phenotype that is a blending of the parental
traits. (2 alleles produce 3 phenotypes.)
Result: Heterozygous phenotype somewhere in
between homozygous phenotype.
Incomplete Dominance
5. 1. Incomplete Dominance
Examples:
Trait: Flower Color
Expressions: Red x
White Pink
RR= Red; RW= pink; WW=
white
straight hair, wavy, curly
7. In codominance, neither allele are dominant;
both are expressed. A cross between
organisms with two different phenotypes
produces offspring with has both phenotypes
of the parental traits shown.
Codominance
8. 2. Codominance
Both alleles contribute to the phenotype.
Example: In come chickens
Black Chicken x White Speckled Chicken
9. YOU tell me which type of
dominance…
Codominance!
12. Test Cross (Back Cross)…
Used to determine whether an individual is
homozygous dominant or heterozygous.
RULES:
Always cross unknown genotype with a homozygous
recessive.
Observe (count) large numbers of offspring to insure
accuracy in determining the unknown genotype.
Then…
if ANY offspring show the recessive trait, the
unknown genotype is heterozygous
if ALL the offspring have the dominant trait, the
unknown genotype is homozygous dominant
