Download Name: LAB 3 ANTH 2101 MENDELIAN TRAITS and INHERITANCE

Name:_____________________
LAB 3
ANTH 2101
MENDELIAN TRAITS and INHERITANCE
Background knowledge
Genes Alleles Dominant Recessive Homozygous Heterozygous
Mendelian Traits
The discrete traits listed below are inherited through the principles of Mendelian genetics.
Record which phenotype you display for each trait by marking the appropriate column
with an X. The photographs supplied will help you determine whether you do or do not
exhibit the trait. After presence/absence data are recorded for each student, total the
number of students displaying each phenotype in the class.
Trait Descriptions:
1. PTC tasting: Phenylthiocarbamide (PTC) is a synthetic form of a naturally occurring
chemical known to suppress the function of the thyroid gland because it blocks iodine
absorption and could lead to goiter. This substance is found in vegetables in the cabbage
family, including kale, brussel sprouts, turnips, cauliflower, etc. The allele for tasting is
dominant. Tasters tend to avoid large amounts of these vegetables; therefore they are less
likely to develop goiter. Put a paper strip toward the back of your tongue. If you are a
taster, the paper will be bitter. If it tastes bad, but not too bad, you may be heterozygous.
2. Tongue folding: Try to turn the tip of your tongue up and back so it touches the center
of your tongue. Most people can only do this if they use the front teeth. The ability to fold
the tongue without use of the teeth is recessive.
3. Widow’s peak: A distinct downward point of the
hairline at the center of the forehead is due to the
expression of a dominant allele. A receding hairline
due to baldness will mask this trait.
4. Attached earlobes: The inheritance of a dominant allele results in earlobes that hang
free rather than attaching directly to the side of the head. Those with attached earlobes are
homozygous recessive for the trait.
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Name:_____________________
LAB 3
ANTH 2101
5. Darwin’s tubercle: This projection or thickening of cartilage on
the outer curve of the ear is due to a dominant allele. The size of
the projection is variable and may be seen on only one ear.
6. Hitchhiker’s thumb: With your fingers curled and
thumb pointing upward, the presence of hitchhiker’s
thumb results in a backward angle of the tip of the thumb
of 45 degrees more. Expression of the trait is recessive.
In people with the dominant allele, the tip of the thumb
bends back only slightly.
7. Bent little finger: Lay your hands palm down on the table and relax your muscles.
Presence of the dominant allele causes the last joint of the little finger to angle slightly
toward the fourth finger. Presence of the dominant allele results in shortening of the
middle bone (the intermediate phalanx) so that it has a triangular shape. This is known as
brachydactyly Type A. Those with straight fifth fingers are homozygous recessive.
8. Mid-digital hair: The presence of hair on the middle segment of the fingers is due to
the presence of a dominant allele. Make fists with both hands. Examine the surface of the
middle finger segments for the presence of any hair. Those with no hair are homozygous
recessive.
9. Facial dimples: Presence of cheek dimples is due to a dominant allele. Smile and have
a classmate note whether you have a dimple. Those who cannot express a facial dimple
on either side are homozygous recessive.
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Name:_____________________
LAB 3
ANTH 2101
10. Short hallux: Compare the length of the big toe to the second toe. Those who have a
big toe that is shorter than the second toe have inherited a dominant allele. Those with a
big toe equal to or longer than the second toe are homozygous recessive.
11. Short index finger: This trait is sex-linked. Lay your hands palm down on the table.
Compare the length of your second (index) finger to the fourth (ring) finger. The allele
for short index finger is recessive but is inherited on the X chromosome. Thus, females
who have the recessive allele on both X chromosomes and males who have one copy on
the X chromosome will express the trait. Females who have only one copy (recessive
allele on one X chromosome) or two dominant alleles will have index fingers that are
longer than the ring fingers.
12. Hand clasping: Fold your hands together by interlacing your fingers. Which thumb
is on top? Try interlacing your fingers the opposite way. It will feel awkward and may
require some thought to accomplish. Those who place the left thumb over the right have
inherited a dominant allele, while those who place the right thumb over the left are
homozygous recessive.
13. Hair whorl: Stand behind your classmate and note which way the hair curves. If the
hair flows clockwise, the individual has at least one dominant allele. Hair that flows
counterclockwise is a recessive trait.
14. Sticky earwax: Note your type of earwax., while flaky grayish earwax is a dominant
allele. Sticky, yellowish earwax is inherited as recessive.
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Name:_____________________
Trait
LAB 3
Present
Absent
ANTH 2101
Total w/
trait
Total w/o
trait
PTC tasting
Tongue folding
Widow’s Peak
Attached Earlobe
Darwin’s tubercle
Hitchhiker’s thumb
Bent little finger
Mid-digital hair
Facial dimples
Short hallux
Short index finger
Hand clasping
Hair whorl
Sticky earwax
Mendelian Inheritance
Now we’ll examine what Mendel found through his famous experiments with the peaplants. Mendel was able to identify that genes acted as discrete (quantum) units through
some very simple cross-breeding experiments with pea plants. Two principles were
illustrated: the Law of Segregation and the Law of Independent Assortment.
The Law of Segregation simply states that alleles of a gene will separate from each
other during reproduction, and maintain their separate identities during reproduction,
even if they are not expressed.
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Name:_____________________
LAB 3
ANTH 2101
The Law of Independent Assortment tells us that two genes on different chromosomes
will segregate into gametes independently of each other.
Punnett Square
To help illustrate Mendel’s laws, we can use a Punnett Square. This segregates possible
alleles from each parent (indicated as row and column labels, each row is equally likely
and each column is equally likely) and will help find the possibilities for any given
combination alleles (shown as the cells in the square, with each cell being an equally
likely offspring genotype). We will use the trait (gene) of tasting for a chemical called
phenylthiocarbamide (PTC). PTC is found in many cruciferous vegetable (brussel sprouts
for example), and follows the rules for Mendelian inheritance. We will use a capital (P)
for the dominant allele, tasting condition and a lower case (p) for the recessive nontasting condition (allele).
The two alleles that are carried by each donor (parent) are separated onto opposite sides
of the square. This shows us that the alleles from each donor can be inherited independent
of one another due to the separation of the chromosomes into a random gamete (Law of
Segregation). If two people who are heterozygous for PTC tasting (Pp) were to produce
offspring, the Punnet square would look like:
From this we can calculate the frequency for each genotype (the combination of alleles)
and each phenotype (the outward expression of those alleles- what you see).
There are three possible genotypes in the offspring from this mating: PP, Pp, and pp. The
ratio of these genotypes is 1:2:1.
From these genotypes only 2 phenotypes are possible: those who can taste PTC and those
who cannot. Those genotypes with the dominant P allele will taste PTC and those with
two recessive alleles (pp) will not taste PTC.
Scenario 1:
A pea plant is heterozygous for seed color. It contains the alleles for yellow and green
seed color. Use the letter “Y” to represent the seed color allele where yellow is dominant
and “y” for the recessive green color.
1. What is the plant’s genotype?
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Name:_____________________
LAB 3
ANTH 2101
2. If yellow is dominant to green, what is the plant’s phenotype?
Scenario 2:
You decide to repeat Mendel’s breeding experiments for seed color in peas. You begin
your experiment with a homozygous yellow plant and a homozygous green plant.
3. What are the genotypes for each plant?
4. Provide a Punnett square for a cross between the two plants.
5. What is/are the possible genotype(s) for the offspring produced by crossing the plants
(this is the first, or F1, generation)?
6. What is/are the possible phenotype(s) for this F1 generation?
Scenario 3
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Name:_____________________
LAB 3
ANTH 2101
Punnett squares can be extended beyond a simple one gene cross. For instance, we want
to find out the possible genotypes and phenotypes for two people who are heterozygous
for PTC tasting (Pp) and who are also heterozygous for attached earlobes (Ee, with free
earlobes dominant). These genes are not linked and are therefore on 2 different
chromosomes. Each person’s genotype is PpEe and their phenotype is PTC tasting and
free earlobes. To complete this Punnet square, you must first find every combination of
alleles for each person. This would be what the gametes would carry at the end of
meiosis. These combinations are listed at the top and on the left for each individual.
There are nine possible genotypes from this cross: PPEE, PPEe, PPee, PpEE, PpEe, Ppee,
ppEE, ppEe, ppee. There are four possible phenotypes from this cross with a frequency of
9:3:3:1.
7. What are the four phenotypes possible from this cross?
8. What is the frequency (percentage) for each of these phenotypes?
Scenario 4:
You are given a female dog that is heterozygous for two loci. It is heterozygous for tail
length and ear shape. The two loci are unlinked (they are on different chromosomes).
Long tails are dominant to short tails and triangular ears are dominant to ovular ears,
9. Use the letter “T” for the tail length gene and “E” for the ear shape gene. What is the
genotype of the dog? What is the phenotype of the dog?
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Name:_____________________
LAB 3
ANTH 2101
10. What are all of the possible gametes this dog can produce (what are all of
combinations of alleles)?
11. Imagine that you mate your dog with a male dog that has a short tail and ovular ears.
What is the genotype of the male dog?
12. Draw a Punnett square to depict the possible offspring that could be produced by
mating the two dogs.
13. What are the genotype frequencies from this mating?
14. What are the phenotype frequencies from this mating?
15. Imagine that your original female dog is now mated with another male dog that is
heterozygous for both traits. Draw a Punnett square to depict the mating.
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