Cytogenetics - Test 5 Flashcards
How many chromosomes does a normal healthy horse have?
64 (32 pairs)
What are the three main characteristics that cytogeneticists use to characterize chromosomes?
bands, length, location of centromeres
What is the first category for organizing and numbering chromosomes?
acrocentric vs. metacentric
How are the arms of metacentric chromosome designated?
p, q
What is the cytogenetics designation for a normal male horse?
64, XY
What is the cytogenetic designation for a male horse with three copies of chromosome 31 (trisomy 31)?
65, XY +31
What is the cytogenetic designation for a mare with a balanced translocation?
64, XX t(2;16)
What technique is used when identifying chromosome location for a gene?
fluorescent in situ hybridization (FISH)
What was Zoo-FISH used to identify?
homology of gene organization between two species
What makes FISH studies possible?
homology of DNA sequences between species
What is the most common cytogenetics abnormality among horses?
X monosomy
Which of the following is the correct designation for X monosomy?
63, XO
What is the most consistent characteristic of a mare with X monosomy?
infertility; they have small inactive ovaries
Which of the following is a cause of XY sex reversal?
loss of the SRY gene
What is another cause of XY sex reversal?
mutation of androgen receptor on autosome
Why are cytogenetic disorders sometimes identified as genetic but not hereditary?
some cytogenetic variants are not inherited since the affected is infertile
Which of the following is a cytogenetic variant that can be inherited?
balanced translocation
What is the phenotypic gender of a horse with XY sex reversal?
female
Why do horses with translocated chromosomes have reduced fertility?
partial failure at meiosis
Why do we think that we only see trisomy for small chromosomes?
trisomy for large chromosomes would cause too many copies of too many genes
What is the large round idem on the right side of this image?
intact nucleus
What is the karyotype of this individual represent?
XY sex reversal
What does the karyotype of this individual represent?
Monosomy X
What does this karyotype represent?
balanced translocation
What does this karyotype represent?
trisomy X
What does this image illustrate?
FISH image for trisomy 30
What does this image represent?
FISH image showing two genes are on the same chromosome
What type of chromosome is this?
metacentric
What is a metacentric chromosome?
centromere located midway between ends of chromosome; separating two arms of chromosome
- submetacentric: centromere between one end and the middle; one longer and one shorter arm
What is a acrocentric chromosome?
centromere located near the end of chromosome
- telocentric: located at the end of chromosome
What does G-banding do?
identify chromosome pair based on banding pattern
What causes a trisomy in horses?
one of the chromosomes fail to separate from partner and both segregate to sperm/egg
- almost always, only small chromosomes
What is a translocation?
piece of one chromosome breaks off and attaches to another
What is mosaicism?
during development of females, one of the X chromosomes gets lost, causing population of cells with X and a population without X
- see two types of cells
abnormal karyotypes used
- ”+” to identify polysomes (extra chromosomes)
- ”/” to designate mosaicism (two populations of cells)
- “t (chromosome #, chromosome #)” to describe translocation
- some others for deletions (del), etc.
What don’t we see in live horses?
- monosomy (loss) of autosomes: too harmful
- trisomy (3 copies) of large autosomes: too harmful
PIC NEEDED FROM QUIZ 16 QUESTION 1
If the dam contributed the allele for K to the offspring, which allele must come from the sire?
M
FISH technology
clone DNA for any gene and label with fluorescent dye
PIC NEEDED FROM QUIZ 16 QUESTION 2
Which sire is disqualified based on the results of AHT4, if we assume the dam is correct?
Neither
PIC NEEDED FROM QUIZ 16 QUESTION 3
Which sire is excluded based on the results for AHT5, if we do not assume the dam is correct?
Neither
63,X or “XO mares”
- most common chromosome abnormality
- sometimes small stature
- small, inactive ovaries
- underdeveloped repro. tract
- may have angular limb deformities/poor conformation
- infertile, but still healthy + don’t have estrous cycles
PIC NEEDED FROM QUIZ 16 QUESTION 4
Which sire is disqualified on the basis of HTG4 if we do not assume the dam is correct?
Neither
65,XXX
- normal looking mare
- inconsistent estrous cycles if cycles at all
- very small ovaries
- underdeveloped uterus
- hypoplastic endometrium
- infertile
- rare
What is included in Disorders of Sexual Development (DSD)
- sex reversal syndrome
- intersex
- hermaphrodism
- pseudohermaphrodism
PIC NEEDED FROM QUIZ 16 QUESTION 5
Which sire is disqualified on the basis of HTG6, assuming the dam is correct?
Sire 1
PIC NEEDED FROM QUIZ 16 QUESTION 6
Which sire is disqualified on the basis of VHL20, assuming the dam is correct?
Sire 1
Gender Reversal: 64, XY, SRY-
female phenotype with male karyotype (64, XY)
- SRY (sex determining region) on Y chromosome that sets in motion developmental characteristics of males
- mutation leading to loss of SRY causes to appear female
- Y chromosome has repeats and inversions that make it possible for loss of SRY meiosis
PIC NEEDED FROM QUIZ 16 QUESTION 7
Which sire is disqualified on the basis of LEX3?
Neither
PIC NEEDED FROM QUIZ 16 QUESTION 8
Overall, which of the two sires is disqualified as being parents of the foal?
Sire 1
DSD (female characteristics) 64,XY sex-reversal syndrome
- normal female external appearance; small, inactive ovaries, non-tubular uterus
- may suspect XO
- some are large in stature
- infertile
PIC NEEDED FROM QUIZ 16 QUESTION 9
Bizarre.. but why not… It might be possible that the foal was switched with another. With regard to just Sire 1, the Dam and the Foal…. What might we discern?
Mating is still excluded
What does the term “genetic load” mean?
The number of recessive disease causing genes carried by an individual
What does term “carrier” refer to with respect to disease genes?
Horse with a single copy of a recessive disease gene, which is inapparent from phenotype
SRY+, XY female
- normal female appearance and external genitalia, but small, blind-ending vagina and hypoplastic uterus
-each had pair of underdeveloped abdominal testes containing seminiferous tubules and germ cells - mutation found for Androgen Receptor Gene
Why might a partially dominant disease gene become common in a population of horses?
Because it may be linked to a desirable gene
Whats another reason that a partially dominant gene may become common in a population of horses?
It is pleiotropic, having both beneficial and deleterious effects and selected by breeders
What is the mode of inheritance for HYPP?
Partial dominant
HYPP appeared as a mutation in the stallion named Impressive. Why did HYPP subsequently become so common among Quarterhorses?
Breeders selected for muscularity caused by this gene
What was the negative aspect of the HYPP gene that caused some breeders to want to control it?
Horses with this gene could experience paralysis during exercise
The molecular basis for HYPP was discovered in 1992. What method was used to discover it?
Candidate gene
Which breeds of horse are affected by severe combined immunodeficiency disease
Arabians
What are the symptoms of SCID?
Absence of immune system; death from opportunistic infections by age 3
If you wanted information about diseases of livestock, which source would be most accurate and specific?
OMIA
Why are some genetic diseases difficult to study?
Breeders do not continue to breed when they occur
Some diseases have multiple genetic causes
Some diseases are caused by more than one gene
Some diseases have variable symptoms (poor phenotypes)
JEB1 and JEB2 occur in Belgian Draft horses and American Saddlebred horses, respectively. They are:
Phenocopies
Founders are individuals who are the first know carriers of a disease gene and the source for future generations. Which of the following diseases does not have a single, known Founder?
ACAN dwarfism
XX sex-reversal syndrome
Male-like external genitalia: testes, no sperm;
ovarian tissue or ovotestes; increased
anogenital length; enlarged clitoris or small
penis; closed vagina; some exhibited stallion
-like behavior
- may have high testosterone levels
- SRY and Y chromosome negative
- normal female karyotype of 64,XX
translocation summary
- rare
- horses can be entirely healthy but have lowered fertility due to problems following meiosis
trisomy summary
- extremely rare
- only seen with small chromosomes (or X)
- if with larger chromosomes, probably result in embryonic lethal effect
