Bio 200 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, ,
sex-linked
a. Look for clues… is there a capital & a lower case letter… two capitals… 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. XX for female; XY for male…
d. X”T” for X-linked dominant… X”t” for X-linked recessive… Y for the male gene
without the trait
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. For sex determination put X first then Y
c. For X-linked X”T” (X-linked dominant) goes before X”t” (X-linked recessive) in
a female carrier
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.
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 phenotype for the recessive trait, you know it HAS TO
BE homozygous recessive.
2. If an organism shows the phenotype for the dominant trait, you know it has at
least one dominant allele.
3. If a third phenotype is observed, expect codominance (both traits show at the
same time—ex. red & white colors are both seen in a Roan cow)
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. Heterozygous x Heterozygous (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. If two parents that have both codominant allele… the result is a 1:2:1 phenotype
ratio
a. Example: codominance
i. Roan cows: RW x RW  1 red : 2 roan : 1 white
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