Hunting the gene

Question 1

What did Hippocrates think? Around what time?

Answer 1

Equal contribution of 2 sexes to genotype of child, merging of gemules from each body part (earlier than Aristotle, around 400 BC)

Question 2

What did Aristotle think? Around what time?

Answer 2

All substance came from man :( (around 330BC)

Question 3

What was special about Mendel’s experiments?

Answer 3

He used true-breeding strains, quantified his results, had good design

Question 4

What did he notice?

Answer 4

That observed inheritance pattern was independent of phenotype studied

Question 5

Give the purple and white flower example to illustrate

Answer 5

see notes

Question 6

What is bad for Hippocrates and Aristotle about Mendel’s experiments?

Answer 6

Hippocrates: emergence of homozygous recessive in F2 eg white flowers
Aristotle: Reciprocal crosses give same results

Question 7

Give eg of yellow and green pea experiment extended to F2

Answer 7

see notes

Question 8

What were Mendel’s 4 propositions resulting from his experiments?

Answer 8

1) genes are particulate in nature. passed generation to generation without changing
2) Each parent carries 2x genes for each character
3) members of a gene pair segregate equally into gametes so each gamete contains 1 member of each gene pair
4) Fertilisation is independent of the genes carried by gamete

Question 9

How did he test his hyptheses?

Answer 9

Cross heterozygous F1 back to homozygous recessive parent and gets 1/2 green 1/2 yellow (in seed example) as expected

Question 10

Describe Mendel’s experiment seeing if seed colour and seed morphology characteristics were inherited independenty

Answer 10

see notes for Punnet square

Predicted ratio of 9:3:3:1 agreed well with Mendel’s ratio of 315:101:108:32

Question 11

Draw diagrams of pollen meiosis

When was this observed?

Answer 11

see notes (was observed in 20th century experiments not by Mendel, Mendel’s ideas purely theoretical and he didn’t observe chromosomes)

Question 12

Who came up with the chromosome theory of inheritance and in what year?

Answer 12

Walter Sutton and Theodore Boveri, 1903

Question 13

What 5 conclusions did Sutton make?

Answer 13

1) Total chromosome composition before meiosis is made up of 2 equivalent chromosome series, 1x paternal and 1x maternal
2) Each chromosome pair maintains individuality throughout cell divisions
3) In meiosis maternal and paternal pair members come together
4) One homologue of each chromosome pair enters each gamete
5) Each pair is distributed throughout gametes independent of other pairs

Question 14

Draw diagram to illustrate Sutton

Answer 14

see notes

Question 15

Which important genetic work happened in Cambridge 1905-1908?

Answer 15

Bateson, Punnett and Saunders worked on inheritance of pollen shape and flower colour in sweet peas (revisiting Mendel)
They saw characters were not inherited independently and parental combinations remain together more often than expected on the basis of random meiotic segregation

Question 16

What did Morgan study and when?

Answer 16

In 1909 he studies inheritance of purple eyes (pr) and vestigial wings (vg) in drosophila

Question 17

Draw out morgan’s cross

Answer 17

see notes

Question 18

Why did Morgan do a backcross?

Answer 18

It is easier to deduce the phenotype of progeny flies

Question 19

What did Morgan suggest based on his experiments?

Answer 19

Gene for purple eyes and vestigial wings are carried on the same pair of homologous chromosomes and when these pair in meiosis there is sometimes exchange of genetic information on chromosomal segments leading to new genetic combinations

Question 20

Draw a diagram representing crossing over

Answer 20

see notes

Question 21

What are chiasmata?

Answer 21

The cytological counterpart of interchromosomal recombination

Question 22

What is chromosomal mapping and what assumptions does it make

Answer 22

Technique used to estimate distance between mutations on the same chromosome, assume crossing over in meiosis occurs at random points along the chromosome length, prob crossing over between 2 markers is proportional to their distance apart

Question 23

If you get value of 13% recombinants you conclude genes are separated by distance of…

Answer 23

13 map units

Question 24

Describe Morgan’s 3 factor mapping experiment

Answer 24

see notes for diagram

Question 25

How can you minimise a compression effect for

mapping?

Answer 25

Build up a genetic map with pairs of markers that are relatively closely linked

Question 26

What is the max map distance between 2 genes?

Answer 26

50%, means genes aren’t linked as unlinked genes are equally likely to occur in parent or recombinant combinations

Question 27

How does recombination frequency change as genes on the same chromosome get further apart?

Answer 27

Tends asymptotically to 50%

Question 28

Draw diagram showing double crossover

Answer 28

see notes