genetics exam 2

barr body

• inactive X chromosomes

  • highly condensed

  • can be observed in stained interphase cells

• arise from random inactivation of either maternal or paternal chromosome

• occur early in embryonic development

  • all cellular descendants have same inactivated chromosome

hermaphrodites

• 2 phenotypes in C. elegans

  • have both testes and ovaries (bisexual)

  • have 2 X chromosomes and no Y

  • eggs fertilized by stored sperm

    • self fertilization

  • majority of offspring are x

    • or males w testes from only 1 X and no Y

      • less than 1%

primary sexual differentiation

• involves only gonads where gametes are produced

secondary sexual differentiation

• involves overall appearance of the organism

• clear differentiation in organs like mammary glands and external genitalia

unisexual

• plants and animals that contain only male or female reproductive organs

• only 1

bisexual

• plants and animals that contain both male and female reproductive organs

• can produce both male and female gametes

differentiation in different tissues

• in maize, diploid sporophyte stage predominates

  • both male and female structures are present on adult plant

• indicates sex determination must occur differently in different tissues of the same plant

xy system

• mammals and some insects

Xo system

some insects

zw system

• reptiles

• birds

• some amphibians and insects

no special sex chromosomes

• plants

• fungi

genetic control of sex determination

• in some organisms it is genetically determined and regulated

• mutant genes can cause sex reversal

  • homozygous, mutations in tassel seed interfere w production and induce production of female structures instead

  • recessive mutations in silkless and barren stalk interfere w development of pistil

    • plants w only male-functioning repro organs

XX/XY mode

• sex determination in lygaeus

• female gam all have an X chromosome

• male gam have either X or Y

• zygotes w 2 X’s result in female

• w/ 1 X and 1 Y result in male

  • 6A + X vs 6A + Y from male

• 1:1 sex ratio

XX/XO mode

• sex determination in protenor

• depends on random distribution of X chromosome into half of male gam

• presence of 2 X in zygote results in female

• presence of only 1 X results in male

  • 6A vs 6A + X from male

• 1:1 sex ratio

Z/W system

• males not always heterogametic sex

  • in some organisms females are

• ex: chickens

  • females are ZW and males are ZZ

mode in drosophilia

• Y chromosome doesn’t determine sex

  • Y needed for fertility

• determined by ratio of X chromosomes to haploid sets of autosomes (A)

• normal female: XX and AA (1:1 or 1.0)

• normal male: XY and AA (1:2 or 0.5)

• X in male is upregulated so transcription lvls = that of XX female

genic balance theory

• w/ respect to primary sex determination

• threshold of maleness is reached when X:A ratio is 1:2 (X:2A)

• presence of additional X (XX:2A) alters balance and results in female differentiation

temperature-dependent

• environment, specifically temp, has profound influence on sex determination

• achieved according to incubation temp of eggs during critical period of embryonic development

  • for all crocodiles, most turtles, and some lizards

  • case 1: low temp = 100% female, high temp = 100% male

  • case 2: opposite

  • case 3: low and high = 100% female, intermediate = various proportions of males

mechanism underlying TSD

• temp affects expression/activity of enzyme, aromatase

  • converts T into estradiol

mode in humans

• females have 2 X

• males have X and Y

• during early embryonic develop, embryo is hermaphroditic

  • gonadal phenotype is sexually indifferent (look same and have potential to form ovaries or testes)

    • triggered by presence of absence of Y

human Y

• SRY: sex-determining region

• MSY: male-specific region

• has at least 75 genes

• PARS on both ends

pseudoautosomal regions

• present on both ends of Y chromosome

• share homology w regions on X

• synapse and recombing w X during mitosis

• presence is critical to segregation of X and Y during male gametogenesis

male-specific region of the Y

• MSY

• nonrecombining region of Y chromosome

• some portions of it share homology w X chromosome

• has euchromatic (functional gene) and heterochromatic (nonfunctioning) regions

sex-determining region Y

• SRY

• located adjacent to the PAR of short arm of Y chromosome

• genes on it become active in XY embryos at 6-8 wks

  • SRY/TDF either expressed or not to form testis/ovaries

testis-determining factor

• TDF

• determines maleness; master switch controlling genes involved in differentiation

  • males w 2 X and no Y, SRY on X

    • translocation from Y to X

  • females w 1 X and Y missing SRY

    • deletion of part of Y

• ptn encoded by a gene in SRY that triggers testes formation

  • located within SRY

• present in all mammals

• gonads develop into testis if SRY/TDF expressed

klinefelter syndrome

• male w extra Xs; phenotypically male

• XXY; 47,XXY; 48,XXXY; 49,XXXXY karyotypes

• tall stature w long arms & legs

• underdeveloped testes and prostate gland

• infertile

• slight breast enlargement, hips rounded, no facial hair

• normal intelligence but may be slow learner

turner syndrome

• XO

• 1/2000 females

• 45, XO karyotype

• female external genitalia and internal ducts

• ovaries are rudimentary

• short stature, underdeveloped breasts, skin flaps on back of neck

• normal intelligence, may have learning disabilities

XXX syndrome

• abnormal presence of 3 X’s

• normal set of autosomes (47,XXX)

• results in female differentiation

• typically normal and unaware

• or underdeveloped secondary sex characteristics, sterility, and mental retardation

• extra X can disrupt normal female development

supermales

• 47, XYY

lyon hypothesis

• explains dosage compensation

• inactivation of X; random choice in each cell of which X is

• occurs in somatic cells at early embryonic stage

  • then passed on to progeny cells by mitosis

• ex: calico cats

  • x-linked; alleles in diff areas of body

  • genetic mosaics

• ex: lack of sweat glands in human females

dosage compensation

• prevents excessive expression of X-linked genes in humans & other mammals

X-inactivation

• N - 1 rule; N = total # of X

• all but one are inactivated

  • regardless of how many a somatic cell posseses

• none in Turner (XO)

• 1 barr body in Klinefelter’s (XXY)

• 2 barr bodies in 47,XXX

• syndromes not normal possibly bc:

  • inactivation does not take place in early stages of development of gonadal tissue

  • not all X inactivated

imprinting

• expression of genes on one homolog but not the other

• chemically modified DNA that gets inactivated, silencing genes

  • creates memory that keeps same homolog inactivated following chromosome replications and cell division

androgen insensitivity

• XY karyotype

• mutation in gene on X that codes for testosterone receptor

  • bodies can’t respond to male hormones (androgens)

• phenotype

  • externally normal female w sparse or absent body hair

  • internally: blind vagina, no uterus, testis present in abdominal cavity

aneuploidy

• variations in chromosome #

  • mistakes in mitosis or meiosis

• when organism gains or loses 1 or more chromosomes and has other than an exact multiple of haploid set

• 2n +- x chromosomes

euploidy

• complete haploid sets of chromosomes are present

• multiples of n

polyploidy

• occurs when more than 2 sets of chromosomes are

• 3n, 4n, 5n, …

• v rare in humans

• occurs in amphibians and lower animals

• common in plants and important for speciation

monosomy

• zygote w only 1 copy of a chromosome

  • 2n - 1

• result of nondisjunction

• for X chromosome occurs in humans

• for any autosome is usually not tolerated in humans & other animals but

trisomy

• zygotes w 3 copies of 1 chromosome

  • 2n + 1

• result of nondisjunction

• more viable than loss

• in autosomes, have severe effects and usually lethal during development

• plants are viable but phenotype altered

haploinsufficiency

• single recessive gene is insufficient to provide life-sustaining function for the organism

• Result of monosomy in autosome (2N - 1)

autopolyploidy

• addition of one or more sets of chromosomes identical to haploid complement of same species

  • multiples of the same genome

• 2 sperms fertilize 1 egg

  • haploid gamete x diploid gamete; triploid

• 3 sperms fertilize 1 egg

  • diploid x diploid; tetraploid

allopolyploidy/amphidiploidy

• combination of chromosome sets from diff species as consequence of interspecific matings

  • hybridization of 2 closely related species

    • contains 4 haploid genomes derived from separate species

• diploid x diploid; tetraploid

chromosomal mutation/abberation

• change in total # of chromosomes

• deletion or duplication of genes or segments of a chromosome

• rearrangements of genetic material w/in or among chromosomes

• can result in form of phenotypic variation or be lethal

nondisjunction

• chromosomes or chromatids fail to disjoin and move to opp poles during meiosis 1 or 2

• usually occur to 1 chromosome

• disrupts normal distrib of chromosomes into gametes

  • results in trisomy or monosomy

XYY condition

• duplication of Y and nondisjunction originates in male

  • X + YY

• occurs in meiosis 2 in Y-bearing cell

  • get sperm w 2 Y (from 2 sister chromatids) and sperm w/o any sex chromosome

    • at fertilization get: XYY or XO

nondisjunction in males in meiosis 1

• sperm w X and Y chromosome

• sperm w no sex chromosome

• at fertilization

  • XXY or XO

down syndrome

• trisomy 21 caused by random error

  • not expected to be inherited

• affected express 6-8 characteristics:

  • prominent epicanthic fold in each eye

  • flat face, round head, short stature w protruding tongue

  • short, broad hands

  • mental retardation

  • avg life span 50 yrs

• 75% occur during meiosis 1

• can come from maternal or paternal but inc w mate

down syndrome critical region

• critical region of chromosome 21 contains genes that are dosage sensitive in this trisomy

• responsible for many of the phenotypes

familial down syndrome

• runs in families

• translocation of bottom part of chromosome 21 onto another chromosome

• one parent contains 14/21, D/G translocation

  • phenotypically normal but has only 45 chromosomes

  • normal chromosome 21 and other translocated to 14

    • fertilization results in embryo w 46 but 3 of 21

restore fertility of sterile hybrids

• natural or induced chromosomal doubling

• produces fertile amphidiploids

colchicine

• disaggregates microtubules, causing nondisjunction of whole set of chromosomes

  • cell fails to divide and instead enters interphase; chromosome # is duplicated

• applied to somatic cells undergoing mitosis

polyploidy cell size

• often larger than cells of diploid relatives

  • ex: flowers and fruits

• as polyploidy inc, gene expression either inc or dec tenfold

• 2 genes that encode G1 cyclins are repressed when ploidy inc

  • cell stays in G1 phase longer and grows to a larger size

reciprocal translocation

• 2 non-homologous chromosomes trade or swap segments w each other in 2 way exchange

• structural chromosome abnormality

• has unsual synapsis config during meiosis

• results in rearrangement of genetic material

non-reciprocal translocation

• single chromosomal segment breaks off and transfers to a non-homologous chromsome in one-way shift

  • no corresponding piece returned

• structural chromosome abnormality

cri du chat syndrome

• deletion of part of chromosome 5

• developmental disorder

duplication

• arise thru unequal crossing over between synapsed chromosomes during meiosis

• or arise thru replication error prior to meiosis

• can change dosage and gene expression, phenotype

rRNA genes

• multiple copies of as it is needed to support ptn synth

• some cells have more copies than others in same individual

  • normal amplification is insufficient

nucleolar organizer region

• single area of chromosome in amphibian where 400 copies of rDNA are present per haploid

• selectively replicated to further inc rDNA copies

• ex of gene amplification

gene duplication

• provide extra copies of gene

  • dup can acquire mutational changes over extended periods w/o hindering fitness of organism

• can evolve into new genes

  • new function can impart adaptive advantage to organisms

inversions

• change linear order of information on chromosome

• crossing over in x heterozygotes can change # of centromeres chromosome has

• x heterzygotes exhibit crossing over less freq in region w x

translocation

• movement of a chromosomal segment to a new location in the genome

alternate segregation

• regarding chromosome organization during meiosis for individuals hetero for reciprocal translocations

• leads to normal and balanced gamete

adjacent segregation

• regarding chromosome organization during meiosis for individuals hetero for reciprocal translocations

• leads to gametes containing duplications and deficiencies

• results in semisterility

  • partial trisomy or monosomy leading to birth defects

  • big impact on repro fitness/evolution

robertsonian translocation

• breaks at extreme ends of short arms of 2 nonhomologous acrocentric chromosomes

• small segments lost, larger fuse at acrocentric

• ex: familial down syndrome

fragile sites

• more susceptible to chromosome breakage when cells are cultured in absence of certain chems

  • ex: folic acid

• could be regions along chromosome where chromatin is not tightly coiled

• strong association btwn x and mental retardation

• link btwn autosomal x and lung cancer

  • FHIT gene on x found altered or missing

linkage

• 2 genes on a single pair of homologs

• no exchange/recombination occurs

  • 2 types of gametes

• exchange occurs between 2 nonsister chromatids

  • get 4 types of gametes

parental/nonrecombinant gamete

• chromatids that don’t cross over

recombinant gamete

• chromatids that cross over

linkage ratio

• affected by genetic distance

• no linkage

  • 4 gametes; 1:1:1:1

    • AB, Ab, aB, ab

• linkage w/o recombination

  • 2:0:2:0

    • AB and ab

• linkage w recombination

  • ratio btwn 1:1:1:1 to 2:0:2:0

single crossover

• recombination is reciprocal

  • equal proportion of genetically different gametes w recombinant chromosomes

    • 0-25% each

  • altogether recombinant gametes can’t be more than 50% of total

• 4 types of gametes

genetic map

• likelihood that 2 genetically linked traits will be separated by a crossover is dependent upon how far apart they are

  • if close together on chromosome, less likely to have a chiasma and be separated

  • if far apart, more likely for alleles to be recombined

• linked genes exist in linear order along chromosome, basis for x

restriction fragment length polymorphism

• RFLP

• example of DNA markers

• several million help give geneticists ability to identify and locate related genes

mendel

• experiments would not have been as informative if he picked traits controlled by genes close together on same chromosomes

complete linkage

• 4 gametes still made

• only 2 types of gametes

• each makes up 50% of total

test cross for linkage

• F1: double heterozygous x double homozygous

• chi square analysis

test cross for linkage on sex chromosomes

• F1: heterozygous female x hemizygous male

• Female: yw y+w+ x Male: y w

map unit

• defined as 1% recombination between 2 genes on a chromosome

• also called centiMorgan (cM)

  • 1 cM = 1% recombination

• supports chromosomal theory that chromosomes contain genes in linear order and are equivalent of mendel’s unit factors

double crossover

• occur less frequently

• 2 single crossovers

• use product law

three point cross

• genetic method to determine gene order

• parent producing crossover gametes must be heterozygous at all loci

  • y w ec

  • y+w+ec+

• other parent should be homozygous (recessive)

  • y w ec

genetic cross only estimation of distance

• as distance btwn 2 genes increases, mapping estimates become less accurate

• recombination underestimates the distance

  • farther apart, greater underestimate

• some areas on chromosomes are hot or cold spots for recombination

• farther apart 2 genes, greater prob of undetected crossovers

  • degree of inaccuracy increases

  • most accurate maps from closely linked genes

interference

• numerical indication of observed rate of DCO compared to expected

• reduces expected # of multiple crossovers when crossover event in one region of chromosome inhibits a second event nearby

• can be quantified using observed DCOs

  • I = 1 - C

• complete when no DCOs occur

• positive if fewer DCOs than expected

  • when 2 genes close together

• negative if more DCOs than expected

coefficient of coincidence

• observed # of DCOs divided by expected # of DCOs

dna markers

• short segments of DNA whose sequence and location are known

• useful landmarks for mapping purposes

conjugation

• transfer of DNA between 2 bacterial cells

  • requires physical contact

    • U-tube experiment (no exchange bc cells can’t pass)

• unidirectional and only some strains can be donor, F+

  • receiving cells, F-

  • occurs in 1×10^-7 cells

• double dna nicked, complement synthesized on both single strands, moves across tube to F-, ligase closes circles

transformation

• introduction of free-form DNA into bacterial cells

  • done in labs to get transgenic animals

• genes close enough to each other to be cotransformed are linked

• extracellular dna binds to cell at receptor site

• dna enters cell adn strands separate

  • 1 strand degraded,1 pairs homologously w host dna

• after, dna recombines w chromosome

  • creates heteroduplex

• 1 round of cell division produces transformed and nontransformed

transduction

• introduction of viral DNA into bacterial cells

  • ex: genetic recomb in bacteria via bacteriophages

• no cell to cell contact (U-tube experiment)

• 
  

minimal medium

• salts

• NH₄⁺

• glucose

complete medium

• minimal medium

• yeast extract/ amino acids

prototrophs

• wild type

• strains that will grow on minimal medium

auxotrophs

• mutant strains that can only grow on minimal medium if it is supplemented w some biochemical

  • complete medium

F+ factor

• donor

• not lost in x strain

• copied into recipient cells

plasmid

• composed of double stranded closed circle of DNA

• exist in multiple copies in cytoplasm

• may contain one or more genes

• use same rep enzymes as host

• distributed to daughter cells

• rep independently of bacterial chromosome

bacterial genome

• haploid DNA

  • no homologous chromosome

  • no masking of phenotype

• no mitosis or meiosis

• chromosome dna is circular

• plasmids—extrachromosomal DNA

• genetic exchanges occur

resistance mutants

• will grow on media containing an antibiotic

• ex: str^R resistant to streptomycin

genetic exchange in prokaryotes

• involves DNA molecule from external source

• replacement of chromosomal info by recombination

• how closely linked 2 genes influences likelihood:

  • both will be transferred to host simultaneously

  • one recombination even will include both genes

F factor plasmids

• confer fertility and contain genes for sex pilus formation on which genetic recombination depends

Hfr strain

• high freq recombination (1×10^-4)

• special types of DNA donors

• F+ plasmid integrated into chromosome

  • becomes part of chromosomal DNA

• genes transferred to F- cells

• conjugation btwn x and F-

  • leads to transfer of genes on the chromosome by recomb

  • rarely complete

  • F+ DNA not transferred

  • F- cells don’t become F+

interrupted mating technique

• used to map genes in E. coli

• conjugation interrupted

  • recipients w various number of bacterial genes

• number of transferred genes inc w amt of time of conjugation

  • transferred replace genes in chromosome by homologous recomb

• genes close to F+ first

F’ DNA

• can shuttle in and out of bacterial chromosome thru recomb

• piece of chromosome may be transferred along w F plasmid

• deleted from one strain and inserted into another

Rec proteins

• essential to bacterial recomb

• mutants deficient in x don’t undergo recomb