Unit 7 - Introduction to Cytogenetics

Question 1

what is cytogenetics?

Answer 1

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

Question 2

when does recombination happen in meiosis?

Answer 2

prophase I

Question 3

when does reduction division happen in meiosis? what is it?

Answer 3

anaphase I (hallmark of meiosis)
-homologs pair, and centromeres remain together so that homologs separate

Question 4

what are 2 types of meotic nondisjunction? how do they occur? examples of if Xm A and B?

Answer 4

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)

Question 5

when does oogenesis begin? when are primary oocytes present?

Answer 5

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)

Question 6

when is meiosis II completed for eggs?

Answer 6

in fertilization; makes an ovum and a second polar body (with less cytoplasm)

Question 7

spermatogenesis VS oogenesis

• when are primary spermatocytes VS oocytes made/found?
• when are gametes produced?
• how many gametes per gametocyte?

Answer 7

• 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

Question 8

homogametic VS heterogametic

Answer 8

females are homogametic (have 2 copies of X Xm)

males are heterogametic (have 1 X and 1 Y)

Question 9

what is the TDF? the SRY? the pseudoautosomal region?

Answer 9

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)

Question 10

what is the default sex developed? what is sex really determined by?

Answer 10

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

Question 11

female sex determination

Answer 11

no TDF or SRY

• ovary develops
• -proliferation of Mullerian ducts
• -regression of Wolffian ducts

Question 12

male sex determination

Answer 12

TDF and SRY

• testis develops
• -inhibition of Mullerian ducts
• –degeneration of Mullerian ducts
• -androgen (testosterone)
• –Wolffian duct proliferation

Question 13

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?

Answer 13

clinically irrelevant

Question 14

what is the Lyon hypothesis? what is a Barr body and how did it prove this?

Answer 14

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

Question 15

when does X inactivation occur? is it random? what does this result in?

Answer 15

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

Question 16

what is somatic mosaicism?

Answer 16

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)

Question 17

when is X inactivation not random?

Answer 17

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)

Question 18

what is the mechanism of X inactivation?

Answer 18

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

Question 19

cytogenetics in fetal loss

Answer 19

• 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

Question 20

postnatal cytogenetics

Answer 20

about 0.6^ of newborns have Xm anomaly

• karyotype analysis can confirm diagnosis of known Xmal disorders
• amgibuous genitalia
• multiple congenital anomalies

Question 21

children and adult cytogenetics

Answer 21

not all Xm abnormalities manifest early in life

• features of a known Xmal disorder
• family history of an Xmal disorder
• mental retardation
• infertility
• malignancies

Question 22

numerical VS structural Xmal abnormalities

Answer 22

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.)

Question 23

what specimens can be used to obtain Xm samples?

Answer 23

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

Question 24

what are the 3 primary landmarks used to identify specific Xm in karyotype analysis? what stage are Xm stuck in during this?

Answer 24

Xm are arrested in metaphase

-use size, centromere position, and banding pattern to identify specific Xm

Question 25

what is the p VS q arm?

Answer 25

P = short arm
Q = long arm
in metacentric Xm, use banding patterns to determine P VS Q

Question 26

metacentric, submetacentric, and acrocentric?

Answer 26

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)

Question 27

what repeat sequence do telomeres have?

Answer 27

TTAGGG

Question 28

what are Xm usually stained with?

Answer 28

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

Question 29

what are Xm polymorphisms?

Answer 29

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