Download Part I: Predicting Genetic Outcomes

Genetic Outcomes
Mechanisms of Genetics
Part I: Predicting Genetic Outcomes
Deoxyribonucleic acid (DNA) is found in every cell of living organisms,
and all of the cells in each organism contain the exact same copy of
that organism's DNA. Because the genetic information is in the form
of a code, the information carried in DNA can be copied and transferred
to offspring. Not only does each species have its own unique code,
but so does each individual within that species. Those unique codes
are what determine the varying traits that are observed from one
individual organism to the next. In other words, the DNA is the
organism's “blueprint”, and each organism's blueprint is slightly different
from the next.
Gregor Mendel, considered the father of genetics, studied pea plants
by crossing them and noting the resulting offspring. He was able to
predict the outcomes of plants with dominant traits vs. recessive traits.
Mendelian crosses are used to predict the possible genetic and
phenotypic outcomes of offspring. That is what you will be doing in this
activity, but you do not have to grow the plants!!
The DNA code, or the information contained in the gene, can be used
to create proteins that specify a particular trait, such as eye color. The
information carried on the DNA is the genotype. The way that information
is expressed, such as blue or brown eyes, is the phenotype.
Remember that an organism's genes are carried on the chromosomes in the nucleus of the cell.
During meiosis, gametes, or reproductive cells, are formed. When an egg and sperm from two
individuals unite, the offspring inherits one set of chromosomes from the mother's egg and one set
of chromosomes from the father's sperm. Therefore, this creates a unique set of chromosomes for
each offspring.
Each chromosome will carry the gene for a specific trait, but in a slightly different version. A
different version of the same gene is called an allele. The combination of alleles, one from the
father and one from the mother, results in a unique genetic combination. It is this unique
combination of alleles in each individual that creates species diversity.
Please continue to the next page.
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Genetic Outcomes
Mechanisms of Genetics
Part I: Predicting Genetic Outcomes, continued
Below you will see two chromosomes from the offspring of true-breeding parents. We call the
parents the P generation and their offspring the F1 generation. True-breeding means that each
parent has two matching alleles for a trait, in this case eye color. If the allele for brown eye color is
represented by B, the true-breeding genotype must be BB (one from each chromosome), and the
phenotype will always be brown eyes. On the other hand, if the allele for blue eye color is
represented by b, the true-breeding genotype has to be bb, and the phenotype will always be blue
eyes. Let's walk through this step by step.
In our diagram below, the F1 offspring has inherited one chromosome from the father (shown in
blue) and one from the mother (shown in pink) . On these two chromosomes, you can see the
specific locus for the gene for eye color. It is this specific location on the chromosome that specifies
the gene for eye color. You can see, however, that each allele of the gene expresses a different
trait, or phenotype. The offspring has inherited the blue allele b from the father and the brown allele
B from the mother. Genetics tells us this particular offspring will have brown eyes. How can you
predict that outcome?
Father
Mother
Go to Part I of your Student Journal.
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Genetic Outcomes
Mechanisms of Genetics
Part I: Predicting Genetic Outcomes, continued
Eye Color Trait Example
Each genotype for a trait may have dominant and/or recessive alleles. The dominant allele is
represented by an upper-case letter (B), while the recessive allele is represented by a lower-case
letter (b). In our example, both parents are homozygous for eye color, which means the individual
carries two of the same allele, one allele for each parent's sex chromosome. Therefore, the
genotype of one parent is BB (homozygous for brown eyes), and the genotype of the other parent
is bb (homozygous for blue eyes). Homozygous individuals always express the phenotype that
matches their alleles. If the parents had a genotype that was heterozygous, that would mean that
they would carry one of each allele for that trait, or Bb. Heterozygous individuals express the
phenotype of the dominant allele, in this case brown eyes.
Our cross between a parent with homozygous dominant eye color BB and a parent with
homozygous recessive eye color bb is shown in the Punnett square below. You can see that all of
the possible combinations result in a dominant B and a recessive b, which means they are all
heterozygous. The dominant trait will be expressed in this phenotype, so all of the offspring will
have brown eyes.
This box represents the
cross of the mother (BB)
with the father (bb). Both
parents are homozygous.
Each B represents one
allele found on one
maternal chromosome.
Each b represents one allele
found on one paternal
chromosome.
BB x
bb
b
b
B
Bb
Bb
B
Bb
Bb
The resulting possibilities represent the F1 generation as they are
offspring of the parents. Each Bb represents the genotype of each of
the four possible offspring produced. In this case, all offspring are
heterozygous.
Answer the Eye Color Example questions in Part I in your Student Journal.
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Genetic Outcomes
Mechanisms of Genetics
Part I: Predicting Genetic Outcomes, continued
The results of Punnett square analyses are expressed as ratios for both genotypes and
phenotypes. In our first example on the previous page, all 4 of the offspring genotypes are Bb, with
no other combinations, so the genotypic ratio would be 4:0. The only phenotype expressed in the
offspring will be brown eyes, with no other choices, so the phenotypic ratio would be 1:0 (or 100%
brown).
If the results of another cross were BB = 1, Bb = 2, and bb = 1, the genotypic ratio would be 1:2:1.
The expression of the phenotype would be 3 offspring with brown eyes and 1 with blue eyes, so
the phenotypic ratio would be 3:1.
Hair Color Trait Examples
Scenario 1
A man has a genotype of Pp. P will be dominant for
purple and p will be recessive for white. This means
his hair color is purple. A woman also has a genotype
of Pp, and her hair color is also purple.
The pair decides to have offspring. The genotype cross
is Pp x Pp. This means it is a cross between two
parents who are heterozygous for hair color.
Hair Color Trait
Phenotype
Allele
Purple
P
(dominant)
White
p
(recessive)
Scenario 2
A man has a genotype of Pp, which means his hair color is purple. A woman has a genotype
of pp, which means her hair color is white. They also decide to have offspring. This is a
cross between a parent who is heterozygous for hair color with a parent who is homozygous
recessive.
In your Student Journal, determine what the hair color of the offspring in the F1 generation
will be for each scenario.
Complete Part I in your Student Journal.
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Genetic Outcomes
Mechanisms of Genetics
Part II: DNA Fingerprinting
In Part I, you explored how to predict genetic outcomes using Punnett squares. Now, you will
investigate a method for matching genetic outcomes with possible sources for that outcome.
What is DNA Fingerprinting?
A DNA fingerprint is not an actual fingerprint like you might imagine. A DNA fingerprint consists
of segments of DNA that are processed and can be used for comparison against the DNA of
another organism.
DNA fingerprinting can be used to determine paternity, in criminal investigations, and in finding
evolutionary relationships. Here's how it works:
The structure of any organism's DNA is the same. All DNA nucleotides have a phosphate group, a
five-carbon sugar, and a nitrogenous base. There are four different nitrogenous bases. The only
difference in the DNA of every living thing is the number and order of the base pairs. Although
there are millions of base pairs in humans, scientists only isolate, process and compare the
repeating sequences found in the genome.
Please continue to the next page.
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Genetic Outcomes
Mechanisms of Genetics
Part II: DNA Fingerprinting, continued
The process of making a DNA fingerprint is called gel electrophoresis.
Below are the steps of the procedure:
1.  A sample of DNA is collected. The sample can
come from hair, saliva, blood, or semen.
2.  Restriction enzymes cut the DNA into smaller
pieces at a specific base sequence. When the
DNA is cut, there are many fragments of different
sizes.
3.  The fragments are put into a gel made of
agarose.
4.  An electric current pulls the fragments across the
gel. The pieces of DNA are sorted by size.
Smaller fragments move farther across the gel
than the larger ones.
5.  The gel is “blotted” with nylon, and the DNA is
transferred onto the nylon.
6.  Radioactive probes are washed onto the nylon.
A film is placed on the nylon and X-rays will be
used to produce an image.
Please continue to the next page.
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Genetic Outcomes
Mechanisms of Genetics
Part II: DNA Fingerprinting, continued
Once the DNA fingerprint is made, comparisons can be made. Let's practice!
Example A. The sample shows blood collected at a criminal investigation. DNA from three
suspects will be compared to the crime scene blood at each numbered probe.
Start with # 1. The blood has a dark band. Suspects 1 and 3 also have that dark band.
# 2. The blood has a thin band. All suspects have the band, too.
# 3. The blood has a dark band. Suspects 1 and 3 have the dark band.
# 4. The blood has a thin band. Suspects 2 and 3 have these bands as well.
Suspect 3 is the closest match because he shares the most bands in common.
Example B. Look at the example to the right.
Is it likely the suspect committed the crime?
Complete Part II in your Student Journal.
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Genetic Outcomes
Mechanisms of Genetics
Part III: Karyotyping
A karyotype is a map of an organism's chromosomes. Chromosomes are extracted from an
organism's cells and then stained. The chromosomes are then arranged and numbered by size,
from largest to smallest. Once the chromosomes are organized by size, they are further paired by
using banding characteristics and location of the centromere. During chromosomal analysis, this
arrangement helps scientists quickly identify chromosomal alterations that could possibly indicate a
genetic abnormality. A karyotype is shown below. Notice that there are 23 pairs of chromosomes
in a normal karyotype. The first 22 pairs are called autosomes. The 23rd pair of chromosomes
indicates the sex of the person.
Sex chromosomes XX = female
Sex chromosomes XY = male
The example shown here is a normal karyotype. There are 23 pairs of chromosomes. There are
no chromosomes missing, and there are no extra chromosomes on any pair.
Analyze the karyotypes in your Student Journal. Use the data table to determine which genetic
abnormality exists.
Complete Part III and the Reflection and Conclusions sections in your Student Journal.
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Genetic Outcomes
Mechanisms of Genetics
Part III: Karyotyping, continued
Genetic Disorder
Down's Syndrome
Chromosome
Abnormality
Extra chromosome on
# 21 (trisomy 21)
Symptoms
Possible heart defects, prone to
respiratory disease, short stature
Usually some degree of mental
retardation
Cri du chat
Deletion of a small portion
of chromosome # 5
Severe mental retardation
Small head with unusual facial features
A cry that sounds like a distressed cat
Turner's Syndrome
Patau Syndrome
Missing one of the sex
chromosomes
(monosomy)
# 23 XO
Genetically female, but do not reach
sexual maturity, sterile
Extra chromosome on
# 13 (trisomy 13)
Serious eye, brain, circulatory defects
Normal intelligence
Cleft palate
Many do not survive past a few months
Edward's syndrome
Extra chromosome on
# 18
(trisomy 18)
Rare disorder, majority of people with
the syndrome die as a fetus
Infants who survive have serious
defects
Short life expectancy
Complete Part II in your Student Journal.
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