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