Genetics Chapter 6

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Chromosome mutations are variations in the number and structure of chromosomes and also play an important role in evolution
Variations in chromosomes may lose or gain parts of induvidual chromosomes and the order of genes within a chromosome may become altered
Metacentric The centromere is located approximately in the middle, and the chromosome has two arms of EQUAL length
Submetacentric The centromere is displaced toward one end, creating a short and a long arm which is represented by p and q
Submetacentric arms P arm: "petitie" arm
Acrocentric The centromere is near one end, producing a long arm and a knob, or satelliite, at the other end
Telocentric The centromere is at or very near the end of the chromosome
The compelete set of chromosomes possesed by an organism is called a karyotype
To determine chromosome structure and viewing is best carried out by which cell stage metaphase, where the chromosomes are lined up
A human karyotype consists of 46 chromosomes
Staining of chromosomes with a special dye called Giemsa reveals G bands
G bands distinguish areas of DNA that are rich in adenine-thymine (A-T) base pairs
Q bands are revealed by staining chromosomes with quinacrine mustard and viewing chromosomes under ultraviolet light
Variation in the brightness if the Q bands results from differences in the relative amounts of cytosine-guanine (C-G) and adenine-thymine base pairs
C band techniques which are regions of DNA occupied by centromeric heterochromatin
R bands are rich in cytocine-guanine base pairs
Chromosome mutations 1) chromosome rearrangement 2) aneuploids 3) polyploids
Chromosome rearrangements alter the structure of chromosomes. For example: a piece of chromosome might be 1) duplicated 2) deleted 3) inverted
In aneuploidy the number of chromosomes is altered: one or more individual chromosomes are added or deleted
in polyploidy one or more complete sets of chromosomes are added.
Chromosome mutations consist of chromosome 1) rearrangements 2) aneuploids 3) polyploids
Chromosome rearrangements are mutations that change the structure of individual chromosomes.
Four basic chromosome rearrangements 1) duplications 2) deletions 3) inversions 4) translocations
Duplication a segement of the chromosome is duplicated
Deletion a segement of a chromosome is deleted
Inversion a segement of the chromosome is turned 180 degrees
Translocation a segement of chromosome moves from one chromosome to a nonhomlogous chromosome or to another place on the same chromosome
Chromosome duplications is a mutationn in which part of the chromosome has been doubled
Chromosome duplication example AB*CDEFG (EF segements) = AB*CDEFEFG
Tandem duplication in which the duplicated region is IMMEDIATELY adjacent to the original segement
Displaced duplication If the duplicated segment is located some distance from the original segment, either on the same chromosome or on a different one
Example of a displaced duplication AB*CDEFG (EF segments) =AB*CDEFGEF
Reverse duplication (when the duplication is inverted) a duplication can be either in the same orientation as that of the original sequence , as in the two preceeding examples or inverted
Example of a reverse duplication AB*CDEFG (EF segments) =AB*CDEFFEG
In an individual heterozygous for a duplication the duplicated chromosome loops out during pairing in prophase I
An individual homozygous for duplication carries the duplication on both homologous chromosomes
An individual heterozygous for a duplication for a duplication has one normal chromosome and one chromosome with the duplication
Duplications have major effects on phenotype
The pairing and synapsis of homologous regions require that one or both chromosomes loop and twist so that these regions are able ot line up
The amount of a particular protein synthesized by a cell is often directly related to the number of copies of its corresponding gene
An individual organism eith three functional copies of a gene often produces 1.5 times as much of the protein encoded by an individual with two copies
Proper gene dosage = critical if the amount of one protein increases which the amounts of others remain constant, problems can result
Segemental duplication defined as duplications greater than 1000 base pairs (1000 bp ) in length
Unequal crossing over produces Bar and double-bar mutations
In chromosomes with Bar 1) unequal crossing over between chromosomes containing teo copies of bar...2) produces a chromosome with three Bar copies (double Bar mutation 3) and a wild-type chromosome
4 percent of the human genome consists of segemental duplications
in the human genome, the average size of segmental duplications is 15,000bp
Segmental duplication arise from processes that generate chromosome duplications such as unequal crossing over
Crossing over between two duplicated sequences located at different postions on a chromosome can generate additional duplications, deletions, and other chromosome rearrangements, which often result in genetic disorders through abnormal gene dosage
Over evolutionary time, segmental duplication often give rise to new genes while the original gene sequence continues to funtion, the segmental duplication may undergo chnage and eventually provide a new function
A chromosome duplication is a mutation that doubles part of a chromosome. In individuals heterozygous for a chromosome duplication, the duplicated region of the chromosome loops out when homologous chromosomes pair in prophase I of meiosis. Duplications often have major effects on the phenotype, possibly by altering gene dosage. Segmental duplications are common within the human genome. Chromosome duplications often result in abnormal phenotypes because: Developmental processes depend on the relative amounts of proteins encoded by different genes
Chromosome deletion loss of a chromosome segment
Example of a chromosome deletion AB*CDEFG ( deletion of EF segement) = AB*CDG
In individuals heterozygous for deletions, the normal chromosome must loop out during the pairing of homologs in prophase I of meiosis to allow the homologous regions of the two chromosomes to align and undergo synapsis.
Effects of deletions 3 types
The heterozygous condition may produce imbalances in the amounts of gene products similar to the imbalances produced by extra gene copies
Normally recessive mutations on the homologous chromosome lacking the deletion may be expressed when the wild type allele has been deleted ( and no londer is present to mask the recessive allele's expression)
Pseudodominace is the expression of a normally recessive mutation
Pseudodominance is an indication that one of the homologous chromosome has a deletion
Some genes must be present in two copies for a normal function When a single copy of a gene is not sufficient to produce a wild type phenotpe, it is said to be haploinsufficient gene
In an individual heterozygous for a deletion the normal chromosome loops out during chromosome pairing in prophase I
When a single copy of a gene is not sufficient to produce a wild-type phenotype it is said to be haploinsufficient gene
A chromosomal deletion is a mutation in which a part of a chormosome is lost. In individuals heterozygous for a deletion, the normal chromosomes loops out during Prophase I of meiosis. Deletions cause recessive genes on the homologous chromosome to be expressed and may cause imbalances in gene products. Question: What is pseudodominance and how is it produced bt a chromosome deletion? ANSWER: Pseudodominance is the expression of a recessive mutation. It is produced when the dominant wild type allele in a heterozygous individual is absent due to a deletion on one chromosome
Effects of inversions Individudal organsims with inversions have neither lost nor gained any genetic material: just the order of the chromosome segemetn has been altered.
An inversion my break a gene into two parts with one part moving to a new location and the other destroying the function of that gene
Even when the chromosome breaks are between genes phenotypic effects may arise from the inverted gene order in an inversion
Many genes are regualted in a position dependent manner and if their postions are altered by an inversion they may be expressed at inappropriate times or in inappropriate tissues, therefore the outcome is referred to as a POSITION EFFECT
Homozygous for inversions no special problems arise
Heterozygous for inversions the gene order of the teo homologs differs and the homologous sequences can align and pair only if the two chromosomes form an inversion loop
Individuals heterozygous for inversions also exhibit reduced recombination among genes located in the inverted region
The frequency of crossing over within the inversion is not actually diminished but when crossing over does take place, the result is abnormal gametes that result are nonviable offspring and therefore no recombinant progeny take place
In an individual heterozygous for a paracentric inversion the chromosomes form an inversion loop during pairing in prophase I
The heterozygote has one normal chromosome and one chromosome with an inverted segement In prophase I of meiosis, the chromosomes form an inversion loop which allows the homologous sequences to align
The individual is heterozygous for an inversion with one wild type unmutated chromosome (AB*CDEFG) and one inverted chromosome (AB*EDCFG)
In Prophase I of meiosis an inversion loops forms, allowing the homologous sequences to pair up
If a single crossover takes place in the inverted region (between C and D) an unusual structure results
The two outer chromatids, which do not participate in crossing over, contain original, nonrecombinant gene sequences
The two inner chromatids, which did participate in crossing over are highly abnormal: each has two copies of some genes and no copies of others
One of the four chromatids now has two centromeres and is said to be a dicentric chromatid
Acentric chromatid lacks a centromere
Anapahse I of meiosis the centromeres are pulled toward opposite poles and the two homologous chromosomes seperate
This action stretches the dicentric chromatid across the center of the nucleus, forming a sturcture called a DICENTRIC BRIDGE
Dicentric bridges eventually break as the two centromeres are pulled futher apart
Spindle fibers do not attach to the acentric fragment and so this fragment does not segregate to a spindle pole and is usually lost when the nucleus re-forms
In the second division of meiosis, the chromatids seperate and four gametes are produced
Two of the gametes contain the original, nonrecombinant chromosomes (AB*CDEDG and AB*EDCFG) and the other two gametes contain recombinant chromosomes that are missing genes therefore these gametes will not produce viable offspring
No recombinant progeny result when crossing over takes place within a paracentric-inversion crossing over still takes place but when it does take place, the resulting recombinant gametes are not viable so no recombinant progeny are observed
In an individual heterozygous for a paracentric inversion the chromosomes form an inversion loop during pairing in Prophase I
In a heterozygous individual a single crossover within a paracentric inversion leads to abnormal genes
Steps and concepts 1) the heterozygote prossesses one wild type chromosome 2) and one chormosome with a paracentric inversion 3) in prophase 1 an inversion loop forms 4) a single crossover within the inverted region 5) results in an unusual structure 6) one of the four chromatids now has two centromeres 7) and one lacks a centromere 8) in anaphse 1 the centromeres separate, stretching the dicentric chromatid, which breaks. The chromosome lacking a centromere is lost. 9) two gametes contain non-recombinant chromosomes: one wild type (normal) and one with the inversion 10) The other two contain recombinant chromosomes that are missing some gene; these gametes will not produce viable offspring
Recombinant gametes are nonviable because genes are either missing or present in too many copies
What happens? 1) The heterozygous possesses one wild type chromosome 2)and one chromosome with a pericentric inversion 3)In prophase I an inversion loop forms 4) if crossing over takes place within the inverted region 5) two of the resulting chromatids have too many copies of some genes and no copies of others 6) The chromosomes separate in anaphase I 7) The sister chromatids separate in anaphase II forming four gametes
Which results in.... -Normal nonrecombinant gametes -nonviable recombinant gametes - nonrecombinant gametes with pericentric inversion
Recombination is also reduced within a pericentric inversion
No dicentric bridges or acentric fragments are produced but the recombinant chromosomes have too many copies of some genes and no copies of others so gametes that recieve the recombinant chromosomes cannot produce progeny
Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants


















Double crossovers in which both crossovers are on the same strands (two strand double crossover) result in functional recombinants
















In an inversion, a segment of a chormosome is turned 180 degrees. Inversions cause breaks in some genes and may move others to new locations. In individuals heterozygous for a chromosome inversion, the homologous chromosomes form a loop in Prophase I of meiosis. When crossing over takes place within an inverted region, nonviable gametes are usually produced, resulting in a depression in observed recombination frequencies.

A dicentric chromosome is produced when crossing over takes place in an individual heterozygous for which type of chromosome rearrangement?
paracentric inversion
Translocation involves the movement of genetic material between nonhomologous chromosomes or within the same chromosome
Nonreciprocal translocation genetic material moves from on chromosome to another without any reciprocal exchange
Consider these two nonhomologous chromosomes: AB*CDEFG and MN*OPQRS If chromosome segment EF moves from the first chromosome to the second without any transfer of segments from the second chromosome to the first, a nonreciprocal translocation has taken place producing AB*CDG and MN*OPEFQRS
reciprocal translocation two way exchange between the chromosomes
A reciprocal translocation between chromosomes: AB*CDEFG and MN*OPQRS =AB*CDQRG and MN*OPEFS
Translocations can phsyically link genes that were formerly located on different chromosomes
These new linkage relations may affect gene expression ( postion effect) genes translocated to new locations may come under the control of different regulatory sequences or other genes that affect their expression
The chromosomal breaks that bring about translocations may take place within a gene and disrupt its function
Deletions accompany translocations
Robertsonian translocation the long arms of two acrocentric chromosomes become joined to a common (1) centromere through a translocation, generating a metacentric chromosome with two very short arms
The smaller chromosome fails to segregate leading to an overall reduction in chromosome number
Robertsonian translocations are the causeof some cases of Down syndrome
Individual heterozygous for a reciprocal translocation:
Original chromosomes: AB*CDEFG and M*NOPQRST and that a reciprocal translocation takes place
producing chromosomes AB*CDQRST and M*NOPEFG
Individual heterozygous for this translocation would possess one normal copy of each chromosome and one translocated copy
Each of these chromosomes contains segments that are homologous to two other chromosomes
When the homologous sequences pair in prophase I of meiosis crosslike configurations consisting of all four chromosomes form
Whether viable or noviable gametes can be produced depends on how the chromosomes in these crosslike connfigurations separate
Only about half of the gametes from an individual heterozygous for a reciprocal translocation are expected to be functional and these individuals exhibit reduced fertility
In translocations, parts of chromosomes move to other non-homologous or to other regions of the same chromosome. translocations can affect the phenotype by causing genes to move to new locations, where they come under the influence of new regulatory sequences, or by breaking genes and disrupting their function.

What is the outcome of a Robertsonian translocation?
One large chromosome and one very small chromosome with two very short arms
Fragile sites Chromosomes of cells grown in culture sometimes develop constrictions or gaps at particular locations
Fragile sites are prone to breakage under certain conditions.
More than 100 fragile sites have been identified on human chromosomes
Fragile X-syndrome a disorder that inculdes intellectual disability
Fragile sites are chromosomal regions susceptible to breakage under certian conditions
Methods that measure variations that they detect are called copy-number variations
Copy-number variations include duplications and deletions that range in length from thousands of base pairs to several million base pairs
Fragile sites are contrictions or gaps in chromosomes that are prone to breakage under certian conditions
Variations in the number of copies of particular DNA sequences (copy-number variations) are surprisingly common in the human genome
Aneuploidy is an increase or decrease in the number of individual chromosomes
Chromosome mutations include changes in the number of chormosomes
Variations in chromosome number can be classified into two basic types: 1) aneuploidy 2) polyploidy
Aneuploidy is the change in the number of individual chromosomes
Polyploidy is the change in the number of chromosome sets
Aneuploidy causes: 1) a chromosome may be lost in the course of mitosis or meiosis for example-its centromere is deleted. Loss of a centromere prevents the spindle fibers from attaching so the chromosome fails to move to the spindle pole and does not become incorporated into a nucleus after cell division. 2) the small chromosome generated by a Robertsonian translocation may be lost in mitosis or meiosis. 3) Aneuploids may arise through nondisjunction
Nonodisjunction the faliure of homologous chromosomes or sister chromatids to seperate in meiosis or mitosis.
nondisjunction leads to some gametes or cells that contain an extra chromosome and other gametes or cells that are mssing a chromosome
Four types of common aneuploid conditions in diploid indiviiduals: 1) nullisomy 2) monosomy 3) trisomy 4) tetrasomy
Nullisomy is the loss of both members of a homologous pair of chromosome. Represented 2n-2, where n refers ti the haploid number of chromosomes. Thus among humans, who normally prossess 2n-46 chromosomes, a nullisomic zygote at 44 chromosomes
monosomy is the loss of a single chromosome, represented as 2n-1. A human monosomic zygote has 45 chromosomes
Trisomy is the gain of a single chromosome
Trisomy is repesented as 2n+1. A human trisomic zygote has 47 chromosomes. The gain of a chormosome means that there are three homologus copies of one chromosome. Most cases of Down Syndrome result from Trisomy chromosome 21
Tetrasomy is the gain of two homologus chromosomes
Tetrasomy is represented as 2n+2 A human tetrasomic zygote has 48 chromosomes. Tetrasomy is not the gain of any two extra chromosomes, but rather the gain of two homologous chromosomes so there will be four homologous copies of a particular chomosome
A human tetrasomic zygote has 48 chromosomes
Tetrasomy is not the gain of any two extra chromosomes
but rather the gain of two homologous chromosomes so tere will be four homologous copies of a particular chromosome
Aneuploidy affects the number of gene copies but not their nucelotide sequences, the effects of aneuploidy are most likely due to abnormal gene dosage
Major execption of aneuploidy

relation between gene number and gene dosage pertains to the genes on the mammalian x-chromosomes.
X-chromosome inactivation in males females ensures males and females recieve the same functional dosage for x-lonked genes
Aneuploidy, the loss or gain of one or more individual chromosomes, may arise from the loss of a chromosome subsequent to translocation or from nondisjunction in meiosis or mitosis. It disrupts gene dosage and often has severe phenotypic effects.

A diploid organism has 2n=36 chromosomes. How many chromosomes will be found in a trisomic member of this species?
Answer: 37


Aneuploids can be produced through nondisjunction
in meiosis I, meiosis II and mitosis
Sex-chromosomes aneuploids most common aneuploidy seen in living humans

Turner syndrome and Klinefelter syndrome result from
aneuploidy of the sex chromosomes
Autosomal aneuploids
resulting in live births are less common than sex chromosome aneuploids in humans probably because there is no mechanism of dosage compensation for autosomal chromosomes
Down syndrome

-the most common autosomal aneuploidy in humans
-also called trisomy 21
-1 in700 births
Primary Down syndrome 92% of human births have 3 full copies of chormosome 21.
A conditon terrmed pimary dow syndrome
familial Down syndrome -4% of people with down syndrome
-46 chromosomes
-extra copy of part of chromosome21 is attached to another chromosome through a translocation
Primary down syndrome is caused by the presence of three copies of chromosome 21
The transloaction of chromosome 21 onto another chromosome results in familial Down syndrome
Familial Down syndrome arises in offspring whose parents are carrise of chromosomes that have undergone a Robertsonian translocation most commonly between chromosome 21 and chromosome 14: the long arm of 21 and the short arm of 14 exchange places
Transloaction carriers persons with this translocation, do not have down syndrome.

Although, they possess only 45 chromosomes, their phenotypes are normal
Translocation carriers are at increased risk for producing children with down syndrome 1) a parent who is a carrier for a 14-21 translocation is normal
2)Gametogenesis produces gametes in these possible chromosome combinations
3) if a normal persn mates with a translocaton carrier
4) two thirds of their offsping will be healty and normal- even the translocation carriers
5) but one-third wll have down syndrome
6)other chromosoma combinaton result in aborted embryos
Trisomy 18 also known as Edward Syndrome -1 in 8000 live births
Trisomy 13 -also known as Patau syndome
-1 in 15000
Many tumor cells have extra or missing chromosomes or both some types are associated with specific chromosome mutations, including aneuploidy and chromosome rearrangements
In humans, sex chromosome aneuploids are more common than autosomal aneuploids. X-chromosome inactivation prevents problems of gene dosage for x linked genes. Down syndrome results from three functional copies of chromosome 21, either through trisomy (primary down syndrome) or a Robertsonian translocation (familial down syndrome)

Why are sex-chromosome aneuploids more common than autoploids in humans and other mammals?
Dosage compensation prevents the expression of additional copies of x-linked genes in mammals, and there is little info in the y chromosome, so the extra copies of the x chromosomes do not have major effects on development. In contrast, there is no mechanism of dosage compensation for autosomes, and so extra copies of autosomal genes are expressed, upsetting development and causing spontaneous abortion or aneuploid embryos
Polyploids include 1) triploids (3n) 2) tetraploids (4n) 3) pentaploids (5n) 4) or even higher numbers of chromosome sets
Polyploidy is common in plants and is a major mechanism by which new plant species and from 70% to 80% of grasses are polyploids
Two major types of polyploidy -autopolyploidy -allopoloploidy
Autopolyploidy in which all chromosomes sets are from a single species
Allopolyploidy in which chromosomes sets are from two or more species
Autoployploidy is due to accidents of meiosis or mitosis that produce extra sets of chromosomes, all derived from a single species
nondisjunction of all chromosomes in mitosis in an early 2n embryos-doubles the chromosome number and produces a autotetraploid (4n)
An autotriploid (3n) may arise when a nondisjunction in meiosis produces a diploid gamete that then fuses with a normal haploid gamete to produce a triploid zygote
Triploids may arise from a cross between an autotetraploid that produces 2n gametes and a diploid that produces (1n) gametes
unbalanced gametes random segregation with various numbers of chromosomes
Allopolyploidy aries from hybrization between two species the resulting po,lyploid carries chromosome sets derived from two or more species
amphidiploid the type of allopolyploid, consisting of two combined diploid genomes
Autopolyploidy can arise through nondisjunction in mitosis or meiosis
Autopolyploidy through mitosis 1) Diploid (2n) early embryonic cell 2) replication 3) separation of sister chromatids 4)nondisjunction (no cell division) 5)autotetraploid (4n) cell
Autoployploidy through meiosis 1) Diploid (2n) cell 2) replication 3) nondisjunction 4) gametes both 2n 5) fertilization of 1n 6)zygotes are triploid (3n) that then fuses with a 1n gamete to produce an autotriploid
in meiosis of an autotriploid, homologous chromosomes can pair or not pair in three ways
Species 1 has 2n=14 and species II has 2n=20. Give all the possible chromosomal numbers that may be found in the following indivudials a) An autopolyploid of speceis I A: b) an autotetraploid of species II A: c) An allotriploid formed from species I and species II A: d) An allotetraploid formed from species I and species II A;
Polyploidy is the pressence of extra chromosome sets: autoployploidy possess extra chromosome sets fromt he same species; allopolyploids possesss extra chromosome sets from two or more species. Problems in chromosome pairing and segregation often lead to sterility in autopolyploids, but many allopolyploids are fertile. Species A has 2n=16 chromosomes and species B has 2n=14. How many chromosomes would be found in an allotriplod of these two species? Answer: 22 or 23
Polyploidy, particularly allopolyploidy, often gives rise to new species and has been particualy important in the evolution in flowering plants
Three basic types of chromosome mutations are: 1) chromosome rearrangements, which are changes in the structure of chromosomes 2)aneuploidy, which is an increase or decrease in chromosome number and 3) polyploidy, which is the presence of extra chromosome sets
Chromosome rearrangments include 1) duplications 2)deletions 3)inversions 4)translocations
In individuals heterozygous for a duplication, the duplicated region will form a loop when homologous chromosomes pair in meiosis. Duplications often have pronounced effects on the phenotype owing to unbalanced gene dosage
Segmental duplications are common in the human genome
In individuals heterozygous for a deletion, one of the chromosomes will loop out during pairing in meiosis.
Deletions may cause recessive alleles to be expressed
Pericentric inversions include the centromere; paracentric inversions do not
In individuals heterozygous for an inversion the homolgous chromosomes form inversion loops in meiosis, with reduced recombination taking place within the inverted region
In translocation heterozygotes the chromosomes form crosslike structures in meiosis
Fragile sites are constrictions or gaps that appear at particular regions on the chromosomes of cells grown in culture and are prone to breakage under certain conditions
Copy- number variations are differences in the number of copies of DNA sequences and include duplications and deletions
These variants are common in the human genome: some are associated with diseases and disorders
Nullisomy is the loss of two homologus chromosomes
monosomy is the loss of one homologous chromosomes
Trisomy is the addition of one homologus chomosome
Tetrasomy is the addition of two homologous chromosomes
Aneuploidy usually causes drastic phenotypic effects because it leads to unbalanced gene dosage
Primary Down syndrome is caused by the presence of three full copies of chromosome 21, whereas familial Down syndrome is caused by the presence of two normal copies of chromosome 21 and a third copy that is attached to another chromosome through a translocation
All the chromosomes in an autopolyploid derive from one species chromosomes in an allopolyploid come from two or more species
Chromosome variations have played an important role in the evolution of many groups of organisms