Mendel's Laws: Human Inheritance of Single Gene Traits
A Brief
Review of Mendel's Work with Garden Pea Plants
In garden pea plants,
there are two character states for pea height, tall and short. Mendel began with pure lines of pea
plants. In plants, pure lines are
strains that consistently yield offspring with the same traits generation after
generation. Mendel grew plants that were
of pure line for tall and plants that were of pure line for short. He crossed a pure line tall parent plant with
a pure line short parent plant (the P generation) to produce hybrids
(the F1 generation [first filial], in simple terms, the
children of the parent plants). He then
obtained a second hybrid generation (the F2 generation
[second filial], in simple terms, the grandchildren of the parent plants) by
crossing the F1 generation plants with each other. The following results were observed…
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|
Crossing |
Results |
|
1st cross |
pure line tall X
pure line short |
100% tall hybrids
(children of the pure line parent plants that were as tall as the parent
plant) |
|
2nd cross |
hybrid tall F1 X
hybrid tall F1 |
¾ tall to ¼ short
plants (the grandchildren of the pure line parent plants) |
Mendel made the following
conclusions…
1. Alleles control an inherited
characteristic & exist in individuals in pairs (You inherit one member of
the pair from your father & one member of the pair from your mother). The two alleles of a pair are the same in homozygous
individuals (e.g. the pure line short plants are homozygous for the character
state of short. Their allele pair is
short/short). The two alleles of the
pair differ in heterozygous individuals (e.g. the hybrid F1
generation inherited a tall allele from one parent and a short allele from the
other parent. Thus their allele pair is
tall/short). The genotype is the
allele combination that produces a character state. The genotype of the hybrid F1
generation is tall/short. The phenotype is the visible, physical
trait. The phenotype of the hybrid F1
generation is tall.
2. LAW OF DOMINANCE: Whenever the two alleles of a pair in a given
individual differ, only one, the dominant one will be expressed. In the hybrid F1 generation, the
plants inherited one tall allele and one short allele. Yet, all of these plants appeared to be
tall. Thus tall is the dominant
allele (the allele that indicates the appearance of heterozygotes). One allele is said to be dominant over
another if a heterozygous individual for that allele has the same appearance as
an individual homozygous for it. The
short allele is the recessive allele (an allele whose phenotype effects
are masked in heterozygotes by the presence of a dominant allele).
3. LAW OF SEGREGATION OF ALLELES: When the gametes (egg & sperm) are formed
by an individual, only one member of each allele pair is included in a
gamete. Recall that gametes are haploid. When the hybrid F1 generation
plants produce gametes, each gamete will receive only one allele for plant
height. So, an egg (or a sperm) will
have an allele for tall or an allele for short, but not both. When the egg unites with the sperm during fertilization,
the sperm will carry one allele for plant height, restoring the allele pair and
the diploid condition.
4. LAW OF INDEPENDENT ASSORTMENT: All of the possible kinds of gametes that can
be formed, will be formed in equal proportions.
Alleles for different traits are inherited independently of each other
if they are located on different homologous gene pairs.
If Mendel had a tall pea
plant, how could he be certain that it was a pure line tall plant?
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The Sad Plight of the Naked Bunnies
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Photo source: users.tamuk.edu/ |
In this activity, you
will examine inheritance in a small population of wild rabbits. Breeders of rabbits have long been familiar
with a variety of genetic traits that affect rabbits. One such trait is the trait for furless
rabbits (naked bunnies, you can imagine their embarrassment). This trait was first discovered in |
Exploring Segregation (the production
of gametes)
In this
exercise, we are going to imagine that we crossed a pure line furred rabbit
with a naked rabbit. This cross produced
10 hybrid bunnies that were furred, but carried the recessive allele. So, from the hybrid F1 (hybrid)
generation of bunnies, we will have a total of 10 alleles for fur and 10
alleles for no fur.
We are
going to simulate this situation by placing 10 white beads (representing the
furred allele) & 10 red beads (representing the furless allele) into a
cup. We will draw one bead at a time
from the cup, without looking, to produce a "gamete" & then
return the bead to the gene pool. Repeat
the process of picking a bead out of the cup & returning it for the number
of times indicated by your instructor.
Each time, record in Table 1 whether the chosen bead is red or
white. When you have completed your
table, add your results to those of other members of the class in the table on
the chalkboard.
Table 1. "Gametes"
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WHITE
|
RED |
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Your Total |
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Class Total |
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When we
do this, we have a one in two (or 50%) chance of drawing a white bead and a one
in two (or 50%) chance of drawing a red bead.
Let's imagine that we drew 200 times at random from the cup. We would expect to form 100 gametes with the
furred (white) allele and 100 gametes with the naked (red) allele.
Does one always get
exactly the fraction expected in gamete production? _____
What could one do to get
closer to the expected ratio?
________________________________________________________________________________________________________________________________
Do a larger number of
choices (the pooled data of the class) more closely approach what is
expected? ____
In a bunny with one
allele for fur and one allele for furless (a heterozygous bunny), what fraction
of the gametes should contain the fur allele?
________
Exploring Fertilization
In this exercise, we will work in pairs. Each partner will represent on hybrid F1
generation bunny capable of producing gametes with furred (white) alleles and
gametes with furless (red) alleles. One
partner will produce the eggs (by drawing one bead from the cup) and the other
partner will produce the sperm (by drawing one bead from the cup). We will use the cup with 10 red beads and 10
white beads, as before. Each time you draw,
you are simulating the union of one egg and one sperm, resulting in the formation
of one offspring. You and your partner
should each pick one bead from the cup as often as your instructor
indicates. Return the two beads to the
cup after each pick & record the red/red, red/white, and white/white
combinations in Table 2. Again, the
class results will be tabulated on the chalkboard. Since in the offspring it usually makes no
difference whether the one allele (white) comes from the male parent or form
the female parent, the two different red/white or white/red combinations are recorded
together.
Table
2. "Genotypes"
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WHITE
/ WHITE
|
WHITE
/ RED |
RED
/ RED |
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Your Results |
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Class Totals |
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In this procedure, you & your partner each have a 50%
chance of drawing a red bead and a 50% chance of drawing a white bead. Every time you both draw, there are four
possible outcomes.
1. Partner 1 might draw a white bead. Partner 2 might draw a white bead. This would produce a homozygous dominant (or
furred) rabbit. The probability of this
happening can be obtained by multiplying the chance of the first event
occurring by the chance of the second event occurring. In other words…
(50% chance of Partner 1
drawing white)(50% chance of Partner 2 drawing white) = 25% chance of
offspring inheriting white / white, or both alleles for the furred
condition.
2. Partner 1 might draw a white bead. Partner 2 might draw a red bead. This would produce a heterozygous rabbit that
would be furred. The probability of this
happening is…
(50% chance of Partner 1
drawing white)(50% chance of Partner 2 drawing red) = 25% chance of offspring
inheriting white / red, or one allele for the furred condition and one allele
for furless condition.
3. Partner 1 might draw a red bead. Partner 2 might draw a white bead. This would produce a heterozygous rabbit that
would be furred. The probability of this
happening is…
(50% chance of Partner 1
drawing red)(50% chance of Partner 2 drawing white) = 25% chance of offspring
inheriting red / white, or one allele for the furred condition and one allele
for furless condition.
Note that outcomes 2
& 3 can be combined because it doesn't matter whether the rabbits inherit
the allele for fur from the mother or the father, it still produces a
heterozygous rabbit that would be furred.
Thus, the chance of producing a heterozygous offspring is 25% + 25% = 50%
chance of white / red
4. Partner 1 might draw a red bead. Partner 2 might draw a red bead. This would produce a homozygous furless or
naked bunny. The probability of this
happening is…
(50% chance of Partner 1
drawing red)(50% chance of Partner 2 drawing red) = 25% chance of
offspring inheriting red / red, or both alleles for the furless
condition and one allele for the furless condition.
Imagine
that we drew 200 times (representing the production of 200 baby bunnies). We could predict that 25% or 50 of the
offspring would inherit white / white, 50% or 100 of the offspring would
inherit white / red, and 25% or 50 of the offspring would inherit red /
red. This would give us a genotypic
ratio of 50 white / white : 100 white / red : 50 red / red. We could simplify this as 1 homozygous furred
: 2 heterozygous furred : 1 homozygous furless.
In the
above situation, we know that furred is dominant to furless. Thus, of the 200 offspring we created, we
would expect 150 to be furred and 50 to be furless. This would give us a phenotypic ratio
of 150 furred : 50 furless. We could
simplify this as 3 furred : 1 furless.
Using
the genotype data from Table 2, transfer the allele combination to Table 3 that
represents the phenotypes of our bunnies.
Table 3. "Phenotypes"
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FURRED (white / white or white / red) |
FURLESS (red / red) |
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Your Results |
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Class Results |
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What ratio of furred to
furless did you and your partner get?
__________________________
How does the expected F2
fur ratio compare to the data you obtained
here?____________________________________________________________________________________________________________________________
How does the pooled data
of the class compare to your data for the expected F2 ratio?
________________________________________________________________________________________________________________________________
Is the dominant or
recessive trait more frequent in an F2 generation?
________________________________________________________________________________________________________________________________
Human Inheritance of Single Gene Traits
Determine your own phenotype for each of the following
traits. For some of the more easily
observed traits you may be able to recall the phenotypes of your parents. If it is possible, determine your own
genotype by comparing phenotypes of your parents and your own phenotype. If you have the dominant phenotype, give both
possibilities for your genotype where it cannot be determined definitely. Record your phenotypes in Table 4 and on the
chalkboard so that the data of the entire class may be tabulated. Record in your table the class results.
Tasting: Does
broccoli taste bitter? Is eating hot peppers intensely painful? Scientific
evidence suggests a genetic basis for food preferences – and it’s all on the
tip of the tongue. We will explore your
ability to taste 3 chemicals.
1.
PTC (phenylthiourea), is an
organic compound that either tastes very
bitter or is virtually tasteless, depending on your genetic makeup. About
70% of people can taste PTC. Food
choice is related to a person’s ability to taste PTC. One study found that non-smokers and those
that don’t often drink coffee or tea are more likely to be able to taste
PTC. The ability to taste PTC (P) is dominant to the inability
(p) to taste this chemical.
2.
Thiourea is another organic
molecule that either tastes very bitter
or is virtually tasteless. A study at Yale University found that
people could be classified into three groups, "supertasters" who could not abide the taste test paper at all,
medium tasters who did not like it
but tolerated it, and nontasters who
could not taste anything. Supertasters
will not consume any bitter foods, like dark-green leafy vegetables, coffee
or chocolate, and represent perhaps 10
to 15% of the population. Nontasters
have higher rates of thyroid disease - and of course thiourea compounds are
antithyroid agents. The ability to taste
thiourea is dominant to the inability to taste it.
a.
Super
tasters are homozygous dominant (TT).
b.
Medium
tasters are heterozygous (Tt).
c.
Nontasters
are recessive (tt)
3.
Sodium benzoate is a type of salt that
may occur naturally in some foods but is more likely to be added as a
preservative to foods. It is used in
small amounts only (usually in acidic foods like sodas and fruit juices)
because too much makes the food taste very bitter. Sodium benzoate tastes differently to different
people. Some may perceive a salty taste
while others may claim the sodium benzoate tastes sour, bitter or sweet. Others may not taste the sodium benzoate at
all. Approximately 75% of people can taste it.
The ability to taste Sodium benzoate (S) is dominant to the inability
(s) to taste this chemical.
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Double-jointed
thumb. A dominant gene determines a condition of
loose ligaments that allows one to throw the thumb out of joint. The homozygous recessive condition determines
tight joints. |
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Widow’s
peak. The presence of a point in the middle of
the hairline is called a widow’s peak (the woman on the left) and is dominant
to the straight hairline (the woman on the right). |
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Hair
shape. There is a lack of dominance in hair
shape. Curly hair is one homozygous
condition and straight hair is the alternate homozygous condition. Wavy hair results from the heterozygous
genotype. |
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Red
hair. Nonred hair is dominant to its recessive allele. |
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Dark
hair. Brunette is dominant to blond hair. |
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Freckles.
Presence of freckles is dominant to absence of freckling |
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Cheek
dimples. The presence of dimples in the cheeks is a
dominant trait. |
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Ear
lobes. In most people around the world, the ear
lobe hangs free (diagram on the left) but in some individuals it is directly
attached to the side of the head (diagram on the right). Attached ear lobe is recessive. Interestingly, in the United States, more
people have attached ear lobes than free ear lobes. |
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Tongue
rolling. Ability to roll the tongue into a U-shape
longitudinally (without the aid of lips) is dominant to the inability to curl
the tongue. |
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Mid-digital
hair. Some people have hair on the second, or
middle joint of the fingers, while others do not. The complete absence of mid-digital hairs
on all fingers is recessive. There
seems to be several dominant alleles which determine whether these hairs grow
on all fingers or only one, two, three or four of the fingers. All are dominant to absence of hairs. These hairs may be very fine and a hand
lens may be required to determine your phenotype for this trait. |
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Hitchhiker’s
thumb. Hyperextensibility of the thumb
(hitchhikers’s thumb) is recessive to straight thumb. This can be determined by examining the
position of your thumbs when they are in a relaxed position. When this condition is present, the usual
position of the thumb is such that it is bent backward toward the wrist;
there may be as much as a 45˚ angle between the two joints. |
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Interlocking
thumbs. When the hands are clasped, some people
will place the right thumb on top while others will place the left thumb on
top. This can be tested quite easily;
usually the interlocking of the fingers will be the same each time a person
does it. Placing the fingers in the
alternate position feels “wrong.”
Evidence indicates that placing the left thumb over the right is
dominant. |
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Bent
little finger. Hold your hands in front of your face with
the palm toward you, pressing the little fingers together. If the two fingers are straight, they will
be parallel to one another throughout their lengths; whereas, in the bent
finger condition the terminal portions flare away from one another. The bent finger is dominant to the straight
finger. |
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Cleft
chin. A cleft chin is, essentially, a
dimple on the chin. It results from
incomplete fusion of the left and right halves of the jaw during fetal
development. The resulting bony
peculiarity results in a cleft chin.
Cleft chins is dominant to chins without a cleft. |
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Eyebrow
position. Some people have abundant hair between the
eyebrows, so that they seem to converge to form one long eyebrow, known as a
unibrow or monobrow. The condition is
known as synorphrys. Connected eyebrows are dominant to
unconnected. |
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Table
4. Tabulation of Human Traits
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GENETIC TRAITS |
YOUR RESULTS |
CLASS TABULATION |
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|
Phenotype |
Genotype |
Phenotype |
Genotype |
# Dominants |
# Recessives |
|
PTC taster Nontaster |
PP or
Pp pp |
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|
Thiourea Supertaster Medium taster Nontaster |
TT Tt tt |
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Sodium Benzoate taster Non taster |
SS or
Ss ss |
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Double-jointed thumb Tight joints |
JJ or
Jj jj |
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Widow’s Peak Straight hair line |
WW or
Ww ww |
|
|
|
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Curly hair Wavy hair Straight hair |
CC CC’ C’C’ |
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Nonred hair Red hair |
NN or
Nn nn |
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Dark hair Blond hair |
MM or
Mm mm |
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Freckles No freckles |
FF or
Ff ff |
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Cheek dimple No cheek dimple |
DD or
Dd dd |
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Free earlobes Attached earlobes |
EE or
Ee ee |
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Tongue rolling present Tongue rolling absent |
RR or
Rr rr |
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Digital hair present Digital hair absent |
MM or
Mm mm |
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Straight thumb Hitchhikers’s thumb |
HH or
Hh hh |
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Left thumb over right Right thumb over left |
LL or
Ll ll |
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Bent little finger Straight little finger |
BB or
Bb bb |
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Cleft chin present Absence |
CC or
Cc cc |
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Unibrow Separate eyebrows |
UU or
Uu uu |
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For how many traits are
you recessive? _____________________
On the basis of the
tabulated results for the entire class, are there any recessive traits which
are more common among the class members than the dominant condition? If so, name them. ________________________________________________________________________________________________________________________________
What difference would
you expect in the tabulation if 1000 students were included in the table?
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