Chapter 8 Genetics

• a change in the total number of chromosomes
• the deletion or duplication of genes or segments of a chromosome
• rearrangements of the genetic material either within or among chromosomes

• variations in chromosome number when an organism gains or loses one or more chromosomes and has other than an exact multiple of the haploid set
• 2n+/- x chromosomes
• commonly causes an abnormal phenotype because it leads to an imbalance in the amount of gene products
• ex. monosomy 2n-1, disomy 2n, trisomy 2n+1

• complete haploid sets of chromosomes are present
• multiples of n
• ex. diploidy 2n, triploidy 3n, tetraploidy 4n

• occurs when more than two sets of chromosomes are present
• ex. 3n,4n,5n

• chromosomal variation
• chromosomes or chromatids fail to dis join and move to opposite poles during meiosis I or II

• the loss of one chromosome to produce a 2n-1 compleement
• monosomy for x chromosome occurs in humans
• monosomy for any of the autosomes is usually not tolerated in humans and other animals

• 2n+1 chromosomes
• trisomy for autosomes are often lethal
• sex chromosome has a less dramatic impact on the phenotype
• three copies of one chromosome is present and so pairing configuartions are usually irregular

three copies of a chromosome are synapsed

an unpaired chromosome

there is one bivalent and one univalent instead of a trivalent

• trisomy of chromosome 21
• 12 to 14 characteristics
• affected individuals express 6 to 8 on average
• usually due to nondisjunction of maternal chromosome 21 during meiosis
• shows an increased incidence with increasing maternal age

• down syndrome occasionally runs in families
• involves a translocation of chromosome 21

diagnostic testing for women who become pregnant late in their reproductive years

trisomy 13

trisomy 19

• trisomies but now monosomies
• suggests that monosomic gametes may be very functionally impaired

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

the combination of chromosome sets from different species as a consequence of interspecific mating

arise from hybridization of two closely related species

produced when the steril hybrid from allotetraploid undergoes a natural chromosomal doubling and produces a fertile amphidiploid

produced by somatic cell hybridization

• the condition in which only certain cells in an otherwise diploid organism are polyploid
• replication and segregation of chromosomes occur without nuclear division

• deletions
• duplications

• inversions
• translocations (reciprocal or nonreciprocal)

• chromosome breaks in one or more places and a portion of it is lost, the missing piece is the deletion
• can occur near one end (terminal deletion)
• interior of the chromosome (intercalary deletion)

the unpaired region of the homolog must loop out of the linear structure into a deletion or compensation loop

results from a segmental deletion of a small terminal portion of the short arm of chromosome 5

• result of unequal crossing over during meiosis or through a replication error prior to meiosis
• may play a role in evolution

organisms have multiple copies of the ribosomal RNA genes (rRNA)

another mechanism to increase the rRNA

results from duplication

• rearrangement of the linear gene sequence rather than the loss of genetic information
• a segment of a chromosome is turned around 180 degrees within a chromosome
• requires two breaks in the chromosome and subsequent reinsertion of the inverted segment
• may arise from chromosomal looping

does not change the relative lengths of the two arms of a chromosome

• includes the centromere
• does change the relative lengths of the two arms of a chromosome

synapsis of inverted chromosomes

one recombinant chromatid is dicentric and one is acentric

two centromeres

lacking a centromere

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

• involves the exchange of segments between two nonhomologous chromosomes
• has an unusual synapsis configuration during meiosis

one that leads to a normal and balanced gamete

one that leads to gametes containing duplications and deficiencies

• involves breaks at the extreme ends of the short arms of two nonhomologous acrocentric chromosomes
• ex. familial down syndrome

are more susceptible to chromosome breakage when cells are cultured in the absence of certain chemicals such as folic acid

• the most common form of inherited mental retardation
• affects 1 in 4000 males and 1 in 8000 females 
• it is a dominant trait
• fragile site FMR-1 has a trinucleotide sequence of CGG
• normal individual has 6-54 repeats and carriers have 55-230
• fragile x has more than 230 repeats
• link between an autosomal fragile site and lung cancer was reported
• FRA3B (fragile site on p arm of chromosome 3) is often altered or missing in cells taken from individuals with lung cancer
• normal protein product of this gene is absent in cells of many other cancers