Genetics Review Answer Key
1. Explain how Mendel’s original theory of inheritance differs from the more realistic explanation that modern
science has developed. Mendel explained that two genes or alleles are inherited for the trait (one from mom
and one from dad) and the dominant allele will always be expressed over the recessive trait. If an organism
contains two recessive alleles, then the recessive allele will be expressed. In reality, genetics and inheritance
are much more complicated than that. Co-dominance, incomplete dominance, sex linked inheritance,
multiple allele inheritance, and polygenic inheritance are all inheritance patterns that do not follow
Mendel’s theory.
2. Explain the difference between dominant and recessive. What do scientists use to show dominant and
recessive traits? Is recessive always the “bad” trait? Dominant traits are more likely to be expressed and are
shown using a capital letter. Recessive traits are only expressed if there is not dominant and is shown using
a lower case letter. Recessive traits are not always bad if the allele benefits the organism in it’s environment.
3. Explain the difference between a phenotype and a genotype. Give examples.
A phenotype is the genes that are expressed or the way an organism looks. A genotype is the genes or alleles
an organism actually has. Phenotype could be brown eyes and the organism’s genotype could be Aa. Brown
eyes are expressed which is the dominant trait but the organism contains a recessive allele that is not being
expressed (a).
4. Explain the difference between heterozygous and homozygous. Give examples.
A heterozygous organism contains two different alleles for the same trait. Aa. A Homozygous organism
contains two of the same alleles for the same trait. AA or aa.
5. What is the difference between autosomes and allosomes?
Autosomes are chromosomes numbered 1-22 in humans and control the physical characteristics of an
organism. Height, eye color, skin are all examples of traits that autosomes control. Allosomes control the
gender of an organism and are the last chromosomes in an organism’s karyotype. XX or XY.
6. Explain the difference between incomplete dominance and co-dominance. Provide examples.
Incomplete dominance occurs when it appears as though the two genes mix to form an intermediate allele
between the dominant and recessive allele. White rose + Red rose = pink rose.
7. Cross F1 x F1.
Y- yellow pea pods, y- green pea pods
P
F1
F2
YY x yy
Yy
YY x yy
Yy
YY
Yy
Yy
yy
?
8. Color blindness is a recessive sex linked disorder. If Charlie is normal (XBY) for color blindness but his
wife Lynsey is a carrier (XBXb) for color blindness, what is the probability that their children will be color
blind?
XBXB
XBY
XBXb
XbY
9. Fill in the appropriate genotypes for the possible gametes that each of the parents will likely produce and the
resultant genotypes that each of the children will likely receive.
SY
Sy
sY
sy
SY
SSYY
SSYy
SsYY
SsYy
Sy
SSYy
SSyy
SsYy
Ssyy
sY
SsYY
SsYy
ssYY
ssYy
sy
SsYy
Ssyy
ssYy
ssyy
10. If a man has type Ai blood and his wife has type Ai, what is the probability of their offspring having type
A?
75%
AA
Ai
Ai
ii
11. If a Black Mouse that is homozygous for coat color BB is crossed with a White Mouse that is homozygous
for coat color bb all of the F1 generation will have black coats. Use the punnett square on the right to
predict what would happen if two organisms from the F1 generation were crossed? B- Black coat color, bwhite coat color
P
F1
F2
BB x bb
Bb
BB x bb
Bb
?
BB
Bb
Bb
bb
12. Eye color in flies is a sex-linked trait. Use the picture below to answer the following questions.
 Are white eyes determined by the X or the Y chromosome? X Chromosome

Is it possible for two red eyed flies to produce a white eyed offspring? Which parent can be a
carrier? Yes. There is a possibility that the female can carry the gene but may not express it. It is
impossible for a male to contain the gene and not express it.
13. The diagram below represents evidence from a crime scene collected and analyzed by forensic scientists.
There were three primary suspects implicated in the case but the DNA indicates that one of the three was
clearly at the scene. How can you identify which of the three suspects was present at the scene?
The DNA sample from the crime scene contains the same genetic material in the same pattern as suspect #2.
The dark bands indicate the presence of a nitrogenous base and the genetic information pulled from suspect #2
contains the same nitrogenous bases at all the same locations as the DNA left at the crime scene.
14. Using the pedigree chart below, explain trends in the presence of the condition and how this condition might
affect future generations. The conditions only to appear in males which suggests that the disease may be
located on the sex chromosomes. We can also see that two individuals that did not exhibit the disease passed
the disease along to their child which suggests that the disease is a recessive condition.