Core lab #7
Core lab #7 worksheet
Remember to go go through the Powerpoint and
the lab video - they will help you understand
how to complete this worksheet. The two files
attached to this assignment, Geneticshelp.doc
and Sex-Linked.doc, will also help!
Mendelian Genetics:
Corn is a great model for exploring Mendelian inheritance.
• Corn’s importance as a food crop means its genetics have been well studied.
• Many contrasting phenotypes have been identified, some of which exhibit Mendelian
inheritance patterns.
• With the proper crosses, Mendelian patterns can be observed in a single ear of corn
because each kernel is the product of an independent pairing of gametes.
1-4) Corn Kernel Color Gene P
Several genes contribute to kernel color in corn. One gene for kernel color that exhibits
Mendelian inheritance is the P gene. This gene has two alleles, P and p. P exhibits
complete dominance, and when present, causes purple kernel color.
The following genotypes are possible, note you only need ONE P to be purple:
i) PP:
ii) Pp:
iii) pp:
1
Core lab #7
1) Given the images above, describe the recessive phenotype.
2) When a true breeding purple (PP) line is crossed with a true breeding yellow line (pp), the
resulting genotype(s) is/are
3) When a true breeding purple line is crossed with a true breeding yellow line, the
resulting phenotypes(s) is/are
4) What are the possible genotypes of the kernels of corn shown below?:
5-7) Single Trait Cross Part 1
Look closely at the ear of corn below:
5) What is the phenotype ratio for the two kernel colors?
a) 1:1
b) 3:1
c) 10:1
choose the closest
6) Which paring is most likely to have produced this ratio?
a) PP x PP
b) Pp x PP
c) PP x pp
d) Pp x Pp
e) pp x pp
2
Core lab #7
(hint: draw the Punnett squares needed to figure out the offspring ratios produced
by each cross and compare them to your answer in question 5)
7) True or false: the parents both have the same phenotype.
8) Hemophilia is a type of bleeding disorder that causes the blood to take a long time to
[clot]. This can cause abnormal bleeding, or bleeding that won't stop. People with
hemophilia have too little—or even none—of a protein in the blood called [clotting
factor]. Hemophilia most often affects males and, in most cases, is inherited (passed
down through families).
Hemophilia is carried only on the X chromosome. It is
called an X linked genetic disorder. A women who is a
carrier for hemophilia has the genetic mutation on one of
her X chromosomes (thus she is HXX). She thus has one
non mutated X chromosome that will usually somewhat
compensate for the defect in the other. It is not uncommon
for women who carry the hemophilia gene to have low
levels of clotting factor and have bleeding problems.
A man (HXY) who has hemophilia has the genetic mutation on his only X
chromosome. He does not have another X chromosome to compensate for the
defect so he will have hemophilia.. What are the possible outcomes from the
following cross shown in the picture below?
9) If you had a female that was color blind, and she mated with a male with normal vision ...
what types of children could they possibly have? Remember, this is a sex-linked problem!
The possible genes on the X chromosome are: XC = normal, and Xc = color blind (Note:
That's a lowercse "c" on the color-blind.) Show your Punnet Square.
3
Speaking of color-blindness, check out the test below (just for fun) ...............
Most of the circles to
the right are nothing
but spots to
someone with color
blindness.. Below
are the correct
answers to what a
person with normal
color vision would
see
The full Ishihara test
consists of a set of
38 plates and tests
in-depth for color
blindness. The plates
here are a small
representative
sample of the whole,
but will help spot the
most common forms
of colorblindness.
Depending on the
color balancing of
your monitor, the
tests may be
somewhat more or
less effective (e.g. if
your monitor is not
rendering the proper
colors).
Results For Ishihara Test(above)
Normal Color Vision
Left
Right
Top
25
29
Middle
45
Bottom
6
Red-Green Color Blind
Left
Right
Top
25
Spots
56
Middle
Spots
56
8
Bottom
Spots
Spots
10) Earlobes. These can be either attached, where the earlobe blends smoothly into the
neck, or unattached, where the earlobe hangs free (see Figure). Unattached
earlobes (U) are dominant to attached (u) so both males and females can have a
genotype of UU, Uu or uu. Widow's peak (W) is dominant to continous hairline (w)
so both males and females can have a genotype of WW, Ww or ww).
If you crossed a male who was homozygous dominant for widow’s peak and had
attached ear lobes with a female who had a continuous hairline and was heterozygous
for unattached ear lobes ….what types of children could they possibly have? Show
your Punnett square. Remember to create 4 types of sperm from the male, with all
combinations of one gene for widow's peak with one gene for hairline...and do the
same for 4 types of eggs.
11) LAST PROBLEM! You have two mice. You know that B=black and b=brown. The gene
for color is on one chromosome, but there is another gene on a different chromosome
(independent assortment) that codes for whether the pigment is ever expressed or not;
C=color pigment is expressed, c=color pigment is NOT expressed. If you mated a male
mouse that was heterozygous for both color and pigment expression with a white female
mouse that had color geneotype (bb), what are the possible phenotypes of the
offspring? Note that the female has to be ccbb because she is white! Show your Punnet
Square for this problem! (Note: use Slide 38 from the Powerpoint as a guide.)
Color expressed-brown
No color expressed
Color expressed-black
Well, you should have the idea that each individual has TWO alleles (or factors) that
influence each trait. We can give an example with: P= allele or factor for purple color,
and p=allele or factor for white color. Since each individual has two factors, the
combinations could be: PP = purple, Pp= purple (note that P is DOMINANT so that
when present even as only one of the two possible factors the flower is still purple, and
pp=while (hey! There is NO P factor, so with both factors for while, the flower is white.
So, given this how do we determine what happens if we cross two plants?
Suppose we have a purple plant (either PP or Pp) and we want to establish which it is!
We can do a text cross, which means we cross it with a white flower (pp). Let’s see the
Punett Square that shows how the “factors” are passed on by the parents, and how the
recombine in the possible offspring.
We know that one of the parents will be pp (white flower), and we want to know if the
other parent is PP or Pp (this information is give in the Powerpoint under Test Cross).
For now let’s see what would happen if the other flower was PP.
Flower #1
Flower #2
pp
PP
P
P
--------------! PP
PP
!
! Pp
Pp
p
p
Hey! ALL the flowers are purple! Now, let’s see what would have happened if the
parent purple flower was Pp rather than PP and we crossed it with a white flower (pp).
pp
Pp
p
P
P
p
-------------!
! Pp
pp
!
! Pp
pp
Wait a second! Half of the offspring are White (pp) compared to the previous cross
where they were ALL purple. So, you can test cross and find out the genotype
(specifically what the combination of factors were) of the purple flower!
The next problem involved what happens when TWO sets of genes, a factor for type of
color AND another factor for whether the first gene is even expressed or not, are
involved. The two genes are on Different Chromosomes, and that means that they are
passed on INDEPENDENTLY of each other. So, we can have a factor for color:
B=black and b=brown. But, we also have a factor that determine whether the color genes
work or not: C=color is expressed and c=color is NOT expressed (hey! No color
expressed and you are a white individual, not black or brown).
Examples of possible mice would be:
BBCC – a black mouse
BbCC – a black mouse
BbCc – a black mouse
bbCc – a brown mouse
BBcc – a WHITE mouse
Etc. (what are all the combinations of two factors for color and two factors for
expression?)
Suppose we have two mice. One is BbCc and the other is also BbCc. Well, we have two
black mice. What kinds of combinations for factors can they pass on? They can pass
ONE factor for color and ONE factor for expression on. So, what are the possible
combinations that can be passed on for each mouse?
BC is one, Bc is another, bC is the third, and bc is the fourth. They thus pass one of the
factors for color and one of the factors for expression on to the next generation. To make
the Punett Square you thus have:
BC
Bc
bC
bc
BC
Bc
bC
bc
-----------------------------------!
!
!
!
!
!
!
!
You need to fill in the Punett Square to make up to possible offspring, each with TWO
factors for color and TWO factors for expression. For example, BC and BC would
combine to make a mouse that was BBCC (black), Bc and Bc would combine to make a
mouse that was BBcc (opps! This mouse has the black color factor, but does NOT have
the factor for color to be expressed so it is WHITE). What kind of possible mice do we
get?
Sex-Linked Genes:
There is yet another, unrelated, special case that means something totally different, yet has a
similar-sounding name (just to confuse freshman biology students?). This is sex-linked genes,
genes located on one of the sex chromosomes (X or Y) but not the other. Since, typically the X
chromosome is longer, it bears a lot of genes not found on the Y chromosome, thus most sexlinked genes are X-linked genes. One example of a sex-linked gene is fruit fly eye color. An X
chromosome carrying a normal, dominant, red-eyed allele would be symbolized by a plain X,
while the recessive, mutant, white-eyed allele would be symbolized by X' or Xw. A fly with
genotype XX' would normally be a female with red eyes, yet be a carrier for the white-eyed
allele. Because a male typically only has one X chromosome, he would normally be either XY
and have normal, red eyes, or X'Y and have white eyes. The only way a female with two X
chromosomes could have white eyes is if she would get an X' allele from both parents making
her X'X' genotype. The cross between a female carrier and a red-eyed male would look like
this:
X
Y
X
XX XY
X'
XX' X'Y
Notice that while there is a “typical” ratio of ¾ red-eyed to ¼ white-eyed, all of the white-eyed
flies are males.
Typically, X-linked traits show up more in males than females because typical XY males only
have one X chromosome, so if they get the allele on their X chromosome, they show the trait. If
a typical XX female is a carrier, 50% of her sons will get that X chromosome and show the
trait. In order for an XX female to exhibit one of these X-linked traits, most of which are
recessive mutations, she would have to have two copies of the allele (X'X'), which would mean
that her mother would have to be a carrier and her father have the trait so she could get one
allele from each of them.
In humans, two well-known X-linked traits are hemophilia and red-green colorblindness.
Hemophilia is the failure (lack of genetic code) to produce certain substance needed for proper
blood-clotting, so a hemophiliac’s blood doesn’t clot, and (s)he could bleed to death from an
injury that a normal person might not even notice. One of the most famous genetic cases
involving hemophilia goes back to Queen Victoria. While both of her parents were perfectly
normal, it is usually assumed that a chance mutation in either the egg or sperm that came
together to make her, caused her to be a carrier for the hemophilia allele (XX') [see the yellow
box, below, for an alternative hypothesis that some people have suggested]. When she grew up,
she married Prince Albert, who was normal XY, so the Punnett square for their marriage would
look like the one just drawn. The Punnett square would predict that ½ of their sons (¼ of their
children) would be hemophiliacs and ½ of their daughters (¼ of their children) would be
carriers. Their children married other royalty, and spread the gene throughout the royal families
of Europe.
Genetics
The work of
Gregor Mendel
Who was this Mendel and what the heck is
he doing in a monastery?
• born in 1822
• trained himself to be a
naturalist early in life
• worked as a substitute
science teacher
• failed the qualifying exams to be
a regular high school teacher!
• joined a monastery in Brunn,
Austria
• sent to Vienna U. to study
science and math
MendelWeb
Mendel’s first published work:
"Versuche über Pflanzen-Hybriden“
or Experiments in Plant Hybridization
was a landmark in clarity and insight!
He figured out the laws of
inheritance…mathematically!!
The work of Gregor Mendel
• worked with pea plants…
…he called
them his
children!
Why pea plants???
There was a long-standing tradition of breeding pea
plants at the monastery where Mendel lived and worked
So…they were readily available and they come in lots
of varieties!
…there were plants with different flower colors, seed
color, flower position etc..
And best of all…
Pea plants can have sex
with themselves
This allowed Mendel to see if
strains were true breeding and to
produce hybrids
How Mendel made hybrids…
He’d then tie little
bags around the
flowers to prevent
contact with stray
pollen.
Mendel’s hybridization experiments…
Monohybrid crosses:
Parental
Generation
F1 generation
True-breeding
purple flower
x
True-breeding
white flower
All purple flowers (the hybrids)
Allowed F1 offspring
to self-fertilize
F2 generation
705 purple
224 white
The results of Mendel’s monohybrid
crosses led him to propose…
1. All organisms contain two “units of
heredity” for each trait (alleles).
2. Dominant and recessive alleles…
…and organisms can have any combination of
the two alleles (2 dominants, 2 recessives
or a mixture 1 dominant and 1 recessive).
3. The Law of Segregation – during
gamete (sperm and egg) formation,
alleles separate randomly into separate
gametes.
A bit of genetic jargon…
phenotype vs. genotype
What the
organism
looks like
What alleles
the organism
has - its
genetic makeup
More jargon…
homozygous vs. heterozygous
P
2 of the
same alleles:
PP or pp
2 different
alleles:
Pp
p
A Punnett
square
A Punnett square…
Gametes from
one parent
p
P
Gametes from
other parent
P
PP
purple
p
Pp
purple
Pp
purple
pp
white
Ratio: 3:1 or ¾ purple, ¼ white
Let’s relate Mendel’s findings to what we now
know about gamete formation
True-breeding
purple flower
Note two chromosomes
in the parent, each with
a gene for color. In
this case, both are P
P
P
all purple
P
P
P
P
True-breeding
white flower
x
P
p
p
p
p
p
p
In the above case, the parent was diploit (had a pair of chromosomes). The
blue chromosome came from dad, the orange from mom. Each chromosome
had a gene for color (P for both). Each chromosome replicates, and either a blue OR an orange gets passed on….NOT both
p
F1 generation
purple hybrid
This adult is diploid (has a pair of
chromosomes)…but note that the
gene for color is NOT the same on
each chromosome! One or the other
is potentially passed on to the next
generation
P
F2
p
P
purple hybrid
p
P P
P
x
p
P
p
P
p
p
p
P
PP - purple Pp - purple Pp - purple
pp - white
p
Based on the previous examples,
try to determine the following…
Determine the phenotypic and genotypic ratios
for each of the following monohybrid crosses.
Aa x Aa
Phenotypic
ratio
3:1 (dom:rec)
Genotypic
ratio
1:2:1 (hd:H:hr)
AA x Aa
1:0 (dom:rec)
1:1:0 (hd:H:hr)
or all dominant
aa x Aa
1:1 (dom:rec)
0:1:1 (hd:H:hr)
AA x aa
1:0 (dom:rec)
0:1:0 (hd:H:hr)
or all dominant
hd = homozygous dominant; H = heterozygous; hr = homozygous recessive
Definitions 1
• Must know these!!!
• Trait—A variable characteristic of organism
• Gene—A segment of chromosomal DNA
controlling a specific trait
• Locus—Chromosomal position where DNA for
a specific gene lives
Definitions 2
• Must know these!!!
• Alleles—Different forms of a gene
– “Flower color” is a gene;
– “Purple” is one flower-color allele
– “White” is another flower-color allele
• Genotype—List of alleles an individual
has at specific genes
– Familiar organisms are diploid (two alleles –
one from father, one from mother)
– Allele pairs may be same (homozygous) or
different (heterozygous)
Definitions 3
• Homozygous—Maternal & paternal alleles same
– Father donates purple-flower allele
– Mother donates purple-flower allele
• Heterozygous—Maternal & paternal alleles
differ
– Father donates purple-flower allele
– Mom donates white-flower allele
Definitions 4
• Phenotype:
– List of traits exhibited by individual
– Doesn’t always represent genotype
• Dominant—Allele that is expressed
100% in heterozygote
• Recessive—Allele that is not
expressed in heterozygote
• Incomplete dominance—heterozygote
displays intermediate trait
We noted in the video that genes are really
segments of a long strand of DNA, with each gene
containing enough DNA to code for a trait
We noted that we have pairs of chromosomes (one from
mom and one from dad…hence we are diploid) and that
each chromosome of the pair has a gene for a particular
trait…hence we have pairs of genes for each
trait….Mendel demonstrate that, and noted how they
interact (dominant, recessive, homozygous,
heterozygous)
A karyotype showing the 23 pairs of human
chromosomes…note on the final pair (23) that if
you get a Y chromosome from dad you are a
male (XY), while if you get an X from dad you
are a female (XX)
Mendel’s Model of Heredity
• Parents transmit discrete physiological
trait information (factors) to offspring.
• Each individual receives two factors that
may code for same, or alternative,
character traits.
• Not all copies of a factor are identical.
– alleles
• homozygous - same alleles
• heterozygous - different alleles
Mendel’s Model of Heredity
• Alleles do not influence each other in any
way.
• Presence of a particular allele does not
ensure its encoded trait will be expressed.
– genotype - totality of an individual’s alleles
– phenotype - physical appearance
Interpretation of Mendel’s
Results
• Notational convention
– P - dominant allele (purple)
– p - recessive allele (white)
• PP - homozygous dominant
• Pp - heterozygous
• pp - homozygous recessive
Interpretation of Mendel’s
Results
• F1 generation
– PP x pp (parental generation) yielded all Pp
offspring
• F2 generation
– Pp x Pp yielded: (1:3:1) ratio
• 1 PP
• 3 Pp
• 1 pp
• Punnett squares (aka “Mendel’s Cross”)
Mendel’s Cross
Mendelian Inheritance
• Mendel’s First Law of Heredity
– (Law of Segregation)
• Alternative alleles of a character segregate
from each other in heterozygous individuals
and remain distinct.
An example…
Testcross: We have a purple flower. Is its gneotype
PP or Pp? We can cross it with a white flower (pp)
and note if we get any white offspring
Note that we used a Punnett Square to show
how gametes are formed (each parent can
donate one of their genes) and how gametes
are reformed to the possible combinations in
offspring
Mendelian Inheritance
• Mendel’s Second Law of Heredity
– (Law of Independent Assortment)
• Genes that are located on different
chromosomes assort independently of one
another
• Remember, each pair of chromosomes
separates independently of each other
pair…thus the genes on those separate
chromosomes are passed on independently
of each other.
Here the Y and y alleles for the gene
that determines color are on DIFFERENT
chromosomes that the R and r alleles
for the gene which determines shape. So,
you can get ALL combinations of ONE of
the color alleles and ONE of the shape
Alleles in the gametes! Note that for
the F1 generation which is YyRr that there
are FOUR different possible types of
gametes produced when ONE gene for
color (Y or y) is paired up with ONE gene
for shape (R or r).
Sex-Linked Genes:
•
There is yet another, unrelated, special case that means something totally
different, yet has a similar-sounding name (just to confuse freshman
biology students?). This is sex-linked genes, genes located on one of the
sex chromosomes (X or Y) but not the other. Since, typically the X
chromosome is longer, it bears a lot of genes not found on the Y
chromosome, thus most sex-linked genes are X-linked genes. One example
of a sex-linked gene is fruit fly eye color. An X chromosome carrying a
normal, dominant, red-eyed allele would be symbolized by a plain X, while the
recessive, mutant, white-eyed allele would be symbolized by X' or Xw. A fly
with genotype XX' would normally be a female with red eyes, yet be a
carrier for the white-eyed allele. Because a male typically only has one X
chromosome, he would normally be either XY and have normal, red eyes, or
X'Y and have white eyes. The only way a female with two X chromosomes
could have white eyes is if she would get an X' allele from both parents
making her X'X' genotype. The cross between a female carrier and a redeyed male would look like this:
Note that X=normal red
eyes, while X’=white eyes! And,
the Y chromosome carries NO
information for eye color
X
X XX
Y
XY
X’ X’X X’Y
Note that if the male gets
An X’ from mom, then he has
White eyes!!!
The consequences!
• Typically, X-linked traits show up more in males than females
because typical XY males only have one X chromosome, so if
they get the allele on their X chromosome, they show the
trait. If a typical XX female is a carrier, 50% of her sons
will get that X chromosome and show the trait. In order for
an XX female to exhibit one of these X-linked traits, most
of which are recessive mutations, she would have to have
two copies of the allele (X'X'), which would mean that her
mother would have to be a carrier and her father have the
trait so she could get one allele from each of them.
More consequences!
• In humans, two well-known X-linked traits are hemophilia and
red-green colorblindness. Hemophilia is the failure (lack of
genetic code) to produce certain substance needed for
proper blood-clotting, so a hemophiliac’s blood doesn’t clot,
and (s)he could bleed to death from an injury that a normal
person might not even notice. One of the most famous
genetic cases involving hemophilia goes back to Queen
Victoria. While both of her parents were perfectly normal, it
is usually assumed that a chance mutation in either the egg
or sperm that came together to make her, caused her to be
a carrier for the hemophilia allele (XX'). When she grew up,
she married Prince Albert, who was normal XY, so the
Punnett square for their marriage would look like the one
just drawn two slides ago. The Punnett square would predict
that ½ of their sons (¼ of their children) would be
hemophiliacs and ½ of their daughters (¼ of their children)
would be carriers. Their children married other royalty, and
spread the gene throughout the royal families of Europe.
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