Genetics Exam 1 - Hughes
Sources of genetic variation in Meiosis
1) crossing over - prophase I 2) random separation of homologous chromosomes - anaphase I
Law of segregation
1) each individual organism possesses two alleles encoding a trait 2) alleles separate when gametes are formed 3) alleles separate in equal proportions Anaphase 1
1) What do you expect to get in the F1 progeny of a cross between homozygous white-eyed female Drosophila and red-eyed males? a. all white-eyed flies b. all red-eyed flies c. half-red eyed, half white eyed flies d. ¾ red-eyed, ¼ white eyed flies. 2) If you performed the reciprocal cross of the question above, what would you get? a. all white-eyed flies b. all red-eyed flies c. half-red eyed, half white eyed flies d. ¾ red-eyed, ¼ white eyed flies.
1c. half-red eyed, half white eyed flies 2b. all red-eyed flies
Tay Sachs disease is an autosomal (chromosome 15) recessive disease. If two heterozygous adults choose to have pre-implantation genetic diagnosis, what proportion of their embryos would also be heterozygous? a. 1/4 b. 1/2 c. 3/4 d. all
b. 1/2
In a cross between genotypes Aabb and aaBb, what proportion of offspring would be Aabb? a. 2/16 b. 4/16 c. 8/16 d. none
b. 4/16
At the human H gene, homozygous hh individuals do not make the precursor for the A or B antigens. So hh individuals are blood type O regardless of their genotype at the unlinked I gene. What is the relationship between the H gene and the I gene? a. Dominance b. Epistasis c. Pleiotropy d. Concordance
b. Epistasis
What most commonly makes an allele recessive? a. Incomplete dominance b. Loss of function c. Gene interaction d. Gain of function
b. Loss of function
During which phase of mitosis do spindle fibers attach to kinetochores? a. prophase b. prometaphase c. metaphase d. anaphase
b. prometaphase
What is the main consequence of the second meiotic division? a. recombination of alleles b. separation of chromatids c. reduction from diploid to haploid d. separation of homologs
b. separation of chromatids
Coat color in cats is affected by a sex-linked gene with two alleles black or orange. Males can be either black or orange, as can females, though females can also be both i.e. tabby. Therefore, the tabby phenotype shows a. codominance b. sex influenced c. variably expressivity d. sex limitation
b. sex influenced
What ensures proper segregation of the non-homologous sex chromosomes? a. synapsis at the sry b. synapsis at the pseudo-autosomal regions c. synapsis at the centromeres d. none of the above; they do not segregate
b. synapsis at the pseudo-autosomal regions
What are the sex/phenotype of offspring from a cross of a homozygous red-eyed male and a white-eyed female Drosophila? a. red eyed males, white eyed females b. white eyed males, red eyed females c. red eyed females, ½ females red eyed, ½ females white eyed. d. red eyed males, red eyed females.
b. white eyed males, red eyed females
Law of Independent assorment
Alleles at different loci separate independently Anaphase I
What is the law of independent assortment?
Alleles at different loci separate independently Anaphase I
Gene
An inherited factor (region of DNA) that helps determine a characteristic i.e. gene for seed shape
How does mitotic anaphase ensure that 2 genetically identical daughter cells are produced?
In mitosis, there is no crossing over. 2 genetically identical daughter cells are produced because during anaphase, the sister chromatids are separated toward opposite poles.
At a mechanistic level, how do two alleles show incomplete dominance?
Incomplete dominance occurs when the phenotype of the heterozygote falls in between the phenotypes of the two homozygotes. Both alleles of the homozygous genotypes are not expressed over one another; rather, an intermediate heterozygote is formed. The trait is neither dominant or recessive
Cytoplasmic inheritance
Inheritance of characteristics encoded by genes located in the cytoplasm. Because the cytoplasm is usually contributed entirely by only one parent, most cytoplasmically inherited characteristics are inherited from a single parent (typically the mom/egg) Chloroplasts and mitochondria contain DNA, too! -frequently exhibit extensive phenotypic variation because no mechanism analogous to mitosis or meiosis ensures that cytoplasmic genes are evenly distributed during cell division
Cell cycle
Interphase - the period in between cell divisions when the cell grows, develops, and functions M phase - mitotic phase, the period of active cell division.. includes mitosis and cytokinesis
Stages of mitosis
Prophase Prometaphase Metaphase Anaphase Telophase *produces only genetically identical progeny *single nuclear division, and usually single cell division *chromosome number in newly formed cells is same as original cell
In a dihybrid cross, between parents YYrr and yyRR, how many different genotypes give a Yellow Round phenotype in the F2? a. 2 b. 3 c. 4 d. 6
c. 4
During G0 of the mitotic cell cycle, in a cell with 4 pairs of homologous chromosomes, how many DNA molecules are there in a cell? a. 2 b. 4 c. 8 d. 16
c. 8
Chiasmata are the visual sign of what process? a. Pairing of homologs b. Equational division c. Crossing over d. Cell division
c. Crossing over
Humans and Drosophila both have X Y sex chromosomes; which of the following reveals that they don't determine sex in the same way? a. XY individuals are male humans but female flies b. XXY individuals are female humans, but male flies. c. X0 individuals are female humans but male flies d. XYY are male humans but female flies.
c. X0 individuals are female humans but male flies
Drosophila has X Y sex determination. Which of the following karyotypes leads to a female fly? a. X0 b. XY c. XXY d. none of the above
c. XXY
How can a sex-limited trait be identified? a. expressed more in females than males b. expressed more in males than females c. expressed in only one sex d. expressed equally in both sexes.
c. expressed in only one sex
In the above picture, what is the most likely explanation for the inheritance of the trait? (picture on definition side) a. autosomal recessive b. autosomal dominant c. x-linked recessive d. y-linked
c. x-linked recessive
SRY Gene
sex-determining region, typically on the y chromosome explains XX males, and it is missing on rare XY females.
Anaphase
-connection between sister chromatids breaks down -sister chromatids separate and move toward opposite spindle poles
Telophase
-after sister chromatids have separated, each is considered a separate chromosome -arrival of chromosomes at the spindle poles -nuclear membrane re-forms around each set of chromosomes -can also include cytokinesis (cytoplasm divides)
Metaphase
-chromosomes become arranged in a single plane, the metaphase plate, between the two centrosomes -centrosomes at the opposite ends of the cell center at the spindle poles
Prophase
-chromosomes condense -each chromosome consists of two sister chromatids attached at the centromere -mitotic spindle forms -spindle grows out from a pair of centrosomes that migrate to opposite sides of the cell
Prometaphase
-disintegration of the nuclear membrane -spindle microtubules enter nuclear region -for each chromosome, a microtubule from one of the centrosomes anchors to the kinetochore of one of the sister chromatids; a microtubule from the opposite centrosome attaches to the other sister chromatid
Genotype
A set of alleles possessed by an individual organism diploid organism w/ a genotype of two identical alleles is homozygous at that locus a genotype consisting of two different alleles is heterozygous at the locus.
Epistasis
A type of gene interaction in which one gene alters the phenotypic effects of another gene at a different locus. i.e. colors of labs B_E_ = black bbE_ = Brown B_ee = yellow bbee = yellow one locus determines type of pigment produced (B - black, b- brown) other locus determines whether or not pigment is deposited in the shaft of the hair (E- allows dark pigment to be deposited, e- prevents deposition of dark pigment)
Cell division in prokaryotes
Binary Fission -begins at a specific place on the circular chromosome called the origin of replication -origins of the 2 newly replicated chromosome move away from each other and toward opposite ends of the cell - a new cell wall forms between the two chromosomes, producing two new cells, each with an identical copy of the chromosome.
Differences between binary fission and mitosis
Both are forms of asexual reproduction in which a parent cell divides to form two identical daughter cells. Binary fission: -prokaryotes -replication begins with Origin of Replication, origins move away from eachother and toward opp. ends of cell, cell wall formed between. -no spindle apparatus -dna replication and separation occur at the same time Mitosis -eukaryotes -spindle apparatus is formed -dna replication is completed in Interphase, before mitosis even begins -prophase/prometaphase/metaphase/anaphase/telophase/cytokinesis
Lethal alleles
Causes death at an early stage of development- often before birth- so that some genotypes do not appear among the progeny. i.e. yellow coat color in mice. A cross between 2 yellow heterozygous mice produces a genotype of 1/4 YY, 1/2 Yy, 1/4 yy but the homozygous YY mice die early in development and do not appear among progeny. Thus, a resulting 2:1 ratio of Yy (yellow) to yy(non-yellow) mice in offspring.
Diploid
Cells that carry two sets of genetic information (2n)
Haploid
Cells that contain a single set of chromosomes (n) i.e. eggs, sperm, spores
Sex influenced traits
Determined by autosomal genes and are inherited according to Mendel's principles, but are expressed differently in males and females. i.e. beards in goats, allele located on an autosomal locus that is dominant in males and recessive in females. -in males a single beard allele is required for expression of this trait, both the homozygote and heterozygote have beards. -in females, two beard alleles are required for the trait to be expressed. the homozygote has a beard while the heterozygote and the other homozygote are beardless.
Sex chromosomes
Differ between males and females (different from autosomes which are the same for males and females) X and Y chromosomes are only homologous at the pseudoautosomal regions, which are essential for X-Y chromosome pairing in meiosis in males.
Genetic imprinting (epigenetics)
Differential expression of a gene depending on whether it is inherited from the male or female parent. i.e. gene for insulin-like growth factor. offspring inherit allele from mom and from dad. paternal copy is actively expressed in the fetus and placenta, but the maternal copy is completely silent. Epigenetics - alterations in DNA that do not included changes in base sequence; often affects the way in which DNA sequences are expressed i.e. methylation of DNA can affect which DNA sequences are expressed
The presence of Barr bodies in female nuclei is a visual representation of dosage compensation; why?
Dosage compensation is a mechanism to equalize the amount of protein produced by X-linked genes and autosomal genes. This is accomplished by random inactivation of one X chromosome in the cells of females. The Barr body is an inactive X chromosome. If a cell contains more than two X chromosomes, all but one of them is inactivated. As a result of X inactivation, female placental mammals are functionally hemizygous at the cellular level for X-linked genes.
Sex limited traits
Encoded by autosomal genes that are expressed in only one sex. The trait has zero penetrance in the other sex. i.e. domestic chickens. cock feathering is an autosomal recessive trait limited to males. same genotype in both males and females, but the phenotype produced by those genotypes differ between males and females. only homozygous recessive males have the cock-feathering phenotype.
How does epistasis arise from interactions among different genes?
Epistasis: The effect of gene interaction is that one gene masks (hides) the effect of another gene at a different locus. In dominance, masking of genes occurs at the same locus. In epistasis, the gene that does the masking is called an epistatic gene and is on a different locus from the gene whose effect is being masked. i.e. if BB produces black pigment, and bb produce brown pigment in labs. There is a second locus, with the dominant allele E, that affects the deposition of the pigment in the shaft of the hair. If ee is recessive, then no hair pigment is deposited.
Interphase
Extended period of growth and development between cell divisions. The nuclear membrane is present and the chromosomes are relaxed. G1- cell grows and proteins necessary for cell division are synthesized G0 - before reaching the G1/S checkpoint, cells may exit the active cell cycle in response to regulatory signals and pass into a nondividing state called G0. G1/S checkpoint - keeps the cell in G1 until the cell has all the enzymes/proteins necessary for DNA replication S - each chromosome is duplicated, DNA synthesis must occur before the cell can proceed to mitosis. Before S phase, each chromosome is unreplicated; after S phase, each chromosome is composed of two sister chromatids G2 - more additional biochemical events necessary for cell division to take place. G2/M checkpoint - near the end of G2. This checkpoint is only passed if the cell's DNA is completely replicated and undamaged.
Linked genes
Genes located close together on the same chromosome -travel together in meiosis, are not expected to assort independently. if completely linked, no crossing over possible
Chromosome structure
Linear DNA molecules are highly folded and condensed. Each DNA molecule is coiled around histone proteins and tightly packed, forming a rod-shaped chromosome. Most of the time, chromosomes are thin and hard to observe, but before cell division they condense further into thick, easily observed structures. centromere - constricted region on the chromosome, contains the kinetochore which is the attachment point for spindle microtubules. telomere - specific DNA sequences and associated proteins located at the tips of whole linear chromosomes, provide chromosome stability origins of replication - sites where DNA synthesis begin
Two locus phenomena Mendel didn't see
Linkage
Gene mapping
Map of the relative distances between genetic loci, markers, or other chromosome regions determined by rates of recombination one map unit = 1% recombination rate
Centromere locations
Metacentric- in the middle Submetacentric- close to the middle Acrocentric- close to the end Telocentric- on the end
Monohybrid cross vs Dihybrid cross
Monohybrid - crosses between parents that differed in a single characteristic (round seed vs wrinkled seed) Dihybrid - crosses between parents that differed in two characteristics (round and yellow seeds vs wrinkled and green seeds)
Twin studies
Monozygotic - genetically identical, single zygote split Dizygotic - two eggs, two different sperm (share 1/2 genes, fraternal) ex. did they develop cancer by 60? 12 monozygotic twins were concordant (same), 15 dizygotic twins were concordant. rheumatoid arthritis? - 32 monozygotic, 6 dyzygotic
Single gene phenomena Mendel didn't see
Not all characteristics are inherited as simple dominant and recessive traits. i.e. Boycott and snail's directions of coiling The phenotype of a snail was determined not by its own genotype, but by the genotype of its mother.
How does cytoplasmic inheritance work?
Not all genetic material is located in the nucleus. Some characteristics are encoded by genes in the cytoplasm. A few organelles, notably chloroplasts and mitochondria, contain DNA. A zygote inherits nuclear genes from both parents, but typically all its cytoplasmic organelles come from only one gamete, usually the egg. Thus, most cytoplasmically inherited traits are present in both males and females and are passed from mother to offspring.
Crossing over
Occurs during Prophase I of Meiosis I. Homologous chromosomes exchange genetic information. Can create recombination of alleles on different chromosomes. Crossing over occurs at the chiasma.
Test cross
One individual of unknown genotype is crossed with another individual with a homozygous recessive genotype for the trait in question
Alleles
One of two or more alternative forms of a gene i.e. seed shape gene can be round or wrinkled
Reciprocal cross
Pair of crosses in which the phenotype of the male and female parents are reversed i.e. in one cross, a tall male is crossed with a short female. in the other cross, a short male is crossed with a tall female
Stages of Meiosis
Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II Most are similar to mitosis with the following exceptions: Prophase I - chromosomes form homologous pairs and crossing over takes place. Synapsis occurs. Tetrads consist of four chromatids. Metaphase I - random alignment of homologous pairs (could be all of dad's on one side, mom's on the other side, or a mix) Anaphase I - separation of homologous chromosomes (sister chromatids remain attached and travel together). separation of randomly aligned chromosomes can lead to genetic variability Interkinesis - period between Meiosis I and II in which the nuclear membrane re-forms around the chromosomes clustered at each pole, spindle breaks down, chromosomes relax. Anaphase II - separation of sister chromatids **consists of two divisions **meiosis causes the chromosome number in the newly formed cells to be reduced by half, haploid gametes (n) **produces 4 cells instead of 2 **produces genetically variable cells
Frequency of recombinants
Recombinant progeny - progeny with new combinations of traits formed from recombinant gametes Frequency of recombinant gametes = half the frequency of crossing over, the maximum proportion of recombinant gametes is 50% recombination frequency = # of recombinant progeny / total number of progeny x 100
Dominance
Refers to an allele or phenotype that is expressed in homozygotes (AA) and in heterozygotes (Aa); only the dominant allele is expressed in a heterozygote phenotype.
Phenotype
The appearance or manifestation of a characteristic
Maternal effect
The genes are inherited from both parents but the offspring's phenotype is determined not by its own genotype, but by the genotype of its mother. i.e. shell coiling in Lymnaea peregra. The allele for dextral (right-handed) is dominant to allele for sinistral (left-handed) but the direction of coiling is determined by the mother's genotype. The direction of the coiling is affected by the way in which the egg cytoplasm divides soon after fertilization, which in turn is determined by a substance produced by the mother and passed to the offspring in the cytoplasm of the egg.
White eye in Drosophila
Thomas hunt morgan Crossed red-eyed female with white-eyed male f1 generation - all red eyes f2 generation - red-eyed females, 1/2 red-eyed males and 1/2 white-eyed males Performed reciprocal cross White eyed female with red-eyed male F1 generation - red-eyed females, white-eyed males F2 generation - 1/2 of females were red-eyed, 1/2 were white-eyed. 1/2 of males were red-eyed, 1/2 were white-eyed. Showed that white-eye trait is x-linked X+X+ = red eyed female X+Xw = red eyed female XwXw= white eyed female X+Y = red eyed male XwY= white eyed male
Co-dominance
Type of allelic interaction in which the heterozygote simultaneously expressed the phenotypes of both homozygotes i.e. blood type. Alleles IA and IB are codominant, while i is recessive. (also shows multiple alleles) Blood type A - IAIA or IAi Blood type B - IBIB or IBi Type AB - IAIB Type O - ii
Incomplete dominance
Type of dominance in which the phenotype of the heterozygote falls in between the phenotypes of the two homozygotes i.e. snapdragons show incomplete dominance. red flowers and white flowers can produce offspring of all shades between red and light pink.
Pedigree conventions
Unaffected person - not colored in Affected person - colored in Male - square Female - circle Carrier - dot Autosomal recessive traits - appear equally in males and females -tend to skip generations -more likely to appear in progeny of related parents Autosomal dominant traits -appear equally in males and females -affected people have at least one affected parent -unaffected people do not transmit the trait
Eukaryotes
Unicellular or multicellular cells with compartmentalized cell structure with components bound by intracellular membranes. -nucleus present -relatively large -multiple linear DNA molecules -DNA complexed w/ histones -relatively large amount of DNA -membrane-bound organelles present -ribosomes present
Prokaryotes
Unicellular organism. -no nucleus -relatively small -usually one circular DNA molecule -DNA not complexed with histones -relatively small amount of DNA -no membrane-bound organelles. -ribosomes present
Sex chromosomes humans
XX - female XY - male XO - Turner syndrome (female traits, short, broad chest) XXY/XXYY/XXXY - Klinefelter syndrome (male traits, small tested, reduced pubic/facial hair) XXX/XXXX/XXXXX - poly-X females (female traits, tall and thin) XYY - XXY males (male traits, tall)
Sex determination mechanisms
XX-XO Sex determination -females are XX -males are XO -determined by which kind of male gamete fertilizes the egg, could have sperm lacking x chromosome XX-XY Sex determination -females are XX -males are XY ZZ-ZW Sex determination -females are ZW -males are ZZ Environmental sex determination -sex determined by environmental factors -i.e. some turtles are female if incubated in warm temperatures, or males if incubated in cold temperatures * sex (in mammals) is actually determined by an individual gene, called the SRY gene located on the Y chromosome- determines male phenotype.
Homologous pair
a pair of chromosomes, one from each parent, that have relatively similar structures and gene values ex: humans have 46 chromosomes- 23 homologous pairs. In males, 22 are homologous and the last pair (x and y chromosomes) are partially homologous
A cross between two genes that lie 12 map units apart is: Ab/Ab x aB/aB; what will be the frequency of aB gametes produced by the F1? a. 0.06 b. 0.12 c. 0.38 d. 0.44
a. 0.06
If one of Mendel's traits had exhibited codominance, how many phenotypes would he have seen in the F2 of a monohybrid cross? a. 3 b. 4 c. 6 d. 9.
a. 3
At what stage during the cell cycle would chromosomes certainly be composed of only one chromatid? a. G1 b. S c. G2 d. M
a. G1
Hemophilia A is an X-linked recessive disease. A man with hemophilia, marries a woman whose mother had hemophilia, but does not herself have the disease. What is the expectation that their offspring will have the disease? a. Half of their sons, half their daughters. b. Half their sons, all their daughters. c. All their sons, half their daughters d. All their sons, none of the daughters e. All of their children will have the disease.
a. Half of their sons, half their daughters.
Where would you most likely find a spindle apparatus? a. Pea cells b. Bacterial cells c. Archaeal cells d. All of the above
a. Pea cells
What stage of meiosis recombines genetic variation of genes that are parts of non-homologous chromosomes? a. Prophase I b. Anaphase I c. Prophase II d. Anaphase II
a. Prophase I
When, in meiosis, does the synapsis occur? a. Prophase I b. Anaphase I c. Prophase II d. Anaphase II
a. Prophase I
In the pedigree, which mode of inheritance is most likely? a. autosomal recessive b. incompletely penetrant c. X-linked recessive d. X-linked dominant
a. autosomal recessive
If a monohybrid cross experiment yields three different phenotypes in the F2, what might be the explanation? a. incomplete dominance b. epistasis c. pleiotropy d. sex linkage e. a or c
a. incomplete dominance
The disease neurofibromatosis may show as a few darker skin patches, or many large skin tumors, this is an example of a. variable expressivity b. incomplete penetrance c. codominance d. epistasis
a. variable expressivity
Sex-linked genes
characteristics determined by genes located on the sex chromosomes (typically x-linked) i.e. color blindness in humans X+ -normal color vision Xc -colorblindness females will only be colorblind if both XcXc, males can be colorblind if they have a single Xc
Two genes are known to be part of the same chromosome 8 map units apart. A cross Ab/Ab X aB/aB is performed; the F1 progeny are then test crossed (with ab/ab). What frequency do you predict for Aabb progeny of the testcross? a. 0.02 b. 0.04 c. 0.08 d. 0.46 e. 0.48
d. 0.46
If you performed a dihybrid cross where there is complete dominance at one gene but incomplete dominance at the other gene, how many phenotypes would you see in the F2 generation? a. 3 b. 4 c. 5 d. 6 e. 9
d. 6
One of Mendel's conclusions was his "law of segregation"; what stage of meiosis is responsible for segregation, when crossing-over plays no role ? a. Metaphase I b. Anaphase I c. Metaphase II d. Anaphase II
d. Anaphase II
Which of the following must happen for a 3:1 ratio in the F2 progeny of a monohybrid cross? a. Two alleles b. One allele dominant to other c. Independent assortment of loci d. a and b e. all of the above
d. a and b
Beginning with a monohybrid cross of two true-breeding parents (e.g. Round vs wrinkled strains of pea), what proportion of the F1 generation will be heterozygotes? a. 1/8 b. 1/4 c. 1/2 d. all
d. all
During mitosis, when do sister chromatids separate? a. prophase b. prometaphase c. metaphase d. anaphase e. telophase
d. anaphase
For a few genes among our thousands, expression of an allele is dependent on which parent it was inherited from. This effect is known as: a. cytoplasmic inheritance b. maternal effect c. anticipation d. imprinting.
d. imprinting.
In the mollusk Crepidula fornicata, sex is determined by whether the animal is on the top or the bottom of the pile; what kind of sex chromosomes are predicted in this snail? a. ZZ/ZW b. X0/XX c. XY/XX d. no sex chromosomes.
d. no sex chromosomes.
Coat color in Labrador retrievers is determined by a gene with 2 alleles where Black (B) is dominant to brown (b), and another gene with 2 alleles where Pigment deposition (E) is dominant to no pigment deposition (e), giving yellow coats. Crossing yellow labs (Bbee) with black labs (BbEe) is expected to produce what phenotypic ratio in the F1 progeny? a. all brown b. all black c. 9 yellow : 4 brown, : 3 black d. 9 black : 7 yellow e. 4 yellow: 3 black: 1 brown
e. 4 yellow: 3 black: 1 brown
In Labrador retrievers, the B locus determines coat color (B, black is dominant to b, brown) and E determines whether pigment is deposited in hair (E, yes, is dominant, e, no pigment, is recessive). If BBee dogs are crossed to bbee dogs, what phenotypic ratio do you predict in the F1? a. 9 yellow, 3 black, 4 brown b. all black c. 9 yellow, 4 black, 3 brown d. 9 black, 4 yellow, 3 brown e. all yellow
e. all yellow
Which of the following characteristics will most precisely distinguish prokaryotes from eukaryotes? a. presence of a cell wall b. size of the genome c. nuclear membrane d. size of the cell
nuclear membrane
Novel phenotypes
observed when the interaction between two loci yields new phenotypes and modified Mendelian dihybrid ratios i.e. peppers - p generation is red x cream. f1 generation- red f2 generation - red, peach, orange, and cream.
In peas, Yellow is dominant to green, and Round to wrinkled. In a dihybrid cross RRyy x rrYY, the F2 is composed of 9 round yellow, 3 round green, 3 wrinkled yellow, and 1 wrinkled green. What is the recombination frequency? a. 1/16 b. 3/16 c. 6/16 d. 8/16
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