cytogenetics 1

human genetics

what chromosomes are acrocentric

13-15, 21, 22

what is the Lyon hypothesis

the lyon hypothesis states that in female mammals, one of the two X chromosomes is randomly inactivated during early embryonic development, leading to dosage compensation between males and females.

what does the inactivated chromosome turn into in x-inactivation

a Barr body

what does XIST do?

XIST is a long non-coding RNA that is responsible for the initiation and maintenance of X-inactivation by coating the inactive X chromosome. Stands for x-inactive-specific-transcript. silences transcription from the chromosome that expresses it

Where do X and Y chromosomes recombine

during male meiosis at the pseudoautosomal regions. (homologous sequence of nucleotides that behave like an autosome by pairing and recombining)

aneuploidy definition

gain or loss of a chromosome(s)

what do aneuploidys mainly arise from?

Non-disjunctions (paired chromosomes fail to separate)

what is mosaicism

different cell populations in one organism (arises through subsequent mutations, we are all mosaics due to somatic mutations)

what is chimerism

when two zygotes fuse so two different cell pops

what three autosomal trisomies are not lethal

13 (Patau), 18 (Edwards), 21 (Down syndrome)

why are trisomies usually lethal and what makes the three common trisomies viable

usually lethal due to additive gene dosage effects. The three viable trisomies are on the smallest chromosomes which have the least amount of genes

how often do monosomies and trisomies occur in zygotes

1 in 20 (but the majority terminate)

how can trisomy 21 arise

nondisjunction of ch 21 during egg or sperm development or 2-4% arise from robertsonian translocation involving chromosome 21

how common is trisomy 21 in live births

1 in 1000

how common is trisomy 13

Patau syndrome is 1 in 10000 live births

how common is trisomy 18

edwards syndrome occurs in 1 in 5000 live births

how does trisomy 13 and 18 arise?

13 arises through nondisjunction or robertsonian translocation. 18 arises only through non disjunction as the chromosome is NOT acrocentric

most severe to least severe viable trisomies (and why)

13→18→21 (largest chromosome to smallest chromosome)

most common to least common trisomies (why)

21→18→13 (least severe is the most common, most severe is the least common)

what is turner syndrome

45,X

how common is turner syndrome

1 in 2500

what condition is (45,X)

turner syndrome

what is klinefelters syndrome

47,XXY

how common is klinefelters syndrome

1 in 600

what condition is 47,XXY

Klinefelter syndrome

what is Jacobs syndrome

47,XYY

How common is Jacobs syndrome

1 in 1000

what condition is XYY

Jacobs syndrome

most to least common gametic aneuploidies

klinefelters→jacobs→turner

most to least severe gametic aneuploides

turner→klinefelters→ jacobs (XYY isn’t as bad cause ychromosomes dont have much genetic material) (turner is most serious because of missing genetic material, not all genes are inactivated on inactivated x-chromosomes

what are mosaic aneuplodies most often due to

anaphase lag (loss of a chromatid in mitosis, chromatid faults to connect to spindle or is delayed in movement toward the spindle)

what is trisomy rescue and how does it happen and what other disorder can it cause

anaphase lag can result in trisomy rescue— loosing one of the trisomic chromosomes, if the two chromosomes left over are from the same parent this is UPD (Uniparental disomy)

what percentage of zygotes will have UPD after trisomy rescue

1/3 will have UPD. 2/3 will have normal biparental contribution)