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Genetics Exam #1
September 30, 2005
(5) Define and distinguish between acrocentric and metacentric chromosomes.
acrocentric chromosomes have centromeres near 1 end
metacentric chromosomes have its centromere near the center
(5) Define and distinguish between leptotene and zygotene.
leptotene is the stage of meiosis where chromatin starts condensing
zygotene is the stage of meiosis when chromosomes begin pairing up
(3) Define interphase.
All stages of the cell cycle not including mitosis (specifically G1, S, and G2)
(3) Define carrier.
A heterozygous individual for a recessive trait- passes on the gene and doesn't show a phenotype
(3) Define cyclin.
family of proteins that vary in their amount according to the cell cycle and regulate it's progress
(3) Define karyotype.
ordered array of metaphase chromosomes
(3) Define polyploid.
more than 2 complete sets of chromosomes-- (-somy represents a single chromosome)
(3) Define zygote.
diploid fertilized egg
(3) Define epistasis.
interaction of two different genes where 1 gene masks the effect of another
(3) Define genetic mosaic.
situation where different cells in an organism's body express different genes
(3) Define mutually exclusive events.
two events which cannot happen at the same time (ARE dependent upon the other)
(3) Define ascus.
sac containing yeast spores
(10) In Mendel's peas, axial flowers are dominant over terminal flowers. Assuming an F1
heterozygous cross, what is the probability that at least 5 of 6 plants have axial flowers?
P(5) + P(6) = P(at least 5 of 6)
6!/(5!1!)*(.75)5*(.25)1 + 6!/(6!0!)*(.75)6*(.25)0
6*.0593 + 1* .1780
0.5338 = 53.4%
(10) In snapdragons, white and red flowers show incomplete dominance and tall vs short plants
show incomplete dominance. If a white medium snapdragon is crossed to a pink medium height
snapdragon, what will be the phenotypes of the offspring and their ratios?
rrTt X RrTt
rT
rt
RT
Rt
RrTT RrTt
RrTt rrTt
1 white tall
1 pink tall
rT
rrTT
rrTt
rt
rrTt
rrtt
2 white medium
2 pink medium
1 white short
1 pink short
(10) In a particular genetic cross, you fine 67 red berry and 33 blue berry plants. Assuming a red
X red mating, would these results be consistent for red dominant over blue for berry color?
why?
if red was dominant, you would expect 75 red and 25 blue berries
(67-75)2/75 + (33-25)2/25 = 2
2 = 64/75 + 64/25 = 3.413 with 1 degree of freedom therefore is consistent with the idea
(10) In soybeans, wide leaves are dominant over narrow leaves, brown beans are dominant over
white beans, and short (should be tall!) plants are dominant over tall (should be short) plants.
When a particular tall, wide leaf brown bean plant is test crossed to a short, narrow leaf white
bean plant, the following offspring are produced:
375 short, wide leaf, brown seed
3 tall, wide leaf, white seed
350 tall, narrow leaf white seed
2 short narrow leaf, brown seed
105 tall, wide leaf, brown seed
40 short, wide leaf, white seed
95 short, narrow leaf, white seed
30 tall, narrow leaf, brown seed
Diagram the two chromosomes of the heterozygous plant and the order of the loci.
Tall Narrow white / short wide Brown
What is the distance between wide leaf and white seed loci in centimorgans?
ie. short wide white, tall narrow brown, both doubles
(40+30+3+2)= 75 /1000 = .075 = 7.5 centimorgans
What is the coefficient of interference in this cross?
other recombination distance = (105+95+2+3)=205 / 1000 = .205
expected number of double recombinants = .205*.075 = .0154 *1000 = 15.4 expected
5 (observed)/15.4 (expected) = .325 (coefficient of coincidence)
1-.325= .675 = coefficient of interference
(10) In neurospora, 200 ordered tetrads were analyzed for a and b and the results show below:
#
1-2
3-4
5-6
7-8
+ +
+ +
- 73
ab
ab
ab
a-b- - + +
67
ab
ab
ab
a+b+
14
a-ba+b- a-b+ a+b+
+ +
13
ab
a-ba+b+ a-b- +
+ +
12
ab
ab
a-ba+b11
a-ba+b+ a+b+ a-b- 10
ab
a+b+ a-ba+b+
What is the distance from the centromere to gene b?
2nd divison for b = 13+11+10 = 34
.5* (13+11+10)/200 = 17/200 = 8.5 centimorgans
What is the distance from gene a to gene b?
2nd division for a = .5*60/200 = 15 centimorgans
15-8.5 = 6.5 centimorgans
(10) Given below is a pedigree for a rare genetic disease:
X
What is the likely form of inheritance for this disease? Why?
autosomal dominant-- shows up every generation including if crossed to 'normals' (ie. rare)
What is the probability that daughter (should be son) X has the disease? Why?
0%-- parents cannot be carriers and don't have the disease, so child must be normal