Bio H – Beyond Mendelian Genetics
BEYOND MENDELIAN GENETICS
Remember 2 of Mendel’s Principles
Principle of Dominance:
Some forms of a gene may be dominant (mask another allele), while others are recessive (hidden
when a dominant allele is present). We call this a Complete Dominance Inheritance Pattern
Principle of Segregation
The two alleles (from a single parent) are separated when gametes are formed. Only one allele
goes into each haploid cell.
Mendel’s principle of dominance is not ALWAYS true. We now know that there are other
dominance patterns, and therefore other phenotypes. Let’s look at a few different patterns.
Inheritance Patterns that DO NOT DEMONSTRATE “Complete dominance”
Incomplete Dominance
Watch YOUTUBE PODCAST https://www.youtube.com/watch?v=QTSsGIZh4iY Beyond
Mendelian Genetics or https://www.youtube.com/watch?v=fQvER3MyI2c (you will probably
need to view both, the first has more types of inheritance patterns, while the second explains how
the patterns work a bit better.
Define Incomplete Dominance and show a picture IN COLOR of an example of incomplete
dominance. Complete incomplete dominance example problem (in the problem set)
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Bio H – Beyond Mendelian Genetics
Co-dominance
Define Co-dominance
Create a picture (in color and NOT from your notes) that shows an example of the phenotype
that occurs when two pure-breeding organisms are cross fertilized
Complete the codominance example problem in the problem set. Don’t do the blood typing
problem yet, we will come back to that.
Sex-linked genes
https://www.youtube.com/watch?v=wryk_O4liz0
Why are fruit flies a good model system for studying genes?
What trait did Morgan use to study Sex-linked genes? How did he figure out that this trait was
sex-linked?
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Bio H – Beyond Mendelian Genetics
What are two confirmed X-linked diseases?
Colorblindness and inheritance of sex-linked traits (review of Mendel’s laws, and intro to sexlinked traits): https://www.youtube.com/watch?v=IJqFk-28G08
Describe the three forms of colorblindness. Who sees in black and white? What common form
of colorblindness is a form of trichromacy?
This VODcast provides examples for each of Mendel’s FOUR laws. What is the example given
for each of the three laws we are studying (dominance, independent assortment, and segregation)
How is the inheritance pattern for sex-linked traits different from the pattern for a complete
dominance trait?
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Bio H – Beyond Mendelian Genetics
What is the notation (how are letters used) to signify an X linked gene?
https://www.youtube.com/watch?v=fQvER3MyI2c.
Explain how letters and superscripts are used to identify the fact that an allele is carried on the X
chromosome and that it is dominant.
All questions below are related to sex-linked genes.
What is the genotype for a woman who is homozygous dominant and normal doesn’t have the
disease) for the hemophilia gene?
What does a female carrier look like (genotype)? What is her phenotype?
Can a female have hemophilia (exhibit characteristics of the disease) ? What needs to occur
(genetically) for this to happen?
What is the genotype of a male carrier?
Why does a male with one hemophilia allele have the disease?
Find the sex-linked example in your problem packet and complete the example.
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Bio H – Beyond Mendelian Genetics
Test cross https://www.youtube.com/watch?v=IK5QW1UfAuI
1. What is the purpose of a test cross?
2. How do I set up a testcross?
According to the youtube video, when would I use a testcross? Do the test cross example
problems.
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Bio H – Beyond Mendelian Genetics
Define Multiple Alleles and provide an example of a multiple allele characteristic.
Watch the Youtube video on blood typing https://www.youtube.com/watch?v=L06TJTMVkBo
(ABO blood typing and Rh groups).
Identify the inheritance pattern (complete dominance, incomplete dominance, codominance for
a) A and O
b) B and O
c) A and B
Now consider the Rh factor. How is this factor inherited?
Watch youtube: https://www.youtube.com/watch?v=KXTF7WehgM8 What are key ideas
covered here (or ideas you better understand) in this youtube, that are NOT in the first video.
Complete the lab on Blood Typing (we will do this in class) and the problem set on blood types
Polygenic Traits
Now let’s turn our attention to traits that show a gradual variation (in color or in size). These are
called polygenic traits and often involve 3 or 4 genes, each having the same two variations.
However the effect of these genes is cumulative. What is the example used on the Youtube
video. How do polygenic traits allow for many different colorations?
https://www.youtube.com/watch?v=QTSsGIZh4iY
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Bio H – Beyond Mendelian Genetics
Dihybrid Crosses (to be shown in class)
It is also possible to look at the probability of inheriting alleles from two independently assorting
genes. When we look at 2 genes at a time, the Punnett square we use reflects ALL possible
combinations of alleles from the parents for both genes. This means that the Punnett square we
use must be much bigger (16 squares). https://www.youtube.com/watch?v=ob9mgWhZRaY
First listen to the vodcast and list important concepts covered by the instructor. Then go back
and create the example.
Key Concepts
Example:
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Bio H – Beyond Mendelian Genetics
VOCABULARY and PROBLEMS
Read through the three previous pages. Define the words (below) in your own words
WITHOUT using the internet (you may use your book). Then do the problems on pages 6 and 7.
Hybrid
Genetics
Cross-pollination
Purebred
Homozygous
Allele
Heterozygous
Genotype
Phenotype
Dominant
Recessive
Characteristic
Trait
Selective breeding
F1 generation
F2 generation
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Bio H – Beyond Mendelian Genetics
Genetics Practice problems:
ALL parts must be completed. Zero credit is given for the “answer” alone.
Part I: Complete dominance
Problem 1: In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross
of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should
have spherical seeds?
a.. Parental Genotypes (Dad’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Problem 2: In dogs, wire hair (S) is dominant to smooth (s). In a cross of a female homozygous
wire-haired dog with a male smooth-haired dog, what will be the phenotype of the F1 generation
(offspring)?
a.. Parental Genotypes (Dad’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
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Bio H – Beyond Mendelian Genetics
Problem 3: Woodrats are medium sized rodents with lots of interesting behaviors. You may
know of them as packrats. Let's assume that the trait of bringing home shiny objects (H) is
dominant to the trait of carrying home only dull objects (h). Suppose two heterozygous
individuals are crossed. What would be the expected genotypic ratio of the progeny (offspring)?
ALSO how many of the progeny bring home shiny objects?
a.. Parental Genotypes (Dad’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
Problem 4: The common grackle is a species of blackbird that is fairly common in the United
States. Suppose that long tails (L) were dominant to short tails in these birds. A female shorttailed grackle mates with a male long-tailed grackle who had one parent with a long tail and one
parent with a short tail. What percentage of the offspring have a genotype of Ll? What
percentage of the offspring would have a long tail?
a.. Parental Genotypes (Dad’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
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Bio H – Beyond Mendelian Genetics
Part II: Incomplete dominance
Problem 1: In Japanese four o’clock flowers, color is inherited by genes that show incomplete
dominance. In such flowers, a cross between a homozygous red (R) flower and a
homozygous white (r) flower will always result in pink flowers. A cross is made between
two pink flowers. What is the probability for each phenotype?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Problem 2: In chickens, feather color is incompletely dominant. A white rooster mated with a
black hen creates chicks with grey feathers. What phenotypic AND genotypic ratios of the
offspring would you expect from the mating of a grey rooster and a black hen?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Draw a picture of the result using colored pencils
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Bio H – Beyond Mendelian Genetics
Part III: Co-dominance
Problem 1: In shorthorn cattle, the hybrid between red and white is roan. Roan means that the
animal has BOTH red hairs AND white hairs. If a roan is bred with a white, what will the
phenotypic ratio be?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
Draw a picture of the result using colored pencils:
c. Answer:
Multiple Alleles (mixture of co-dominance and complete dominance)
Problem 2: Blood type in humans is determined by the presence of 2 of 3 possible alleles. The
alleles are IA, IB and iO. IA is completely dominant to iO. IB is completely dominant to iO. IA and
IB are co-dominant with each other. Suppose a female who is homozygous for Type A blood
marries a male who is heterozygous for type B blood. Their first child is type AB and their
second child is type O. Do both these children belong to the same father? Explain.
a. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
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Bio H – Beyond Mendelian Genetics
Part IV: Sex-linked Problems
Problem 1: The ability to see color is determined by a gene found on the X chromosome. When
this gene is defective, it causes colorblindness. A colorblind person can not distinguish between
certain colors, such as red and green. The good news is that the “normal” allele is dominant to
the colorblind allele. Suppose a Colorblind Female marries a normal male. Can they have any
male children who can see color? Can they have any female children who can see color?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
Problem 2: Hemophilia is a disease where the blood does not clot very well. The mutated gene
that causes this disease is found on the X chromosome. Again, the mutation is recessive and the
normal allele is dominant. Suppose you have a Homozygous normal female and a male with
hemophilia. What is the probability of getting a healthy child (male or female)? What fraction
of the male children will be healthy?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
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Bio H – Beyond Mendelian Genetics
Part V: Test Crosses
Problem 1: A florist knows the up-coming holiday will put a large demand on red roses (R). He
wants to make sure that his Red rose plant is PURE so that he doesn’t get a lot of white flowers
(r) when he pollinates the plant.
a. What possible genotypes might his red plant be?
b. What genotype should he cross his rose with to identify its genotype?
c. Draw out BOTH possible punnett squares below:
d. By looking at the offspring of the test cross, how will he tell if his plant is pure?
Problem 2: A dog breeder knows he can sell a beagle that is PURE for a “slim neck” for more
than he can sell a hybrid. A hybrid has a chance of producing the recessive “thick neck” beagle,
which is considered undesirable. He crosses his “slim neck” beagle with a “thick neck” beagle
and 8 puppies are born. 5 are “slim neck” and 3 are “thick neck”.
a. Can he determine if his beagle is pure or not?
b. If so, how? If not, why not?
Problem 3: A farmer wants to produce all yellow corn. He knows that sometimes corn comes
out yellow, sometimes it comes out white and sometimes it comes out with both yellow and
white kernels. He has heard that test crosses can help his case. To make sure that his plant can
only produce yellow kernels, he crosses his yellow plant with a yellow/white plant. He gets a
mixture of yellow and yellow/white offspring. He decides his plant is NOT pure for the yellow
allele.
a. Did the farmer have to perform a test cross to determine the genotype of
his corn? Explain?
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Bio H – Beyond Mendelian Genetics
VI. Dihybrid Crosses (to be demonstrated in class)
Problem 1: In mice the gene for coat color as two forms. The allele for dark coat is
dominant to the allele for albino. There are two forms for the gene controlling whiskers,
as well, straight is dominant to bent.
Cross two mice that are heterozygous for both these genes.
a. What is the genotype of each mouse?
b. What possible gametes can each mouse make (remember each gamete
contains one allele for EACH trait being considered)?
c. Draw out the Punnett square for this cross
d. What proportion of the offspring are albino?
e. What proportion would have bent whiskers?
f. What is the phenotypic ratio of ALL the offspring? (be sure to include the
traits!)
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Bio H – Beyond Mendelian Genetics
Problem 2: If a maize plant heterozygous for the alleles for pigmy and crinkly-leaf (both
recessive to normal size of plant and normal leaf) is self-pollinated and 208 seeds are
subsequently collected and germinated, how many would you expect to show:
a. crinkly leaves?
b. normal size?
c. normal leaves and normal size?
d. normal leaves and pigmy size?
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Bio H – Beyond Mendelian Genetics
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