ch 17 genetics terms
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the genome can be remodeled on a larger scale by alterations to chromosome struccture or by changes in the number of copies of chromosomes in a cell. The can be dectected by microscopy, by genetic or molecular analysis, or by a combination of techniques. Many chromosome mutations cause abnormalities in cell and organismal function
two types of changes in chromosome number
- - aberrant euploidy: changes in WHOLE chromosome sets
- - aneuploidy: changes in PARTS of chromosome sets
Organisms that have more or fewer than their normal number of sets of chromosomes (in humans, we are diploidy, 2n, so 1n or 3n would be aberrant euploids)
a type of aberrant euploidy in which a normally diploid species has only one chromosome set; different from haploid, who naturally only have one set. Normally it is lethal. Viable indidivudal are sterile.
In generaly, the higher the ploidy level the ___ the size of the organism
LARGER; in aberrant euploids there is often a correlation between the number of copies of chromosome set and the size of the organism--polyploids are often larger and have larger component parts than their diploid relatives
Usually sterile or highly infertile on account of unpaired chromosomes at meiosis. Instead of normal pairing, there is formation of aneuploid gametes and offspring. The three chromosomes can either pair as a trivalent (|||) or as a bivalent plus a univalent (|| + |)
autotetraploids arise by the doubling of a 2n complement to 4n. This doubling can occur spontaneously, but it can also by induced artifically by applying chemical agents, such as colchicine.This disrupts spindle formation in metaphase and anaphase, preventing the migration of chromatids after the centromere split. A single 4n cell is thus created, rather than two 2n cells
chromosome sets are from two or more closely related species. In nature, allopolyploidy seems to have been a major force in the evolution of new plant species. An example is bread wheat (6n), composed of two sets each of three ancestral genomes
changes in PARTS of chromosome sets;
the failure of the disjunction process, in whcih two chromosomes or chromatids incorreclty go to one pole and none to the other. This results in aneuploid gametes. It occurs spontaneously. Appears to go awry in meiosis I more often
Trisomy 21: Down syndrome. XYY, XXX, XXY (Klinefelter syndome
XO: turner syndrome; sterile females, short in stature, and often have a web of skin extending between the neck and shoulders.
gene dosage effect
amount of gene product is proportional to the number of copies of the gene; gene balance is needed for normal development
mechanism to generate equal expression genes in XY males and XX femaltes
nonallelic homologous recombination (NAHR)
a type of rearrangement in which crossing over between repetitive (duplicated) DNA segments occurs.
chromosomal rearragnement can be produced by what two basic mechanisms?
- chromosome breakage and rejoining (deletion, deletion and duplication, inversion, translocation)
- crossing over between repetitive DNA
A deletion is simply the loss of a part of one chromosome arm. A small deletion within a gene, called in intragenic deletion, inactivates the gene. A more severe deletion is which several to many genes are missing, multigenic deletion, can produce lethality in offspring of homozygous inbreeding. Even some hetero offspring will not survive. This is on account of gene imbalance.
unmasking of recessive alleles in area of deletion
the failure of the corresponding segment on the normal homolog to pair, creating a visible loop
duplicate region located adjacent to each other
extra copy (duplication) located elsewhere in the genome
duplications much bigger than the simple sequence repeats. The duplicated unitis range from 10-15 kb in length and encompass whole genes and the regions in between
segment of chromosome is flipped (abcdef--> abdcef); do not affect overall amount of genetic material, so they don't result in gene imbalance
one normal and one abnormal chromosome set are present; the main diagnostic features are inversion loops, reduced recombinant frequency (nearly 0), and reduced fertility (how reduced depends on the size of the inversion) because of unbalanced or deleted meiotic products
inversion spans the centromere. Recombinant products have low viability because of duplications/deletions. A crossover in the loop gives rise to chromosomes containgin duplications and deletions
centromere is outisde the inversions. Recombination generates acentric fragment and dicentric bridge. Recombinant products are invialbe because of deletions. A crossover in the loop gives rise to chromosomes containing deletions
In paracentric inversion, crossing over within the inversion loop at meiosis causes a connection of homologous centromeres
the lost portion that results in paracentric inversion when the crossing over creates the dicentric bridge.
switching of acentric fragments between nonhomologous chromosomes
reciprocal translocation heterozygotes
diagnosed by semi sterility, half the gametes are inviable or generate inviable zygotes; or by pseudolinkage, genes appear linked when they are normally on separate chromosomes
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