2. ANEUPLOIDY
Having too many or too few chromosomes compared to a normal genotype
Aneuploid organisms have unbalanced sets of chromosomes due to an excess or deficiency of individual chromosomes
This creates an imbalance among the genes and causes an abnormal phenotype or even death

3. ANEUPLOIDY Most autosomal aneuploids are incompatible with life
Autosomal nondisjunction occurs at a reasonable high frequency in humans but the zygote does not develop
Aborts spontaneously soon after conception
Humans do not tolerate aneuploidy well, especially monosomies
Excess of chromosomes is tolerated better than a deficiency so trisomies are more viable than monosomies

4. TRISOMY 3 COPIES OF A SINGLE CHROMOSOME
A few types of trisomic zygotes are capable of survival
Trisomy 21
Trisomy 18
Trisomy 13
Only those trisomies involving the smallest or heterochromatic chromosomes are able to survive at all

5. Chromosome 22 is a gene jungle with 545 genes
Chromosome 21 has been described as a desert harboring a million base pair stretch with only 225 genes
Chromosome 22 is a gene jungle with 545 genes

6. DOWN SYNDROME TRISOMY 21
1866, Dr. John Langdon Down noticed that about 10% of the residents at his asylum resembled each other and could be easily distinguished from the rest of his patients
Took geneticists another 90 years to determine the correct human chromosome number and it was not until 1959 that it was known that individuals with Down Syndrome have 3 copies of the smallest chromosome

9. DOWN SYNDROME TRISOMY 21 Most common and best known genetic disorder
Accounts for about 10% of all mentally disabled children
Occurs in about 1/750 live births in all ethnic groups
Nearly all cases occur just once within a given family
Only about 2-5% of cases run in families

10. DOWN SYNDROME TRISOMY 21
Babies with this disorder grow slowly and have poor muscle tone
Lack of muscle tone gives them a floppy appearance
Individuals are generally shorter than average
All parts of their bodies are shortened due to poor skeletal development, including the hands and fingers

11. Affected individuals have
DOWN SYNDROME TRISOMY 21
Affected individuals have
Sparse hair
a protruding tongue and thick lips
Their hands have an irregular pattern of creases
Their joints are loose and they have poor reflexes
Motor development is delayed

13. DOWN SYNDROME TRISOMY 21
1/2 of the children born with Down syndrome are born with severe heart malformations
15% of the babies die in their first year from heart abnormalities
Many others die before the age of 5
Those who do live beyond the fifth year of life have an average life expectancy of 50 years

14. DOWN SYNDROME TRISOMY 21
Leukemia is times more prevalent in people with Down syndrome than in the general population
Adult males have poorly developed genitals and are sterile
Adult females can have children
1/2 of their children are born with Down syndrome

15. DOWN SYNDROME TRISOMY 21
Maturation is delayed but following puberty there is a rapid onset of aging
Those over age 40 develop the black fibers of amyliod proteins in their brains
These proteins are associated with Alzheimers
The chance of a person with Trisomy 21 developing Alzheimer’s disease is 25% compared to 6% in the general population

16. TRISOMY 21
Karyotyping has shown that not all of chromosome 21 has to be present in triplicate to produce Down syndrome
In a few rare individuals, the only extra chromosomal material is the distal half of the long arm of chromosome 21.
This region houses most of the genes including a gene for an enzyme involved in aging and a leukemia causing gene

17. ABNORMAL CHROMOSOME STRUCTURE
Structural chromosomal defects include missing, extra, or inverted genetic material within a chromosome or exchanged parts of a chromosome

19. TRANSLOCATION
In translocations, nonhomologous chromosomes exchange parts
2 types
Robertsonian translocation
Reciprocal translocation

20. ROBERTSONIAN TRANSLOCATION
Robertsonian translocation the short arms of 2 different acrocentric chromosomes break, leaving sticky ends that then cause the 2 long arms to adhere
A new large chromosome forms from the long arms of the two different chromosomes
This individual may produce unbalanced gametes

22. Fig

24. ROBERTSONIAN TRANSLOCATION
A carrier parent with 45 chromosomes one of which is the combined 14q21q, will produce 3 kinds of offspring
Phenotypically and karyotypically normal
Phenotypically unaffected translocation heterozygote
Translocation Down syndrome individual
The risk of a carrier having a child with Down syndrome is 15%
The condition is not related to age

25. MATERNAL EFFECT
1930 a maternal age effect was shown to be a factor in most cases, not the number of children in a family
Frequency of Down syndrome births increases with advancing maternal age
Women over 35 years of age produced about 25% of all the babies born with Down syndrome

27. FEMALE MEIOSIS
The effect may be a consequence of the long delay between prophase I and the first meiotic division in human oocytes
Human oocytes begin meiosis before the female is born
They remain in a state of suspended animation until the girl hits puberty some years later
Egg ovulated by a 45 year old female has been in suspended animation for 45 years
Plenty of time for something to go wrong

28. TRISOMY Trisomy 18 Edward Syndrome 1/6000 live births
Trisomy 13 Patau Syndrome
1/12,000 live births
Both lead to early death by 2.5 months
Nearly all cases die during the first year of life
All suffer from profound mental retardation.