NONDISJUNCTION


= a meiotic error that results in a gamete with an abnormal number of chromosomes (< 23 or >23)

This can happen in one of two ways:

1.     matched pair(s) of chromosomes fail to separate during _______________________

2.     sister chromatids are not pulled apart during ____________________________

If the abnormal gamete is subsequently involved in a fertilization event, the resulting embryo will inherit an abnormal number of chromosomes (more or less than the normal 46 chromosomes). This usually results in deformities so severe that the embryo dies (accounting for 20% of all _______________________________).  

Click here for more info on the inheritance of too few or too many chromosomes

Embryos with <46 Chromosomes

When nondisjunction results in an embryo that is ______________ one or more chromosomes, the embryo usually develops deformities that are so severe that the embryo dies.
There is only one exception:  inheritance of a single X chromosome, without an accompanying X or Y (individual also has the normal amount of 44 non-sex chromosomes)

Turnerís Syndrome (karyotype is written as: 45, X or, if the individual is a mosaic, as: 45X/46XX) (= females with ____________________ chromosome)  Click here for more info

       Usually results in fetal death; only about ________________ survival rate.

       This is the only case in which a human can survive with only 45 chromosomes.

       Individuals that survive to birth may have heart, kidney or thyroid problems and may possess a thick, webbed neck, but many appear as normal females until puberty.

       Have normal intelligence, but some with mild learning disabilities.

        ________________________________________ at puberty (absence of menstruation, no pubic hair, and little or no breast development.  Estrogen therapy resolves these problems.

       Lacking true ovaries (only streaks of undeveloped ovarian tissue and no oocytes), these individuals are nearly always (99% of cases) _________________.

       Commonly of short stature (only 4 Ĺ - 5 ft. tall), but can grow taller if hormone therapy is administered during puberty.

       Can occur as a _______________________ after fertilization, whereby some cells contain the chromosomal anomaly but others contain the normal complement of 46 chromosomes.   Results from nondisjunction during mitosis, rather than during meiosis. Percentages of cells with the different karyotypes can vary widely among individuals and does not necessarily account for differences in the severity of developmental abnormalities. Click here for more info on mosaics   Here is another interesting link that compares mosaics to chimaeras.

       Frequency of occurrence: 1/2500 live female births

Embryos with >46 Chromosomes

When nondisjunction results in an embryo that inherits one or more __________ chromosomes, the embryo usually develops deformities that are so severe that the embryo dies. Those that do survive to birth are usually severely physically deformed and rarely survive past infancy.
There are a few exceptions:

Downís Syndrome (also known as ___________________________)  Click here for more info
= possession of an _______________________________________ (the individual inherits three copies of chromosome #21 instead of two) (If male, the karyotype is written as: 47, XY, +21. If female: 47, XX, +21)

o      About 95% of Downís Syndrome cases are caused by trisomy 21; 3-4% are caused when all or part of chromosome 21 has been translocated to another chromosome.  Click here for more info

o      Can also occur as a mosaicism after fertilization (see above).

o      Common characteristics: mild to severe learning disabilities, bone deformities, stunted growth, abnormal facial features (e.g. flat nose, small mouth), speech and hearing problems, small hands and feet, small ears, decreased muscle tone, and shortened maximum life expectancy (~55 yrs)

o      Life expectancy and quality of life are strongly correlated with type of educational training and physical therapy, as well as the age at which training and therapy begins.

o      Frequency of occurrence: about 1 in 1000 live births overall, but frequency increases with maternal (and possibly paternal) _________.  Here is a link that explains paternal effect.

The remaining cases of chromosomal anomalies involve individuals that inherit one or more extra ___________ ____________________________ [additional X or Y chromosome(s)].  Physical and mental disabilities resulting from one extra sex chromosome are typically quite ___________ (with the most common problem being either low fertility or infertility).

Sex Chromosome Variations Ė incidence rates are 1/500-1000 live births  Click here for more info

Triple-X Syndrome (47, XXX) (= females with an ___________________ chromosome) Click here for more info

o      Results from nondisjunction of sex chromosomes in either the fatherís or motherís gametes (incidence rates are equal among the sexes)

o      Can also occur as a mosaicism after fertilization (see above).

o      Majority of affected individuals are unaffected (physically and mentally normal). Although some have mild learning disabilities (such as speech and language delays and dyslexia), others have no problems at all and excel as honor students.

o       _____________________ and menstrual irregularities may occur, but not in most cases

o      Adult stature is usually taller than average, with a tendency to be overweight

o      Not "more feminine" than normal females

o      Frequency of occurrence: 1/350-1000 live female births; the most common sex chromosome disorder in females

Klinefelterís Syndrome (47, XXY) (= males with an ___________________ chromosome) Click here for more info 

o      Results from nondisjunction of sex chromosomes in either the fatherís or motherís gametes (incidence rates are equal among the sexes)

o      Can also occur as a mosaicism after fertilization (see above).

o      Physical appearance is normal, at least until puberty

o      Although some have mild learning disabilities (first manifested as speech delay in toddler years and, later, as expressive language problems), others have no problems at all, excelling in school and living their lives without ever even suspecting that they have an additional chromosome (often diagnosed only when seeking diagnosis for infertility problems)

o      Sexual maturity at puberty may appear normal; however, ________________________ occurs in most cases (due to low sperm production), muscular development is less pronounced and facial, pubic and body hair may be sparse or absent (all due to low levels of testosterone); some breast development is also common (~1/3 of cases)

o      Usually tall and thin before and during puberty, then tall (ave is 6 ft Ĺ in) with tendency to be overweight in adulthood

o      Testosterone therapy increases strength and muscle size, promotes the growth of facial and body hair, and improves libido

o      Frequency of occurrence: 1/500-1000 live male births; the most common sex chromosome disorder in males

Double-Y Syndrome (47, XYY) (= males with an ___________________ chromosome)  Click here for more info

o      Results from nondisjunction during _________________________ of ___________________________ in the fatherís spermatocyte (a normal egg is fertilized by an abnormal YY-bearing sperm). NOT due to polyspermy as your book.

o      Can also occur as a mosaicism after fertilization. (see above).

o      Sexual maturation at puberty is normal, although many have severe persistent acne, and IQ is often on the low side, but still within the normal range

o      ____________________ _______ ___________________ (and offspring are usually chromosomally normal).

o      Adult stature is tall and thin (usually greater than 6 ft tall)

o      Not "more masculine" than normal males

o      Incidence of this condition tends to be higher in prison populations than in normal populations, but there is no clear evidence linking this condition to higher levels of aggressive behavior

o      Frequency of occurrence: ~1/1000 live male births, but may be more common (since most affected men show no obvious symptoms and are therefore not likely to be tested)

Inheritance of more than one extra sex chromosome (e.g. 48, XXXX; 48 XXXY, 48 XXYY, 49 XXXXX; 49 XXXXY, 49 XXXYY)

o      Very rare. The extra sex chromosomes do not accentuate the sexual characteristics, and the individual is usually infertile. Usually, the severity of ___________________________________ increases with the number of extra sex chromosomes present.

o      Can also occur as a mosaicism after fertilization (see above).


Inheritance of a complete extra set of 23 chromosomes
(46+23 = 69 chromosomes; called triploidy and written as either 69 XYY, 69 XXY, or 69 XXX)

Can occur in two ways:

1.     nondisjunction of an entire set of chromosomes during Meiosis I of gametogenesis: the sperm or egg ends up with ______ instead of 23 chromosomes

2.     _______________________- two normal sperm simultaneously fertilize one normal egg

Click for question review