genetics 1

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  1. Important aspects of eukaryotic cells
    • Contains nucleus surrounded by nuclear envelope that separates DNA from other components
    • DNA is closely associated with histones to form tightly packaged chromosomes( DNA is negatively charged and histones are positively charged)
  2. Chromatin
    Complex of DNA bound with histone proteins
  3. Conundrum
    Chromatin allows the DNA to fit inside the nucleus but makes it inaccessible to enzymes needed for molecular processes
  4. Viruses
    • Outer protein coat surrounding nucleic acid
    • Have close relationship with the genes of their host, which makes them useful for studying the genetics/gene regulation of the host organism
    • Not all viruses have DNA
    • Because they need to use the cellular machinery of the host, they need similar genetic info as the host
  5. Usefulness of bacteria and viruses
    • Growth is easy and they require little space
    • Rapid reproduction and large numbers of progeny
    • Haploid genomes allow for direct phenotypic expression/copy of genes
    • Techniques for isolating and manipulating genes were developed in bacteria
  6. Prokaryotic replication
    • Reproduce by replicating their circular chromosome
    • Begins at the origin of replication, a new cell wall forms between the copies and then the cell divides=2 identical cells
  7. Karyotype, Homologous pair, Allele
    • Complete set of chromosomes in metaphase I
    • Two chromosomes that are alike in structure and size and carry genetic information for the same set of hereditary characteristics
    • Alternative form of a gene found on homologous chromosomes
  8. Replication in eukaryotes
    • homologous pairs
    • 23 pairs, including sex chromosomes
    • reproduce by sexual reproduction
  9. Diploid, Haploid
    • cells that carry two copies of each gene
    • cells that carry one copy of each gene(reproductive cells=sperm, egg, spores)
  10. A functional eukaryotic chromosome has three main elements
    • Centromere-spindle microtubules attach here, kinetochore assembles on here, allows for chromosome movement during cell division
    • Pair of Telomeres-ends of linear chromosomes, stabilize ends, plays a role in limiting division and aging and cancer
    • Origins of replication-sites where DNA synthesis begins, lots of these, depends on chromosome length
  11. Telomeres and Kinetochores
    • Protein binds on kinetochore
    • Problems occur when telomeres aren't full replicated
    • Shortening of telomeres=aging
    • If kinetochores are damaged then microtubules can't attach=not proper cell division, wouldn't go past checkpoint
  12. Sister chromatids&why two copies?
    • Copies of a chromosome held together at the centromere
    • In preparation for cell division, each chromosome replicates(s phase), making a copy of itself.
  13. MSAT
    • metacentric
    • submetacentric
    • acrocentric
    • telocentric
  14. Eukaryotic cell cycle
    Consists of interphase and M-phase
  15. Interphase
    • extended period of growth&development between cell divisions
    • Lots of activity=DNA is being synthesized, RNA and proteins being produced, biochemical reactions taking place
    • Includes checkpoints that regulate the cell cycle by allowing or prohibiting cell division(necessary to prevent cells with damaged or missing chromosomes from proliferating)
  16. Three phases of Interphase
    • G1=gap 1, cell grows and all proteins needed for cell division are synthesized
    • G1/S checkpoint=holds cell in G1 until all enzymes needed for DNA replication have been made, before reaching this the cell may exit the active cell cycle and enter into G0(stable inactive state)
    • S=DNA synthesis, chromosomes duplicate
    • G2=gap 2, additional necessary biochemical events occur
    • G2/M checkpoint=passed only if cell's DNA is undamaged
  17. M phase
    • Sister chromatids separate and the cell undergoes division
    • PPMATC
    • Prophase-chromosomes condense and mitotic spindle forms
    • Prometaphase-nuclear envelope disintegrates, spindle microtubules anchor to kinetochores
    • Metaphase-chromosomes align
    • Anaphase-sister chromatids separate
    • Telophase-chromosomes are poles, nuclear envelope reforms
    • Cytokinesis-cytoplasm divides
  18. What would happen if the kinetochore was absent/damaged or the spindle microtubules did not properly attach to the kinetochore?
    The chromosomes would not separate properly and would most likely result in unequal separation-nondisjunction
  19. Mitosis vs. Meiosis
    • Mitosis=somatic non-differentiated cells, development&tissue repair, chromosome number is maintained. ONE division
    • &daughter cells are identical
    • Meiosis=germ cells established in reproductive organs, gamete production, chromosome number is reduced to 1/2&daughter cells are different. TWO divisions
  20. Meiosis I
    • Prophase=chromosomes condense, homologous chromosomes synapse, crossing over takes place
    • Prometaphase=nuclear envelope breaks down, mitotic spindle forms
    • Metaphase=homologous pairs line up on metaphase plate
    • Anaphase=two homologous pairs line up next to each other and separate and move to opposite poles
    • Telophase=chromosomes at spindle poles
    • Cytokinesis=cytoplasm divides to produce two cells(each with half the original number of chromosomes)
    • Interkinesis(some cells)=spindle breaks down, chromosomes relax, nuclear envelope reforms
  21. Meiosis II
    • Prophase=chromosomes condense
    • Prometaphase=spindle forms and nuclear envelopes disintegrates
    • Metaphase=Individual chromosomes line up on metaphase plate
    • Anaphase=Sister chromatids separate and move toward spindle poles
    • Telophase=Chromosomes arrive at spindle poles, spindle breaks down, nuclear envelope reforms
    • Cytokinesis=cytoplasm divides.
  22. Crossing over
    • Exchange of genes between non sister chromatids
    • Shuffles alleles on the same chromosome into new combinations
    • Basis for recombination that creates new combinations of alleles on a chromatid
    • Genetic variation
  23. Random distribution of chromosomes
    • Occurs in anaphase I after random alignment in metaphase I
    • Genetic variation
    • Maternal and paternal chromosomes
    • Shuffles alleles on different chromosomes into new combinations
    • Many different combinations of chromosomes in the gametes are possible depending on how they align and separate
  24. Spermatogenesis
    • Spermatogonium(2n)
    • Enter prophase I
    • Becomes primary spermatocyte(2n)
    • Completes meiosis I
    • Produces 2 secondary spermatocytes(n)
    • Completes meiosis II
    • Each spermatocyte produces 2 spermatids(n) that mature into sperm
  25. Oogenesis
    • Oogonium(2n)
    • Enter prophase I
    • Becomes primary oocyte(2n) (before birth)
    • Completes meiosis I. Ovulation occurs
    • Secondary oocyte is produced(n) with first polar body
    • Completes meiosis II(occurs if egg comes into contact with sperm)
    • Ovum is produced(n) with second polar body
  26. Meiosis in the female
    • Oogenesis produces a single mature gamete
    • During oogenesis, cytokinesis is unequal
    • Formation of sperm is continuous in males
    • Formation of female gametes is discontinuous
    • Oogenesis begins before birth in prenatal development(meiosis is arrested and stalled in prophase I, a female is born with primary oocytes arrested in prophase I)
  27. Meiosis in female animals
    • Before ovulation, rising hormone levels stimulate one or more primary oocytes to recommence meiosis
    • First division of meiosis is completed and a secondary oocyte is ovulated from the ovary
    • When the outer layer of the secondary oocyte is penetrated, second meiotic division takes place, nuclei fuses, and zygote is formed.
  28. Mendel's experiments
    • Demonstrated careful and purposeful experimental methodology
    • Used things that were easy to crossbreed
    • Used true breeding
    • Followed seven characters, each represented by two contrasting, discontinuous traits(distinct)
    • Kept accurate quantitative records
    • Subjected his findings to statistical analysis
    • Exemplary scientific methods and technique
  29. Seven characteristics of pea plants
    • no display of variation
    • each represented by two contrasted, discontinuous traits
    • seed color, shape, coat color, pod color, pod shape, flower position, and stem length
  30. Gene, Locus, Genotype, Heterozygote, Homozygote, Phenotype/trait, Character
    • Gene=genetic factor(region of DNA) that helps determine a characteristic
    • Locus=specific place on a chromosome occupied by an allele
    • Genotype=set of alleles that an individual organism possesses
    • Heterozygote=an individual organism possessing two diff. alleles at a locus
    • Homozygote=an individual organism possessing two of the same alleles at a locus
    • Phenotype=the appearance of a character
    • Character=an attribute or feature
  31. Mendel's monohybrid crosses
    • Prohibited self-fertilization
    • Each plant was genetically pure
    • Crosses between parents that differed by a single characteristic, round&wrinkled seeds
    • First generation=P, offspring=F1
    • Allowed F1 to self fertilize and produced F2.
    • Both traits emerged, concluded that traits don't blend
  32. What did the monohybrid crosses reveal?
    • Each plant must possess two genetic factors encoding a characteristic (2 alleles)
    • The two alleles in each plant separate when gametes are formed, only one goes into each gamete.
    • Dominance-Only one trait is observed in the heterozygous progeny
    • Two alleles of an individual plant separate with equal probability into the gametes
  33. Dominant&recessive
    • Dominant=traits that appeared unchanged in the heterozygous offspring
    • Recessive=traits that disappeared in the heterozygous offspring
    • When present together, the recessive allele is masked by the dominant allele.
  34. Principle of segregation
    • Each individual diploid organism possesses two alleles for any particular characteristic, the two alleles segregate into gametes in equal proportions
    • Confirmed this principle by allowing the F2 plants to self-fertilize and produce an F3 generation then went through more rounds of self-fertilization
    • 2/3 produced round&wrinkled
    • 1/3 produced only round offspring
  35. The concept of dominance
    • When two different alleles are present in a genotype, only the trait encoded by one of them is observed in the phenotype.
    • Confirmed this by allowing the F2 to self fertilize and produce an F3 generation, then went through more rounds of self-fertilization
    • 2/3 produced round&wrinkled
    • 1/3 produced only round offspring
  36. Punnett square
    • determines the results of a genetic cross
    • tested the theory of dominant traits by crossing a F1 heterozygous tall plant with a parental homozygous short plant
    • this is called a backcross, F1 x P
    • Each cell contains an allele from each of the gametes that generates the genotype of the progeny
  37. Probability in genetics
    • A tool used to express the likelihood of the occurrence of a particular event
    • # of times an event occurs divided by the number of all possible outcomes
    • The probability of an event can be determined by knowing something about how the event occurs or how often it occurs
  38. Independent&mutually exclusive
    • Independent=outcome of one event must not influence the outcome of another(multiplication rule)
    • Mutually exclusive=one event excludes the possibility of the occurrence of the other event(addition rule)
  39. Albinism
    • Recessive
    • Causes reduced pigmentation in the skin, eyes, and hair
  40. Binomial expansion
    • Useful when trying to figure out different combinations of children and their porbabilities
    • Use if we want to know the probability of having five children with albinism and three with normal pigmentation
    • Equation=(p+q)n
    • P=probability of one event
    • Q=probability of another event
    • n=number of times this event occurs
  41. Incomplete dominance
    • An additional phenotypic ratio obtained when dominance is lacking
    • When a trait displays this, the genotypic and phenotypic ratios are the same because each genotype has its own phenotype
  42. Genetic symbols
    • superscripts and subscripts are sometimes added to distinguish between genes, represents dominant alleles at different loci
    • a slash is used to distinguish alleles present in an individual genotype, EL / ES
  43. Independent assortment
    • Alleles at different loci assort independently of each other
    • dihybrid crosses revealed this
    • extension of the principle of segregation
  44. Dihybrid crosses
    • Two characteristics
    • Mendel always obtained a 9:3:3:1 ratio in the F2
    • Allele encoding color separated independently of the allele encoding seed shape (RY, ry, rY, Ry)
  45. Does independent assortment relate to meiosis?
    • principle of independent assortment applies to characters encoded by loci located on different chromosomes
    • based on behavior of chromosomes during meiosis
    • pairs of homologous chromosomes separates independently of all other pairs in anaphase I
    • genes located on the same chromosome do not sort independently
  46. Why use branch diagrams?
    • convenient way of organizing all the combinations of characteristics
    • can be used to determine phenotypic/genotypic ratios for any number of characteristics
    • can also be used to determine phenotypes and expected proportions of offspring from a dihybrid testcross
  47. Dihybrid testcross
    • useful genetic tool for analyzing genetic crosses
    • cross an individual or unknown genotype with an individual that is homozygous recessive for the genotype to reveal genotype of first individual
  48. Chi-square tests
    • evaluates the role of chance in producing deviations between observed and expected values
    • indicates the probability that the difference between
    • observed value and expected value are due to chance
    • used to determine the significance of the outcomes of scientific experiments or incidence of medical conditions
    • must be applied to numbers of progeny not proportions
  49. Probabilities of the chi square test
    • if the probability is high then we assume that chance alone produced the difference(null hypothesis is true)
    • if the probability is low then we assume that some other factor produced the difference(null hypothesis is false)
  50. Chi-sqaure equation
    • x2= sum(observed-expected)2/expected
    • ( chi-square value)+(chi-square value)=overall chi- square
    • degrees of freedom=number of ways in which the expected classes are free to vary(number of phenotypes), n-1
    • significant difference=some other factor than chance is responsible for the deviation
    • probability less than .05=chance is not repsonsible
  51. Extension to mendel's principles
    • The inheritance of characteristics encoded by genes located on the sex chromosomes
    • These characteristics and the genes that produce them are called sex-linked
    • X&Y chromosomes and genes play a role in sexual phenotypes
  52. Two processes that sexual reproduction consists of
    • alternation of haploid and diploid cells
    • meiosis produces haploid gametes
    • fertilization produces diploid
    • the term sex refers to sexual phenotype
    • parents contribute genes to produce an offspring that is genetically distinct from both parents
  53. chromosomes and sex
    • x&y chromosomes separate into different cells in sperm formation
    • x chromosomes separate into different cells in egg formation
    • sex is inherited like other genetically determined characteristics
    • the sperm always determines the sex of an unborn child
    • sex is determined by sex chromosomes
    • individual genes are responsible for sexual phenotypes also
  54. autosomes
    • non-sex chromosomes
    • same for male and female
  55. grasshoppers
    • females are xx and males are x
    • for females, the 2x separates
    • for males, 1 gamete with x, the other gamete with no x
    • males are heterogametic
    • females are homogametic
  56. heterogametic&homogametic
    • hetero=two difference types of gametes with respect to the x chromosome
    • homo=gametes are all the same with respect to the x chromosome
  57. human sex determination
    • y chromosome is acrocentric
    • x and y can pair during meiosis because they have small pseudoautosomal regions that carry the same genes on the telomeres (only homologous there)
    • males and females have different gamete size because the egg supplies more genetic information, it is a source of nutrients, RNAs, and proteins that aid in early development
  58. ZZ-ZW sex determination
    • birds, snakes, butterflies, fishes, and some amphibians
    • females are ZW(heterogametic) and males are ZZ(homogametic)
  59. Haplodiploidy
    • no sex chromosomes, depends on the number of chromosome sets found in the nucleus of each cell(n or 2n)
    • females are diploid and males are haploid
    • males produce sperm by mitosis because they are haploid
    • females produce eggs by meiosis
    • females develop from fertilized eggs(genetic info from mom&dad)
    • males develop from unfertilized eggs(genetic info only from mom)
  60. genic sex determining systems
    • some plants and protozoans
    • no obvious differences in chromosomes of males&females
    • organisms in which the genotypes of one or more loci determine the sex of an individual
    • sex is determined by individual genes in genic and chromosomal sex determination
  61. environmental sex determination
    • sex is determined fully or in part by environmental factors
    • example is marine mollusk, sequential hermaphroditism(can be both male and female but not at the same time)
  62. fruit flies
    • fruit flies have 8 chromosomes, 3 pairs of autosomes and one pair of sex chromosomes
    • females have 2x and males have 1x1y, but y does not determine maleness
    • sex is determined by the balance between the genes on the autosomes and the genes on the x chromosome-genic balance system
    • x=female producing effects
    • autosomes=male producing effects
  63. sex determination in fruit flies
    • determined by an x:a ratio, the number of x divided by the number of haploid sets of autosomal chromosomes
    • ratio of 1.0=female
    • ratio of 0.5=male
    • <0.5=weak&sterile meta males
    • between 0.5&1.0=intersex
    • >1.0=developmental problems, metafemales
  64. SRY gene
    • presence of the SRY gene on the y chromosome determines maleness
    • the primary determinant of maleness but other genes play a role in fertility and development of sex differences
    • XX males with no Y chromosome=small portion of y attached to another chromosome
    • phenotypes that result from the abnormal numbers of sex chromosomes illustrate the importance of the y chromosome in human sex determination
  65. turner syndrome
    • 45, X
    • under-developed female sex characteristics
    • 1 in 3000 female births
    • short stature, low hairline, webbed neck, normal intelligence, most are sterile(2x chromosomes are important in fertility)
  66. klinefelter syndrome
    • 47, XXY
    • under-developed male characteristics
    • 1 in 1000 male births
    • small testes, reduced facial and pubic hair, taller than normal, sterile, and often normal intelligence
  67. poly-x syndrome
    • 47, XXX
    • triplo-x syndrome
    • 1 in 1000 female births
    • tall and thin, few are sterile but most menstruate regularly, mental retardation is slightly greater
    • 48, XXXX or 49, XXXXX=very rare, normal female anatomy but cognitively impaired and have physical problems
  68. Role of sex chromosomes
    • x chromosome contains genetic info essential for both sexes
    • male determining gene is located on the y chromosome
    • absence of y results in a female phenotype
    • genes affecting fertility are on the x&y chromosomes
    • additional copies of x may upset normal development in males and females
  69. Testosterone
    • early in development, all humans have undifferentiated gonads and male and female reproductive traits
    • 6 weeks after fertilization, a gene on the y becomes active
    • neutral gonads develop testes and secrete testosterone and mullerian inhibiting substance
    • testosterone induces the development of male characteristics
    • mullerian inhibiting substance=causes degeneration of the female reproductive tract
    • In absence of SRY, neutral gonads become ovaries&female features develop
  70. Androgen insensitivity syndrome
    • external female sexual characteristics&psychological orientation-cells contain x&y chromosome
    • unaware of condition until fail to reach menarche
    • vagina ends blindly, no ovaries, uterus, oviducts, has testes that produces normal levels of testosterone
    • for testosterone to have an effect, must bind to androgen receptor, receptor is defective in females
  71. What AIS proves
    • human sexual development is influenced by more than the SRY gene
    • most people carry genes for both female and male characteristics-those with AIS have the capacity to have female characteristics even though they have a y chromosome
    • genes for male and female secondary sex characteristics are present on autosomes, not sex chromosomes
    • key to fe/maleness lies in the control of gene expression
  72. sex linked characteristics and genes on x&y
    • sex linked characteristics determined by genes on sex chromosomes
    • x&y linked characteristics are due to the expression of genes located on the x/y chromosome
    • y chromosome has little genetic info, most sex linked characteristics are x linked
  73. white eyes in drosophila
    • thomas morgan
    • pure red eyed x white eyed=white eyes are a recessive trait
    • F1 x F1=lacked white eyed females, not a recessive trait. suggested that the locus affecting eye color was on the X
    • white eyed x red eyed male=all females had red eyes, all males had white. white eyes are x-linked
    • hypothesized that white eyed females possessed 2X and 1
  74. Non-disjunction in drosophila
    • most of the time 1X and 1Y go into one gamete and other X into another gamete
    • nondisjunction occurs when 2X chromosomes do not separate and go into the same gamete
    • nondisjunction=failure of homologous chromosomes/sister chromatids to separate in mitosis or meiosis
    • white eyed females in F1 result from nondisjunction of the X chromosome in an XXY female
  75. association between genotype and chromosomes
    • gave evidence that sex-linked genes are located on the X chromosome
    • confirmed chromosome theory of inheritance
  76. X linked color blindness
    • blue on chromosome 7 and red&green close together on x chromosome. encoded by different loci
    • most common=red&green pigments. recessive, x-linked
    • females are only color blind when they inherit color blind alleles from both parents
    • males are color blind if they inherit a color blind allele from their mother=more common in males
  77. Dosage compensation
    • females have two copies of every x linked gene and males only have one copy=amount of protein produced from x-linked genes differ between sexes
    • protein conc. plays a role in development but is overcome by dosage compensation=amount of protein produced by x linked genes is equalized
    • humans inactivate genes on one of the X's, other animals double male activity
  78. Barr bodies
    • condensed, dark, stained bodies in nuclei of cells from femal cats
    • placental mammals inactivate genes on one or more of the x chromosomes via RNA called xist that binds the DNA on the x chromosome to be inactivated and alters chromatin structure
  79. lyon hypothesis
    • barr body was an inactivated x chromosome
    • in females, one of the two x chromosomes become inactive-random
    • if a cell contains more than 2x chromosomes then all but one if inactivated
  80. tortoise shell cats
    • x inactivation causes patchy distribution, a single x linked locus determines the presence of orange or non-orange
    • males are hemizygous&can be black or orange but not both
    • females can be black, orange, or tortoiseshell
    • each orange patch is a clone of cells derived from a cell in which the black/orange allele was inactivated
  81. x-inactivation
    • because of x inactivation, females are functionally hemizygous(one copy) at the cellular level for x linked genes
    • in females that are heterozygous at an x-linked locus, 50% of the cells will express one allele and 50% will express the other allele=proteins encoded by both alleles are produced
    • after an x becomes inactive, it remains inactive and is inactive in all somatic cells it produces, neighboring cells have the same x inactivated which produces a patchy pattern=mosaic
    • occurs early in development
  82. y-linked characteristics
    • exhibit a distinct pattern of inheritance&only present in males
    • 2/3 of y is heterochromatin with no active genes& 1/3 is euchromatin with few genes present
    • y linked genes are poorly understood
    • much of the chromosome is nonfunctional so mutations accumulate, can be used to track paternity
    • males possessing same set of mutations are often related
  83. Coat color and lethal dominance
    • coat colors of mice followed same patterns of inheritance that mendel observed in pea plants, but was never able to obtain a true breeding homozygous yellow mouse
    • yellow allele is a recessive lethal during development but dominant for coat color
    • proved that mendel's principles alone are not sufficient to explain inheritance of all genetic characteristics
  84. incomplete dominance and codominance
    • dominance is not universal&does not affect inheritance, just phenotype
    • the heterozygote has an intermediate phenotype between the two homozygotes(range of colors)
    • the phenotype of the heterozygote expresses the phenotypes of both homozygotes(example=MN locus encoded one type of antigen on rbc)
  85. penetrance
    • incomplete penetrance=the genotype does not always produce the expected phenotype(polydactyly)
    • penetrance=the percentage of individuals having a particular genotype that express the expected phenotype
    • caused by dominant allele
    • can be fully penetrant or not fully penetrant
  86. expressivity
    • the degree to which a character is expressed
    • incomplete penetrance and variable expressivity are due to the effects of other genes and to environmental factors that can alter or completely suppress the effect of a particular gene
  87. control of gene expression
    • the presence of a gene does not guarantee its expression
    • every cell has the same dna inside
    • utilization and expression of the gene is what mkaes one cell type distinct from another cell type
  88. multiple alleles
    • more than two alleles are present at one loci
    • example=duck feather patterns. restricted > mallard > dusky
    • mendel's principle of segregation applies to this still
    • codominance&multiple alleles together=blood type AB
  89. gene interaction
    • some times genes do not act independently in their phenotypic expression, the effects of genes at one locus depend on the presence of genes at other loci
    • occurs when genes at multiple loci determine a single phenotype(not allelic)
    • products of genes at different loci combine to produce new phenotypes that are not predictable from single locus effects
    • genes at two or more loci interact to produce a single characteristic
  90. epistasis, epistatic gene, hypostatic gene
    • epistasis=one gene can mask or hide the effect of another gene at a different locus(not allelic genes)
    • epistatic gene=gene that does masking
    • hypostatic gene= gene whose effect is masked
  91. recessive vs. dominant epistasis
    • recessive=the presence of 2 alleles inhibits the expression of an allele at a different locus
    • dominant=a single copy of an allele is required to inhibit the expression of an allele at a different locus
  92. Chromosome Variation
    • most species have a characteristic number of chromosomes, each with a distinct size and structure, all tissues have the same set of chromosomes
    • chromosome mutations=variations in chromosome number does arise and variations in chromosome structure arises as individual chromosomes may lose or gain parts and the order of genes may be altered
    • plays role in evolution
  93. types of chromosome mutation
    • Chromosome rearrangement(duplication) 2n=6
    • Aneuploidy(trisomy) 2n+1=7
    • Polyploidy(autotriploid) 3n=9
  94. Chromosome rearrangements
    • alter structure of the chromosome
    • example=a piece of a chromosome is duplicated, deleted, or inverted
    • duplications, deletions, inversions, and translocations
  95. Aneuploidy
    • the number of individual chromosomes is altered
    • example=one or more individual chromosomes are added or deleted(down syndrome-trisomy 21)
  96. Polyploidy
    • one or more complete sets of chromosomes are added
    • example=a polyploid is any organism that has more than 2 sets of chromosomes
  97. chromosome duplications
    • a mutation in which a part/segment of the chromosome has been doubled
    • an individual homozygous for a duplication carries the duplication on both homologous chromosomes
    • an individual heterozygous for a duplication carries the duplication on one chromosome-not both(problems arise in prophase I of meiosis during chromosome pairing b/c they are not homologous for their entire length, will twist and loop for pairing)
    • can detect this by visualization
    • alters phenotype by altering amount of particular proteins
  98. Chromosome deletions
    • a mutation in which a part/segment of the chromosome has been lost or deleted
    • individuals heterozygous for deletions, the normal chromosomes must loop out during pairing in meiosis I
    • phenotypic consequence depends on the genes that are located in the region deleted
    • lethal when homozygous b/c all copies of the gene are missing, if heterozygous an imbalance in the gene product may result in developmental/physiological problems
    • recessive alleles may be expressed when a wild type allele is deleted
  99. Chromosome Inversions
    • a mutation in which a part of the chromosome has been inverted(a segment is turned 180 degrees)
    • paracentric=does not include centromere
    • pericentric=does include centromere
    • have no lost/gained genetic information, just altered the order of genes
    • can have profound phenotypic effects-gene can break in 2
    • homozygous=no problems arise during meiosis, homologous chromosomes can pair and separate normally
    • heterozygous= gene order of homologs differ and can only align&pair if the 2 chromosomes form an inversion loop in prophase I, single crossover w/ paracentric inversion leads to abnormal gametes(1 of 4 chromatids has 2 centromeres&1 lacks a centromere and is lost)
    • two chromosomes are NRC and the other two are nonviable
  100. Chromosome translocations
    • the movement of genetic material between nonhomologous chromosomes or within the same chromosome(a segment moves to a nonhomologous chromosome or to another place on the same chromosome)
    • nonreciprocal=genetic material moves from one chromosome to another w/o reciprocal exchange
    • AB•CDEFG and MN•OPQRS--->
    • reciprocal=a two way exchange of genetic material between chromosomes
    • AB•CDEFG and MN•OPQRS--->
  101. The bar phenotype in drosphila
    • Example of duplication
    • leads to a reduced number of facets in the eye, resulting in a smaller and bar shaped eye
    • results from a small duplication of the X chromosome(more duplications=smaller eye)
    • inherited as an incompletely dominant x-linked trait
    • homozygous females have smaller eyes than heterozygous females
    • three copies=double bar
    • due to unequal crossing over due to improper alignment of chromosomes that results in a duplication generating process
  102. Unbalanced gene dosage
    developmental processes often require the interaction of many genes in precise amounts-a difference in the relative amounts can result in developmental problems
  103. Pseudodominance
    the expression of a normally recessive mutation-an indication that one of the homologous chromosomes has been deleted
  104. Haploinsufficient gene
    • when a single copy of a normal gene is not sufficient to produce a wild type phenotype
    • some genes must be present in two copies for normal function
  105. The notch phenotype
    • Example of deletion
    • due to a series of x-linked wing mutations in drosphila
    • notch deletions behave as dominant mutations- when heterozygous for the notch deletion, flies have notched wings
    • notch locus=haploinsufficient
  106. Position effect
    • many genes are regulation in a position-dependent manner
    • altered position may result in expression at inappropriate times or tissues
    • loss of molecular control-promoters, enhancers, silencers
  107. Pericentric inversion&human evolution
    • inversions may have played an important role in human evolution
    • G-banding patterns reveal that several human chromosomes differ from chimpanzees by only a pericentric inversion
  108. Translocations can affect a phenotype in several ways
    • position effect=genes translocated to new locations may be under the control of different regulatory sequences or other genes affect their expression
    • disruption of gene function=chromosomal breaks that generate translocations may disrupt gene function
  109. Robertsonian Translocations
    • deletions frequently accompany translocations
    • long arms of two acrocentric chromosomes become joined to a common centromere through a translocation, creating a metacentric chromosome w/ 2 long arms and another chromosome w/ 2 short arms(not metacentric)
    • took place in a human ancestor(we are related to primates)
  110. Fragile sites
    • chromosomal sites that are prone to breakage
    • a number of fragile sites have been identified in humans
    • example=fragile x syndrome: x linked inheritance is more common in males that is the result of trinucleotide repeats(CGG)
    • other fragile sites exist that are not composed of TNRs, these are not completely understood
  111. Copy number variations(CNVs)
    • detailed information about DNA sequence found on individual chromosomes is available b/c of the human genome project
    • geneticists can examine the # of copies of specific DNA sequences present in a cell and detect duplications, deletions, and other chromosome rearrangements
    • the variations they detect are called CNVs
    • common, each person may possess as many as 1000 CNVs with no observable phenotypic effect
    • causes a number of diseases and disorders- associated w/ osteoporosis, autism, schizophrenia
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genetics 1
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