X Inactivation in Female Mammals

• During fetal development, one female X chromosome become inactive and lies along the inside of the nuclear membrane

• Which X chromosome inactivates is random and varies from cell to cell, however during mitotic division the same X chromosome remains inactive

• The inactive X chromosome is called a Barr body

• Females consist of a mosaic of two types of cells; those with active and those with inactive X chromosomes

X Inactivation in Female Mammals

Tortoiseshell cats show mosaicism with patches of orange and black fur. Therefore, all tortoiseshell cats are female.

Alterations of Chromosome Number

• Nondisjunction – members of a pair of homologous chromosomes fail to move apart properly during meiosis I or sister chromosomes do not separate during meiosis II.

• One gamete receives two of the same type of chromosome and the other receives no copy.

Nondisjunction

• Aneuploidy – an abnormal number of chromosomes

• Monosomic – a chromosome is missing (2n - 1)

• Trisomic – a chromosome is present in triplicate (2n + 1)

• Polyploidy – an organism with two or more complete chromosome sets–Triploidy – 3n–Tetraploidy – 4n

Alterations of Chromosome Number

Alterations of Chromosome Structure

Caused by a breakage of a chromosome

Human Disorders Due to Chromosomal Alterations

• Down’s syndrome affects 1 out of 700 births in the US

• Results from a trisomy of chromosome 21 (2n+1)

• Characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation

• Individuals are also more prone to develop leukemia and Alzheimer's disease; these diseases are located on chromosome 21

• Risk – 0.04% for women under 30

1.25% for women over 30

Human Disorders Due to Chromosomal Alterations

Down’s syndrome

Human Disorders Due to Chromosomal Alterations

Klinefelter’s Syndrome

– XXY sex chromosomes

– Individual usually sterile

– Have both male and female secondary sexual characteristics

– Occurs in offspring of mother’s over 40 years old.

Human Disorders Due to Chromosomal Alterations

XYY Condition

– Males who have an extra Y chromosome

– Tend to be very tall and have subnormal intelligence

– 1 in 1000 births

Human Disorders Due to Chromosomal Alterations

Turner’s Syndrome

– Monosomy X chromosome; individual is female

– 1 in 5000 births

– Usually sterile

Human Disorders Due to Chromosomal Alterations

Triple X Syndrome

– Trisomy – XXX chromosomes

– 1 in 1000 births

– Usually a healthy female

Human Disorders Due to Chromosomal Alterations

• Trisomy chromosome 8

– Mental retardation, skeletal deformities, internal organs malformed

– 1 in 3,000 births

• Patau Syndrome

– Trisomy 13

– Malformations of major organs

– Usually die within 1 year

– 1 in 7,500 – 10,000 births

– 88% maternal; 12% paternal

Human Disorders Due to Chromosomal Alterations

• Edward Syndrome

– Trisomy 18

– Thin, frail children with short eyelids, low set ears, webbed fingers & toes, and skin folds.

– In males, the testes do not descend

– 20-30% die in the first month

– 90% die by the age of 1

– 1 in 5,000 births

– Affects girls more than boys

Human Disorders Due to Chromosomal Alterations

Cri du Chat– Deletion of the short arm of chromosome 5– High pitched cry, low birth weight– Difficulties sucking and swallowing– Mental retardation– Most die in infancy –

those that live are severely delayed in development

– 1 in 50,000 live births