Non Disjunction & Down Syndrome

Non disjunction

the failure of homologous chromosomes or sister chromatids to separate properly during meiosis

When can non disjunction occur?

Anaphase 1

Anaphase 2 

Why can non disjunction occur in anaphase 1

when homologous chromosomes fail to separate

when can non disjunction occur in anaphase 2

when sister chromatids fail to separate

What is down syndrome caused by?

non disjunction

Down syndrome is caused by ___________ where an individual has 3 copies of chromosome 21 instead of the usual two

trisomy 21

When does this extra chromosome arise? 

From non disjunction during meiosis in one of the parent’s gametes

When gamete  with an extra chromosome 21  fuses with a normal gamete   

What does this result in?

this results in a zygote with 47 chromosomes instead of 46 

effects of down syndrome

physical traits

health challenges

cognitive effects

characteristics of down syndrome: physical traits

distinctive facial features

short stature and low muscle tone

characteristics of down syndrome: health challenges

heart defects

hearing and vision problems

characteristics of down syndrome: cognitive effects 

mild to moderate intellectual disability 

delayed developmental milestones 

Why can non disjunction occur? 

Maternal age
Environmental Factors 
Cellular Errors

How does maternal age effect non disjunction?

the risk of non disjunction rises significantly with maternal age, especially after age 35

How does environmental factors effect non disjunction?

exposure to certain chemicals or radiation

How does cellular errors effect non disjunction?

issues with the spindle fibers or the cohesion proteins that hold chromatids together can lead to non disjunction

Implications of non disjunction: 

Klinefelter Syndrome —- xxy 
Turner Syndrome — xo 

Implications of non disjunction: Klinefelter syndrome

an extra sex chromosome

Implications of non disjunction: Turner syndrome

A missing sex chromosome