3. INTRODUCTION
Certain species of lower Eukaryotes such as algae
and fungi which spend most of their lifecycle in
haploid state, have also been used in mapping
studies.
Fungi can also reproduce sexually by the fusion of
two haploid cells to create a diploid zygote (2n).The
diploid zygote can then proceed through meiosis to
produce four haploid cells or spores known as
Tetrad. In some, meiosis is followed by mitosis to
form 8 spores known as Octad.
3
5. ORGANISM USED FOR THE STUDY OF
THE TETRAD ANALYSIS
Lower eukaryotes such as algae and fungi are
used for the study of tetrad analysis .
Especially in fungi, Sachharomyces cerevisiae,
Coprinus lagopus, Chlamydomonas reinhardtii
(Tetrads) and
Neurospora crassa ,Aspergillus nidulans
(Octads) are used extensively.
5
6. ADVANTAGES
Haploid in nature.
Produce very large numbers of progeny.
The life cycles of these organisms tend to be short.
Potential for analyzing all of the products from each
meiotic division.
6
7. TYPES OF TETRAD
Two types of tetrad :-
Ordered tetrad:- when some species of fungi
produce a very tight ascus that prevents spores
from randomly moving around. This can create a
linear tetrad also called ordered tetrad.
Ex . Neurospora crassa .
Unordered tetrad:- when the ascus provides
enough space for the tetrads or octads to
randomly mix together, this type of tetrad
arrangement is called unordered tetrad.
Ex. Sachharomyces cerevisiae.
7
8. ANALYSIS OF ORDERED TETRAD
The product of meiosis are contained in an ordered array of
spores. Each mature ascus contains eight ascospores in
four pairs,each pair represnting one of the products of
meiosis. The ordered arrangement of meiotic product
makes it possible to map each gene with respect to its
centromere; i.e. to determine the recombination frequency
between a gene and its centromere.
Two cases are possible depending on whether or not there
is a crossover between the locus and its centromere.
1.first division segregation (FDS)
2.secound division segregation(SDS)
8
9. 1.FDS – in the absence of crossing over between a gene and its
centromere, the alleles of the gene must separate in the first meiotic
division , this separation is called FDS.
2. SDS(I):- cross over occur between the gene and its centromere.
9
A
a a
A
A
a
A
a
4
4
a
A
A
a
A
a
A
A
a
a
a
a
A
A
A
A
a
a
2
2
2
2
Meiosis I Meiosis II Mitosis
Meiosis I
Meiosis II Mitosis
10. 2. SDS(II):- cross over occur .
10
A
a
A
a
A
a
a
A
A
a
a
A
A
a
A
4
2
2
Map distance= Recombinant progeny
Unit of map distance is centimorgan (cM)
Total progeny
X 100
Recombinant progeny = ½ No of SDS progeny
Meiosis I Meiosis II
Mitosis
11. 11
Problems:-
We have given the following data, calculate gene distance between
gene and centromere.
a A A a a A
a A A a a A
a A a A A a
a A a A A a
A a a A a A
A a a A a A
A a A a A a
A a A a A a
34 48 5 4 6 3
B b b B B b
B b b B B b
B b B b b B
B b B b b B
b B B b B b
b B B b B b
b B b B b B
b B b B b B
123 108 15 12 18 24
Map distance= Recombinant progeny
Total progeny
X 100
12. ANALYSIS OF UNORDERED TETRAD
Unordered tetrad analysis can be used to map
genes in dihybrid crosses.This analysis can
determine if two genes are linked or assort
independently.
Three pattern of segregation are possible in the
tetrad when two pairs of alleles are segregating.
1.Parental ditype:- 4:4
2.Non- parental ditype:- 4:4
3.Tetratype:- 2:4:2
12
13. A B
a b
A B
A b
Ba
A B
A B 4
4a b
A b
A b
Ba
4
4Ba
13
I Parental ditype (PD)
II Non Parantal Ditype (NPD)
Meiosis I
Meiosis II
Mitosis
Meiosis I Meiosis II
Mitosis
a b
a b
a b
A B
14. 14
A B
a b
A b
a B
a b
A B
A b
a B
a b
2
2A B
A b
aB
a b
III Tetratype (TT)
Meiosis I Meiosis II
Mitosis
A B
4
Map distance= Recombinant progeny
Total progeny
X 100
Recombinant progeny = NPD+1/2 TT
15. 15
Problems:-
We have given the following data, calculate gene distance between two genes.
AB AB ab ab 1750
AB Ab aB ab 220
AB aB Ab ab 20
Ab aB Ab aB 10
Lys- His- Lys+ His- Lys- His- Lys+ His+ Lys- His- Lys- His+ Lys- His-
Lys- His- Lys+ His- Lys- His- Lys+ His+ Lys- His- Lys- His+ Lys- His-
Lys- His- Lys+ His- Lys- His+ Lys+ His- Lys+ His+ Lys+ His- Lys+ His+
Lys- His- Lys+ His- Lys- His+ Lys+ His- Lys+ His+ Lys+ His- Lys+ His+
Lys+ His+ Lys- His+ Lys+ His+ Lys- His+ Lys+ His+ Lys+ His- Lys+ His-
Lys+ His+ Lys- His+ Lys+ His+ Lys- His+ Lys+ His+ Lys+ His- Lys+ His-
Lys+ His+ Lys- His+ Lys+ His- Lys- His- Lys- His- Lys- His+ Lys- His+
Lys+ His+ Lys- His+ Lys+ His- Lys- His- Lys- His- Lys- His+ Lys- His+
136 2 23 93 121 1 24
Auxotroph +
Prototroph –
OR
Wild type +
Mutant –
16. REFERENCE
16
Griffiths, A. J. F., et al. "Introduction to genetic
analysis, p 351–392." (2015)
https://ocw.mit.edu/courses/biology/7-03-genetics-
fall-2004/lecture-notes/lecture8.pdf
https://www.ncbi.nih.gov/books/NBK21972/
