12) TH Morgan's work on linkage

Question 1

What was the problem of Mendel’s principles?

Answer 1

1) Principle of segregation: Pattern of chromosome segregation seen during mitosis does not match the pattern displayed by alleles
Anser: Meiosis
2) More genes than there are chromosomes= Goes against principle of independent assortment, no segregation in mitosis and there are too many alleles

Mendel: One phenotype is determined by one gene, gene contains 2 alleles= During inheritance, one allele goes to daughter cells
BUT: Cytology= Complete copy all of all information in the cell (mitosis) goes to each daughter cell=

Question 2

What is meiosis?

Answer 2

Mitosis: Each daughter receives all the alleles present in the parent

Meiosis: Gamete cells only receive half of the alleles present in the parent

Question 3

What are the 3 main differences between mitosis and meiosis?

Answer 3

1) Happen in different places= Mitosis= Somatic cells, Meiosis= Germ cells

2) Mitosis: 1 cell division
Meiosis: 2 cell divisions= One immediately after the other
Mitosis: 2 daughter cells
Meiosis: 4 daughter cells
Both mitosis and meiosis start with same amount of DNA in cell= 2n, 4c
Mitosis: 2n 2c daughter cells
Meiosis: 1n, 1c gametes

3) Mitotic prophase: Homologous chromosomes (one from mum, one from dad) don’t associate with each other during mitotic prophase

Meiotic prophase: First division= DO associate with each other, each line contains 1 member of a homologous pair . Line up in pairs= 2 chromatids from mum, 2 chromatids from dad line up side by side

Question 4

What are the stages of meiosis?

Answer 4

SAME G1, S and G2 phases
Meiosis I: Happens after G2
1) Prophase: Chromosomes condense BUT homologous chromosomes also pair up + cross over occurs, homologous chromosomes move slightly apart and homologous pairs move to equatorial plane + Nuclear membrane breaks down

2) Metaphase: Homologs pairs lined up on equatorial plane
3) Anaphase: Homologous chromosomes separate= Each daughter nucleus is haploid
4) Telophase: Nuclear envelope reforms

Result of M1: Produces 2 daughter cells, but is haploid= Only has one from one parent

M2: Divides again, sequence happens again= Result is 4 gametes

Useful diagram in lecture using Aa

Question 5

What did Morgan and Sturtevant find out?

Answer 5

If 2 gene loci are on the same chromosome (i.e. eye colour and wing length)= Alleles will always be inherited together= Linked

If found on the same chromosome: 3:1 ratio
If not and are independent: (9:3:3:1) ratio

If you had relatively limited number of chromosomes= Each chromosome must contain more then one gene
Observed: Actual ratio is between them= Wing length and eye colour are on the same chromosome

Proposed: Alleles were found together on chromosomes (linked) but that those chromosomes were quire fragile and could break apart

Question 6

What is genetic mapping?

Answer 6

The likelihood of crossover between any 2 genes gives some idea of how far apart they are

The further apart any 2 genes were on a chromosome, the more likely they were to be separated by random chromosomal breaks and crossovers

Can use frequency of linkage breaking breaking between 2 alleles as rough indication of how away those alleles were from each other

Question 7

What is the physical basis for crossing over?

Answer 7

During Meiosis I, Prophase I:
Homologous chromosomes pair up= double helix that makes up one sister chromatid in one homologous chromosome can unwind and invade the double helix that makes up a sister chromatid in other homologous chromosome= Holliday Junction

Can be cut in ways:

1) Non-crossover: Important in DNA repair
2) Crossover: Swap bits of chromatid= New crossover products= Can generate new combinations of alleles