Genetics Problems—“Helpful Hints”
1. Determine & write out the traits to be used.
2. Know how the alleles are expressed, e.g., dominant, recessive, codominant,
incomplete dominance, sex-linked
a. Look for clues… is there a capital & a lower case letter… two capitals… a
“base” letter w/ 2 different superscripts… is gender/sex mentioned…
3. If you are not given them, choose a letter symbol for each trait.
a. Use capital letters for the dominant alleles, and lowercase letters for the
recessive alleles.
b. Two capitals for codominant traits…
c. a “base” letter w/ 2 different superscripts for incompletely dominant traits
d. XX for female; XY for male…
4. Determine the phenotypes & genotypes of the parents. Keep in mind which traits
are dominant, recessive, etc. when you do this.
a. Write dominant alleles first.
b. If looking at more than one trait… keep alleles for each gene together (ex.
Put both alleles for hair color together & both alleles for eye color
together).
5. Determine the gametes that can be produced by each parent. Remember, each
gamete has to get one allele of each gene (one letter from each trait).
a. If looking at only one trait, each individual letter represents a gamete.
b. If looking at two traits…. You must figure out the “2 letter” combination of
alleles that will go into the gamete.
i. Use foil or a tree diagram….
6. Cross the parents’ gametes using random fertilization to combine the gametes in
all possible combinations – In other words, make a Punnett square (which is the
easiest way to do this).
7. Determine the genotypes and phenotypes of the offspring. You may be asked to
do this as a ratio.
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Useful Tips to Remember
1. If an organism shows the recessive trait, you know it is homozygous recessive.
2. If an organism shows a dominant trait, you know it has at least one dominant
allele.
3. If a third phenotype is observed, expect either codominance (both traits show at
the same time—ex. red & white colors are both seen in a Roan cow) or
incomplete dominance (both traits blend together like paint colors to form a 3rd
trait--ex. 4 o’clock flowers… red & white blend to form pink flowers.)
4. If a trait is expressed mostly in either males or females, suspect sex-linkage.
a. X-linkage is most common & the trait will appear mostly in males.
5. Monohybrid crosses (ex. Bb x Bb) result in 3:1 phenotype ratios (ex. 3 Brown: 1
blue), and 1:2:1 genotype ratios (ex. 1 BB : 2 Bb: 1 bb).
6. Dihybrid crosses (ex. BbEe x BbEe) result in 9:3:3:1 phenotype ratios (ex. 9 brown
hair, brown eyes : 3 brown hair, blue eyes : 3 blonde hair, brown eyes : 1 blonde
hair, blue eyes).
7. Incomplete dominance and codominance result in 1:2:1 phenotype ratios.
a. Example: incomplete dominance
i. 4 o’clock flower: CRCW x CRCW  1 red : 2 pink : 1 white
b. Example: codominance
i. Cows: RW x RW  1 red : 2 roan : 1 white
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