Lecture 18 Class: Genes and alleles
Starter:
Recessive and dominant alleles, test crosses:
What is the best modern definition of why white pea flowers are white?
A. They produce a white pigment
B. They are homozygous for the recessive allele
C. Their allele produces a nonfunctioning product
Which is the best modern definition of true-breeding?
A. Containing no DNA
B. All gametes contain the same allele
C. Parents are heterozygote
D. Parental phenotype remains the same after multiple generations
What is the best modern biology definition for why, in purple pea flowers,
the F1 looks like the dominant parent?
A. The single copy of the working allele produces enough transcript to
produce the same amount of pigment
B. The purple pigment is stronger than the white pigment
C. An F1 contains two alleles for the same gene, but only the dominant
gene undergoes transcription
Explain why, in simple dominance, the F1 hybrid looks like the dominant
phenotype but is able to produce recessive phenotype offspring. Use
gametes, alleles and the production of anthocyanin in your explanation.
Law of Segregation
 Genes have alleles
 Each gamete gets __________________________
Punnett Squares:
 Show possible ________________________

Show possible ________________________
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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
Probability determines inheritance:
• Multiplication rule: events together
• Addition rule: either event occurring
A man and woman are heterozygous for tongue-rolling (R), which is dominant over non-rolling.
(Draw a Punnett Square)
1. What is the likelihood that their ________________________ will be a non-roller?
2. What is the likelihood that their ________________________ will be a non-roller?
3. What is the likelihood that their __________________________ will be non-rollers?
4. 4. What is the likelihood that _________________________________ will be non-rollers?
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Test Crosses
To identify the genotype of a yellow-seeded pea plant as either
homozygous dominant (YY) or heterozygous (Yy), you could do a test
cross with plants of genotype _______.
A. Yy
B. YY
C. Yy
Piazza Question
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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