Biology 105: Introduction to Genetics
Midterm Winter 2010 EXAM
Name ______
Student ID ______
Before starting, write your name on the top of each page
Make sure you have all pages
You can use the back-side of the pages for scratch, but we will not grade answers written on the back-side of the page.
Unsolved fractions are acceptable answers
Part I Definitions
15 points ------
Part II Short Answer
25 points ------
Part III True or False
14 points ------
Part IV Multiple Choice
22 points ------
Part V Extended Calculation
25 points ------
Total ------
Part1 (15 points)
Definitions
1 MeiosisI
During this stage the chromosomes have replicated and the homologs pair, recombine and segregate from one another
2 Mendels law of independent assortment
Different genes assort independently from one another during gamete formation.
3 Map unit
A measure of distance between two genes. Based on the frequency of meiotic recombination that occurs between them. Defined as #recom/total progeny x 100%
4 Hemizygous
Gene present in one copy in a diploid organism
Human males are hemizygous for genes on the X-chromosome
5 Aneuploid: Having a chromosome number that is not a multiple of the haploid number for the species
Part2 (25 points)
Short answers
1 Blood groups A and B are dominant over o and A and B are codominant to one another.
On Halloween night two children are born to two couples.
Mother M and husband N have one child
Mother R and husband S have one child
Due to heavy partying amongst the nurses and doctors, no one remembered which baby belonged to which mother.
It was necessary for all individuals concerned to submit to a blood test the next day.
BabyX is type O
BabyY is type A
MotherM is type A and her husbandN is type A
MotherR is type AB and her husbandS could not be tested (he was recovering at home from the Halloween party).
With this information, determine which baby belonged to which couple (explain your reasoning).
Baby X belongs to mother M and father N
2 An X-linked dominant allele of a gene causes high blood pressure in humans. A man with high blood pressure marries a woman with normal blood pressure. What proportions of their sons will have high blood pressure?
H= high blood pressure h= normal blood pressure
Cross is H/Y x h/h
Progeny H/h females and h/Y males. Answer is zero.
3 A human disease afflicted a family as shown in the accompanying pedigree.
Deduce the most likely mode of inheritance of this single gene trait.
X-linked dominant
4 Recombination frequency between two genes far apart on the same chromosome is around 50%. So too is recombination frequency between two unlinked genes. How would you differentiate between the two cases?
Use Additional Markers and measure recombination frequencies between the genes and these markers
5 Assume right-handedness (R) dominates over left-handedness (r) in humans, and that brown eyes (B) are dominant over blue (b). A right-handed, blue-eyed man marries a right-handed, brown-eyed woman. One of their two children is right-handed/blue-eyed, while the other is left-handed/brown-eyed. What are the genotypes of the husband and wife?
Realize that the father exhibited the dominant right-handedness trait and the recessive blue-eyed trait. Thus his genotype is best given as R_bb.
The first wife was right-handed and brown-eyed (dominant): R_B_.
The marriage yielded one right-handed, blue-eyed child (R_bb) and one left-handed, brown-eyed child (rrBb).
The only way to get a left-handed child is if both parents offered a left-handed allele, r. Therefore both parents must have been heterozygous with respect to handedness (i.e., father = Rr; and wife = Rr).
The only way to get a blue-eyed child is if both parents offered a blue-eyed allele, b. Therefore the mother must be heterozygous (Bb) with respect to eye color. The father's eye color genotype was bb.
Therefore the genotypes must have been as follows:
Father: Rrbb
First Wife: RrBb
6 A farmer maintains two true-breeding lines of strawberries-a red colored strain and a yellow colored strain. He crosses these two strains and in the F1 obtains plants that produce orange berries. He crosses the F1 to themselves in the hope of obtaining a truebreeding orange berry. In the F2 only half the plants are orange. The rest are either red or yellow. Explain this pattern of inheritance.
Incomplete dominance
7 The karyotype of a child is shown below. What event during meiosis can best explain the child’s observed karyotype. In which parent did this event occur.
meiosis II in father
8 The karyotype of a child is shown below. What event during meiosis can best explain the child’s observed karyotype. In which parent did this event occur.
Nondysjunction in meiosis II in mother or meiosisI in father
Part3 (14 points)
True/False Indicate whether the following statements are true or false.
1 In the wild there lives a hypothetical ant with only one chromosome. A sub-species of this ant is a triploid. Mating between two triploid ants results in diploid, triploid and tetraploid progeny in the ratio 1:2:1
TRUE
2 Mendel identified seven traits that assorted independently. The only explanation for this behavior is that the genes controlling these traits reside on different chromosomes.
False: genes far apart on the same chromosome also assort independently
3 Individuals homozygous for a recessive mutation in the p53 gene develop skin cancer at an early age. An individual known to be heterozygous for the p53 mutation develops skin cancer. Analysis reveals that the cancerous cells are homozygous for the p53 mutation. The only explanation for this is that a mutation occurred in the remaining normal p53 allele in these cells
False- second mutation, chromosome loss or mitotic recombination could produce this result.
4 GeneA, geneB, geneC and geneD reside on chromosome2 of flies. Genes A and B are separated by 10 m.u. Similarly genes C and D are separated by 10 m.u. One concludes from this data that the same physical distance separates the genes along the chromosome.
False: recombination frequencies vary depending on location along chromosomes
5 Wild-type (red eye) male flies are crossed to white eyed females. The eye color gene resides on the X-chromosome. One out of every 2000 female progeny flies is white eyed. These females are most likely the result of a rare chromosomal non-dysjunction during meiosis.
True
6 A true breeding double dominant strain of corn (AABB) is crossed to a true breeding double recessive strain. The F1 plants are then test crossed. If the A locus is linked to the B locus then we should expect more Ab and aB gametes compared to AB and ab gametes in the F1 plants.
False. The Ab and aB gametes are recombinant gametes and should be less than the parental gametes.
7 Crossing over always involves a significant loss of chromatin and this is the source of genetic variation
False
Part4 (22 points)
Multiple choice
1 Normal mitosis takes place in a diploid yeast cell of the genotype A/a B/b.
Which genotype is most likely to be present in the daughter cells?
A A B
B a b
C A/A B/B
D A/a B/b
E a/a b/b
DDD
2 Some of these events occur in both mitotic and meiotic cells while some of these events occur only in meiosis. Circle the event that occurs only in meiosis
A Sister Chromatid formation
B Spindle formation
C DNA replication
D Chromosome movement
E Synapsis
EEEEE
3 Which of the following does not occur during meiosisI
A recombination
B replication of homologous chromosomes
C separation of homologous chromosomes
D pairing of homologous chromosomes
E separation of sister chromatids
EEEE
4 An organism with three pairs of homologous chromosomes (Xx, Yy, Zz) can produce _____ unique combinations of chromosomes in its gametes
A 3
B 4
C 6
D 8
E 12
DDDD
5 Drosophila homozygous for the mutation called delta have abnormally curved wings. Drosophila homozygous for the wild type allele of this gene have normal wings. The delta mutation behaves as a
A lethal dominant
B co-dominant
C incomplete dominant
D all of the above
E Cannot determine
EEEE
6 Red/green color blindness is controlled by an X linked gene in humans. A normal man and woman marry. The fathers of both of these individuals are color blind but the mothers of both are homozygous normal. What is the probability that their first child will be color blind
A 1/2
B 1/3
C 1/4
D 1/8
E 0
CCCCCCC
7 An allele was identified in human populations that produce hairy ears. The trait is passed exclusively from father to son. On which chromosome is the gene most likely to reside
A X
B Y
C autosome
D either sex chromosome
E polytene chromosome
BBBBBB
8 A gene for examania (E) resides on chromosome13. There are two alleles for this gene- EA and EF. An EAEA father and an EAEF mother produce a child with the genotype EAEFEF.
This was due to
A Non-dysjunction in meiosisI in the father + normal dysjunction in the mother
B Non-dysjunction in meiosisII in the father + normal dysjunction in the mother
C Non-dysjunction in meiosisI in the mother + normal dysjunction in the father
D Non-dysjunction in meiosisII in the mother + normal dysjunction in the father
E normal dysjunction in mother + normal dysjunction in the father
DDDDDD
9 A tall parent and a short parent produce F1 offspring that when self-fertilized produce a range of offspring heights, some taller that the tall P1 and some shorter than the short P2. Which set of parental genotypes could best account for these data
A AABBCC x aabbcc
B AABB x aabb
C AABBcc x aabbCC
D AA x aa
E BB x bb
CCCCCC
10 Human cells normally have 46 chromosomes (44 autosomes and 2 sex chromosomes). During metaphase of meiosis I the number of DNA molecules present in a human cell are
A 92
B 46
C 23
D 44
E 2
AAAAAAA
11 How many types of gametes will a diploid individual of the genotype AaBBCcDDEEFfGG produce
A 2
B 4
C 8
D 16
E 32
CCCCCCExtended calculations
1
A true breeding mutant mouse strain exhibits two different traits (A and B). When a male from the true breeding mutant strain is crossed to a true breeding wild-type female, all of the female F 1 progeny exhibit both traits whereas all of the male F 1 progeny look wild type.
What is the mode of inheritance of the two traits?
X-linked dominant
Male=XabY x female= X++X++
F1= XabX++ females and X++Y males
The male and female F1 mice described above are crossed to one another to produce F2 progeny. Of the male F2 progeny, 40% have both traits, 40% are wild type and the rest of the F2 males have only one trait or the other. What fraction of the female F2 progeny would you expect to have both traits?
40%
A------B (P) A------B (40%)
A------B (R) A------+ (10%)
+------+ (R) +------B (10%)
+------+ (P) +------+ (40%)
Since A and B are dominant, XabX++ will show both traits i.e. 40%
What is the map distance (in cM) between the genes for the two traits?
20CM 10+10/100=20CM
2 As a student in a fly lab you perform a screen for mutations that affect bristle formation. From your screen you isolate one recessive mutation that result in mutant flies with no bristles called nub1. You want to map nub1. You map these relative to two known recessive markers on chromosome 2– vestigial wings (vg) and purple eyes (pr).
You perform the cross:
nub1 pr vg flies crossed to homozygous wild-type flies.
You then take the phenotypically wild-type females, cross them to nub1 pr vg homozygous recessive males and analyze the phenotypes of the F2 progeny.
The results are as follows:
Nub1 cross data
Phenotype progeny No.
nub1 pr vg 404
nub1+ pr+ vg+ 413
nub1 pr+ vg+ 3
nub1+ pr vg 4
nub1 pr+ vg 53
nub1+ pr vg+ 50
nub1 pr vg+ 35
nub1+ pr+ vg 38
Total 1000
What is the order of the three genes on the chromosome?
Pr-nub1-vg
What is the distance between the three genes (in map units)
Pr---nub1 is 11
Nub1---vg is 8
10