Chromosome Abnormalities

I. Chromosome TermsA. Autosome – chromosomes that are not involved in

determining the sex (gender) of an individual. (22 pairs of autosomes in humans

B. Sex chromosomes – determine the sex of an organism

1. In humans

a. male is XY

b. female is XX

2. In other organisms the number and structure vary

a. ex. Grasshopper female XX, male XO (O = absence)

b. birds – male XX, female XO

C. Monosomy – having only 1 chromosome of a set (1 missing)

D. Trisomy – having 3 chromosomes of a set (1 extra)

II. Errors in Meiosis & Chromosomal MutationsA. Non-disjunction – the unequal separation of

chromosomes during meiosis

1. Can happen in Meiosis I – during separation of homologous (like) chromosomes

2. Can happen in Meiosis II – during separation of homologues (identical chromosomes)

3. Results in either monosomy or trisomy

4. Is most common in older parents (especially females)

B. Deletion – section of a chromosome is broken off completely

C. Duplication – chromosome fragment attaches to its homologous chromosome; causing it to carry 2 copies of a certain set of genes.

D. Inversion – the chromosome piece reattaches to the original chromosome but in a reverse orientation.

E. Translocation – a piece of chromosome reattaches to a non-homologous chromosome. Ex. A pieces from chromosome 9 is moved to chromosome 22.

Deletion Duplication Inversion

Robertsonian Translocation

Reciprocal Translocation

III. Karyotypes

A. A karyotpe is an organized profile of a person’s chromosomes.

B. In a karyotype, chromosomes are arranged and numbered by size, from largest to smallest.

C. This arrangement helps scientists quickly identify chromosomal alterations.

D. To make a karyotype, scientists take a picture of someone’s chromosomes (while a cell is in mitosis), cut them out and match them up using size, banding pattern and centromere position as guides.

Male

Female

IV. Chromosomal Disorders

A. Trisomy Disorders – disorders in which an individual has an extra (or piece of an extra) chromosome. (3 chromosomes)

1. Down Syndrome (Trisomy 21) – caused by the presences of all or part on an extra 21st chromosome. It is named after John Langdon Down, the British doctor who described it in 1866. Individuals with the syndrome have characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation. Individuals have a shorter life span and are prone to leukemia and Alzheimer’s disease. The incidence of Down Syndrome can be correlated with the age of the mother. Down Syndrome can be identified during pregnancy or at birth and affects 1 out of every 700 live births.

2. Edward’s Syndrome – caused by an additional copy of chromosome 18. Individuals with the syndrome have an elfin appearance with small nose and mouth, a receding lower jaw, abnormal ears, and they cannot properly bend their fingers. Individuals also suffer from severe mental retardation. It occurs about 1 in every 10,000 live births. Most of the affected individuals are female and 80 to 90% of them die by age 2.

3. Patau Syndrome – a syndrome in which a patient has an additional chromosome 13. The extra chromosome disrupts the normal course of development, causing the characteristic features of Patau syndrome which are cleft palate, hare lip, congenital heart defects, polydactyly (extra fingers) and severe mental retardation. Like all non-disjunction diseases the risk of disease in the offspring increases with maternal age at pregnancy. Patau syndrome affects aproximately 1 in 15,000 live births.

Edward’s Syndrome

Patau Syndrome

4. Klinefelter’s Syndrome – is a condition caused by an extra X sex chromosome. The principal effect is small testes development and reduced fertility. A variety of other physical and behavioral differences and problems are common, though severity varies and many boys and men with the condition have few detectable symptoms. It is the second most common extra chromosome condition and is named after Dr. Harry Klinefelter, an endocrinologist who first described it in 1942. The condition exists in roughly 1 out of every 500 to 1,000 males.

5. Triple X Syndrome – syndrome characterized by the presence of an exta X chromosome in each cell of a human female. Occurs at a frequencey of 1/1,000 female live births. No specific abnormalities are usually associated with this condition. The vast majority of women who have this condition are normal mentally and physicllay and are fertile. Most women with XXX who are fertile do not tend to pass the extra X to their offspring. It is also possible to inherit 4 or 5 X’s. With each extra X, there tends to be a corresponding increase in mental handicap, a chance of being sterile, and a lack of secondary sex characteristics.

Triple X Syndrome

6. Jacob’s Syndrome – A chromosome aberration which is causesd by non-disjunction of the Y chromosome during the second phase of meiosis giving a 47 XYY karyotype. Occurrences is 1/1,000 live male births. Men with this karyotype are tall and have low mental ability. Most men with an extra Y chromosome lead normal lives, but 4% of men in prison are XYY..

B. Monosomy Disorders – disorders in which an individual is missing a chromosome or part of a chromosome.

1. Turner’s Syndrome – syndrome in which only one X chromosomes is present. Women with Turner’s syndrome can live normal lives, though they are unable to bear children. The phenotype of this female includes short stature, short broad neck, and a broad chest. Intelligence does not seem to be affected. This can occur in about 1 per 2,700 births and is not linked to maternal age.

Geneticist who has Turner’s Syndrome

2. Cri du Chat Syndrome – caused by the deletion of a portion of chromosome 5. It is characterized by a cat-like-cry (in half the infants), a small cranium, congenital hear disease, and severe mental retardation. The incidence of this syndrome is 1/1,000,000 live births.

3. Angelman Syndrome – a rare neuro-genetic disorder name after a British pediatrician, Dr. Harry Angelman, who first described the syndrome in 1965. AS is caused by a deletion or inactivation of critical genes on the maternally inherited chromosome #15. The syndrome is characterized by intellectual and developmental delay, speech impediment, sleep disturbance, unstable jerky gait, seizures, hand flapping movements, frequent laughter/smiling and a happy demeanor. Sometimes referred to as “Puppet man syndrome” . The sister syndrome is called Prader-Willi Syndrome, and is caused by loss of paternal genes.

Cri du Chat Syndrome

Angelman Syndrome

Actor Colin Farrell’s son James suffers from Angelman Syndrome.

Prader-Willi Syndrome

Prader-Willi syndrome (PWS) is a chromosomal disorder with symptoms

that include learning difficulties, obesity and behavioural problems

The End