1. Thomas Cline
practicing developmental geneticist
(Drosophila melanogaster)
former Berkeley undergrad (‘64-68)
Office hours:
Monday 5-6pm (door 5:15p)
Friday 4-5 pm
11 Koshland Hall

2. Topics for today:
recombination (genetic maps)
complementation
(bacterio)phage as an experimental system
famous geneticists
Reading for today and Monday:
Chapter 7 (and skim Chapter 8)
Problems: Chp 7, 16-24

3. genetics: epigenetics: From your textbook’s glossary:
the science of heredity
epigenetics:
study of states of gene functionality
that are not encoded within the DNA sequence
but that are still heritable from one generation to the next.

4. X genetics: genes epigenetics: study of states of gene functionality
From your textbook’s glossary:
genetics: X
the science of heredity
genes
epigenetics: study of states of gene functionality
that are not encoded within the DNA sequence
but that are still heritable from one generation to the next.

5. From your textbook’s glossary:
genetics: the science of genes
gene:
basic unit of biological information;
specific segment of DNA in a discrete region of a chromosome
that serves as a unit of function
by encoding a particular RNA or protein.
(only in cis)
encodes what
what about aspects relevant to
when?
where?
how much?

6. What do we do with the Drosophila Dscam gene?
basic unit of biological information;
specific segment of DNA in a discrete region of a chromosome
that serves as a unit of function
by encoding a particular RNA or protein.
(only in cis)
What do we do with the Drosophila Dscam gene?
(Down syndrome cell adhesion molecule)
…encodes 38,000 different proteins (via alternative pre-mRNA splicing)
(Drosophila has “only” 14,000 genes)

7. a difference that makes a difference w/ respect to biological function
gene:
basic unit of biological information;
specific segment of DNA in a discrete region of a chromosome
that serves as a unit of function
by encoding a particular RNA or protein.
(only in cis)
information:
a difference that makes a difference
w/ respect to biological function
“nonsense” DNA sequence
(encodes nothing…
…such as DNA between genes?)
gene DNA sequence
(encodes particular RNA or protein)
VS.

8. “nonsense” DNA sequence gene DNA sequence VS.
(encodes nothing…
…such as DNA between genes?)
gene DNA sequence
(encodes particular RNA or protein)
VS.
This is NOT the difference that really concerns geneticists.
(although we will touch on it today)
genetic map

9. alleles: The difference that really concerns geneticists.
alternative forms of a single gene
DIFFERENT forms of a single gene
If you understand the concept of allelism,
you will understand what genetics is all about
and what geneticists mean by genes

10. (with respect to seed color)
Mendel defined alleles (but it is not a particularly useful definition):
given: Heritable character difference (e.g. seed color)
IF:
pure-breeding yellow
pure-breeding green X
hybrid (yellow)
hybrid (yellow) X
(1:2:1)
(genotype)
pure-breeding
pure-breeding yellow
pure-breeding green
hybrid yellow
yellow
(phenotype)
AND NOTHING ELSE
(with respect to seed color)
This particular seed color difference
is due to a difference
in (the alleles of) a single gene
Then:
Y & y are alleles; hybrid yellow is Y/y

11. (with respect to seed color)
pure-breeding yellow
pure-breeding green X
hybrid (yellow)
hybrid (yellow) X
(1:2:1)
pure-breeding yellow
pure-breeding green
hybrid yellow
AND NOTHING ELSE
(with respect to seed color)

12. …and by extension the allele responsible
for the difference between
the recessive and dominant characters

13. (with respect to seed color)
hybrid (yellow) X
pure-breeding yellow
pure-breeding green
hybrid yellow
AND NOTHING ELSE
(with respect to seed color)
(1:2:1)
Y vs. y are alleles as defined by a segregation test
(1) unchanged in the hybrid
(2) segregate during meiosis
The gene defined as the unit of segregation during meiosis
(…not with respect to function)

14. Problem:
What if there are really TWO genes involved
… that are very closely linked?
AB/AB X ab/ab
AB/ab
How hard do we have
to look before deciding
we must be dealing with
a single gene difference?
self
almost all: AB/AB AB/ab ab/ab
but occasionally rare recombinants (“non-parentals”)
such as: Ab/ab

15. Problem with the segregational definition of allelism
(= definition of the gene as the unit of segregation)
arose in the case of multiple alleles
difference between pb. whiteeosin vs. white+ = one gene diff. = alleles
difference between pb. white-1 vs. white+ = one gene diff. = alleles
difference between pb. white-1 vs. whiteeosin = one gene diff. = alleles
(all based on Mendel’s test for segregation)
Hence, two different alternative mutant forms
(we = orange, & w-1 = white)
of one wildtype gene: white+ (= red)

16. What’s the problem with multiple alleles?
w-1/weosin X w-1/Y
very rarely: w+/Y (hardly “unchanged in the hybrid”)
Isn’t this just a revertant?
y- w-1 sn-/
y+ we sn+
y+ w+ sn-
only from heterozygous moms
2) always associated
with recombination of
flanking genetic markers
and a specific nonparental
arrangement of those markers!

17. w+ w-1 we Must not be “true” alleles
(true alternative forms of the same gene)
but rather
“pseudoalleles”
genetic map: yellow w-1 we singed
white-A white-B
genes as “beads on a string”

18. Must not be “true” alleles
w w we
Must not be “true” alleles
but rather
“pseudoalleles”
Didn’t pass the smell test
two genes?
genetic map: yellow w-1 we singed
white-A white-B
genes as “beads on a string”

19. The first example of “pseudoallelism” was lozenge:
Gene A
Gene B wt
lz(g)
lz(BS) wt
Aa Bb
And many more cases followed in flies,
and even more in micro-organisms
where one had much greater mapping resolution power

20. The cis-trans (complementation) test, 1949: lozenge (M. Greene)
Two different recessive mutants, both with the same phenotype (small eyes and fused facets).
Are they mutations in the same gene?
Make two different fly lines and compare their phenotypes.
Cis: Trans: wt wt wt
lz(g)
lz(BS)
lz(g)
lz(BS) wt

21. If flies are normal, then mutations are in different genes.
Cis: Trans: wt wt wt
lz(g)
lz(BS)
lz(g)
lz(BS) wt
This is a control experiment.
The flies will be wild-type
regardless of whether
BS and g are in the
same gene or not.
If flies are normal, then
mutations are in different genes.
If the phenotype is still mutant,
then BS and g must be in the
same gene!!!
A test for allelism (a definition of the gene)
based on the PHENOTYPE OF THE HYBRID
(function, not segregation & NOT MENDEL!)

22. genetic map based on segregation frequencies
meiotic recombination can separate
the parts of a “gene” like white or lozenge
How does a genetic map of
regions within a gene (as defined by the complementation test)
compare with that for
regions between genes (as defined by the complementation test)?
do true “segregational alleles” exist,
and if so,
how do they compare with “functional alleles”?

23. Who said it?

24. genetic map based on segregation frequencies
meiotic recombination can separate
the parts of a “gene” like white or lozenge
How does a genetic map of
regions within a gene (as defined by the complementation test)
compare with that for
regions between genes (as defined by the complementation test)?
do true “segregational alleles” exist,
and if so,
how do they compare with “functional alleles”?

25. Fig. 7.20
p228

26. American Naturalist, v56 p32 (1922)…
=bacteriophage