Medical Genetics Wk 5 Flashcards
name three types of mutations
mutations that affect the number of chromosomes in the cell (genome mutations)
mutations that alter the structure of individual chromosomes (chromosome mutations)
alter individual genes (gene mutations)
what is aneuploidy
Genome mutations are alterations in the number of intact chromosomes (called aneuploidy) arising from errors in chromosome segregation during meiosis or mitosis.
name two chromosome anomalies
numerical: aneuploidy, polyploidy
structural: deletions, insertions, isochromosome
what is an isochromosome
an abnormal chromosome that has two identical arms due to duplication of one arm and loss of the other. Isochromosomes are found in tumors and in some girls with
Turner syndrome
Causes of Aneuploidy
Deletion of centromere during mitosis and meiosis
Robertsonian translocation
Nondisjunction during meiosis and mitosis
what is triosmy
Trisomy (‘three bodies’) means the affected person has 47 chromosomes instead of 46. Down syndrome, Edward syndrome and Patau syndrome are the most common forms of trisomy.
Children affected by trisomy usually have a range of birth defects, including delayed development and intellectual disabilities.
The addition of an extra chromosome usually occurs spontaneously during conception. The cause of this is unknown and prevention is not possible. The most important risk factor for trisomy disorders is maternal age.
common types of autosomal trisomy
trisomy 21 downs syndrome
trisomy 18 edwards syndrome
trisomy 13 patio syndrome
sex chromosome aneuploidy
SCAs as a whole are relatively common, with an estimated incidence of approximately 1 in 440 births and are characterized by the loss or gain of one or more sex chromosomes. The most well-known SCAs include Turner syndrome (monosomy X), Klinefelter syndrome (XXY), 47,xxx syndrome. 47,xyy condition.
Klinefelter
also known as the XXY condition, is a term used to describe males who have an extra X chromosome in most of their cells.
not inherited
Mosaic Klinefelter syndrome (46,XY/47,XXY) is also not inherited.
Triple X
presence of an extra X chromosome in each cell of a female
47 XYY syndrome
an extra copy of the Y chromosome in each of an individual’s cells.
have normal production of the male sex hormone testosterone and normal male sexual development, and they are usually able to father children.
Turner syndrome
occur if one of the sex chromosomes is partially missing or rearranged rather than completely absent.
Some women with Turner syndrome have a chromosomal change in only some of their cells, which is known as mosaicism.
Crossing over
Chromosomes in eukaryotes contain large numbers of genes, whose locations are fixed along the length of the chromosomes.
■■ Unless separated by crossing over, alleles on the same chromosome segregate as a unit during gamete
formation.
■■ Crossing over between homologs during meiosis creates recombinant gametes with different combinations of alleles that enhance genetic variation.
■■ Crossing over between homologs serves as the basis for the construction of chromosome maps. The greater the
distance between two genes on a chromosome, the higher the frequency of crossing over is between them
what are chromosome maps
indicate the relative locations of genes on the chromosome
what does chromosome mapping entail
determine the order of genes in a chromosome
determine how far apart genes are on the chromosome
construct a map of the chromosome
what is crossing over
Crossing Over Serves as the Basis for Determining the Distance between Genes in Chromosome Mapping
Mechanism of crossing over: It is responsible for recombination between linked genes and takes place during meiosis i.e. after the homologous chromosomes have undergone pairing and before they begin to separate.
types of crossing over
single crossing over- formation of a single chiasma between non sister chromatids of homologous chromosomes.
involves two linked genes
double crossing over- formation of two chiasmata between non sister chromatids of homologous chromosomes. involves three linked genes
multiple crossing over- occurrence of more than two crossing overs between non sister chromatids of homologous chromosomes
Differences between crossing over and linkage
crossing over:
it leads to the separation of linked genes
it involves the exchange of segments between non-sister chromatids of homologous chromosomes
it increases variability by forming new gene combinations
linkage:
it keeps genes together
it involves individual chromosomes
it reduces variability
what are chromosome maps
Chromosome maps can be prepared by genetical or cytological methods
Genetical method: This is the general method and is based upon cross over data. The resulting map is the linkage map. Linkage map (cross over map or genetical map) map be defined as a line on which the relative positions of genes proportional to the amount of crossing over between them is represented.
Genetic mapping of chromosomes is based on the following assumptions:
a) The genes are arranged in a linear order.
b) Crossing over is due to breaks in the chromatids
c) Crossing over occurs by chance and is at random
d) The percentage of crossing over between the genes is an index of their distance apart.
Map distance
Recombination frequencies between the linked genes are determined from appropriate testcrosses. These percent frequencies are used as map units for preparing linkage maps.
cytological maps
it is possible to construct map of chromosomes showing the actual physical location of gene in a chromosome. Such maps are called cytological maps of chromosomes. The work on cytological maps also confirm the theory of linear arrangement of genes in chromosomes.
Determining the Gene Sequence during Mapping
the study of single crossovers between two linked genes provides a basis for determining the distance between them.
the discovery that multiple crossovers occur between the chromatids of a tetrad has facilitated the process of producing more extensive chromosome maps.
look at last slide of ppt lect 5 about the exercise
