Download Study Guide for Genetics Test #127

KEY Study Guide for Genetics Test
What to study:
#127
Name __________________________
Textbook: pages 76-93; 96; 102-103; 110-121
Binder: #112-#127
Essential Question: How are traits handed down from one generation to the next?
Vocabulary:
heredity
trait
genetics
gene
allele
dominant
recessive
hybrid
heterozygous
purebred
homozygous
chromosome
sex chromosomes
sex-linked genes
cystic fibrosis
hemophilia
pedigree chart
genetic counseling
sex-linked traits
Huntington Disease
carrier
karyotype
Down Syndrome
Skills:
 Differentiate between genotypes that are homozygous and heterozygous; hybrid and purebred
 Given an individual’s genotype, identify its phenotype
 Set up the appropriate Punnett Square from a word problem (dominant/recessive, sex-linked,
incomplete dominance, or co-dominance)
 Complete a Punnett Square and express probabilities for different genotypes and phenotypes
 Analyze a pedigree chart to determine genotypes
 Trace the transmission of a disease through multiple generations on a pedigree chart
 Draw a Punnett Square to show how sex is determined in humans
 Analyze a karyotype to determine sex and any chromosome abnormalities
Concepts:
 the relationship between cell, nucleus, chromosome, gene, DNA base pairs
DNA base pairs make up the “genetic code” on genes, which are found on chromosomes.
Chromoomes are found in the nucleus of the cell.
 why our chromosomes come in pairs and how each one of a pair is inherited
We have 2 chromosomes of each pair because we get 1 copy from mom and one from dad.

how genes are arranged in chromosome pairs
The same genes are found in the same order on chromosome pairs.

the difference between the sex chromosomes and the rest of the pairs of chromosomes
In females they are just another matching pair, XX. In males, they have one copy of the X
chromosome and one copy of the much smaller Y chromosome.

how the combination of recessive and dominant alleles result in a phenotype
Dominant alleles show up in the phenotype even if there is just one copy. The only way to show a
recessive phenotype (for the first 22 pairs of chromosomes) is to have 2 copies of the recessive
allele. In sex-linked traits, a male can show the recessive trait if he has just 1 copy of the recessive
allele because he doesn’t have another X chromosome that could have an allele to mask it.
 different types of genetic transmission: dominant/recessive, sex-linked, incomplete dominance,
co-dominance, multiple alleles, multigenic traits


how many genes humans have in each cell 20,000-30,000
how cystic fibrosis and other diseases caused by recessive alleles are inherited
CF is inherited when a person receives a copy of the recessive allele from each parent.

how Huntington disease and other diseases caused by dominant alleles are inherited
HD is inherited when a person receives one copy of the dominant genet. It can come from either
parent.

how hemophilia and other sex-linked diseases are inherited
Hemophilia and red-green colorblindness are inherited by a gene on the X chromosome. A male
can get it just be inheriting 1 affected allele from his mom. A female would have to get 2 recessive
alleles, from both her mother and her father. That’s why these conditions are more common in
males.
 which type of inheritance patterns have carriers and which don’t and why
Recessive disorders and sex-linked disorders. Both males and females can be carriers for
recessive alleles, but only females can be carriers for sex-linked traits. Males either have it or
don’t.
 the cause of Down Syndrome an extra 21st chromosome

how probability is used in genetics word problems to predict outcomes
You use a Punnett square to set up all the possible outcomes. Then the probability by dividing
the outcome in question by the total number of outcomes. It can be expressed as a fraction or a
percentage.
Answer the following questions in complete sentences, skipping a line between questions. If there
is a question you can’t figure out, write down where in your binder and book you looked for it.
Questions:
1. Which is larger, a DNA base pair or a gene? Which is larger, a chromosome or a gene?
A gene is bigger that a DNA base pair because it is made of hundreds or thousands of DNA base
pairs. A chromosome is bigger than a gene. There are between 25,000-30,000 genes found on our
23 pairs of chromosomes so the average would be about 1,000 genes per chromosome.
2. In Bikini Bottom, Sponge Bob’s cousin Sponge Rob is heterozygous for eye shape. Which trait
must his phenotype show, the dominant trait or the recessive trait? How do you know?
If he is heterozygous that means he have one recessive allele and one dominant allele. Since the
dominant allele will mask the recessive one, he will show the dominant trait.
3. For a trait that is definitely caused by just one gene, can two people with the same genotype have a
different phenotype? If so, give a specific example using one of the human traits we’ve studied in class
(Gene Wheel, 4 People 5 Traits, a genetic disease, etc.) If not, explain why not.
This is a tricky question. If you have the same genotype that means you have the same alleles for
a trait so you should show the same phenotype. For example, if two people both had Ff for
earlobes, they should both show free earlobes.
However sometimes the environment and/or unknown factors influence how genes are expressed.
For example, identical twins sometimes have slightly different traits even though they have
exactly the same genes (for example, one has a cleft chin and the other doesn’t).
4. For a trait that is definitely caused by just one gene, can two people with the same phenotype have a
different genotype? If so, give a specific example using one of the human traits we’ve studied in class
(Gene Wheel, 4 People 5 Traits, a genetic disease, etc.). If not, explain why not.
Yes. You can’t tell the difference between a person who is homozygous dominant for a trait and
a person who is heterozygous. They both will show the dominant trait. So two people that both
have a widow’s peak could have Ww or WW.
5. What is the difference between co-dominance and incomplete dominance? Give an example of
each.
In codominance both alleles of a trait are seen in the offspring together. Example: chickens who
show both the white feathers from one parent and the black feathers from another parent.
Incomplete dominance is when the offspring show a blend of the parents’ alleles. For example, a
purebred red flower crossed with a purebred white flower would result in pink flowers.
6. Draw a Punnett Square and set up a cross between 2 parents who are carriers for Cystic Fibrosis.
(A = normal gene. a = CF gene)
Then copy each of the following statements. Label each one TRUE or FALSE. If the statement is
false, change it to make it true.
a. There is a 50% chance that the parents would have child with CF. FALSE; 25%
b. There is a 50% chance that the parents would have a carrier TRUE
c. There is a 50% chance that the parents would have an unaffected child (not a carrier,
without the disease.) FALSE; 25%
d. These parents will have 4 kids and one will have CF. False; Not necessarily…If these
parents have 4 children, there is a 25% probability that each child will have CF
e. If these parents had 4 kids, 2 would have to be carriers FALSE: It is likely but not the
only outcome possible.
f. It is possible that these parents could have 4 children, all with CF. TRUE (but unlikely)
7. A dog karyotype shows a total of 78 chromosomes. Sex is determined in the same way as it is in
humans (the last pair is the sex chromosomes, either XY or XX.)
a. How many pairs of non-sex chromosomes are there in a dog?
78 – 2 sex chromosomes = 76 76 divided by 2 (to make pairs)= 38 pairs
b. How many chromosomes did the dog inherit from its mother? From its father?
39 from mom, 39 from Dad for a total of 78 chromosomes
8. Which type of allele can “skip” a generation in a pedigree chart? How can it do this?
A recessive allele can “skip” a generation. In the first generation, one grandparent could show
the recessive trait but be married to someone with a dominant allele. Their child could show the
dominant trait but be heterozygous. If that person marries another person who is heterozygous
for the trait, each of these 2nd generation parents could pass on a recessve allele to a child in the
3rd generation. Therefore the trait “skipped” from the grandparent through the heterozygous
parent to the child. A sex-linked recessive allele can also pass from an affected grandfather
through a carrier mother to an affected grandson
9. Compare the number of chromosomes in a human muscle cell to the number of chromosomes in a
human egg or sperm cell. Is there a difference? If so, why? If not, why not?
Muscle cells have the full amount, 46. The egg cell and sperm cell only have 23. They only
contribute 1 of each pair of chromosomes so when they join, the zygote (which grows into a
human) will have 46 chromosomes in 23 pairs.
10. Of these 4 genetic diseases listed, which one(s) is/are caused by a mutated gene and which one(s)
is/are caused by a difference in the number or structure of chromosomes: Cystic fibrosis, hemophilia,
Down syndrome, and Huntington disease.
Mutated gene: cystic fibrosis ( recessive gene); hemophilia (sex-linked gene); Huntington disease
(dominant gene)
Different # of chromosomes: Down syndrome (3 copies of chromosome pair #21)
11. Which of the following things can be told by looking at a human karyotype?
a. sex YES by looking at the 23rd pair of chromosomes XX= female XY= male
b. age- NO
c. eye color- NO
d. Down syndrome status- YES, if they have 3 copies of 21st gene
e. height- NO f. habitat- NO
12. Draw a picture of a homologous pair of chromosomes. Give them 5 genes, two of which are
heterozygous and 3 of which are homozygous.
A
B
C
D
e
a
B
c
d
e
13. A male fruit fly has the recessive trait for eye color, red. A female is heterozygous with black eyes.
Draw a Punnett Square and predict the probability of genotypes and phenotypes of their offspring.
50% Bb
50% bb
50% black eyes
50% red eyes
14. Why do some kids resemble their biological parents? Why don’t all kids resemble their biological
parents?
Some kids resemble their biological parents because they inherit their genetic information from
them. Half the chromosomes come from one parent and half from the other. Therefore, the genes
that code for proteins that control things like jaw shape or hair color or eye shape are inherited
from their parents.
Many traits are caused by multiple alleles so the child might be a blend or fall in a range for that
trait. Height and skin color are an example of this.
Also, children sometimes receive 2 recessive alleles from parents who show a dominant trait so
they might show a trait that is not found in either parent.
There is so much we don’t know about gene expression- what causes a particular gene to be
turned on or off. It’s very complex!
15. What was the main contribution of Mendel to the field of genetics?




Mendel concluded that there are individual “factors” (which we now know as genes)
which control the inheritance of traits.
these factors come in pairs, one inherited from the mother and one from the father
sometimes one factor in each pair can mask the other
traits from parents don’t always blend in their offspring; offspring can have a totally
different trait (phenotype)
16. Draw a Punnett Square and show a cross between a woman who is a carrier for red-green
colorblindness and a man who is color blind.
a.
b.
c.
d.
What is the probability of having a child who is red-green colorblind? 50%
What is the probability of having a child who is a carrier for red-green colorblindness? 25%
What is the probability of having a child with normal vision, not a carrier? 25%
What is the probability of having a daughter with normal vision, not a carrier? 0%
17. Draw a 3 generation pedigree that shows how cystic fibrosis can be handed down.
Answers will vary but will often show 2 carriers having an affected child.