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BIOL 1F90
Experiment #5: Mendelian Genetics: Yeast Dihybrid Cross

Abstract:
The purpose of this experiment, involves monohybrid, F1-generation, and F2generation dihybrid crosses performed on a the yeast medium
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cerevisiae is referred to as R, expressing the colour white whereas the
recessive allele is stated to as r expressing the colour red
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cerevisiae is
capable of synthesizing an amino acid known as tryptophan
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The life cycle of Saccharomyces
cerevisiae alternates between a haploid phase and a diploid phase
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red colour, using a monohybrid cross on YED
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red colour and tryptophan independent production vs
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The
differences in colony colour change48 hours of incubation took place in order to
witness changes in the phenotypes
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This helped understand as to
why the pattern which was correlated with Mendelian’s law of segregation
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Slight inaccuracies were observed after 48 hours of
incubation
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Introduction
Saccharomyces cerevisiae is also known as baker’s yeast
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 A haploid cell is a cell that
is formed during meiosis containing one set of unpaired chromosomes
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The amount of chromosomes S
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Colony

colour changes can be observed after as early as 24 hours of incubation when
yeast is developed on various mediums such as YED agar or MIN agar
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S
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The
dominant T allele is recognized as being tryptophan-independent meaning they
do not require another medium to supply extra source to support the synthesis
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A monohybrid process involves the crossing
of one trait using two alleles
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This law helps to
explain why not all offspring resemble the parental generation
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Mendel’s law of independent
is the behavior of chromosomes during meiosis
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A monohybrid cross using S
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Colony colour alterations will be observed 48
hours following the incubation
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The second dihybrid cross of F2 generation taking place in YED will occur also
observing colony colour changes and tryptophan independence/dependence
seen 48 hours following the incubation
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No changes were made in this experiment
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Figure 1, shows the yeast penetrated for LED culture plate after 48 hours
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Figure 2, shows the yeast penetrated for MIN culture plate after 48 hours
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Table 2: Experiment showing both the phenotypic results for the dihybrid cross
obtained and the predicted phenotypes
Experimental Results
Alpha 1

Alpha 2

Alpha 3

Alpha 4

A1

White (+)

White (+)

White (+)

White (+)

A2

White (+)

Red (+)

Red (+)

Red (+)

A3

White (+)

White (-)

White (-)

White (+)

A4

White (+)

Red (-)

White (-)

Red (+)

Expected Results
Alpha 1

Alpha 2

Alpha 3

Alpha 4

A1

White (+)

White (+)

White (+)

White (+)

A2

White (+)

Red (+)

White (-)

Red (+)

A3

White (+)

White (+)

White (+)

White (-)

A4

White (+)

Red (+)

White (-)

Red (-)

Table 2 shows that the experiment results verses predicted results differ
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“Rr” would have
“R” replace the smaller “r” and become white because white is dominated
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Discussion:
i)

An allele is an alternative form of a gene, which inhabits a certain place
on a chromosome
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A dominant allele is
shown in the phenotype where as the recessive is masked
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In this
experiment, the dominant allele in S
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The dominant allele involved in tryptophan

ii)

iii)

iv)

synthesis is represented by T and is tryptophan-independent
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The predicted phenotypic ratio for the outcome of this dihybrid cross
was 9:3:3:1 but my observations showed that there is 13 white and 3
red
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The Mendelian Law followed by these laws, 1:
principle of segregation, where two members of a gene pair that are
called alleles separate from each other in the formation of gametes
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In other words, the students would get a phenotypic ratio of
2:1:1 like with a monohybrid cross, so we know that the independent
assortment is working because we get a 9:3:3:1 instead of 2:1:1
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These results were expected because the results obey
Mendelian’s law regarding the monohybrid cross
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The Phenotype is a recognizable or physical trait that alleles express
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The genotype can be looked at as a blue print passed from one
generation to the next through mitosis for diploids and meiosis for
haploids
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This essentially means that they are
opposite in that one is a homozygous or true breed, and the other is a
heterozygous cross
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This is what helps prevent them from turning red,
this is due to the feedback inhibition on the adenine biosynthetic
pathway
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 If we were to streak out the yeast on
the YED culture plate, it should start to look pink after a couple of days
growth on the YED culture plate
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There is a tremendous disadvantage to the yeast to
make all red pigment
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This is because in a strong
variety for any mutants that may occur that would prevent the red
pigment from being formed
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Depending on the presence or absence in the
media, it is not a red molecule initially, but is oxidized to become a red

v)

pigment in an aerobic environment
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Regarding the dihybrid cross performed in this experiment the fraction
of the F2-generation has a genotype homozygous which is 3/16
genotypes are homozygous for each characteristic; RRTT, rrTT, and
RRtt
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13/16 of the genotypes represent
heterozygous for both characteristics with a variation in 7 genotypes
and 3 phenotypes
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The resulting phenotypes would be red tryp-independent red
tryp-dependent, white tryp-dependent, and white tryp-independent
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White
tryp-independent was the most prevalent phenotype because the white
colour allele (R) and the tryp-independent allele (T) were both
dominant over the opposing recessive alleles r, and t
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The
inaccuracy of some of the data could be a result of the transferring
over of the previous medium when collecting the yeast
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There was complete dominance of the dominant
R allele for the colour of white, and the T allele for tryptophan the independent
growth as observed on both agar mediums where the yeast was grown
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References:
A Dihybrid Cross
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d
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phys
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edu/gene/bb3
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(2014) BIOL 1F90 Lab Manual 2014-2015 Ed
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Catherines, ON
Methods and materials used in this experiment can be found in the BIOL 1F90
Laboratory Manual (Martin, 2014-2014, Experiment # 2 Protein Quantification,
pages 2 - 5
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(n
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Biochemical Mutants in the Smut Fungus Ustilago Maydis
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ncbi
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nih
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(n
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Retrieved January 2, 2015, from http://
www
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org/AE/AEPC/WWC/1994/yeast

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