Unit 4: The Chromosomal Theory of Inheritance and Meiosis
The proof that Mendel's factors are physical chromosomes, and the cell divisions whose choreography is that proof in motion.
8 topics · 28 questions
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The Chromosomal Theory of Inheritance
- How it worksHow did Sutton and Boveri connect chromosome behavior during meiosis to Mendel's laws of inheritance?
- Worked exampleTrace how Morgan's white-eyed fly experiment proved a specific gene sits on a specific chromosome.
- Ties togetherHow does the chromosomal theory of inheritance connect Mendel's abstract factors to physical chromosomes visible under a microscope?
Your turn1 of 2
Sutton and Boveri noticed that homologous chromosomes pair up and then separate into different gametes during meiosis. What specifically about this observation convinced them chromosomes were the physical carriers of Mendel's abstract hereditary factors?Mitosis and Chromosome Behavior
- How it worksHow do sister chromatids line up and separate during the phases of mitosis?
- Common mistakeWhy doesn't mitosis ever reduce a cell's chromosome number the way meiosis does?
- Going furtherWhat happens to the daughter cells if the spindle fails to attach to one chromosome during mitosis?
- Worked exampleTrace the chromosome and DNA content of a cell with 4 chromosomes through G1, S phase, and mitosis.
Your turn1 of 2
A cell enters G1 with 18 chromosomes, each a single chromatid. After S phase, each chromosome has been replicated into two sister chromatids. How many total chromatids are present in the cell just before mitosis begins?Meiosis I: Homolog Pairing and Segregation
- How it worksHow does synapsis line up homologous chromosomes so they can separate accurately in meiosis I?
- How it worksWalk me through how homologous chromosomes physically separate to opposite poles during anaphase I.
- Common mistakeWhy doesn't meiosis I separate sister chromatids the way mitosis and meiosis II do?
Your turn1 of 2
Before a cell can accurately segregate its chromosomes in meiosis I, homologous chromosomes must first find each other and pair up tightly (synapsis). Why is this pairing step necessary at all?Meiosis II and the Haploid Products
- How it worksHow does meiosis II split sister chromatids to finally produce four haploid cells?
- Common mistakeWhy doesn't meiosis II reduce the chromosome number the way meiosis I does?
- Going furtherWhat happens to the four haploid products of meiosis when they come from a male germ cell versus a female germ cell?
Your turn1 of 2
A cell finishes meiosis I with a haploid chromosome number of 17 (each chromosome still made of two sister chromatids). After meiosis II splits the sister chromatids apart, how many chromosomes does each of the four final haploid cells contain?Meiosis and the Punnett Square as the Same Fact
- How it worksHow does each allele pair separating during meiosis I generate exactly the gamete ratios a Punnett square shows?
- Ties togetherHow does watching chromosomes segregate during meiosis explain why a monohybrid cross gives a 3 to 1 ratio?
- Worked exampleFind the gamete types produced by meiosis in a Bb individual and match them to a monohybrid Punnett square.
- Common mistakeWhy isn't the Punnett square just a math trick unrelated to what chromosomes are physically doing?
- Going furtherWhat happens to a Punnett-square prediction when the gene in question is actually linked to a second gene nearby?
Your turn1 of 2
A Bb individual undergoes meiosis, producing gametes that are half B and half b, exactly matching the probabilities used to build a monohybrid Punnett square. When this Bb individual is crossed with another Bb individual and they have 151 offspring, how many are expected to be heterozygous (Bb)?Independent Assortment During Meiosis I
- How it worksHow does the random orientation of homologous pairs at the metaphase plate generate independent assortment?
- Worked exampleFind the number of genetically distinct gametes possible from independent assortment in a human cell with 23 chromosome pairs.
- Ties togetherHow does independent assortment during meiosis explain the 9:3:3:1 ratio from a dihybrid cross?
Your turn1 of 2
A cell has 17 chromosome pairs, each orienting randomly and independently at the metaphase plate during meiosis I. Ignoring crossing over, how many genetically distinct gamete types are possible from independent assortment alone?Nondisjunction in Meiosis I Versus Meiosis II
- How it worksHow does nondisjunction of homologs in meiosis I produce a different gamete outcome than nondisjunction of sister chromatids in meiosis II?
- Common mistakeWhy doesn't nondisjunction always come from meiosis I even though that is where homologs separate?
- Worked exampleFind the chromosome count in each of the four gametes when nondisjunction occurs during meiosis I instead of meiosis II.
- Worked exampleTrace the chromosome counts in all four gametes when nondisjunction occurs during meiosis II instead of meiosis I.
Your turn1 of 2
A species has a normal haploid chromosome number of 18. If nondisjunction of one homologous pair occurs during meiosis I (both homologs go to the same pole), what is the chromosome count in each of the two resulting gametes that receive the EXTRA chromosome?Gametogenesis: Spermatogenesis Versus Oogenesis
- How it worksHow does unequal cytoplasm division during oogenesis produce polar bodies instead of four equal eggs?
- Going furtherWhat happens to a polar body's genetic material once it is discarded during oogenesis?
- Common mistakeWhy doesn't a polar body ever develop into a viable egg on its own?
Your turn1 of 2
Spermatogenesis converts one primary spermatocyte into four functional sperm, but oogenesis converts one primary oocyte into only one functional egg. Both start from a single diploid cell and go through the same two meiotic divisions. Why the difference in usable output?