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Terms in this set (64)
What is the incidence of Down Syndrome?
1 in 800 births
What is the single most common genetic cause of moderate MR?
Down syndrome survival
20-25% conceptuses survive to birth
What percentage of DS is due to Tri21?
95% (90% from Mat, usually M-I; 10% from Pat, usually M-II)
What percentage of DS is due to Robertsonian Translocation?
14;21 Translocation Carrier Karyotype
Risk of DS child for 14;21 Translocation Carrier
15% if maternal, less than 1% if paternal
In what % of 14;21 DS is one parent a carrier?
21;22 Translocation carrier Karyotype
Risk of DS child for 21;22 Translocation Carrier
21;21 Translocation/Isochromosome Recurrence Risk
What % of individuals with DS are mosaic?
about 2%, usually milder features but there is a range
Recurrence Risk of DS?
(if no translocations)
1.4% for women under 30
age-based for 30+
Trisomy 18 (Edward syndrome) features
MR, FTT, severe heart malf, hypertonia, prom occiput, receding jaw, low set ears, rocker bottom feet
Incidence of Tri18
1 in 7500 births
What % of Tri18 spontaneously abort?
What % of Tri18 are female
(perhaps preferential survival)
Incidence of Tri13 (Patau syndrome)
1 in 15,000-20,000 births
Karyotypes of Tri13
80% Trisomy 13
20% 13;14 translocation (55% of time, parent is carrier)
What is the prevalence of cyto visible deletions?
1 in 7000 births
What type of mutation causes cri-du-chat?
deletion of 5p
(mostly sporadic, 10-15% offspring of transloc carrier)
Features of Cri-du-chat
mewing cry, microceph, hypertelorism, epicanthal folds, micrognathia, mod to severe MR
*role in up to 5% of heart defects, 40% of TOF
How do X and Y chromosomes avoid nondisjunction?
by pairing at meiosis by pseudoautosomal regions at ends of p arms
What is responsible for whether a fetus develops into a male or female?
Testicular Determining Factor (TDF) in SRY; present in males of all species
acts with SOX and SIP-1
Reproductive Development: Testis-Determining factor (TDF) switches to male pathway
leydig cells: product testosterone
Sertoli: cause Mullerian regression
Wolffian(mesoneph) ->vas deferens
5a-reductase->DHT->phallus & Scrotum
Reproductive Development: no TDF present - Ovarian development is default
Wolffian ducts regress
Urogenital folds stay unfused
Sex Reversal by SRY
Sex-Determining Region Y: present in XX males, lacking in XY females (1 in 20,000 births each)
Inactive X: promoter region of genes modified by methylation to silence; 10-15% of genes stay active
Xq13: X-inact center (XIST gene, expressed from inact X)
Collective Incidence of X-Linked MR
1 in 500-1000
Incidence of Sex Chromosome Abnormalities
1 in 400
1 in 1000 male births
Infertile, tall, long limbs, narrow shoulders and chest, relatively small genitalia
Origin of Klinefelter
50% error in Pat Meiosis I
50% error in Mat Meiosis (most Me-I)
Variants of Klinefelter
15% are mosaic
Incidence and Features
1 in 1000 male births
frequent behavior problems and possible lower IQ
Trisomy X (47,XXX)
Sometimes taller, possible lower IQ, behavior problems
Almost all due to Mat Meiosis I errors
Turner Syndrome features
webbed neck, lymphedema of hands/feet at birth, short stature, webbed neck, delayed sexual maturation, broad chest, gonadal dysgenesis (streak gonads)
Turner Syndrome Incidence
1-2% of conceptuses
99% abort spontaneously
*usually due to sperm lacking X
both ovarian and testicular tissue is present, very rare
What are the causes of true hermaphrodite?
46,XX (may be AR)
What causes gonadal dysgenesis in Sex-reversed 46,XY females?
SRY or duplication of DAX1 on Xp21.3 (encodes a TF that plays dosage-sensitive role in determination of gonadal sex
What causes gonadal dysgenesis in camptomelic dysplasia?
mutations in SOX9 gene on 17q, AD with usually lethal skeletal malformations; ~75% of XY are sex reversed (absence of SOX9, testes fail to form)
What causes gonadal dysgenesis in WT1 mutation carriers?
TF involved in interactions between Sertoli and Leydig cells. Mutations disrupt normal testicular development
What is the theory behind lack of ovary development vs maintenance?
it is believed that 2Xs necessary for maintenance (ex. Turners: nl early development of ovaries, but cannot maintain)
What is Pseudohermaphroditism?
psuedo = have gonadal tissue of only one sex that matches chromosomal constitution
What is Female Pseudohermaphroditism?
46,XX with nl ov tissue but with ambig or male external genitalia
aka 46,XX DSD
What are the causes of female pseudohermaphroditism?
Congenital adrenal hyperplasia (CAH)
teratogens (high androgen levels)
What are the features of Congenital Adrenal Hyperplasia (CAH)?
Defects in enzymes that are required for cortisol biosynthesis; Accounts for ½ of all cases presenting with ambiguous external genitalia; Excessive production of androgens cause masculinization of external genitalia in female infants
What is the most common type of CAH?
1 in 25,000 births
(75% have salt-losing type)
What are causes of Male Pseudohermaphroditism?
45X/46XY mosaic(range from nl M to turners)
What is 5α-reductase deficiency?
male pseudohermaphroditism; enzyme responsible for converting the male hormone testosterone to its active form hihydrotestosterone
What is androgen insensitivity syndrome?
aka testicular feminization
complete: female extern, no body hair or uterus
incomplete: amb genitalia, breasts, no uterus
1 in 20,000 births; usually raised as female, but infertile
What is hypogonadotropic hypogonadism?
infertile, sexually infantile, nl internal genitalia, micropenis in males
What are the causes of hypogonadotropic hypogonadism?
hypopituitarism, Kallman syndrome
What are the causes of 46,XX males?
aka 46,XX testicular DSD
80% due to Y material on X
Autosomal recessive forms
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