Medical Genetics Wk 5

Question 1

name three types of mutations

Answer 1

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)

Question 2

what is aneuploidy

Answer 2

Genome mutations are alterations in the number of intact chromosomes (called aneuploidy) arising from errors in chromosome segregation during meiosis or mitosis.

Question 3

name two chromosome anomalies

Answer 3

numerical: aneuploidy, polyploidy

structural: deletions, insertions, isochromosome

Question 4

what is an isochromosome

Answer 4

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

Question 5

Causes of Aneuploidy

Answer 5

Deletion of centromere during mitosis and meiosis
Robertsonian translocation
Nondisjunction during meiosis and mitosis

Question 6

what is triosmy

Answer 6

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.

Question 7

common types of autosomal trisomy

Answer 7

trisomy 21 downs syndrome
trisomy 18 edwards syndrome
trisomy 13 patio syndrome

Question 8

sex chromosome aneuploidy

Answer 8

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.

Question 9

Klinefelter

Answer 9

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.

Question 10

Triple X

Answer 10

presence of an extra X chromosome in each cell of a female

Question 11

47 XYY syndrome

Answer 11

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.

Question 12

Turner syndrome

Answer 12

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.

Question 13

Crossing over

Answer 13

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

Question 14

what are chromosome maps

Answer 14

indicate the relative locations of genes on the chromosome

Question 15

what does chromosome mapping entail

Answer 15

determine the order of genes in a chromosome
determine how far apart genes are on the chromosome
construct a map of the chromosome

Question 16

what is crossing over

Answer 16

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.

Question 17

types of crossing over

Answer 17

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

Question 18

Differences between crossing over and linkage

Answer 18

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

Question 19

what are chromosome maps

Answer 19

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.

Question 20

Map distance

Answer 20

Recombination frequencies between the linked genes are determined from appropriate testcrosses. These percent frequencies are used as map units for preparing linkage maps.

Question 21

cytological maps

Answer 21

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.

Question 22

Determining the Gene Sequence during Mapping

Answer 22

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.

Question 23

look at last slide of ppt lect 5 about the exercise