STUDY GUIDE
CHAPTER 6.3-6.5 and 7: Human Genetics
1. Define the following terms:
o Dominant – the gene or allele that hides a recessive allele. It takes over the recessive
allele and the trait will be expressed.
o Recessive – the gene or allele that is hidden when in the presence of a dominant allele.
The only time a recessive trait will show is when there are 2 recessive alleles.
o Heterozygous – means 2 different
o Homozygous – means 2 of the same
o Pure – means 2 of the same. It is the same as saying homozygous.
o Gene – a trait or characteristic. They are found on chromosomes. There are 2 types:
dominant and recessive.
o Allele – is an alternate form of a gene. All alleles come in pairs because 1 comes from
the mother and 1 comes from the father. Alleles can be dominant or recessive.
o P generation, F1 generation, and F2 generation
P generation – represent the parents.
F1 generation – represent the first offspring from the P generation.
F2 generation – represent the second generation of offspring. They come from
the F1 generation.
2. What is the difference between homozygous dominant, heterozygous, and homozygous
recessive?
Homozygous dominant – 2 alleles that are the same and dominant (example AA)
Heterozygous – 2 alleles that are different from one another (example Aa)
Homozygous recessive – 2 alleles that are the same and recessive (example aa)
3. Define the term heredity? Is the passing on of genes or traits from parents to offspring.
4. State the difference between genotype and phenotype?
Genotype – represents the genes that an individual has. You can not see an individual’s
genes. They are made up of 2 alleles (1 from mom and 1 from dad) that are represented by
letters.
Phenotype – represents the physical characteristics that can be seen on an individual.
o How many possible genotypes exist and what are they?
3 possible genotypes
homozygous dominant, heterozygous, and homozygous recessive.
5. How are dominant and recessive genes written (think about the letters)?
Dominant genes – are written in capital letters.
Recessive genes – are written in lowercase letters.
6. What does a punnett square show? It shows the possible genotypes and phenotypes that an
organism may get.
o What do the letters outside the punnett square box represent?
The parent’s genotype and phenotype or the alleles that are found in the ova and
sperm.
o What do the letters inside each individual box represent?
The possible genotypes and phenotypes that an organism may get.
o Does it show observed results or expected results?
Expected results
o Explain what probability means?
It is the likelihood or chance that something can happen.
o Explain the difference between observed results and expected results?
Observed results – are the results that can be seen, it is what actually happened.
Expected results – are the possible results that may happen. It is the likelihood or
possibility that something may happen.
7. Where are chromosomes found in a cell? Inside the nucleus
8. Where are genes found in a cell? On chromosomes
9. All genes and alleles come in pairs.
10. What combination of sex chromosomes results in a male and a female?
Male = XY
Female = XX
11. How many blood types exist? 4
o What are the possible genotypes for a person with blood type A, blood type B, blood
type AB and blood type O?
Blood type A = AA or AO
Blood type B = BB or BO
Blood type AB = AB
Blood type O = O
o Do blood types demonstrate co-dominance or incomplete dominance? Co-dominance
12. Explain the difference between co-dominance and incomplete dominance?
Co-dominance – is when both alleles are dominant, therefore both alleles show and share
each other’s traits.
Incomplete dominance – is when neither allele is dominant, therefore neither allele will
show. One will see a trait that is a combination of both traits. A trait that is in
between or in the middle of both traits.
13. List the steps to completing a punnett square?
Step 1: draw the squares
Step 2: Put mom and dad's genotype outside of the squares. Mom at the top and dad along
the side
Step 3: Bring the top alleles down into the boxes below.
Step 4: Bring the side alleles over to the side boxes.
o What are the genotypic and phenotypic ratios of a Punnett square?
Genotypic ratios – represent the # of possible genotypes in the punnett square
Phenotypic ratios – represents the possible physical traits in the punnett square
14. Practice completing punnett squares.
1. D = Black hair
The parents are DD X Dd
d = Blonde hair
D
D
D
d
DD
Dd
DD
Dd
What are the offspring genotypic ratios?
2DD and 2Dd
2. E = 5 fingers
e = 6 fingers
What are the offspring phenotypic ratios?
4: Black hair or 100%
The parents are Ee x Ee
E
e
What are the offspring genotypic ratios?
1EE, 2Ee, 1ee
E
EE
Ee
e
Ee
ee
What are the offspring phenotypic ratios?
3: 5 fingers
1: 6 fingers
3. Two parents are crossed with the following genotypes: (mom) AO x (dad) AB.
- What type of dominance does this example demonstrate? Co-dominance
- What is Mom’s blood type? A
- What is Dad’s blood type? AB
- Complete a punnett square to determine the possible blood types that their first child
may have.
A
O
A
AA AO
B
AB BO
- Include the possible genotypic and phenotypic ratios for the child?
Genotypic ratio
Phenotypic ratio
1 AA, 1 AO, 1 AB, 1 BO
2: A blood types
1: AB blood type
1: B blood type
15. In humans, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a blueeyed woman and they have three children, two of whom are brown-eyed and one of whom is
blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s
genotype? What are the genotypes of the children?
B
b
b
Bb
bb
b
Bb
bb
Man’s genotype: Bb
Possible genotype of the child: 2:Bb
2:bb
What will the child’s phenotype possibly be? 50% blue or 50% brown
16. In dogs, there is a hereditary deafness caused by a recessive gene, “d.” A kennel owner
has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so
the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does
not wish to use him for breeding so that the deafness gene will not be passed on. This can be
tested by breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate
these two possible crosses. In each case, what percentage/how many of the offspring would
be expected to be hearing? deaf?
Heterozygous
Homozygous Dominant
d
d
d
d
D
Dd
Dd
D
Dd
Dd
d
dd
dd
D
Dd
Dd
Hearing puppies:
2: Dd
Deaf puppies:
2:dd
Hearing puppies: 4: Dd
Deaf puppies: 0:dd
17. Now, using Punnett square(s), show how two hearing dogs could produce deaf offspring.
D
d
D
DD
Dd
d
Dd
dd
18. For each genotype, indicate whether it is heterozygous (HE) or homozygous (HO). If it is
homozygous also state whether it is homozygous dominant or recessive.
AA _HO dominant_
Bb _HE ___
Cc _HE___
Dd _HE___
Ee __HE__
ff __HO recessive__
GG __HO dominant__
HH __HO dominant__
19. For each of the genotypes below, determine the phenotype.
Purple flowers are dominant to white
flowers
PP
_____Purple______________________
Pp
___Purple________________________
pp
___white________________________
Round seeds are dominant to wrinkled
RR
_____round______________________
Rr
___round________________________
rr
___wrinkled________________________
20. Who is Gregor Mendel and why is he important to the field of genetics.
Father of genetics experimented with pea plants and determined that genes/traits get passed
on from parent to offspring.
21. Compare and contrast.
Chromosomes #1-22
Autosomes
Body cells
Chromosome #23
Sex chromosomes – determine gender
XX or XY
22. What is a karyotype, and how can it be used to study human chromosomes?
A picture that shows Autosomes, sex chromosomes, and homologous pairs of chromosomes.
Amniocentesis is when a sample of fetal fluid is taken to create a karyotype of a fetus to look
for any chromosomal abnormalities.
23. Give an example of a sex-linked recessive trait. Why is it more likely that a male be
colorblind than a female?
Color-blindness or hemophilia.
Males only need one colorblind gene, and females need two
Males cannot be carriers, but females can be.
24. How many chromosomes are found in a human somatic/body cell? _46_______ In a
human gamete? ___23___________
25. A fruit fly has a body (somatic) cell with 8 chromosomes. How many chromosomes are in
its gametes? _______4_________
26. Explain X chromosome inactivation.
In females one of the two X chromosomes is randomly turned off by a process called X
chromosome inactivation.
*********Know how to complete a Punnett square for codominance, incomplete
dominance and sex-linked.
****Use your practice worksheets from class*****