Lecture 18 Basics: Genes and Alleles
Basic vocabulary
Gene:
Allele:
Homologous chromosomes:
Homozygous vs heterozygous
Genotype:
Phenotype:
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More vocabulary:
P (Parental) generation:
Gamete:
F1 generation:
F2 generation:
True-breeding:
Dominance
As Mendel understood it:
Offspring are purple
Therefore, the “big P” purple allele is
dominant to the “little p” white allele.
Dominance Explained:
Pigment that causes purple color: anthocyanin
2 Genes: transcription factor on one chromosome, enzyme gene on another chromosome
The “Big P” allele
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The “Little p” allele
Homozygous PP
Homozygous pp
Heterozygous Pp
Dominant allele:
Recessive allele:
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Punnett Squares
Calculating Probabilities:
Multiplication rule:
Q: What is the probability that two pea parents that are heterozygous for flower color will
have a homozygous recessive offspring?
Addition rule:
Q: What is the probability that two pea parents that are heterozygous for flower color will
have a homozygous recessive offspring?
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18.5 The Test cross: determines genotype in an organism showing the dominant trait
Given an unknown:
Solution:
Offspring if PP:
Offspring if Pp:
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Practice problems:
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?
________________
2. In guinea pigs, short hair is dominant to long hair (S,s). Use a Punnett square to determine the
following.
a. One guinea pig has long hair, the other is a heterozygote.
What proportion of the offspring will have short hair?
________________
b. A litter of guinea pigs all have short hair. One parent has short hair,
the other long hair. What is the genotype of the short haired guinea pig?
________________
3. In humans, being a tongue-roller is dominant over non-roller (R, r). A man who is a non-roller marries a
woman who is a heterozygous for tongue-rolling.
a. What is the father’s phenotype?
________________
b. What is the father’s genotype?
________________
c. What are the possible gametes the father can give his offspring?
________________
d. What are the possible gametes the mother can give her offspring?
________________
e. What is the probability their child will be a tongue-roller?
________________
f.
What is the probability that their first two children will be tongue-rollers?
______________
A decent set of more questions can be found here:
http://www.iusd.org/uhs/apbiology/Unit%20Resources/Unit%205/Genetics%20Problems.pdf
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Class Notes:
Learning Goal: Explain Mendelian genetics using modern cell biology explanations.
Use your new understanding to create “new” definitions that explain using mutation, gene products, and
pigment.
Why are white pea flowers white?
Old: They are homozygous for the recessive allele
New: The recessive allele produces a nonfunctioning gene product
What is true-breeding?
Old: Parental phenotype remains the same after multiple generations
New:
In purple flowers, why does the F1 generation all look like the dominant parent?
Old: Purple is dominant to white
New:
Learning Goal: Solve difficult monohybrid cross and probability problems quickly and correctly.
In (oversimplified) humans, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a
blue-eyed woman and they have three children, two of which are brown-eyed and one is blue-eyed.
What is the genotype of the man?
A. bb
B. BB
C. Bb
Melanistic jaguars are black versions of the regular, spotted jaguar. The black trait is dominant. In 2006, a
black jaguar named Diablo was inadvertently mated to a female lion named Lola at a wildlife sanctuary.
The offspring were a black female and a spotted male jaglion. What was the genotype of Diablo?
A. Homozygous dominant
B. Homozygous recessive
C. Heterozygous
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The following problem is an example of a dihybrid cross.
Parental genotypes: GGww x ggWW
What are the genotypes of the F1 generation?
_______________
What is the probability that the F2 generation will have at least one dominant allele for each trait?
Answer (broken into steps)
a. What are the genotypes of the two parents?
________________________
_________________________
b. What is the probability of the offspring having at least one G allele?
c. What is the probability of the offspring having at least one W allele?
d. Do you add or multiply these probabilities? Explain why.
What is the probability that the F2 generation will have at least one dominant allele
for each trait?
________________
Apply what you’ve learned.
For the parents below:
AaBBCcdd x aaBbccDd
What is the probability that the offspring will have at least one dominant allele
for each trait?
________________
For the same parents, what is the probability that the offspring will be a aaBBccdd?
________________
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