Chromosomal Abnormalities I

Question 1

Describe the events in meiosis that produce genetic variability among offspring.

Answer 1

Random segregation of homologous chromosomes (2^23 possibilities)
Recombination between homologous chromosomes

Question 2

Compare and contrast mitotic and meiotic cell divisions.

Answer 2

Mitosis
A. one round of chromosome segregation, resulting in daughter cells identical in chromosomal content to the parental cell
B. DNA replication precedes each round of chromosome segregation
C. no pairing of homologous chromosomes
D. infrequent recombination
E. centromeres on paired sister chromatids segregate at each anaphase
F. occurs in somatic cells and in germ line precursor cells prior to entry into meiosis
Meiosis
A. two rounds of chromosome segregation without an intervening round of DNA replication
B. parental cells must be diploid and the chromosome number is halved in the resultant cells
C. requires the pairing of homologous chromosomes and recombination for its successful completion
D. centromeres on paired sister chromatids divide only at anaphase II in a normal meiosis
E. occurs only in the germ line

Question 3

Describe the relationship between meiotic recombination (crossovers) and chromosome nondisjunction

Answer 3

Crossovers are essential for tethering homologous chromosomes during the first meiotic division

1. Nondisjunction events are related to the positioning of chiasmata; crossover events that occur too near or too far from the centromere increase chromosome nondisjunction. Distal = less effective in ensuring appropriate spindle attachment and separation; centromere-proximal or excessive #s of exchanges lead to entanglement of paired homologs
2. Nondisjunction events are also related to the frequency of crossover events. The reduction or absence of recombination events increases the likelihood of nondisjunction.

Question 4

Differentiate between the reproductive consequences of nondisjunction events in meiosis I versus meiosis II.

Answer 4

Meiosis I nondisjunction = 100% abnormal gametes (2 will be 2n, 2 will be 0)
Meiosis II nondisjunction = 50% abnormal gametes (1 will be 2n, 2 will be normal, and 1 will be 0)

Question 5

What are the 3 most common trisomies?

Answer 5

13 = Patau syndrome
18 = Edwards syndrome
21 = Down syndrome
XXY = Klinefelter syndrom

Question 6

Describe trisomy 13

Answer 6

Patau syndrome:
• Characteristic facies, severe intellectual disabilities
• Congenital malformations: holoprosencephaly, facial clefts, polydactyly, renal anomalies, congenital heart disease, omphalocele, dermal defects

Question 7

Describe trisomy 18

Answer 7

Edwards syndrome:
• Intrauterine growth retardation, characteristic facies, severe intellectual disabilities, hypertonicity (clenched hand, narrow hips)
• Congenital malformations – valvular heart disease, posterior fossa anomalies, seizures, CNS maldevelopment, diaphragmatic hernias, renal anomalies

Question 8

Describe trisomy 21

Answer 8

Down syndrome:
• Short stature, intellectual disabilities, hypotonia, dysmorphic feat (characteristic facies, short fingers w/ trans palmar crease, clinodactaly, wide “sandle grip”)
• Congenital malformations – congenital heart disease, endocardial cushion defects, duodenal atresia and other GI anomalies, Hirschprung disease.

Question 9

Describe 47 XXY

Answer 9

Klinefelter syndrome
• Tall stature, hypogonadism, gynecomastia, infertile, under-dev. secondary sex char, language impairment, impulsive
• Phenotypically male, but no gonad development

Question 10

Describe 45 X

Answer 10

Turner syndrome
• Short stature, webbed neck, edema of hands and feet, broad shield-like chest, narrow hips, renal and cardiovascular anomalies, gonadal dysgenesis (failure of ovarian maintenance).

Question 11

XYY syndrome

Answer 11

Normal male
• Increased risk of behavioral and educational problems, delayed speech and language skills
• Not associated with criminality, as was originally hypothesized

Question 12

Paternal error in meiosis I causes:

Answer 12

XXY, Klinefelter
XXYY
XXYY –
XY XY - -

Question 13

Paternal error in meiosis II causes

Answer 13

XYY Syndrome
XXYY
XX YY
X X YY -