Chromosome Number Prestudy

Question 1

What is a nondisjunction?

Answer 1

error in cell division whereby improper segregation leads to abnormal chromosome number (monosomy or trisomy).

Question 2

When can nondisjunction occur?

Answer 2

in any cell division during Meiosis I, Meiosis II, or mitosis

Question 3

What will nondisjunction during mitosis cause?

Answer 3

mosaicism: mixture of normal cells with mutated cells with chromosomal abnormalities

Question 4

What is the main risk factor for meiosis I nondisjunction?

Answer 4

age

Question 5

What is euploidy?

Answer 5

normal chromosome number (haploids = 23, diploids = 46)

Question 6

What is aneuploidy?

Answer 6

any abnormality of chromosome number that is not a multiple of 23.
Example: monosomy (n=45)
trisomy: n = 47

Question 7

What is polypoidy?

Answer 7

abnormalities in number of chromosomes with multiples of 23.

Ex: tripoidy n = 69, tetraploidy n=92

Question 8

When is triploidly most common?

Answer 8

at fertilization if two sperm fertilize one egg

Question 9

What is the most common mechanism of tetraploidy?

Answer 9

after DNA duplicates, but cells fail to divide

Question 10

What is ISCN? Give an example of normal genotype?

Answer 10

international system for human cytogenetic nomenclature.

Normal: 46,XX or 46, XY

Question 11

How will a monosomy affect an autosome? Trisomies?

Answer 11

all monosomies are lethal in autosomes. Most trisomies are lethal. (potential for live birth- 13, 18, 21)

Question 12

What happens with aneuploidy in sex chromosomes?

Answer 12

tolerate aneuploidy better than autosomes do, but at least one X is required for survival

Question 13

Which is usually the least severe disjunction?

Answer 13

mosaicism, because the phenotype is less severe due to normal cell lines mixed with abnormal instead of all cells being abnormal

Question 14

How common is aneuploidy in female eggs? Sperm?

Answer 14

about 20% in eggs

about 1-2% in sperm

Question 15

What is most likely outcome of aneuploidy in a pregnancy?

Answer 15

failure of the pregnancy

Question 16

How common are spontaneous abortions/miscarriages?

Answer 16

15% of recognized pregnancies end in miscarriage

Question 17

How often do fertilized eggs fail to come to term?

Answer 17

about 50% of fertilized eggs will fail

Question 18

What is the most common cause of miscarriage?

Answer 18

chromosome imbalance

Question 19

What is the most common chromosomal abnormality involved in miscarriage? What is the most common class of chromosomal abnormality?

Answer 19

45, X is single most common at 20% (turner’s syndrome)

trisomies together make up most common class. (nearly 55%)

Question 20

What are the viable trisomies?

Answer 20

21, 18, 13

Question 21

What are the common clinical findings for trisomy 21?

Answer 21

down syndrome usually has mental retardation, flat facial profile, prominent epicanthal folds, simian crease, duodenal atresia (intestinal obsturction) and congential heart defects

Question 22

What is the most common cause of trisomy 21?

Answer 22

nondisjunction during maternal meiosis I.

Usually only 2-4% mosaic and 5% translocation

Question 23

What is Edwards syndrome?

Answer 23

Trisomy 18. Severe mental retardation, prenatal growth deficiency, heart defects, rocker bottom feet, clenched hands, usually die before age 1.
Usually from maternal nondisjunction

Question 24

What is the ISCN nomeclature for trisomy 18 in a female?

Answer 24

47,XX,+18

Question 25

What is Patau Syndrome?

Answer 25

Trisomy 13. Abnormal midface, eye and forebrain. Polydactyly and midline defects. rapidly fatal

Question 26

What is the Positive Exposure project?

Answer 26

project that tries to promote genetic diversity as a good thing, not something to hide away. Photographs children with abnormalities and tries to make them feel good about themselves

Question 27

What will happen to an individual with no X chromosomes?

Answer 27

they will die it is lethal. Usually sex chromosome abnormalities are not as serious.

Question 28

What is turner syndrome?

Answer 28

Only have single X chromosome. short stature, ovariag dysgenesis, congenital heart defect, lymphatic problems. normal intelligence.

Question 29

What is the karyotype for Turner Syndrome?

Answer 29

45,X.

Question 30

What is the only monosomy associated with human live birth?

Answer 30

45,X turner syndrome

Question 31

What is the most common cause of turner syndrome?

Answer 31

PATERNAL nondisjunction (50% of the time it is meiotic nondisjunct and 80% of that is paternal)
Mosaicism is also pretty common (40%)

Question 32

Will advanced maternal age significantly increase risk for Turner Syndrome?

Answer 32

no, turner syndrome is mostly paternal

Question 33

Is turner syndrome often fatal for fetuses?

Answer 33

Yes. over 99% of fetuses die before birth. After birth they have high chance of living.

Question 34

What is Klinefelter Syndrome?

Answer 34

47, XXY

Question 35

What is the most common cause of Klinefelter syndrome?

Answer 35

half of the time it is paternal nondisjunct

half of the time it is maternal and has a maternal age risk effect

Question 36

Does Klinefelter syndrome produce mental retardation?

Answer 36

no, but it does typically drop IQ by about 10-15 points

Question 37

When is Klinefelter usually diagnosed?

Answer 37

when puberty doesnt occur normally or the man is infertile later in life

Question 38

What is triple X syndrome?

Answer 38

47, XXX. Essentially normal phenotype, mild learning problems

Question 39

What usually causes triple X syndrome?

Answer 39

maternal nondisjunction.

Question 40

What is 47,XYY syndrome and what are its phenotypes?

Answer 40

essentially normal phenotype, but tall and has educational and behavioral problems

Question 41

What is the most common cause of 47,XYY syndrome?

Answer 41

always from paternal meiosis II nondisjunction