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Mendelian Genetics and Exceptions
Name:

__________________________ is the father of genetics.

He grew __________________________ and studied many different characteristics. (seed color,
flower color, seed shape, pod shape & color)

How did he determine his plants were pure bred? (experiment 1)

What happened when he crossed two pure breeding plants with different traits? (experiment 2)

Summarize the results of crossing two hybrids. (two hybrid purple flowers)
CONCEPTS LEARNED FROM MENDEL
_______________________ - Alternate forms of a gene. (P) or (p)
________________________– The allele that is expressed. (P)
________________________ – Allele is present, but unexpressed unless both alleles are recessive.(p)
________________________– Both alleles are the same (PP or pp)
________________________ – Alleles are different. (Pp)
________________________ - alleles separate, when the F1 generation produces gametes.
________________________ - a chart used to determine expected probabilities of a genetic cross.
Rules for completing a genetic cross: (cross two hybrid purple flowers)
1.
2.
3.
4.
5.
A monohybrid cross is a simple Mendelian cross that involves a single trait. The expected offspring can
be predicted using a punnett square. When completing punnett squares, the mother’s and father’s
genotypes will be listed on the sides of the square. The probability of genotypes and phenotypes can be
determined.
A mother has a pure dominant genotype (RR) for rolling her tongue. The father is hybrid (Rr) for this
trait. A punnett square will show the possible offspring for this single trait.
Genotypic ratio: ___________________________
Phenotypic ratio: ___________________________
A man, heterozgous for tasting PTC, is crossed with a woman who can also taste PTC. However, their
15 year old daughter can not taste PTC. How can this be possible? Prove with a punnett square.
Genotypic ratio: ___________________________
Phenotypic ratio: ___________________________
Are you taster or non-taster? _________ What are your possible genotypes?____________________
How could you determine you genotype?
ANOTHER HYPOTHESIS:
Are 2 traits always passed on together, or are they dependent of one another?
Does the gene that determines whether a seed is round or wrinkled in shape have anything to do with
the gene for seed color? Must a round seed always be yellow?
Conclusion:
_____________________________________ – genes for different traits segregate
independently of one another during gamete formation.
Doing a dihybrid cross can prove this. A dihybrid cross occurs between two individuals
that differ in two characteristics.
Cross a pure breeding round yellow plant (RRYY) with a wrinkled green plant (rryy)
Define the trait:
R=
Y=
r=
y=
What are the possible gamete combinations (RRYY)? ______ ______ ______ ______
What are the possible gamete combinations (rryy)? ______ ______ ______ ______
------------
Gratio:
Pratio:
Now complete the cross of two F1 plants.
What are their genotypes? _____________ x ______________
What are the possible gametes? ______ , ______, ______, ______
------------
Gratio:
Pratio:
Practice with Gametes:
What is the genotype of a wrinkled yellow(YY) pea plant? _________________
What are the possible gametes?
What is the genotype of a Round (Rr) yellow pea plant? ___________________
What are the possible gametes?
What are (is) the possible genotype(s) for a wrinkled green pea plant? ________________________
What are the possible gametes?
Eventhough Mendel did a tremendous job teaching us about genetics there are some
exceptions to his work. Some alleles are neither dominant nor recessive, and many
traits are controlled by multiple alleles or multiple genes.
The following are exceptions to Mendel’s work.
Exceptions to Mendel’s Rules:
Incomplete dominance –
Snapdragon flower represent incomplete dominance. Neither Red (RR) or White (rr) is
dominant. Cross a red snapdragon with a white flower.
Genotypic Ratio:
Phenotypic Ratio:
Now Cross a pink flower with a red flower?
Genotypic Ratio:
Phenotypic Ratio:
Codominance-
The alleles for curly hair and straight hair are examples of alleles for a trait that are
codominant. Individuals with curly hair are homozygous for curly hair alleles. Individuals
with straight hair are homozygous for straight hair alleles. Individuals who are heterozygous,
with one of each allele have wavy hair, which is a blend of the expressions of the curly and
straight hair alleles.
HCHC – Curly Hair
HSHS – Straight Hair
What is the result when two Wavy hair people have kids?
___ ___ - Wavy Hair
Sex-linked
Male =
Female =
1. If a “normal” male and female have children, what is the probability of having a boy?
2. Hemophilia is known as bleeder’s disease and is inherited as a recessive trait carried on the X
chromosome.
Define the trait:
Normal female = _____
Carrier female = _____
Affected female = _____
Normal Male = _____
Affected Male = _____
Cross a Carrier female with a normal male.
What are the following chances?
# of males with hemophilia? _____
# of carrier females? _____
# of normal males? _____
# of females with hemophilia? _____
3. Colorblindness is a sex-linked recessive trait carried on the x chromosome. If a colorblind male is
crossed with a homozygous, normal woman, what are their possible offspring?
SEX INFLUENCED –
Early balding (pattern baldness) in humans. Heterozygous men (b+/b) lose their hair; heteroyzgous
women do not have significant hair loss. Homozygous men or women (b/b) become bald. The trait is
therefore dominant in men, recessive in women. (We used b to designate the mutant baldness allele even
though the allele is dominant in males.)
1. Cross and homozygous bald male with a heterozygous female.
DEFINE THE TRAIT
b+b+ = ________________________
b+b (male) = _______________________
b+b (female) = _______________________
bb = __________________________
2. Your mother carriers the trait for baldness and your dad is not bald, and shows no signs of balding.
What are the chances that you go bald? Answers vary depending on your gender.
MULTIPLE ALLELES-
Blood type is determined two alleles on the homologous No. 9 chromosome. The four phenotypes are
type A, B, AB, and O.
Complete the following chart for possible blood types.
Phenotype
Genotype
Antiobodies
A Blood
Antigen in Blood
cells
A
B
% distribution in
US
42
B Blood
B
A
7
A and B
None
2
none
A and B
48
AB Blood
O Blood
1. A person with type O blood has a child with a person with type AB blood. What are the possible
genotypes of their children?
Parents ________ X ___________
% type A - _____
% type B - _____
% type AB - _____
% type O- _____
2. From problem one, which parent would be the ideal blood donor for the children in problem 1?
3. Can a person with a type O blood have a child with type AB blood? Explain
4. I have type A blood and my brother has type AB blood and my sister is type 0 blood? What are the
possible genotypes of my parents?
Polygenic inheritance –
Human skin color is a good example of polygenic (multiple gene) inheritance. Assume that three
"dominant" capital letter genes (A, B and C) control dark pigmentation because more melanin is
produced. The "recessive"alleles of these three genes (a, b & c) control light pigmentation because lower
amounts of melanin are produced.
Imagine the following couple:
AaBbCc (female)
1. Give the gametes for these parents. Female:
Gametes ABC
x
AaBbCc (male)
Male:
ABc
AbC
Abc
aBC
aBc
abC
abc
ABC
6
5
5
4
5
4
4
3
ABc
5
4
4
3
4
3
3
2
AbC
5
4
4
3
4
3
3
2
Abc
4
3
3
2
3
2
2
1
aBC
5
4
4
3
4
3
3
2
aBc
4
3
3
2
3
2
2
1
abC
4
3
3
2
3
2
2
1
abc
3
2
2
1
2
1
1
0
2. a. What is the genotype of the darkest child they could have?
__________________________________
b. What are the odds of producing such a child? __________
3. a. What is the genotype of the lightest child they could have?
_________________________________
b. What are the odds of producing such a child? __________
4. What are the odds of having a child that is heterozygous for all three genes? _________