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Bio CP1
Chapter 11: Introduction to Genetics Part 2
Section 11-2: Probability & Punnett Squares
General Terms:
• Zygote –
• Homozygous –
• Ex:
• Heterozygous –
• Ex:
Practice
Identify each of these genotypes as being homozygous or heterozygous.
GG _________________
Ss ____________________
Yy _________________
Vv ____________________
Kk _________________
Identify each of these genotypes as being homozygous dominant, homozygous recessive, or
heterozygous.
ee _________________
CC ____________________
QQ _________________
pp ____________________
Ll _________________
Suppose that the I allele codes for orange fins and the i allele codes for yellow fins.
The heterozygous genotype: ____
The homozygous dominant genotype: ___
The homozygous recessive genotype: ___
A fish with yellow fins must have a ________________ genotype.
A fish with orange fins could have either a _________________ genotype or a ______________
genotype.
Genetics and Probability

Because not every gamete a parent makes is the same, which alleles are passed to the offspring
are random.
An example, using coins:
Flipping two coins, what are the chances of getting heads on both flips?
__ in 4 possible outcomes would be both heads (HH). __/4 = _____%
Flipping two coins, what are the chances of getting heads on just one flip?
__ in 4 possible outcomes would be just one heads (Hh). __/4 = __/__ = _____%
Flipping two coins, one with two heads and one normal coin, what are the odds of getting heads
both times?
__ in 4 possible outcomes would be both heads (HH). __/4 = __/__ = _____%

o

Step-By-Step Instructions
Sample problem: What are the chances that a heterozygous brown-eyed father and a blue-eyed mother
would have a blue-eyed child? (Use letters B/b)
1.

2.
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Dad’s gametes = __ and __
Mom’s gametes = __ and __
3.
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4.
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5.

6.
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7.
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For instance, if two of the four zygotes are AA, the probability of genotype AA is 2/4 = ½ = 50%
8.
The Problem:
Practicing with Punnett Squares
Parents – Tt and tt
Parents – TT and Tt
Offspring:
Offspring:
Tall:
Tall:
Short:
Short:
Show a monohybrid cross, with all genotype and phenotype probabilities, for parents who are HH and
hh.
Show a monohybrid cross, with all genotype and phenotype probabilities, for parents who are Ll and Ll.
If Dad’s genotype is Rr and Mom is homozygous recessive, what are the odds of having homozygous
dominant offspring?
If both parents are heterozygous, and they have ten offspring, approximately how many of those
offspring would you expect to be homozygous recessive?
Suppose black fur is dominant and white fur is recessive. Two parents, one with black fur and one with
white fur, have many offspring. Roughly half of their babies are black-furred, anf half are white-furred.
What were the genotypes of the parents?
(Hint: when a question asks you to figure out parental genotypes, make test crosses, Punnett
Squares for every possibility, then see which one gives you offspring results that fit.)
Pedigrees
Pedigree =
KEY
7’s daughter is __
1’s great grandson-in-law is __.
Amongst 4’s nieces and nephews, __ is older than __.
Both of them are _______.
Section 11.3: Exploring Mendelian Genetics
Mendel’s Fourth Conclusion!
• 4.
•
• Example: If Dad has brown eyes (Bb) and a widow’s peak (Ww), his genotype is BbWw.
• Half of his sperm will get the dominant B allele from the eye color gene.
• But that does NOT force those sperm to also get the dominant W allele from the other
gene. Some of them will get B and W, some of them will get B and w.
• He will make four different gametes: BW, Bw, bW, and bw.
Dihybrid Crosses
Dihybrid Cross =
• Cross: RRYY X rryy [homozygous dominant round yellow peas (RRYY) and homozygous
recessive wrinkled green peas (rryy)]
• Result: F1 offspring were all RrYy (heterozygous round yellow peas).
• Mendel let the F1 plants self-pollinate (RrYy x RrYy)
F2 results
•
Law of Dominance =
Law of Segregation =
Law of Independent Assortment =
Patterns of Inheritance
•
•
Polygenic Traits Example: Eye color, in reality, doesn’t just come from one B/b gene.
There’s a gene for blue vs green tint, a gene for dark vs light, a gene for tan
vs gold tint, a gene for color density, a gene for outlying vs inlying rings…
and your exact eye color depends on all of them.
Multiallelic Traits Example: In cats, coat color is partly determined by the browning gene, which has THREE
alleles. B is the allele for black, b is the allele for chocolate brown, and bl is the allele for cinnamon
brown.
Possible genotypes for this gene are: BB, Bb, Bbl, bb, blbl, and bbl.
If you cross a Bb cat with a blbl cat, what are the offspring genotype probabilities? ________
Incomplete Dominance –
Ex:
Practice: If B is the allele for blue feathers, and Y is the allele for yellow feathers, what are
phenotype probabilities if you cross a blue-feathered bird with a yellow-feathered bird?
Codominance –
Ex:
Show the Punnett Square for a BB x BW cross:
Sex-Linked Traits –
Sex chromosomes Autosomal chromosomes –
Practice
Identify each example as displaying sex-linkage, incomplete dominance, codominance, multiple alleles,
or polygenic traits.
1. A heterozygous wombat has striped brown and grey fur. ___________________
2. The gene for flagella length has alleles for very long, long, medium, and short flagella. ___________
3. Skin color is determined by the melanin gene, carotene absoprtion gene, carotene deposition gene,
and several genes that affect the number and thickness of skin capillaries. _____________________
4. Crossing a blue-nosed Dad with a red-nosed Mom produces purple-nosed babies. _______________
5. Crossing a yellow-eyed Dad with a white-eyed Mom produces white-eyed sons, but not all daughters
have white eyes. ___________________