Question 1

Two genes are considered linked when there are more F₂ progeny with recombinant genotypes than parental genotypes in the offspring of a dihybrid testcross.

Accepted Answer

Two genes are considered linked when there are more offspring with parental genotypes than recombinant genotypes in the offspring of a dihybrid testcross, indicating that the genes are physically close to each other on the same chromosome and do not assort independently.

Question 2

Suppose the map for a particular human chromosome interval is:
 a——1 m.u.——b--1 m.u.——c--1 m.u.——d——1 m.u.——e——1 m.u.——f
In a man heterozygous for all six genes, what fraction of his sperm would be recombinant in the a-f interval?

Accepted Answer

The total map distance between genes a and f is 5 m.u. (1 m.u. between each pair of adjacent genes). Since 1 m.u. corresponds to a 1% recombination frequency, the total recombination frequency in the a-f interval is 5%. Therefore, 5% of the sperm will be recombinant in the a-f interval.

Question 3

Twin spotting provides evidence of what genetic event?

Accepted Answer

Twin spotting provides evidence of mitotic recombination, which can lead to the exchange of genetic information between sister chromatids during cell division, resulting in genetically distinct patches of cells.

Question 4

A coefficient of coincidence of 0.5 in a region of three genes means that half as many double crossovers were observed as would have been expected if crossovers in the two intervals were independent.

Accepted Answer

A coefficient of coincidence (COC) of 0.5 indicates that the observed number of double crossovers is half of the expected number if crossovers in each interval were independent, reflecting interference in the crossover process.

Question 5

If the p value corresponding to a given χ² value and number of degrees of freedom is lower than 0.05, then the null hypothesis is rejected and it can be said with some confidence that the two genes being evaluated are linked.

Accepted Answer

A p-value lower than 0.05 typically indicates that there is a statistically significant difference from the null hypothesis, which in this context suggests that the two genes being evaluated are indeed linked, as the chance occurrence of the observed data (or more extreme) under the null hypothesis is less than 5%.

Question 6

Mitotic recombination occurs between homologous chromosomes.In which of the following would you not expect to encounter mitotic recombination?

Accepted Answer

Mitotic recombination is a process that occurs in eukaryotic organisms, which have a nucleus and undergo mitosis. E. coli, being a prokaryote, does not undergo mitosis and therefore does not experience mitotic recombination. Tobacco plants, Homo sapiens, and Drosophila melanogaster are all eukaryotes and can undergo mitotic recombination.

Question 7

A linkage group includes all of the genes on a chromosome, including genes that are so far apart from each other on a chromosome that they assort independently during meiosis.

Accepted Answer

A linkage group refers to all genes located on the same chromosome. Even if genes are far apart and assort independently due to crossing over, they are still part of the same linkage group.

Question 8

The hypothesis that predicts no linkage between genes is known as the null hypothesis.

Accepted Answer

The null hypothesis typically predicts no effect or no relationship between variables, which in the context of genetics, would include predicting no linkage between genes, assuming they assort independently according to Mendel's law of independent assortment.

Question 9

A female mouse from a true-breeding wild-type strain was crossed to a male mouse with apricot eyes (ap)and grey body (gy).The F₁ mice were wild-type for both traits.When the F₁ were interbred, the F₂ were distributed as follows: Females $$\begin{array} { | l | l | } \hline \text { all wild type } & 200 \ \hline \end{array}$$ Males $$\begin{array} { | l | r | } \hline \text { wild type } & 91 \ \hline \text { apricot } & 11 \ \hline \text { grey } & 9 \ \hline \text { apricot, grey } & 89 \ \hline \end{array}$$ Which of the following statements is correct?

Accepted Answer

The distribution of phenotypes, especially the difference in phenotype distribution between males and females, suggests that the traits are X-linked. The fact that all females are wild type indicates that they must receive one wild-type allele from their father, suggesting that the traits are linked to the X chromosome, which males only have one of. The recombination frequency (11 apricot + 9 grey out of 200 total males) indicates a 10% recombination rate, which corresponds to 10 map units apart on the X chromosome.

Question 10

Chiasmata are structures that show where recombination occurred between sister chromatids.

Accepted Answer

Chiasmata are structures that show where recombination has occurred between homologous chromosomes, not sister chromatids.

Question 11

Chiasmata can be seen through a light microscope and are sites of recombination.

Accepted Answer

Chiasmata are visible under a light microscope as X-shaped structures where homologous chromosomes remain attached to each other, indicating sites of crossover and genetic recombination during meiosis.

Question 12

Another name for a chromosome is a ________, because it contains alleles that are often inherited together.

Accepted Answer

A linkage group refers to a group of genes that are located on the same chromosome and tend to be inherited together because they are physically linked. This makes "linkage group" synonymous with a chromosome in the context of inheritance.

Question 13

The diploid garden pea plant has 14 chromosomes.The haploid fungus Neurospora crassa has 7 chromosomes.Neither organism has separate male and female individuals.Therefore, the number of linkage groups in these two organisms is

Accepted Answer

The number of linkage groups is equal to the haploid chromosome number.

Question 14

Genes that are not syntenic are not linked.

Accepted Answer

Genes that are not syntenic are located on different chromosomes or far apart on the same chromosome, meaning they are not physically linked and do not tend to be inherited together.

Question 15

Large sectors suggest a mitotic recombination event late in the growth of a yeast colony.

Accepted Answer

Large sectors in a yeast colony suggest a mitotic recombination event early in the growth of the colony, allowing the resulting genotype to proliferate through a significant portion of the colony's development.

Question 16

The genetic distance from one end of a linkage group and the other may exceed 50 m.u.because the distances between many gene pairs are added together to make the map.

Accepted Answer

Genetic distance in a linkage group can exceed 50 map units (m.u.) because it represents the sum of distances between multiple gene pairs along a chromosome, and recombination rates between distant genes can accumulate beyond 50%.

Question 17

Sectors with a different phenotype in an otherwise uniform yeast colony may be evidence of mitotic recombination.

Accepted Answer

Mitotic recombination can lead to genetic changes in individual cells within a colony, resulting in sectors with different phenotypes in an otherwise uniform yeast colony.

Question 18

In some fungi, such as the bread mold Neurospora crassa, the arrangement of spores in the ascus directly reflects the order in which they were produced during meiosis.What is the collection of spores produced by meiosis in Neurospora crassa called?

Accepted Answer

The collection of spores produced by meiosis in Neurospora crassa is called an ordered tetrad or ordered octad, reflecting the specific sequence of their formation during meiosis.

Quiz 5: Linkage, Recombination, and the Mapping of Genes on Chromosomes

Two genes are considered linked when there are more F2 progeny with recombinant genotypes than parental genotypes in the offspring of a dihybrid testcross.

Suppose the map for a particular human chromosome interval is: a——1 m.u.——b--1 m.u.——c--1 m.u.——d——1 m.u.——e——1 m.u.——f In a man heterozygous for all six genes, what fraction of his sperm would be recombinant in the a-f interval?