Lecture 8

Question 1

recombination

Answer 1

the exchange of genetics material by homologous chromosomes during meiotic prophase

(changes the cis- trans orientation os alleles in different genes)

Question 2

recombinant chromosomes are referred as:

Answer 2

non-parental chromosomes
(because the parent does not have this cis- trans orientation)

Question 3

linkage group

Answer 3

genes of the same group are part of the linkage group

number of linkage groups corresponds to the haploid chromosomes number (n)

Question 4

significance of “crossing over”

Answer 4

crossing over serves as the basis of determining the distance between genes in chromosome mapping

Question 5

“crossing over”

Answer 5

the exchanging of genes between 2 homologous chromosomes’ non-sister chromatids

Question 6

chiasmata

Answer 6

X shaped intersections that are points of genetics exchange (singular- chiasma)

Utilized for “crossing over”

Question 7

% of offspring resulting from recombinant gametes depends on:

Answer 7

the distance between the two genes on the chromosome

Question 8

mu

Answer 8

map unit, is defined at 1 percent recombination between two genes on a chromosome

often called centimorgans (cM) and are relative distances, not exact ones

Question 9

SCO(single cross over)

Answer 9

alters linkage between two genes only if the crossover occurs between those two genes

Question 10

2 genes that are located close to each other along a chromosome:

Answer 10

Are less likely to form chiasma, and therefore less likely to have crossing over.

Question 11

cis coupling

Answer 11

alleles on the same chromosome

Question 12

repulsion

Answer 12

alleles on opposite chromosome

Question 13

recombination frequency formula

Answer 13

r= sum of non- parental progeny/ total progeny of test cross

Question 14

Mendel’s law of Independent assortment is violated by:

Answer 14

genetic linkage

Question 15

linked gene ration tend to more toward ___ is dominant alleles are in cis

Answer 15

3:1

Question 16

interlocus distance

Answer 16

distance between 2 loci on a single chromosome

The degree of crossing over in proportional to interlocus distance

Question 17

two factor mapping

Answer 17

done with a test- cross, the dihybrid is constructed such that non- parental phenotypes are easy to identify

used to identify non- parental phenotypes

Question 18

three factor mapping

Answer 18

determines gene order, and calculates recombination interference

ALWAYS determines middle gene first

helpful in simplifying gene mapping

Question 19

Because crossing over occurs at the ________ stage of the cell cycle, notice that each single crossover involves only
______ of the _______ chromatids.

Answer 19

four-strand, two, four

Question 20

Assume that the genes for tan body and bare wings are 15 map units apart on chromosome II in Drosophila. Assume also that a tan-bodied, bare-winged female was mated to a wild-type male and that the resulting F1 phenotypically wild-type females were mated to tan-bodied, bare-winged males. Of 1000 offspring, what would be the expected of wild-type offspring, and in what numbers would they be expected?

Answer 20

425
Because:
Total- wild type = total parental type
1000- (100x.15)= 850

parental type/2 = total with mother phenotype
850/2 = 425

Question 21

The cross GE/ge × ge/ge produces the following progeny: GE/ge 404; ge/ge 396; gE/ge 97; Ge/ge 103. From these data, one can conclude that there are ________ map units between the G and E loci.

Answer 21

20 map units

Because, gE/ge total and Ge/ge total = 20% of the total, which directly correlates to % of map units

Question 22

Genes X, Y, and Z are linked. Crossover gametes between genes X and Y are observed with a frequency of 25%, and crossover gametes between genes Y and Z are observed with a frequency of 5%. What is the expected frequency of double crossover gametes among these genes?

Answer 22

1.25%

Because:
Double crossover event is calculated by probabilities of single crossover events:

0.25 x 0.5 = 0.0125

Question 23

Assume that the genes from the previous example are located along the chromosome in the order X, Y, and Z. What is the probability of recombination between genes X and Z?

Answer 23

30%

Because:
Recombination frequencies between linked genes along a chromosome are additive, so the recombination frequency between genes X and Z is 25 + 5 = 30.

Question 24

Crossing over during meiosis occurs between alleles on _____ chromatids.

Answer 24

nonsister

Question 25

If two genes on the same chromosome exhibit complete linkage, what is the expected F2 phenotypic ratio from a selfed heterozygote with the genotype a + b + ⁄⁄ ab?

Answer 25

3:1 Because: there is no crossing over there will be three wild- type and one is mutant, also the wild type is dominant to recessive genotype.

Question 26

Complete linkage means:

Answer 26

no crossing over or recombination

Question 27

What essential criteria must be met in order to execute a successful mapping cross?

Answer 27

Recombination must occur in the sex that displays genetic heterogeneity. One parent should be heterozygous and the other homozygous recessive for all the genes being mapped. The genotypes of the offspring must be readily determined from their phenotypes.

Question 28

An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). If these three genes were all assorting independently, how many genotypic and phenotypic classes would result in the offspring, and in what proportion, assuming simple dominance and recessiveness in each gene pair?

Answer 28

8, 1:1:1:1:1:1:1:1

Question 29

An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). Assuming the three genes are so tightly linked on a single chromosome that no crossover gametes were recovered in the sample of offspring.