Yeast gene names are three letters followed
by a number and are written in italics.
i.e. URA3
(the third gene identified that is required for uracil biosynthesis)
CDC44 (the 44th
cell division cycle mutant that was identified)
The one exception to this nomenclature
is the yeast mating type locus which is designated;
Mat a
: haploid a type cells
Mat a
: haploid a
type cells
Mat a/ a
: diploid cells
Normal, unmutated, functional alleles of genes are designated as wild type, and can have the + superscript. i.e. CDC44 +
If a gene is not listed in the genotype if a strain, it is assumed to be wild type.
Mutant genes are followed by an allele
number that specifies the mutation.
i.e. cdc44-1 (first
mutant allele, happens to be a point mutation at amino acid 513)
lys2-*301 (encodes for a Lys2 protein containing a 301 amino acid
deletion)
Capital letters signify dominant genes,
lower case letters for recessive genes
i.e. LEU2
dominant wild type allele
SUP4-1 dominant mutant
allele
cdc44-1 recessive mutant
allele
If a chromosomal copy of a gene has been
deleted and replaced with another gene, it is indicated with two colons
(::) as follows;
CDC44::LEU2
(CDC44 gene is absent and LEU2 gene is located where CDC44 was)
However, if a gene is placed next to another
gene on a chromosome without destroying its function, it is indicated as
follows;
cdc44-1.URA3 (URA3 gene has
been integrated next to cdc44-1 gene without destroying cdc44-1
or URA3 function)
Plasmids genotypes are written in parantheses (URA3 CDC44)
For diploid cells, both copies of the genes are indicated as follows;
Mat a/ a
CDC44 ura3-52 leu2-1
cdc44-1 ura3-52 LEU2
Proteins are written in normal type, with
the first letter capitolized and have the letter p following gene name,
i.e. Cdc44p
= protein encoded by wild type CDC44 gene
Cdc44-1p = protein encoded
by mutant cdc44-1 gene
Phenotypes are written in normal type,
and can have r/s +/- designations
i.e. Ura- = strain cannot
grow in absence of uracil (Ura auxotroph)
ts- =
strain cannot grow at elevated temperatures (35°C)
Mmss = strain is sensitive to MMS
Plates
YPD = rich media made from yeast extract
(supports growth of all strains)
SC = synthetic complete media (synthetic
media with all nutrients and amino acids etc)
SD = synthetic deficient media
(has only salts and sugars)
SC - His = synthetic media with all the
nutrients and amino acids to grow except histidine
For our purposes, SC = SD + His, Lys, Leu, Ura, Ade, Tyr, Trp
i.e. therefore SC - Ura = SD + His, Lys, Leu, Ade, Tyr, Trp
I. Segregation of 2 unlinked genes
Genotype Phenotype Class
Mat a cdc44-1
LEU2 cs-, Leu+ Parental
crossed with
Mat a CDC44
leu2-1 cs+, Leu- Parental
gives diploid Mat a/ a
cdc44-1 LEU2 cs+,
Leu+
CDC44 leu2-1
upon sporulation can give haploids like
(remember meiosis is one round of DNA replication followed by two rounds
of chromosome segregation). Each spore gets one copy of each gene.
Tetrads Genotype Phenotype
Class Type
1 cdc44-1 LEU2 cs-, Leu+
Parental Ditype
cdc44-1 LEU2 cs-, Leu+
“
CDC44 leu2-1 cs+, Leu-
Parental
CDC44 leu2-1 cs+, Leu-
“
2 cdc44-1 leu2-1 cs-,
Leu- Non-Parental Ditype
cdc44-1 leu2-1 cs-,
Leu- “
CDC44 LEU2 cs+, Leu+
Non-Parental
CDC44 LEU2 cs+, Leu+
“
3 cdc44-1 LEU2 cs-, Leu+
Parental Tetratype
cdc44-1 leu2-1 cs-,
Leu- Non-Parental
CDC44 LEU2 cs+,
Leu+ Non-Parental
CDC44 leu2-1 cs+, Leu-
Parental
4 cdc44-1 leu2-1 cs-,
Leu- Non-Parental Tetratype
cdc44-1 LEU2 cs-, Leu+
Parental
CDC44 leu2-1 cs+, Leu-
Parental
CDC44 LEU2 cs+, Leu+
Non-Parental
5 cdc44-1 LEU2 cs-, Leu+
Parental Tetratype
cdc44-1 leu2-1 cs-,
Leu- Non-Parental
CDC44 leu2-1 cs+, Leu-
Parental
CDC44 LEU2 cs+, Leu+
Non-Parental
6 cdc44-1 leu2-1 cs-,
Leu- Non-Parental Tetratype
cdc44-1 LEU2 cs-, Leu+
Parental
CDC44 LEU2 cs+, Leu+
Non-parental
CDC44 leu2-1 cs+, Leu-
Parental
In all, ratio is: 1 Parental
Ditype: 1 Non-parental Ditype: 4 Tetratype
If genes are unlinked, then each
one of these cases is equally likely (alleles segregate randomly).
Overall, each class of phenotype (i.e. cs-, Leu+) accounts for 6/24
or 25% of progeny as would be expected (0.5 x 0.5).
II. Segregation of two linked genes
Tetrads Genotype Phenotype Class Type
1 cdc44-1 LEU2 cs-, Leu+
Parental Ditype
cdc44-1 LEU2 cs-, Leu+
“
CDC44 leu2-1 cs+, Leu-
Parental
CDC44 leu2-1 cs+, Leu-
“
If the CDC44 and LEU2 genes are tightly linked to one another (physically very close to each other on the chromosome) then the resulting phenotypes of the spores would be the same as in the parental strains. Only a recombination event between the CDC44 and LEU2 genes could give rise to a different phenotype of spore (i.e. cs+, Leu+). Therefore, the further away the genes are, the more likely a recombination event, the more tetratype products of meiosis.
In all ratio is:
many Parental Ditypes: very few Non-parental ditypes: few tetratypes