Chromosomes and Linkage

Question 1

What problem does chromosome theory pose to Mendelian genetics?

Answer 1

Many genes are on same chromosome, therefore problem with independent assortment

Question 2

Who crossed sweet peas to test for linkage?

Answer 2

Bateson and Punnet

Question 3

What traits with Bateson and Punnet use in their sweet pea cross?

Answer 3

Purple/red flower color, and Long/short pollen grains

Question 4

What P1 cross did Bateson and Punnet use?

Answer 4

PP LL x pp ll

Question 5

In the Bateson Punnet sweet pea cross, after F1 was self-fertilised, what was unusual about the F2?

Answer 5

The phenotypes were not 9:3:3:1 ratio

Question 6

In the Bateson Punnet sweet pea cross, how did the F2 ratios differ from the Mendelian 9:3:3:1?

Answer 6

There was an excess of parental and deficiency of recombinant phenotypes

Question 7

Who continued to investigate linkage after the Bateson Punnet cross?

Answer 7

Thomas Hunt Morgan

Question 8

What organism did THM work on?

Answer 8

Drosophila

Question 9

What characteristics did THM use to study linkage in drosophila?

Answer 9

Purple eyes (prpr) and vestigial wings (vgvg)

Question 10

What cross did THM use in his drosophila linkage study?

Answer 10

(prpr vgvg) x (pr+pr+ vg+vg+)

Question 11

What was different about THM’s F1 cross from that of Bateson and Punnet?

Answer 11

He crossed the F1 against a test-cross of homozygous recessive flies, i.e. a back-cross

Question 12

Why did THM do his back-cross as daughter-father rather than son-mother?

Answer 12

There is no crossing over in male drosophila

Question 13

What was the expected ratio for THM’s drosophila back-cross?

Answer 13

1:1:1:1, according to independent assortment

Question 14

What did THM actually see in his drosophila back-cross?

Answer 14

Parental excess and recombinant deficiency, i.e. linkage

Question 15

In THM’s drosophila back-cross, what second test did he do after the first back-cross?

Answer 15

(pr+pr+ vgvg) x (prpr vg+vg+)

Question 16

What was the point of THM’s second drosophila test?

Answer 16

To make sure that the results for mthe first test were not due to lethality of the mutant/wild-type combination

Question 17

Leptotene:

Answer 17

First subphase of Meiosis 1 metaphase, chromosomes become visible as beaded threads

Question 18

Zygotene:

Answer 18

Second subphase of Meiosis 1 metaphase, each chromosome seeks out its pair, zipped up into tetrads

Question 19

Pachytene:

Answer 19

Third subphase of Meiosis 1 metaphase, the tetrads shorten and wrap around each other

Question 20

Diplotene:

Answer 20

Fourth subphase of Meiosis 1 metaphase, tetrads open up after exchanging material by forming chiasmata, crossing between sister and non-sister arms

Question 21

Diakinesis:

Answer 21

Fifth subphase of Meiosis 1 metaphase, contraction of chromosomes and movement towards metaphase plate

Question 22

Coupling:

Answer 22

Wild-type traits are linked, mutants are linked

e. g.
- —Pr+—————Vg+—-
- —Pr—————–Vg—–

Question 23

Repulsion:

Answer 23

Wild-type is linked with mutant, and vice-versa

e. g.
- —Pr+—————Vg—–
- —Pr—————–Vg+—

Question 24

Who used the drosophila incomplete-linkage studies to invent linkage mapping?

Answer 24

Sturtevant