Download Genetic Traits

Genetic
Traits
Topic
The traits of offspring are determined by genetic instructions received from
the mother and the father.
Introduction
Traits are passed down from parent to offspring through genetic material in
cells called deoxyribonucleic acid (DNA). Therefore, the DNA that an
individual inherits determines his or her characteristics. Within cells,
molecules of DNA form structures called chromosomes. The instructions for
specific traits are contained on sections of chromosomes called genes.
Offspring receive genes from both their mother and their father for each
genetic trait. Many traits are determined by multiple genes, but some traits
are regulated by only two genes. Often, one of these genes is dominant over
the other. A dominant gene is one that masks the appearance of a recessive
gene. For this reason, a dominant trait will appear in an individual even if
only one of the genes is present. For a trait carried on a recessive gene to
show up, both genes in the pair must carry the trait.
The combination of genes for a specific trait is referred to as an allele.
Gene combinations may be heterozygous or homozygous. A homozygous
allele for a trait can be homozygous dominant, carrying two dominant
genes, or homozygous recessive, with two recessive genes. An allele with
a heterozygous combination of genes carries one dominant and one
recessive gene.
Most genetic traits are carried on body chromosomes, or autosomes, but a
few are carried on sex chromosomes. Each individual possesses only one
pair of sex chromosomes: XX for a female and XY for a male. Of the two
types, the X chromosome carries many more traits than the Y
chromosome.
Consider an imaginary scenario in which a new species of insect has
recently been discovered. Because it can be found buzzing around
chocolate, the new species has been nicknamed the “chocolate fly.”
Genetically, chocolate flies are simple animals. Each has only four
autosomes in addition to its two sex chromosomes.
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GENETIC TRAITS
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In chocolate flies, only a few genes have been located and researched.
The gene for eye color is found on the X chromosome. Since the Y
chromosome lacks genes, X chromosomes determine eye color in
chocolate flies. The rest of the genes that have been studied in chocolate
flies are located on the four autosomes. Each of these genes carries traits
for color. For example, a set of genes on chromosome 1 codes for wing
color, and pink wings are dominant to gray wings. A set of genes on
chromosome 2 carries the traits for body color. The gene for white body
color is dominant to the gene for blue body color. Chromosome 3 carries
genes for head color, and a yellow head is dominant to an orange one. The
genes on chromosome 4 are responsible for the color of the insects’ legs.
The trait for brown legs is dominant to the trait for black legs.
In today’s lab, craft sticks represent the chromosomes of chocolate flies,
and dots represent traits (see Data Table 1). Red craft sticks correspond to
X chromosomes, and green craft sticks stand for Y chromosomes. White
dots drawn or taped on the craft sticks indicate genes for white eyes, and
purples dots indicate the genes for purple eyes. Wood-tone craft sticks are
numbered from 1 to 4 to represent the flies’ autosomes. The traits, and
their representative craft sticks, are detailed in Data Table 1. Remember
that a male fly has 1 X chromosome, 1 Y chromosome, and four sets of
autosomes. A female fly has two X chromosomes and four sets of
autosomes.
Data Table 1: Craft sticks and dots represent
traits on chocolate flies
Chromosome
pair
Traits
Combination of craft sticks
X chromosome
eye color
1 red craft stick with white dot
Y chromosome
eye color
1 green craft stick
Autosomes 1
wing color
2 craft sticks labeled 1, one with a pink
dot, the other with a gray dot
Autosomes 2
body color
2 craft sticks labeled 2, one with a white
dot, the other with a blue dot
Autosomes 3
head color
2 craft sticks labeled 3, one with a yellow
dot, the other with an orange dot
Autosomes 4
leg color
2 craft sticks labeled 4, one with a brown
dot, the other with a black dot
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GENETIC TRAITS
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Time Required
60 to 75 minutes
Materials
✒
✒
✒
✒
pencil
paper
4 empty brown paper bags (such as lunch bags)
1 small paper bag that contains craft sticks representing
chromosomes of a male chocolate fly (prepared earlier by
the teacher)
✒ 1 small paper bag that contains craft sticks representing
chromosomes of a female chocolate fly (prepared earlier by
the teacher)
Safety Note
Please review and follow the safety guidelines.
Procedure
1. In each lab group, assign one lab partner the role of male chocolate
fly and the other the role of female chocolate fly.
2. From the teacher, obtain a paper bag that contains craft sticks
representing chromosomes for either a male or female chocolate
fly. For the purposes of this lab, the chromosomes of both the male
and female flies are heterozygous for each autosomal trait. In
addition, both the male and female flies have white eyes. The
combinations of chromosomes for each gender of fly are detailed
in Data Table 2.
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GENETIC TRAITS
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Data Table 2: Combinations of chromosomes for
male and female flies
Sex of
Sex
Eye color on X
chocolate chromosomes chromosome
(E = white;
fly
e = purple)
Wing color on
autosome 1
(W = pink;
w = gray)
Body color on
autosome 2
(B = white;
b = blue)
Head color on
autosome 3
(H = yellow;
h = orange)
Leg color on
autosome 4
(L = brown
l = black)
Male
XY
white
(XEY)
pink
(Ww)
white
(Bb)
yellow
(Hh)
brown
(Ll)
Female
XX
white
(XEXe)
pink
(Ww)
white
(Bb)
yellow
(Hh)
brown
(Ll)
3.
The male chocolate fly empties the “chromosomes” in his paper
bag on the lab table. Each male should match his autosomal alleles
together by number. The two sex chromosomes should also be
paired (Figure 1).
paper sack of "male chromosomes"
te
ola
oc
h
c
ale fly
3
1
2 2
1
two number 1
craft sticks
with dots
3
red
craft stick
with dot
m
green
craft stick
two number 2
craft sticks
with dots
4 4
two number 4
craft sticks
with dots
two number 3
craft sticks
with dots
Figure 1
4.
The male transfers one craft stick (chromosome) from each of the
matching pairs into one of the empty paper bags. The remaining
crafts sticks should be placed in the other empty paper bag, then the
tops of both bags rolled down. This step in the procedure represents
meiosis of the male chromosomes. Meiosis is the type of cell
division in which the number of chromosomes is reduced by half.
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GENETIC TRAITS
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5. The male places the two bags behind his back, then mixes them up
by switching hands several times. The female partner then chooses
a hand to select a bag. The selected bag should be set aside on the
desk top. The other bag can be stored out of the way under the
desk or lab table.
6. The female lab partner repeats steps 3 through 5, letting the male
select a bag. This step in the procedures represents meiosis of the
female chromosomes.
7. Each partner pours the contents of the two chosen bags onto the
lab table to represent the combination of male and female
chromosomes, or fertilization. These contents represent the genes
of their offspring. Partners match up the colored craft sticks to find
the sex of the baby fly, then match the autosomal sticks to find out
the baby fly’s body characteristics. Partners record these results on
Data Table 3 in the row labeled “Fly offspring 1.”
8. Replace the chromosomes in the male and female paper sacks,
then repeat steps 3 through 7 to find the genes for the second
offspring. Record these data on Data Table 3 in the row labeled “Fly
offspring 2.”
9. Repeat this entire process six more times until you have produced a
total of eight offsprings and completed Data Table 3.
Data Table 3: Chromosomes of male, female, and offspring
Fly
Sex
offspring chromosomes
1
2
3
4
5
6
7
8
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Eye color
(E)
Wing color
(W)
Body color
(B)
Head color
(H)
Leg color
(L)
GENETIC TRAITS
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Analysis
1.
2.
3.
4.
Did any of the female offspring have purple eyes? Did any of the
male offspring have purple eyes? Based on your knowledge of
genetics, explain which of these two combinations would have been
impossible to achieve.
Some inheritable diseases are transmitted on sex chromosomes.
Explain why such diseases are more often transmitted through the
mother’s family than through the father’s.
Assume that a pair of chocolate flies has eight offspring. Use a
Punnett square to determine the expected number of offspring that
would have each of the following traits: pink wings, white body,
yellow head, black legs. A Punnett square is a simple chart that
shows the possible combinations of parental genes. The chart is
shaped like a window with four panes. The genes of one parent are
written across the top of the Punnett square, and the genes of the
other parent are written along the side (Figure 2).
Use a Punnett square to answer the following: Of four offspring,
how many would you expect to have orange heads if one parent
had a yellow head (Hh) and the other parent had an orange
head (hh)?
genes of another parent
genes of one parent
A
a
A
AA
Aa
a
Aa
aa
Figure 2
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Punnett square
GENETIC TRAITS
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What’s Going On?
In this lab the parents are heterozygous (one dominant and
one recessive gene) for the four autosomal traits. That
means that 75 percent of their offspring express the
dominant traits. The female parent has white eyes, but she
also carries a gene for purple eyes on one of her X
chromosomes. White eyes are expressed because the gene
for white is dominant over purple. The male has white eyes
because he has a gene for white eyes on his X
chromosomes, but his Y chromosome does not carry a gene
for eye color. All of the female offspring of this couple will
have white eyes, although some of them will also possess
the recessive gene for purple eyes. Male offspring may have
white or purple eyes.
Want to Know More?
See Our Findings.
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OUR FINDINGS
GENETIC TRAITS
Notes to teacher: Before the experiment, prepare paper bags of
“chromosomes” for each student. On the day of the experiment, pair
each student with a lab partner. One student will receive a paper bag
containing craft sticks that represent the chromosomes of a male
chocolate fly, and the other student will receive a bag containing craft
sticks that represent the chromosomes of a female chocolate fly. To
prepare the sex chromosomes, use markers or paint to color a supply of
the craft sticks red and a smaller supply of craft sticks green. Each
mating pair (set of two students) will need three red sticks and one green
stick. On two of the red craft sticks, draw a purple dot, or tape a purple
dot punched out from construction paper. This indicates the presence of
the gene for purple eye color. Use Wite-Out™ to paint a white dot on the
other red craft stick indicating the presence of the gene for white eyes.
White eye color is dominant to purple. No genes will be present on the
green craft sticks.
Each lab group should receive:
four
four
four
four
craft
craft
craft
craft
sticks
sticks
sticks
sticks
labeled
labeled
labeled
labeled
“1”
“2”
“3”
“4”
For each student, create a bag of craft sticks to represent his or
her genome.
Each male bag should contain the following (see the table of male
chromosomes) :
A. One set of sex chromosomes, made from a red craft stick
(X chromosome) with a white dot (gene for white eyes) and
1 green craft stick (Y chromosome)
B. Four sets of autosomes
Set 1: Two #1 craft sticks, one with a pink dot and the other with
a gray dot
Set 2: Two #2 craft sticks, one with a white dot and one with a blue
dot
Set 3: Two #3 craft sticks. Place a yellow dot on one and an orange
dot on the other
Set 4: Two #4 craft sticks Place a brown dot on one and a black dot
on the other
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2
Data Table: Male Chromosomes
Chromosome pair
Traits
Combination of craft sticks
Sex chromosomes
Eye color
one red craft stick with white dot and
one green craft stick
Autosomes 1
Wing color
two #1 craft sticks, one with a pink dot, the
other with a gray dot
Autosomes 2
Body color
two #2 craft sticks, one with a white dot,
the other with a blue dot
Autosomes 3
Head color
two #3 craft sticks, one with a yellow dot,
the other with an orange dot
Autosomes 4
Leg color
two #4 craft sticks, one with a brown dot,
the other with a black dot
Each female bag should contain the following (see the Table of female
chromosomes):
A. One set of sex chromosomes, made from two red craft sticks, one
with a white dot on it and the other with a purple dot
B. Four sets of autosomes
Set 1: Two craft sticks labeled “1” that represent wing color, one with
a pink dot and the other with a gray dot
Set 2: Two craft sticks labeled “2” that represent body color, one with
a white dot on one of the craft sticks and a blue dot on the
other
Set 3: Two craft sticks labeled “3” representing head color, one with
a yellow dot and the other with an orange dot
Set 4: Two craft sticks labeled “4” representing leg color. Place a
brown dot on one and a black dot on the other
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3
Data Table: Female Chromosomes
Chromosome pair
Traits
Combination of craft sticks
Sex chromosomes
Eye color
one red craft stick with white dot and
one red stick with a purple dot
Autosomes 1
Wing color
two #1 craft sticks, one with a pink dot,
the other with a gray dot
Autosomes 2
Body color
two #2 craft sticks, one with a white
dot, the other with a blue dot
Autosomes 3
Head color
two #3 craft sticks, one with a yellow dot,
the other with an orange dot
Autosomes 4
Leg color
two #4 craft sticks, one with a brown dot,
the other with a black dot
Analysis question 3 asks students to prepare a Punnett square. If
students are not familiar with this technique, go over it before the
experiment.
Analysis
1. No, yes. A female offspring cannot inherit purple eyes because she
inherits a gene for white eyes from the father’s X chromosome.
2. The males would either inherit the X chromosome that carries the
normal gene from the mother or the one for the diseased trait from
the mother. That X chromosome would be paired with the father’s Y
chromosome (which would not carry a gene) dictating whether the
male offspring inherited the disease or not.
3. pink wings—6
white body—6
yellow head—6
black legs—6
4. 50 percent
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Data Table
Subject
Sex
Eye color (E) Wing color (W) Body color (B) Head color (H) Leg color (L)
chromosomes
Father
XY
White
(XEY)
Pink
(Ww)
White
(Bb)
Yellow
(Hh)
Brown
(Ll)
Mother
XX
White
(XEXe)
Pink
(Ww)
White
(Bb)
Yellow
(Hh)
Brown
(Ll)
Fly
offspring
1
Answers will vary on the rest of the data table
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SAFETY PRECAUTIONS
Review Before Starting Any Experiment
Each experiment includes special safety precautions that are relevant
to that particular project. These do not include all the basic safety
precautions that are necessary whenever you are working on a
scientific experiment. For this reason, it is necessary that you read
and remain mindful of the General Safety Precautions that follow.
Experimental science can be dangerous, and good laboratory
procedure always includes carefully following basic safety rules. Things
can happen very quickly while you are performing an experiment.
Materials can spill, break, or even catch fire. There will be no time after
the fact to protect yourself. Always prepare for unexpected dangers by
following the basic safety guidelines during the entire experiment,
whether or not something seems dangerous to you at a given moment.
We have been quite sparing in prescribing safety precautions for the
individual experiments. For one reason, we want you to take very
seriously every safety precaution that is printed in this book. If you
see it written here, you can be sure that it is here because it is
absolutely critical.
Read the safety precautions here and at the beginning of each
experiment before performing each activity. It is difficult to remember a
long set of general rules. By rereading these general precautions every
time you set up an experiment, you will be reminding yourself that lab
safety is critically important. In addition, use your good judgment and
pay close attention when performing potentially dangerous procedures.
Just because the text does not say “be careful with hot liquids” or
“don’t cut yourself with a knife” does not mean that you can be
careless when boiling water or punching holes in plastic bottles. Notes
in the text are special precautions to which you must pay special
attention.
GENERAL SAFETY PRECAUTIONS
Accidents caused by carelessness, haste, insufficient knowledge,
or taking an unnecessary risk can be avoided by practicing safety
procedures and being alert while conducting experiments. Be sure to
check the individual experiments in this book for additional safety
regulations and adult supervision requirements. If you will be working
in a lab, do not work alone. When you are working off site, keep in
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SAFETY PRECAUTIONS
2
groups with a minimum of three students per group, and follow school
rules and state legal requirements for the number of supervisors
required. Ask an adult supervisor with basic training in first aid to carry
a small first-aid kit. Make sure everyone knows where this person will
be during the experiment.
PREPARING
• Clear all surfaces before beginning experiments.
• Read the instructions before you start.
• Know the hazards of the experiments and anticipate dangers.
PROTECTING YOURSELF
• Follow the directions step-by-step.
• Do only one experiment at a time.
exits, fire blanket and extinguisher, master gas and
• Locate
electricity shut-offs, eyewash, and first-aid kit.
• Make sure there is adequate ventilation.
• Do not horseplay.
• Keep floor and workspace neat, clean, and dry.
• Clean up spills immediately.
• If glassware breaks, do not clean it up; ask for teacher assistance.
• Tie back long hair.
• Never eat, drink, or smoke in the laboratory or workspace.
not eat or drink any substances tested unless expressly
• Do
permitted to do so by a knowledgeable adult.
USING EQUIPMENT WITH CARE
• Set up apparatus far from the edge of the desk.
• Use knives or other sharp-pointed instruments with care.
• Pull plugs, not cords, when removing electrical plugs.
• Clean glassware before and after use.
• Check glassware for scratches, cracks, and sharp edges.
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SAFETY PRECAUTIONS
3
• Clean up broken glassware immediately.
• Do not use reflected sunlight to illuminate your microscope.
• Do not touch metal conductors.
• Use alcohol-filled thermometers, not mercury-filled thermometers.
USING CHEMICALS
• Never taste or inhale chemicals
• Label all bottles and apparatus containing chemicals
• Read labels carefully.
chemical contact with skin and eyes (wear safety glasses, lab
• Avoid
apron, and gloves).
• Do not touch chemical solutions.
• Wash hands before and after using solutions.
• Wipe up spills thoroughly.
HEATING SUBSTANCES
• Wear safety glasses, apron, and gloves when boiling water.
• Keep your face away from test tubes and beakers.
test tubes, beakers, and other glassware made of Pyrex™
• Use
glass.
• Never leave apparatus unattended.
• Use safety tongs and heat-resistant gloves.
your laboratory does not have heat-proof workbenches, put your
• IfBunsen
burner on a heat-proof mat before lighting it.
care when lighting your Bunsen burner; light it with the airhole
• Take
closed, and use a Bunsen burner lighter in preference to wooden
matches.
• Turn off hot plates, Bunsen burners, and gas when you are done.
flammable substances away from flames and other sources
• Keep
of heat.
• Have a fire extinguisher on hand.
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SAFETY PRECAUTIONS
4
FINISHING UP
• Thoroughly clean your work area and any glassware used.
• Wash your hands.
careful not to return chemicals or contaminated reagents to the
• Be
wrong containers.
• Do not dispose of materials in the sink unless instructed to do so.
up all residues and put them in proper containers for
• Clean
disposal.
of all chemicals according to all local, state, and federal
• Dispose
laws.
BE SAFETY CONSCIOUS AT ALL TIMES!
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