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Questions and Answers
Which of the following errors most likely produces Down syndrome?
Which of the following errors most likely produces Down syndrome?
- Nondisjunction during meiosis I
- Nondisjunction during either meiosis I or II in the female gamete (correct)
- Nondisjunction during meiosis II
- Fertilization errors
What condition is characterized by females having only one X chromosome?
What condition is characterized by females having only one X chromosome?
Turner syndrome
Two products of the second meiotic division have both the maternal and paternal chromosomes of a set when nondisjunction occurs during meiosis I.
Two products of the second meiotic division have both the maternal and paternal chromosomes of a set when nondisjunction occurs during meiosis I.
True (A)
What happens if nondisjunction occurs during meiosis I?
What happens if nondisjunction occurs during meiosis I?
What happens if nondisjunction occurs during meiosis II?
What happens if nondisjunction occurs during meiosis II?
Offspring from a cross between a tetraploid and a diploid member of the same species can undergo sexual reproduction.
Offspring from a cross between a tetraploid and a diploid member of the same species can undergo sexual reproduction.
Which of the following statements about allopolyploid individuals is true?
Which of the following statements about allopolyploid individuals is true?
How many chromosomes would be found in an allopolyploid plant if its parents had diploid numbers of 4 and 6 respectively?
How many chromosomes would be found in an allopolyploid plant if its parents had diploid numbers of 4 and 6 respectively?
Crossing over during prophase I of meiosis occurs between alleles on sister chromatids.
Crossing over during prophase I of meiosis occurs between alleles on sister chromatids.
Which of the following statements about gamete formation during meiosis is false?
Which of the following statements about gamete formation during meiosis is false?
What is the expected F2 phenotypic ratio from a selfed heterozygote with the genotype a + b + / ab?
What is the expected F2 phenotypic ratio from a selfed heterozygote with the genotype a + b + / ab?
Two genes that are separated by 10 map units show a recombination percentage of 10%.
Two genes that are separated by 10 map units show a recombination percentage of 10%.
What is the expected frequency of double crossover gametes among three linked genes X, Y, and Z?
What is the expected frequency of double crossover gametes among three linked genes X, Y, and Z?
What is the probability of recombination between genes X and Z if the map distances are 25% and 5%?
What is the probability of recombination between genes X and Z if the map distances are 25% and 5%?
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Study Notes
Down Syndrome and Nondisjunction
- Down syndrome, or trisomy 21, incidence rises with maternal age due to nondisjunction during meiosis.
- Nondisjunction results in gametes with incorrect chromosome numbers, either having an extra or missing chromosome.
Turner Syndrome
- Turner syndrome occurs in females with only one X chromosome, leading to underdeveloped ovaries.
- Males lacking an X chromosome generally do not survive, making this condition lethal.
Nondisjunction in Meiosis
- Nondisjunction during meiosis I leads to gametes with either two copies of a chromosome or none, while the remaining products have the normal set.
- Nondisjunction in meiosis II results in 50% normal haploid gametes, 25% with an extra chromosome, and 25% missing one.
Tetraploid and Diploid Crosses
- Crossing a tetraploid with a diploid results in triploid offspring, which cannot reproduce sexually due to uneven chromosome sets.
Allopolyploidy
- Allopolyploid individuals may be sterile and are not formed by simultaneous fertilization of an ovum by two sperm or by colchicine exposure.
- The process generally involves mating between different species, leading to hybrid vigor but potential sterility.
Chromosome Counts
- An allopolyploid plant from diploid parents with counts of 4 and 6 would have a total chromosome count of 5.
Crossing Over in Meiosis
- Crossing over occurs between nonsister chromatids during prophase I, not between sister chromatids.
Gamete Formation and Linkage
- Recombinant gametes have new allele combinations not present in the parent cell, while complete linkage only results in parental gametes.
- Parental gametes may still be formed if crossing over occurs, with a ratio of 3:1 expected for selfed heterozygotes with complete linkage.
Mapping and Recombination
- A recombination percentage of 10% corresponds to a distance of 10 map units between genes.
- For linked genes X, Y, and Z with crossover frequencies of 25% (X and Y) and 5% (Y and Z), the expected frequency of double crossover gametes is 1.25%.
Probability of Recombination
- The probability of recombination between genes X and Z is calculated as 30%, combining the frequencies of single crossovers observed (25% + 5%).
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