Question 1

Match the missing words in the following phrase. Genes are [A] if they are on the same chromosome and are [B] if they are on different chromosomes. Genes that are on different chromosomes segregate [C] at meiosis whereas genes on the same chromosome are more frequently inherited [D].

Accepted Answer

a. A = linked
b. B = unlinked
c. C = independently
d. D = together

Question 2

Which structure that forms during meiosis leads directly to the formation of recombinant chromosomes?

Accepted Answer

Crossover is the process in which homologous chromosomes exchange genetic material during meiosis. This leads to the formation of recombinant chromosomes, which carry a unique combination of alleles from both parental chromosomes. Therefore, crossover is the structure that directly leads to the formation of recombinant chromosomes. Microtubules and centromeres are important structures for chromosome movement during meiosis, while sister chromatids are identical copies of a chromosome that separate during meiosis.

Question 3

Fill in the blank. In a cross between parents with the genotypes A/a;B/b and a/a;b/b, where the A and B loci are unlinked, __% of the offspring will have a phenotype that differs from the phenotype of both parents.

Accepted Answer

50

Question 4

A cross between two true-breeding strains (P₀ generation) with genotypes A/A;b/b and a/a;B/B results in an F₁ genotype of A/a;B/b. If this is test-crossed to a/a;b/b, which of the following offspring have parental phenotypes? Please select all that apply.

Accepted Answer

Parental strains are those in the parental (P₀) generation. A/a;b/b has the same phenotype as the first parent (A/A;b/b) and a/a;B/b has the same phenotype as the second parent (a/a;B/B). The remaining two offspring have phenotypes that differ from both parents and are termed recombinants.

Question 5

A series of crosses to test whether two genes in Drosophila flies are sex-linked and on the same chromosome, and which alleles are dominant, are described below. Match each conclusion with the piece of evidence supporting it. A cross between a true-breeding black, apterous (no wings) female parent and a true-breeding wild-type male parent produced 100% F₁ offspring with wild-type phenotype in both sexes. A test cross of an F₁ male with the female parent produced 45% wild-type flies, 45% black, apterous flies, 5% flies with a black body and normal wings, and 5% apterous flies with a normal colored body. a. The two genes are autosomal b. Normal wings is dominant c. The two genes are on the same chromosome

Accepted Answer

The answer of A series of crosses to test whether...

Question 6

What does 10 centiMorgans (cM) represent?

Accepted Answer

10 centiMorgans (cM) represents a 10% recombination frequency between two genes on the same chromosome.

Question 7

In an experiment, two homologous chromosomes with alleles as shown below (original homologous chromosomes) underwent meiosis and to product four chromosomes (resultant chromosomes). How many crossovers appear to have occurred during meiosis?

Accepted Answer

The answer of In an experiment, two homologous chromosomes with...

Question 8

Genes that are located close together on a homologous chromosome are more likely to be inherited together than genes that are further apart.

Accepted Answer

Genes that are close together on a chromosome are less likely to have a crossover occur between them than genes that are further apart, and therefore genes that are closer together on a chromosome are more likely to be inherited together as a block.

Question 9

Which of the following are necessary characteristics of balancer chromosomes? Please select all that apply.

Accepted Answer

Balancer chromosomes are used to maintain (often lethal) alleles in a population. It must therefore be easy to distinguish the heterozygote from the two possible homozygotes, and this is normally achieved using marker genes that are co-inherited with the alleles. In order to ensure that the target allele is always inherited with the marker, crossing-over must be suppressed between the target allele and the marker. This is usually done by inverting parts of the chromosome. Balancer chromosomes are only relevant in organisms with homologous pairs of chromosomes (i.e., not haploids). Balancer chromosomes still require a centromere, in order to be able to segregate at meiosis.

Question 10

An experiment was conducted with Drosophila flies carrying recessive white-eye and yellow-body alleles. The white and yellow genes are on the X-chromosome. The parental (P₀) individuals were homozygous white-eye, normal body female and red-eye, yellow-body male. F₁ females had a wild-type phenotype and F₁ maleshad normal bodies and white eyes. F₁ individuals were then mated. What phenotypes are expected in the F₂ generation? Please select all that apply.

Accepted Answer

The white and yellow genes are on the X-chromosome, and so are linked. The parental X-chromosomes have w/w y⁺/y⁺ (female) and w⁺/_ y/_ (male). The + symbol denotes wild-type. This means that here, the w allele is linked to the y⁺allele and the w⁺allele is linked to the y allele. F₁ females are w⁺/w y⁺/y. F₁ males are w/_ y⁺/_. Therefore, the majority of F₂males will be w/_ y⁺/_ (white-eye, normal body) or w⁺/_ y/_ (red-eye, yellow body). Rarely, a crossover will occur between the white and yellow genes to give males with w⁺/_ y⁺/_ (wild-type) or w/_ y/_ (white-eye, yellow body). This means that very few males (w⁺/_ y⁺/_) will have a wild-type phenotype. 50% of males have white eyes. The majority of F₂ females will be w⁺/w y+/y (wild-type) or w/w y⁺/y⁺(white-eye, normal body). Rarely, a crossover will occur between the white and yellow genes to give females with w⁺/w y⁺/y⁺(wild-type) or w/w y⁺/y (white-eye, normal body). Together, ~50% of female flies are wild-type and 50% are white-eye, normal body. Thus, 100% of females have a normal body and 50% have white eyes.

Question 11

When genes are on the same chromosome, the alleles on one of the two homologous chromosomes are considered to be in trans.

Accepted Answer

Alleles on the same homologue are considered to be in cis. Alleles on different homologues are considered to be in trans. For example, if Drosophila y+ and w+ alleles are on the same chromosome, then the wild-type alleles are in cis.

Question 12

The maximum map distance that can be observed in a cross concerning two loci is ___ map units.

Accepted Answer

The answer of The maximum map distance that can be...

Question 13

What is a tetrad

Accepted Answer

A tetrad is formed during meiosis and consists of two homologous chromosomes, each made up of two sister chromatids, thus forming a group of four chromatids.

Question 14

Two genes can be syntenic and unlinked.

Accepted Answer

Genes are syntenic if they are physically located on the same chromosome. Linkage relates to the probability that alleles are co-inherited. If two genes are very far apart on a chromosome such that a crossover always occurs between them, then they are unlinked. Therefore it is possible for a two genes to be syntenic and unlinked.

Question 15

Syntenic blocks are sections of chromosomes that have remained largely intact during evolution. For example, human chromosome 4 corresponds to syntenic blocks on mouse chromosomes 3, 5, and 8. Which of the following statements about synteny are true? Please select all that apply.

Accepted Answer

Syntenic blocks tell us about the evolutionary history of organisms. Genes that are found in the same block in multiple organisms are likely to have been in the same block throughout evolutionary history, but not necessarily in the same arrangement. The ancestor of human and mice probably had the same syntenic blocks, but not in the same chromosomal arrangement as is seen in either human or mouse today. Crossovers do not generally have an effect on the arrangement of syntenic blocks and crossovers can potentially occur anywhere on a chromosome (with some limitations).

Question 16

Which of the following statements about double-crossovers are true? Please select all that apply.

Accepted Answer

The further apart two loci are on a chromosome, the more likely they are to have one (or multiple) crossovers between them. Double-crossovers between two loci are not recognized as crossovers, which leads to an underestimation of the recombination rate between two loci. The map distance between two loci is related to recombination frequency (1% recombination = 1 map unit), so if recombination rate is underestimated then map distance is also underestimated. The probability of two crossovers occurring between two loci is lower than the probability of a single crossover occurring between the same two loci.

Question 17

Ascomycete fungi are valuable models for studying recombination. Which of the following are advantages of these systems? Please select all that apply.

Accepted Answer

Haploids cannot undergo normal meiosis. All other options are true. In ascomycete fungi, the haploid spores produced after meiosis are held together, so researchers can isolate and analyze the four products in the knowledge that they were produced by the same meiotic event. Haploid fungal spores can grow (no fertilization is necessary) and therefore they can be analyzed directly. This simplifies the genetic analysis and dominance/recessive relationships do not need to be taken into account, so test crossing is not needed.

Question 18

A Drosophila parent fly heterozygous for alleles for black body, dumpy wings, and purple eyes was test-crossed to a fly that was homozygous recessive for all three genes. The genes are all on the same chromosome. The offspring had the following phenotypes in the following numbers:
$\begin{array}{|l|c|}
\hline \text { Phenotype } & \text { Number of offspring } \
\hline \text { wild type } & 225 \
\hline \text { black body, purple eyes, dumpy wings } & 238 \
\hline \text { dumpy wings } & 122 \
\hline \text { black body, purple eyes } & 135 \
\hline \text { black body, dumpy wings } & 15 \
\hline \text { purple eyes } & 19 \
\hline \text { purple eyes, dumpy wings } & 6 \
\hline \text { black body } & 5 \
\hline
\end{array}$ What were the genotypes of the chromosomes in the parent? Please select all that apply.

Accepted Answer

The answer of A Drosophila parent fly heterozygous for alleles...

Question 19

A Drosophila fly heterozygous for alleles for black body, dumpy wings, and purple eyes was test-crossed to a fly that was homozygous recessive for all three genes. The genes are all on the same chromosome. The offspring had the following phenotypes in the following numbers:
$$\begin{array} { | l | c | } 
\hline \text { Phenotype } & \text { Number of offspring } \
\hline \text { wild type } & 225 \
\hline \text { black body, purple eyes, dumpy wings } & 238 \
\hline \text { dumpy wings } & 122 \
\hline \text { black body, purple eyes } & 135 \
\hline \text { black body, dumpy wings } & 15 \
\hline \text { purple eyes } & 19 \
\hline \text { purple eyes, dumpy wings } & 6 \
\hline \text { black body } & 5 \
\hline
\end{array}$$ What is the order of the genes on the chromosome?

Accepted Answer

The answer of A Drosophila fly heterozygous for alleles for...

Question 20

Alleles for hairy body, curled wings, and glass eyes are on the same Drosophila chromosome. There are 23.5 map units between hairy and curled and 13 map units between curled and glass. One percent of offspring from a test cross had double crossovers. Was crossover interference observed?

Accepted Answer

Interference is calculated as 1 - (observed double crossovers / expected double crossovers). Expected double crossovers = (23.5% * 13%) / 100 = 3.055%. Observed = 1%. Interference = 1 - (1% / 3.055%) = 67%.

Quiz 9: The Locations of Genes on Chromosomes: Linkage and Genetic Maps