L4 Gamete Genotypes, Linkage & Allele Interactions

[T4] What is the major difference between the products of mitosis and meiosis?

Mitosis produces two daughter cells that are genetically the same as each other and the parent cell, whereas meiosis produces four gametes that can be genetically different from the parent and from each other.

[T4] How many rounds of nuclear division occur in mitosis?

One.

[T4] How many daughter cells are produced by mitosis?

Two.

[T4] What should the genotypes of daughter cells produced by mitosis be relative to the parent cell?

The same as the parent cell and the same as each other.

[T4] How many gametes are produced from one parent cell completing meiosis?

Four.

[T4] Are gametes produced by meiosis necessarily genetically identical to each other?

No. Meiosis can produce genetically different gametes.

[A4] Why can meiosis generate genetically different gametes?

Processes including independent assortment of homologous chromosomes and crossing-over/recombination can create different combinations of alleles.

[IT4] On the old-exam chromosome diagram on pages 14–16, what phase of the cell cycle is the cell in?

G1.

[IT4] What feature of the chromosome diagram on pages 14–16 indicates that the cell is in G1 rather than G2?

The chromosomes are unreplicated.

[A4] If all chromosomes in a cell are unreplicated and the cell has not yet entered S phase, which interphase stage is it in?

G1.

[IT4] On the old-exam chromosome diagram on pages 17–18, what is the ploidy of the cell?

Diploid.

[IT4] On the old-exam chromosome diagram on pages 19–20, what is the haploid number of the cell?

n = 3.

[A4] A diploid cell contains 6 total chromosomes. What is its haploid number?

n = 3.

[A4] A cell is 2n = 6. How many homologous chromosome pairs does it contain?

Three homologous pairs.

[A4] A G1 cell contains 2.4 pg of DNA. How much DNA will the cell contain after DNA replication?

4.8 pg.

[IT4] In the page 21–22 old-exam question, a G1 cell contains 2.4 pg of DNA. How much DNA is present in the entire cell during anaphase of mitosis?

4.8 pg.

[A4] Why does DNA amount double between G1 and anaphase of mitosis?

DNA is replicated during S phase before mitosis.

[A4] Does separation of sister chromatids during anaphase immediately halve the amount of DNA in the entire still-undivided cell?

No. Until cytokinesis separates the cell, the entire cell still contains the replicated amount of DNA.

[IT4] On pages 23–24, why can the proposed daughter cell not be a product of mitosis?

It is haploid even though the parent is diploid, and it contains a recombinant allele combination; mitotic daughters should retain the parent's genotype/ploidy and crossing-over/recombination does not occur in mitosis.

[A4] Can a diploid parent cell normally produce a haploid daughter cell through mitosis?

No. Mitosis should preserve ploidy.

[A4] Can normal mitosis generate new allele combinations through crossing-over and recombination?

No.

[A4] If a proposed mitotic daughter has a genotype different from the parent because of recombination, what conclusion should you make?

It could not normally have been produced by mitosis.

[T4] What are the two main learning objectives for predicting gamete genotypes in Lecture 4?

Predict the number of different gamete genotypes that can be produced and state what those possible gamete genotypes are.

[T4] What equation is introduced for calculating the number of possible gamete genotypes when genes are on different chromosomes?

2^n.

[T4] In the gamete-genotype equation 2^n, what does n represent?

The number of genes for which the parent is heterozygous.

[A4] A parent is heterozygous for two genes located on different chromosomes. How many possible gamete genotypes can be produced across multiple meiotic events?

2^2 = 4.

[A4] A parent is heterozygous for three genes located on different chromosomes. How many possible gamete genotypes can be produced across multiple meiotic events?

2^3 = 8.

[A4] A parent is heterozygous for four genes located on different chromosomes. How many possible gamete genotypes can be produced across multiple meiotic events?

2^4 = 16.

[A4] Does a homozygous gene increase n in the 2^n gamete-genotype calculation?

No. Only heterozygous genes are counted in n.

[A4] For genotype A1/A2;E1/E2, how many genes are heterozygous?

Two.

[A4] For genotype A1/A2;E1/E2, how many possible gamete genotypes can be produced across multiple meiotic events if the genes are on different chromosomes?

Four.

[A4] For genotype A1/A2;E1/E2, what are the four possible gamete genotypes if the genes are on different chromosomes?

A1;E1, A1;E2, A2;E1, and A2;E2.

[T4] How many alleles of each gene does a gamete receive?

One allele of each gene.

[A4] Can a gamete produced from A1/A2;E1/E2 have the genotype A1 alone?

No. It must receive one allele of the A gene and one allele of the E gene.

[A4] Can a gamete produced from A1/A2;E1/E2 have A1;A2?

No. A gamete receives one allele of each gene, not both homologous alleles of the same gene.

[A4] Why are A1;E1, A1;E2, A2;E1, and A2;E2 valid gamete genotypes from A1/A2;E1/E2?

Each contains one A allele and one E allele.

[T4] What process allows different combinations of alleles on different chromosomes to enter gametes?

Independent assortment of homologous chromosomes at Metaphase I.

[IT4] On page 33, what process is illustrated by the two different possible alignments of homologous chromosomes at Metaphase I?

Independent assortment.

[IT4] For Alignment #1 on pages 33–35, what two gamete genotypes are produced?

A1;E1 and A2;E2.

[IT4] For Alignment #2 on pages 36–37, what two gamete genotypes are produced?

A1;E2 and A2;E1.

[IT4] Considering both Metaphase I alignments on pages 33–38 across multiple meiotic cells, what four gamete genotypes are possible?

A1;E1, A1;E2, A2;E1, and A2;E2.

[A4] Why can genetically identical parent cells undergoing meiosis produce different gamete genotypes?

Homologous chromosome pairs can orient differently at Metaphase I because of independent assortment.

[A4] Does the orientation of one homologous chromosome pair at Metaphase I determine the orientation of another homologous pair?

No. Their orientations assort independently.

[A4] What separates during Anaphase I of meiosis?

Homologous chromosomes.

[A4] What separates during Anaphase II of meiosis?

Sister chromatids.

[A4] Why can different Metaphase I alignments produce different combinations of alleles in gametes?

Different homologs move toward different poles during Anaphase I, changing which maternal/paternal chromosomes are inherited together.

[A4] For genotype A1/A2;B1/B2;E1/E1 with the genes on different chromosomes, which genes count toward n in 2^n?

A and B.

[A4] For genotype A1/A2;B1/B2;E1/E1, what is n in the 2^n equation?

n = 2.

[A4] For genotype A1/A2;B1/B2;E1/E1, how many possible gamete genotypes can be produced across multiple meiotic events?

2^2 = 4.

[A4] For genotype A1/A2;B1/B2;E1/E1, what are the four possible gamete genotypes?

A1;B1;E1, A1;B2;E1, A2;B1;E1, and A2;B2;E1.

[A4] Why does every gamete from A1/A2;B1/B2;E1/E1 contain E1?

The parent is homozygous E1/E1 and therefore has only the E1 allele to contribute.

[A4] Which alleles can vary among gametes from A1/A2;B1/B2;E1/E1?

The A and B alleles because the parent is heterozygous at those genes.

[IT4] On page 43, use the chromosome/genotype diagram for A1/A2;B1/B2;E1/E1 to identify the possible gamete genotypes.

A1;B1;E1, A1;B2;E1, A2;B1;E1, and A2;B2;E1.

[A4] A parent is A1/A1;B1/B2;C1/C1. How many possible gamete genotypes can be produced if the genes are on different chromosomes?

2^1 = 2.

[A4] A parent is A1/A2;B1/B1;C1/C2;D1/D1. How many possible gamete genotypes can be produced if the genes are on different chromosomes?

2^2 = 4.

[A4] A parent is homozygous for every gene being considered. How many possible gamete genotypes can be produced from those genes?

One.

[T4] What is the maximum number of gametes produced by one parent cell completing one meiosis?

Four.

[T4] What is the maximum number of different gamete genotypes that one parent cell can produce in a single meiosis?

Four.

[A4] If 2^n = 8 possible gamete genotypes exist for an individual, can one parent cell undergoing one meiosis produce all eight?

No. One meiosis produces only four gametes, so at most four different gamete genotypes can come from that one parent cell.

[A4] Why may multiple parent cells undergoing meiosis be required to produce every genotype predicted by 2^n?

Each individual meiosis produces only four gametes and has only one particular Metaphase I chromosome orientation.

[IT4] On pages 46–47, what is the maximum number of different gamete genotypes that can be produced by one parent cell undergoing meiosis?

Four.

[IT4] On pages 49–55, why are multiple parent cells needed to produce all eight possible gamete genotypes shown?

Different parent cells can have different homologous chromosome alignments at Metaphase I, whereas one meiosis produces only four gametes.

[A4] If an individual can theoretically make 8 gamete genotypes across many meioses, what is the maximum number of gametes produced by one single meiotic event?

Four.

[A4] Does 2^n always tell you how many different genotypes appear among the four gametes produced by one single parent cell?

No. It gives the possible genotype set across meiotic events for the conditions described; one meiosis can produce only four gametes.

[IT4] Pages 50–54 show several different Metaphase I orientations. What changes between these cells?

Which homolog carrying each allele is positioned toward each pole.

[IT4] Why do the different chromosome alignments on pages 50–54 generate different subsets of the possible gamete genotypes?

Each alignment sends different combinations of homologous chromosomes to the two poles during Meiosis I.

[T4] What does it mean for two genes to be unlinked in this lecture?

They are on different chromosomes.

[T4] What does it mean for two genes to be physically linked?

They are located on the same chromosome.

[IT4] On page 58, what is the major difference between the unlinked-gene diagram and the linked-gene diagram?

The unlinked genes are on different chromosomes, whereas the linked genes are on the same chromosome.

[T4] Why can unlinked genes produce different allele combinations through meiosis?

Homologous chromosomes assort independently at Metaphase I.

[T4] Why do linked alleles tend to travel together into gametes?

They are physically located on the same chromosome.

[A4] If A1 and E1 are physically linked on one homolog and A2 and E2 are linked on the other, what are the parental allele combinations?

A1;E1 and A2;E2.

[T4] What is meant by a parental or non-recombinant allele combination?

An allele combination that was already present together on one of the parent's homologous chromosomes.

[T4] What is meant by a recombinant allele combination?

A new combination of maternal and paternal alleles produced through crossing-over and recombination.

[IT4] On pages 59–60, if A and E are linked and crossing-over does not recombine the loci, how many different gamete genotypes are produced by the cell?

Two.

[IT4] On pages 59–60, if the linked parental combinations are A1;E1 and A2;E2 and no relevant recombination occurs, what gamete genotypes are produced?

A1;E1 and A2;E2.

[A4] If linked genes do not undergo recombination between their loci, do all four combinations A1;E1, A1;E2, A2;E1, and A2;E2 necessarily appear?

No. Only the parental combinations are produced in the example.

[A4] If crossing-over exchanges only chromosome regions beyond both linked loci and does not separate the alleles, does it create recombinant allele combinations for those genes?

No.

[A4] What matters about the location of a crossover when considering two linked genes?

Whether the crossover occurs in a position that recombines the alleles at those loci.

[T4] When is crossing-over/recombination especially important for predicting gamete genotypes?

When two or more genes are physically linked on the same chromosome.

[A4] If the genes being considered are on different chromosomes, does crossing-over create additional genotype combinations beyond those already possible through independent assortment?

No, not in the Lecture 4 example.

[A4] For unlinked A and E genes in an A1/A2;E1/E2 parent, what four gamete genotypes are possible whether or not crossing-over occurs?

A1;E1, A1;E2, A2;E1, and A2;E2.

[IT4] On pages 56–57, does crossing-over change the set of possible gamete genotypes when A and E are on different chromosomes?

No. The same four genotype combinations are possible.

[IT4] On page 61, the A and E genes are linked and crossing-over occurs at X. What gamete genotypes can be produced?

A1;E1, A1;E2, A2;E1, and A2;E2.

[IT4] On pages 61–62, which gamete genotypes are parental and which are recombinant if the original linked combinations are A1;E1 and A2;E2?

Parental: A1;E1 and A2;E2. Recombinant: A1;E2 and A2;E1.

[T4] What effect can crossing-over have on linked genes?

It can create recombinant chromatids containing new combinations of alleles.

[A4] If A1 and E1 are originally linked together and A2 and E2 are linked together, what recombinant combinations can crossing-over produce?

A1;E2 and A2;E1.

[A4] Why can linked genes with crossing-over sometimes generate the same genotype categories as unlinked genes?

Recombination can create the additional allele combinations that independent assortment produces for unlinked genes.

[T4] Even if linked genes with crossing-over produce the same four genotype categories as unlinked genes, what can differ?

Their frequencies; the genotypes may not occur in equal frequency.

[A4] Does obtaining four possible genotypes from linked genes mean the genes behave exactly like unlinked genes in frequency?

No. The lecture notes that the genotype types can be the same but not necessarily occur at equal frequencies.

[A4] Why can one meiosis with crossing-over potentially produce four different gamete genotypes?

The four chromatids can contain different parental and recombinant allele combinations before separating into four gametes.

[IT4] Given a diagram of linked genes and a crossover point, what should you determine before predicting gamete genotypes?

Determine which non-sister chromatids exchange DNA and whether the crossover occurs between the relevant gene loci.

[A4] A crossover occurs between two linked loci. What should you look for in the resulting chromatids?

New recombinant combinations of the alleles on the participating non-sister chromatids.

[A4] A crossover occurs outside the interval containing two linked loci. Will it necessarily recombine those two alleles?

No.

[T4] What are the two Lecture 4 outcomes you should be able to determine for gametes?

The number of different gamete genotypes possible and the identities of those genotypes.

[T4] What is an autosome?

Any chromosome other than a sex chromosome.

[T4] How many autosomes do humans have in a typical diploid somatic cell according to the lecture?

44 autosomes, or 22 pairs.

[T4] What is an autosomal gene?

A gene located on an autosome.

[T4] What chromosomes are described as the human sex chromosomes in this lecture?

X and Y.

[T4] What role are the X and Y chromosomes described as having?

They determine an individual's sex.