14- Numerical Chromosomal Abnormalities

Question 1

two mechanisms in meiosis introducing genetic variation

Answer 1

independent assortment and recombination

Question 2

purpose of meiosis 1

Answer 2

introduce genetic variation into haploid non-identical daughter cells

pairing of homologous chromosomes to form complexes, undergo genetic exchange through independent assortment and recombination

Question 3

purpose of meiosis 2

Answer 3

important to produce 4 haploid cells with a unique combination of genetic material

Question 4

what is independent assortment?

Answer 4

the random chance event during metaphase 1 when homologous chromosomes segregate randomly, leading to a variety of possible combinations in the resulting gametes

Question 5

when does independent assortment occur?

Answer 5

meiosis metaphase 1

Question 6

what is recombination?

Answer 6

the exchange of genetic material between paired homologous chromosomes

Question 7

when does recombination occur?

Answer 7

prophase 1

Question 8

describe the process of recombination

Answer 8

bivalent structure forms with aligned homologous chromosomes, held together by synaptonemal complex

crossing over involves reciprocal breaking & re-joining of homologous chromosomes = occurs during meiosis I, results in exchange of chromosomal segments between non-sister homologous chromatids

after crossing over, chiasmata are visible points where chromatids of homol. chromosomes remain attached – physical exchange of genetic material occurs at the chiasmata

Question 9

what are recombinant chromosomes?

Answer 9

have undergone crossing over, mix of genetic material from paternal and maternal chromosomes

Question 10

what are non-recombinant chromosomes?

Answer 10

maintain original combination of alleles from both parental chromosomes, no crossing-over/ exchange of genetic material

Question 11

summarise meiosis 1

Answer 11

homologous pairs of chromosomes align as 23 bivalents, allowing for chiasma formation for recombination

spindle fibres pull apart homologues from one another, still resulting in chromosomes with sister chromatids

daughter cells have 23 chromosomes, each with 2 chromatids

Question 12

summarise meiosis 2

Answer 12

chromosomes align as 23 independent chromosomes on the cell equator, and the sister chromatids are pulled apart

daughter cells have 23 chromosomes, one of each chromatid

Question 13

three types of chromosomes

Answer 13

metacentric
submetacentric
acrocentric

Question 14

what are metacentric chromosomes?

Answer 14

p and q arms of even length - e.g. chromosomes 10, 16, 17, 18

Question 15

what are submetacentric chromosomes?

Answer 15

p arm shorter than q arm

chromosomes 4-12. 19, 20, X chromosome

Question 16

what are acrocentric chromosomes?

Answer 16

long q, short p arms - p arm has no unique DNA, and q arms have identical genes

chromosomes 13, 14, 15, 21, 22, Y chromosomes

Question 17

what does polyploid mean?

Answer 17

multiple of the haploid number

Question 18

what does aneuploid mean?

Answer 18

chromosome number that isn’t an exact multiple of the haploid number due to missing/ extra chromosomes - trisomy, monosomy

Question 19

what is meiotic non-dysjunction?

Answer 19

errors in the segregation of homologous chromosomes during meiosis; occurs when pairs of homologous chromosomes fail to separate during meiosis I

Question 20

describe what happens during meiosis I non-disjunction

Answer 20

both chromosomes of a homologous pair go into one daughter cell (trisomy) whilst the other cell lacks a chromosome (monosomy)

both daughter cells affected, resulting in aneuploid cells

Question 21

describe what happens during meiosis II non-disjunction?

Answer 21

initial correct chromosome separation in meiosis I - in meiosis II both chromatids of a chromosome go into one cell

imbalance of chromosomes in gametes - some zygotes may be trismic, others may be monosomic for the same chromosome

Question 22

what is trisomy 13?

Answer 22

Patau’s syndrome

Question 23

what trisomy is Edward’s syndrome?

Answer 23

trisomy 18

Question 24

what trisomy is Down’s syndrome?

Answer 24

21

Question 25

what are trisomies a result of?

Answer 25

meiotic non-disjunction causing chromosomal aneuploidies (autosomal and sex chromosomal)

Question 26

aneuploidy of Turner syndrome - describe?

Answer 26

45 chromosomes, monosomy in females lacking an X chromosome

Question 27

aneuploidy of Triple X syndrome - describe?

Answer 27

47 chromosomes, XXX in females with an extra X chromosome

Question 28

aneuploidy of Klinefelter’s syndrome - describe?

Answer 28

47 chromosomes, XXY in males with an extra X chromosome

Question 29

what is mosaicism? under what circumstance would it occur?

Answer 29

the presence of two or more genetically different cell lines derived for a single zygote - a mix of diploid and aneuploid cells in an individual

would occur with meiotic II non-disjunction as only a portion of cells are affected by aneuploidy for a particular chromosome
- monosomic cells die out, trismic cells are maintained and continue dividing throughout the cell cycle

Question 30

what is trismic rescue/ anaphase lag?

Answer 30

occurs after non-disjunction where the cell recognises the incorrect chromosomal number and eliminates one copy – can result in mosaicism, with some cell being diploid and others being trismic

Question 31

clinical impact of trismic rescue and related mosaicism?

Answer 31

random chromosome is eliminated - can result in a cell with two maternal chromosomes or one paternal & one maternal chromosome

mosaic phenotype = less severe condition and symptom presentation

difficult to assess and predict the severity & c
clinical relevance