Human Genetics Activity
Diploid individuals, like humans, inherit chromosomes in pairs – one from Mom and one
from Dad. Therefore, we also inherit pairs of alleles, alternate forms of a gene, one on a
chromosome contributed by Mom and the other on a homologous chromosome contributed by
Dad. The pair of alleles is always located at the same position on the homologous chromosome
pair. The location of an allele on a chromosome is called the gene locus (pl. loci). This pair of
alleles may be identical or they may differ in the characteristic of the trait they express. An
identical pair of alleles coding for a trait are said to be homozygous whereas a pair of different
allele coding for a trait are a heterozygous allele pair. In the dominance and recessive pattern
of inheritance, a dominant allele will mask or prevent the expression of a recessive allele if
they occur together. It is customary to assign a dominant allele a capital letter of the
characteristic of the trait it expresses while the recessive allele is given the same letter
lowercase.
The alleles possessed by an individual for a specific trait are called the genotype of the
individual, whereas the phenotype of the individual describes the expression of the genotype,
i.e. the physical and physiological traits of the individual.
In the illustration above, for example, there are two homozygous allele pairs (PP and aa)
and one heterozygous allele pair (Bb). The genotype for the traits in question is PPaaBb. The
phenotype is the expression of this genotype, for example, pink, inactive, bushy.
The alleles for autosomal characteristics are carried on the autosomal (nonsex)
chromosomes. If individuals are homozygous dominant (AA) or heterozygous (Aa), their
phenotype will be the dominant characteristic. If individuals are recessive (aa), their phenotype
will be the recessive characteristic.
Activity 1: Human Autosomal Dominant and Recessive Inheritance
Procedure:
1. For this activity, you will need a lab partner to help you determine your phenotype for
a number of traits.
2. Record your phenotypes by circling them in the first column of Table 1.
3. Identify the genotype(s) for each phenotype. Use correct symbols as determined by
the dominant characteristic.
4. Circle your probable genotype. If you have a recessive phenotype, you know your
genotype. If you have the dominant phenotype, you may be able to determine
whether your genotype is homozygous or heterozygous by recalling the
phenotype of your parents, siblings or children. If not, circle both dominant
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genotypes.
5. Record the number of individuals in class with each phenotype and calculate the
phenotypic ratio for each trait, dominant to recessive phenotypes. To determine
the phenotypic ratio, simply divide the number of each phenotype by the smallest
of the two numbers. Example: In pea plants, green pea color is dominant to yellow
pea color. You count 15 green and 8 yellow peas. Phenotypic ratio = 15/8: 8/8 =
1.9 : 1 green : yellow.
Table 1: Autosomal Dominant and Recessive Human Traits
Trait
PTC Tasting
Possible
Possible
Phenotypes
Genotypes
(Dominant followed
by Recessive)
Taster
TT, Tt
Non-taster
Earlobe type
tt
Unattached
Attached
Skin Pigmentation
Freckles
No freckles
Hairline type
Widow’s peak
Straight hairline
Mid-digit Hair
Present
Absent
Thumb hyperextension Present
(Hitchhiker’s thumb)
Absent
Little Finger Shape
Bent
Straight
Finger Interlocking
Chin dimple
Left thumb over
right thumb
Right thumb over
left thumb
Dimple present
Dimple absent
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Number in
Class
Phenotypic
Ratio
(dominant to
recessive)
Table 1 Continued
Trait
Tongue Rolling
Possible
Phenotypes
(Dominant followed
by Recessive)
Roller
Possible
Genotypes
Number in
Class
Phenotypic
Ratio
(dominant to
recessive)
Non-roller
Arm folding
Left on top
Right on top
Cheek dimples
Present
Absent
Iris pigmentation
Pigmented
Non-pigmented
Blood type
A
B
AB
O
Trait Descriptions:
PTC Some individuals detect a distinct bitter taste from a chemical, phenylthiocarbamide (PTC), while
others do not taste it at all. A dominant allele (T) seems to confer the ability to taste low concentrations
of PTC. Place a paper strip impregnated with PTC on your tongue and allow it to remain there for about
ten seconds. If you are a taster, you will know it; if you’re not sure, then you are a non-taster (tt).
Earlobe type In most people the earlobes hang free, but when a person is homozygous for a
certain recessive allele, the earlobes are attached directly onto the side of the head.
Skin pigmentation Possessing freckles is dominant over not possessing freckles.
Hairline type If your hairline forms a distinct point in the center of your forehead, you have what is
known as widow's peak. It results from the action of a dominant allele. Absence of a widow’s peak
indicates a straight hairline, produced by a pair of recessive alleles.
Mid-digital hair The complete absence of hair from the middle phalanx of all fingers is recessive to the
presence of mid-digital hair on one or more fingers. There seems to be a number of alleles that
determine whether hair grows on one, two, three, or four fingers.
Thumb hyperextension This characteristic, caused by a recessive allele, is known more exactly as
"distal hyperextensibility of the thumb". Homozygote recessives can bend the distal joint of the thumb
back until there is almost (but not quite) a 90 degree angle between the two phalanges. There is some
variation in expressivity, because occasionally it is found in only one thumb.
Little finger shape A dominant allele causes the last joint of the little finger to bend inward toward the
fourth finger. Lay both hands flat on the table, relax your muscles, and observe your little fingers.
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Finger Interlocking When the fingers are interlocked, some people place the left thumb on top of the
right, while others place the right thumb over the left. Studies suggest that placing the
left over the right is due to a dominant allele, while the right thumb on top is due to a recessive allele.
Chin dimple An indentation in the center of the chin is a chin dimple. A chin dimple is dominant over
lack of a chin dimple.
Tongue rolling A dominant allele results in the ability to roll the tongue upward into a
U-shape.
Arm folding Fold your arms across your chest. Having the left arm positioned on top is the dominant
characteristic whereas having the right arm positioned on top is the recessive characteristic.
Cheek dimples Possessing cheek dimples is dominant to lack of cheek dimples. Variable expressivity
can cause one or both cheeks to have dimples.
Iris pigmentation Homozygosity for the recessive gene results in a lack of pigment in the front part of
the eye’s iris, therefore a blue layer at the back of the iris shows through. This produces blue eyes. A
dominant allele of this gene causes pigment to be deposited in the front layer of the iris and masks the
blue to a varying degree. Other gene pairs determine the nature and density of the pigments that
produce brown, hazel, violet, green, etc. eye colors. However, for this activity, we are only interested in
the presence or absence of such pigment, i.e. blue verses all other colors.
Blood Type is a result of antigen proteins present on the corpuscles. The possible genotypes are AA, AO
(type A); BB, BO (type B); AB (type AB); and OO (type O).
Questions:
1. If an individual exhibits the recessive phenotype, can you be certain of the individual’s
genotype? Explain.
2. If an individual exhibits a dominant phenotype, can you be certain of the individual’s
genotype? Explain.
3. Are dominant phenotypes the most common in a population? Use evidence to support your
answer.
4. Emily and the members of her immediate family have attached earlobes. Her father
has unattached earlobes. What is the genotype of her father?
What are the possible genotypes of Emily’s mother?
5. Jim does not have a bent little finger, but his parents do. What is the expected phenotypic
ratio for little finger shape among all of Jim’s parents’ children.
6. Henry has mid-digit hair. Is it possible that both of his parents possess mid-digit hair?
Could both of his Henry’s parents lack mid-digit hair? Explain.
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