Unit 7 - Introduction to Cytogenetics Flashcards
what is cytogenetics?
the science that combines methods and findings of cytology and genetics to study heredity at the cellular level
-one of the oldest fields of genetics
when does recombination happen in meiosis?
prophase I
when does reduction division happen in meiosis? what is it?
anaphase I (hallmark of meiosis) -homologs pair, and centromeres remain together so that homologs separate
what are 2 types of meotic nondisjunction? how do they occur? examples of if Xm A and B?
isodisomy - 2 Xm from same source (duplication of 1 Xm)
- nondisjunction happens in meiosis II
- get 2 N cells (A1/B2 or A1/B2), 1 N-1 cell (B1), and 1 N+1 cell (A2/A2/B1)
heterodisomy - 2 different Xm, but same homologs
- nondisjunction happens in anaphase I
- get 2 N+1 cells (A1/A2/B2, A1/A2/B2; disomic) and 2 N-1 cells (B1 and B1; nullosomic)
when does oogenesis begin? when are primary oocytes present?
oogenesis starts in developing fetus
- by 3rd month of gestation, primary oocytes are present
- they reach dictyotene (prophase I) by birth, and remain so until ovulation
- at ovulation, the oocyte completes meiosis I, and becomes secondary oocyte (gets most of the cytoplasm) and the first polar body (usually degrades, may divide again to become secondary polar bodies)
when is meiosis II completed for eggs?
in fertilization; makes an ovum and a second polar body (with less cytoplasm)
spermatogenesis VS oogenesis
- when are primary spermatocytes VS oocytes made/found?
- when are gametes produced?
- how many gametes per gametocyte?
- primary spermatocytes made throughout life, while primary oocytes are all present at birth
- male gametes made contiually, but female only once a month
- 4 equal male gametes but only 1 female gamete per original gametocyte
homogametic VS heterogametic
females are homogametic (have 2 copies of X Xm)
males are heterogametic (have 1 X and 1 Y)
what is the TDF? the SRY? the pseudoautosomal region?
TDF - testis determining factor
SRY - sex determining region of Y
pseudoautosomal region - region on short arms of X and Y Xm that engage in recombination (normal meiosis I exchange)
what is the default sex developed? what is sex really determined by?
female (absence of any other signal causes female to develop)
-sex determination is due to genes on the X, Y, and autosomes, and occurs very early in development
female sex determination
no TDF or SRY
- ovary develops
- -proliferation of Mullerian ducts
- -regression of Wolffian ducts
male sex determination
TDF and SRY
- testis develops
- -inhibition of Mullerian ducts
- –degeneration of Mullerian ducts
- -androgen (testosterone)
- –Wolffian duct proliferation
what happens if there is gain or loss of key genes, or loss of one of the sex Xm after the sex of an individual has been determined?
clinically irrelevant
what is the Lyon hypothesis? what is a Barr body and how did it prove this?
1 random X is inactivated in somatic cells of females
- supported by staining that showed Barr body (condensed X Xm)
- total number of Barr bodies should equal the total number of X Xm minus 1
when does X inactivation occur? is it random? what does this result in?
3-7 days after fertilization, so that determination of a normal female (requiring 2 active X Xm) can still occur
- it is random, but once established, it is not reversible in somatic tissue
- results in dosage compensation
what is somatic mosaicism?
heterozygous female will have subpopulations of cells within body
- some cells express traits from maternal X, others from paternal X (tortoise shell cat)
- males will express only traits from maternal X (black or yellow)
when is X inactivation not random?
non-random X inactivation happens if a damaged X is preferentially inactivated
-skews distribution so alleles on other X are always expressed, which can cause clinical problems (Duschenne muscular dystrophy in females)
what is the mechanism of X inactivation?
epigenetic modification (no gene mutations occur)
- methylation starts at XIST locus (X inactivation center), and spreads along length of Xm
- several sites escape inactivation, including pseudoautosomal region
- inactive X must be reactivated at meiosis, so that active Xs are transmitted to offspring
cytogenetics in fetal loss
- 1:13 conceptuses with Xmal abnormality –> 6/1000 live born
- 15% of recognized pregnancies end in spontaneous abortion –> 80% in first trimester
- -of these, 60% are Xmal
- –of these, 52% are autosomal trisomies
postnatal cytogenetics
about 0.6^ of newborns have Xm anomaly
- karyotype analysis can confirm diagnosis of known Xmal disorders
- amgibuous genitalia
- multiple congenital anomalies
children and adult cytogenetics
not all Xm abnormalities manifest early in life
- features of a known Xmal disorder
- family history of an Xmal disorder
- mental retardation
- infertility
- malignancies
numerical VS structural Xmal abnormalities
numerical - change in total number of Xm in the st (gain or loss of Xm)
structural - change in size/shape of one or more Xm in the set (deletion, translocation, etc.)
what specimens can be used to obtain Xm samples?
blood - easiest and least painful, but requires WBC only
amniotic fluid and chorionic villi - prenatally
bone marrow - usually for oncology studies
tissue - skin biopsy in living individual, but usually used if deceased
what are the 3 primary landmarks used to identify specific Xm in karyotype analysis? what stage are Xm stuck in during this?
Xm are arrested in metaphase
-use size, centromere position, and banding pattern to identify specific Xm
what is the p VS q arm?
P = short arm
Q = long arm
in metacentric Xm, use banding patterns to determine P VS Q
metacentric, submetacentric, and acrocentric?
metacentric - centromere is equidistant from both ends
submetacentric - centromere is closer to one end than another
acrocentric - modified short arms with stalks containing multiple copies of rRNA genes capped by modified telomere (satellites)
what repeat sequence do telomeres have?
TTAGGG
what are Xm usually stained with?
Giemsa or Wright stain
- mild trypsinization before staining weakens the DNA-PRO interactions to yield a defined pattern of alternating light/dark regions (G-banding)
- each pair of Xm has unique band pattern schematically represented in ideograms
what are Xm polymorphisms?
normal variations that exist in human population
- must be distinguished as different from abnormality
- presence of 2+ alternative structural forms for an Xm within a population
- inherited as Mendelian characters, and can be traced through pedigrees
- variation usually not associated with clinical disease
