When Meiosis goes wrong… 1.10.12. What can go wrong during meiosis?

When Meiosis goes wrong…

1.10.12

What can go wrong during meiosis?

What can go wrong during meiosis?

• Nondisjunction – The failure of homologous chromosomes or non-sister chromatids to separate during anaphase I or anaphase II

What can go wrong during meiosis?

• Nondisjunction – The failure of homologous chromosomes or non-sister chromatids to separate during anaphase I or anaphase II– One gamete ends up with two copies of a

chromosome, the other with zero– Mitosis then spreads this chromosomal error

throughout the organism

Alteration of Chromosome #

• Trisomy = 3 copies of a particular chromosome• Monosomy = 1 copy

Alteration of Chromosome #

• Trisomy = 3 copies of a particular chromosome• Monosomy = 1 copy

• Polyploidy = double/triple/etc. of entire genome (whole set of chromosomes)– Kills animals (prevents development), but makes

plants bigger and stronger– Can occur from 2 sperm fertilizing 1 egg or by a

diploid gamete

Alteration of Chromosome #

• It is common for humans to have an irregular # of chromosomes, but usually disastrous

• 1/3 of all pregnancies end in spontaneous abortions (natural miscarriage), and ½ of these are due to nondisjunction

Down’s Syndrome (Trisomy 21)

Down’s Syndrome (Trisomy 21)

• 3 copies of chromosome #21• 1 in 500 children in U.S.• Flattened face, short stature, mental

retardation, shortened lifespan, heart defects, sterile

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45 1 in 30

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45 1 in 30– Under 50

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45 1 in 30– Under 50 1 in 11

Down’s Syndrome (Trisomy 21)

• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45 1 in 30– Under 50 1 in 11

• Trisomy 13, 15, and 18 also occur, but die shortly after birth. All other human trisomies fail to develop

Nondisjunction of Sex Chromosomes

• Far less serious • Why?– Y chromosome carries very few genes– X chromosome will mostly shrivel up into an

inactive state if an extra is present (Barr body)

Klinefelter’s Syndrome (XXY)

• 1 in 2,000 births• Underdeveloped male sex organs• Feminine secondary characteristics• Sterile

XYY

• 1 in 1,000• Normal looking male• Tall with relatively severe acne

XXX

• 1 in 1,000• Tall female with irregular menstrual cycle• Otherwise normal, needs karyotype (picture of

chromosomes) for diagnosis

Turner’s Syndrome (XO)

Turner’s Syndrome (XO)

• 1 in 2,000 female births• Only viable monosomy (the only chromosome

that humans can survive with only one copy of)

Turner’s Syndrome (XO)

• 1 in 2,000 female births• Only viable monosomy (the only chromosome

that humans can survive with only one copy of)

• Normal until puberty• Short, sterile, no secondary sex characteristics,

normal intellect, treatable with hormone therapy