Biology 123 SI- Dr. Raut’s Class
Session Seventeen- 3/30/15
1. Review Morgan’s experiment. What happened and why?
The wild type has a gray body and normal wings and the double mutant has a black body and
vestigial wings. A test cross was performed and resulted in an F1 dihybrid with a gray body and
normal wings proving that the wild type was dominant. They then bred a fly from the F1
generation with a double mutant. The expected results and true results are below.
Gray, normal: Black, vestigial: Gray, vestigial: Black, normal
Expected not linked: 1:1:1:1
Expected linked: 1:1:0:0
Recorded: 965:944:206:185
Why are the results so far from what was expected? The genes are linked; however, the
recombinants are possible due to crossing over.
2. Assuming the aristae length is at map unit 5 and body color is at map unit 48.5, what is the
percentage of recombinant phenotypic offspring? (assume that a heterozygous wild-type fly with
short aristae and a black body is test-crossed with a homozygous mutant fly)
43.5% recombination frequency
3. Briefly explain why men more often suffer from X-linked disorders, and give some examples
of X-linked diseases.
While women have two X chromosomes, men only have one. This means that if a recessive
disorder is carried on the X chromosome, then a man carrying the gene will always be affected.
Women will only be affected by the disease if they have the diseased trait on both X
chromosomes.
4. Briefly explain X inactivation.
Since women have two X chromosomes, they inactivate one X chromosome which condenses
into a bar body. This can produce some very interesting phenotypes. An example of which is the
calico cat. I have attached a picture of my cat Lola as a reference. Since the coat colors are
caused by the different X chromosomes being expressed in different cells, calico cats are always
females.
5. Nondisjunction leading to more than two complete sets of chromosomes
is_________polyploidy______. Do animals survive this? Do plants?
If polyploidy occurs, in an animal embryo, miscarriage will occur. Plants however can often
exist in a polyploidy state. However, nondisjunction in only one pair of chromosomes does lead
to many different human conditions.
6. List and explain the four different ways that chromosome structure can be altered.
Deletion-removes a chromosomal segment.
Duplication- a segment is repeated
Inversion-a segment is reversed within a chromosome
Translocation- a segment from one chromosome is switched with a segment from another
nonhomologous chromosome
7. Briefly explain each of the following genetic disorders: Down’s Syndrome, Turner Syndrome,
Klinfelter Syndrome, and Cri du Chat.
Down’s syndrome is also called trisomy 21 and is the result of a nondisjunction leading to a
trisomy in the 21st chromosome.
Turner syndrome is caused by the monosomy of the X chromosome in females and causes them
to be sterile.
Klinfelter syndrome occurs in a male with two chromosome resulting in XXY and causes them
to be sterile.
Cri du Chat is the result in the deletion of a specific segment in the fifth chromosome.
8. What is genomic imprinting?
In genomic imprinting, the phenotype is depending upon which parent the allele is inherited
from.
9. From which parent is organelle DNA inherited? Explain why.
Organelle DNA, which includes mitochondria, chloroplasts, and some plant plastids, is always
inherited from the mother. This is because during fertilization only the nucleus of the sperm is
transferred into the egg, and therefore, all of the organelles are from the mother.
10. What does Chargaff’s Rule state?
Chargaff’s rule states that the base composition varies between species, and that within a species
the number of bases A and T are equal and the number of bases G and C are equal.
11. What model is used currently for how DNA is replicated: conservative, Semi-conservative,
or dispersive? What does this mean?
DNA is semi-conservative. This means that each daughter strand has one strand from the parent
and one that was replicated from the parent strand.