Why is interference important? Why do we calculate it in a three-point cross? Why don't we calculate interference in a two-point cross?

Species A has $2 n=10$ chromosomes. Species B has $2 n=40$ chromosomes. On average, will two randomly selected genes from species A be more likely, less likely, or equally likely to assort independently than two randomly selected genes from species B? Explain your reasoning.

Priscilla Lane and Margaret Green studied the linkage relations of three genes affecting coat color in mice: mahogany (mg), agouti (a), and ragged (Rg). They carried out a series of three-point crosses, mating mice that were heterozygous at all three loci with mice that were homozygous for the recessive alleles at these loci (P. W. Lane and M. C. Green. 1960. Journal of Heredity 51:228–230). The following table lists the progeny of the test crosses :

$$\begin{array}{cccr} a &\hspace{1cm} R g & \hspace{1cm}+ &\hspace{1cm} 1 \\ + & + & m g & 1 \\ a & + & + & 15 \\ + & R g & m g & 9 \\ + & + & + & 16 \\ a & R g & m g & 36 \\ a & + & m g & 76 \\ + & R g & + & 69 \\ \text { Total } & & & \frac{69}{213} \end{array}$$

a. Determine the order of the loci that encode mahogany, agouti, and ragged on the chromosome, the map distances between them, and the coefficient of coincidence and the interference among these genes.
b. Draw a picture of the two chromosomes in the triply heterozygous mice used in the test crosses, indicating which of the alleles are present on each chromosome.

Explain how to determine, using the numbers of progeny from a three-point cross, which of three linked loci is the middle locus.

A geneticist selects for increased body weight in a population of fruit flies that she is raising in her laboratory. She measures body weight in her population and selects the five heaviest males and the five heaviest females and uses them as the parents for the next generation. From the progeny produced by these parents, she selects the five heaviest males and five heaviest females and mates them. She repeats this procedure each generation. The average body weight of flies in the original population was $1.1$ mg. The flies respond to selection, and their body weight steadily increases. After $20$ generations of selection, the average body weight is $2.3$ mg. However, after about $$ generations, the response to selection in subsequent generations levels off, and the average body weight of the flies no longer increases. At this point, the geneticist takes a long vacation; while she is gone, the fruit flies in her population interbreed randomly. When she returns from vacation, she finds that the average body weight of the flies in the population has decreased to $2.0$ mg.\ Provide an explanation for why the response to selection leveled off after $20$ generations. Why did the average size of the fruit flies decrease when selection was no longer applied during the geneticist’s vacation? $$

Both $HCO_3^-$ and $HS^-$ are amphoteric. Write an equation to show how each substance can act as an acid and another equation to show how each can act as a base.

Suppose that you want to sequence the following DNA fragment:
5'-TCCCGGGAAA-primer site-3'
You first use PCR to amplify the fragment, so that there is sufficient DNA for sequencing. You carry out dideoxy sequencing and then separate the products of the sequencing reactions by gel electrophoresis. Draw the bands that should appear on the gel from the four sequencing reactions.

Suppose that you want to sequence the following DNA fragment:
5'-TCCCGGGAAA-primer site-3'
You first use PCR to amplify the fragment, so that there is sufficient DNA for sequencing. You carry out dideoxy sequencing and then separate the products of the sequencing reactions by gel electrophoresis. Draw the bands that should appear on the gel from the four sequencing reactions.

Young children are egocentric. Provide some examples of children’s egocentric thinking.

Why is the frequency of recombinant gametes always half the frequency of crossing over?

The locations of six deletions have been mapped to a Drosophila chromosome, as shown in the following deletion map. Recessive mutations a, b, c, d, e, and f are known to be located in the same region as the deletions, but the order of the mutations on the chromosome is not known.

Chromosome

Deletion 1 $\rule{1cm}{0.4pt}$
Deletion 2 $\rule{1cm}{0.4pt}$
Deletion 3 $\rule{1cm}{0.4pt}$
Deletion 4 $\rule{1cm}{0.4pt}$
Deletion 5 $\rule{1cm}{0.4pt}$
Deletion 6 $\rule{1cm}{0.4pt}$

When flies homozygous for the recessive mutations are crossed with flies heterozygous for the deletions, the following results are obtained, in which “m” represents a mutant phenotype and a plus sign (+) represents the wild type. On the basis of these data, determine the relative order of the seven mutant genes on the chromosome

$$\begin{equation} \begin{array}{ccccccc} &{\text { Mutations }} \\ \text { Deletion } & \boldsymbol{a} & \boldsymbol{b} & \boldsymbol{c} & \boldsymbol{d} & \boldsymbol{e} & f \\ 1 & \mathrm{~m} & + & \mathrm{m} & + & + & \mathrm{m} \\ 2 & \mathrm{~m} & + & + & + & + & + \\ 3 & + & \mathrm{m} & \mathrm{m} & \mathrm{m} & \mathrm{m} & + \\ 4 & + & + & \mathrm{m} & \mathrm{m} & \mathrm{m} & + \\ 5 & + & + & + & \mathrm{m} & \mathrm{m} & + \\ 6 & + & \mathrm{m} & + & \mathrm{m} & + & + \end{array} \end{equation}$$

A group of geneticists are interested in identifying genes that may play a role in susceptibility to asthma. They study the inheritance of genetic markers in a series of families that have two or more asthmatic children. They find an association between the presence or absence of asthma and a genetic marker on the short arm of chromosome 20 and calculate a lod score of 2 for this association. What does this lod score indicate about genes that may influence asthma?

Shown below are eight DNA sequences from different individuals.

$$\begin{array}{cccc} {\text { Nucleotide Position }} \\ \hline \mathbf{1} & 5 & 10 & 15 \end{array}$$

Sequence $1 \ldots$ T C T G G A T C A T C A C A T ...
Sequence 2... A C A G C A T C A T T A C G T...
Sequence $3 \ldots$ T C A G G A T C A T T A C A T ...
Sequence $4 \ldots$ T C A G G A T C A T T A C A T ...
Sequence $5 \ldots$ A $C$ A $\mathrm{G} C$ A T C A T T A C G T...
Sequence 6... T C T G G A T C A T C A C A T ...
Sequence 7... T C A G G A T C A T T A C A T ...
Sequence $8 \ldots$ A C A G C A T C A T T A C G T...

a. Give the nucleotide positions of all single-nucleotide polymorphisms (SNPs; nucleotide positions where individuals vary in which base is present) in these sequences.
b. How many different haplotypes (sets of linked variants) are found in these eight sequences?
c. Give the haplotype of each sequence by listing the specific bases at each variable position in that particular haplotype.

Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recessive traits in cucumbers, the genes for which are linked on the same chromosome. A cucumber plant heterozygous for all three traits is used in a testcross, and the following progeny are produced by this testcross:

$$\begin{array}{lllr} S & U & T u & 2 \\ s & u & T u & 70 \\ S & u & T u & 21 \\ s & u & t u & 4 \\ S & U & t u & 82 \\ s & U & t u & 21 \\ s & U & T u & 13 \\ S & u & t u & \frac{17}{230} \end{array}$$

a. Determine the order of these genes on the chromosome.
b. Calculate the map distances between the genes.
c. Determine the coefficient of coincidence and the interference among these genes.
d. List the genes found on each chromosome in the parents used in the testcross.