Chromosomal Mutations Changes in Chromosome Number or Structure

Alterations in Chromosome Number Polyploidy: one or more extra sets of chromosomes Aneuploidy: gain or loss of one chromosome or a small number of chromosomes

Aneuploidy Arises by Non-disjunction Non-disjunction = failure of homologues or chromatids to separate during meiosis Normal Meiosis Non-disjunction in Meiosis I in Meiosis II

Human Chromosomal Aneuploids Autosomal Aneuploids Down Syndrome Trisomy 21 Edward Syndrome Trisomy 18 Patau Syndrome Trisomy 13 Trisomy: three copies of one chromosome

Human Autosomal Abnormality Down Syndrome Trisomy 21 Three copies of chromosome 21 How can Down Syndrome occur? Eg. Egg with 2 copies of #21 (24 chromosomes) + Sperm with 1 copy of #21 (23 chromosomes) = Embryo with 3 copies of #21 (47 chromosomes)

Karyotype for Down Syndrome Physical Features Eye fold Palm Crease

Incidence of Down Syndrome Increases with Maternal Age 100 200 300 400 10 20 30 40 50 Number per 1000 Births Figure: FIGURE 12.20 Title: Down syndrome frequency increases with maternal age Caption: The increase in frequency of Down syndrome after maternal age 35 is quite dramatic. Age of Mother (years)

Human Chromosomal Aneuploids Sex Chromosome Aneuploids Turner Syndrome 45, XO Triplo-X 47, XXX Klinefelter Syndrome 47, XXY XYY Syndrome 47, XYY Sterile female Fertile female Sterile male Fertile male

Human Sex Chromosome Abnormality Turner Syndrome XO One copy of X No second sex chromosome How can Turner Syndrome occur? Eg. Egg with 0 copies of X (22 chromosomes) +Sperm with 1 copy of X (23 chromosomes) = Embryo with 1 copy of X (45 chromosomes)

Karyotype for Turner’s Syndrome Non-functional Ovaries From Adult Female with Turner’s Syndrome Normal uterus, tubes and ovaries

Human Chromosomal Aneuploids How can XYY Syndrome occur? One Copy of the X chromosome Two Copies of the Y chromosome Eg. Egg with 1 copy of X (23 chromosomes) + Sperm with 2 copies of Y (24 chromosomes) = Embryo with XYY (47 chromosomes)

Chromosome Structure Changes Description Deletion Loss of a chromosomal segment Duplication Repeat of a chromosomal segment Translocation Movement of chromosomal segment to non-homologous chromosome Inversion Reversal of a chromosomal segment (rotated 180o)

Chromosome Deletion in Humans Cri-du-chat syndrome is correlated with a deletion at the end of chromosome 5.

Chromosome Duplication in Humans Small duplications in chromosome 15 cause no symptoms Large duplication (with inversion) causes seizures and mental retardation

Chromosome Translocation in Humans Reciprocal Translocation involves exchange between two non-homologous chromosomes Reciprocal translocation between chromosomes 2 and 20 causes Alagille Syndrome

Chromosome Translocation in Humans Robertsonian Translocation involves a fusion of the long arms of two different chromosomes Translocation Down Syndrome involves a Robertsonian Translocation between chromosomes 14 and 21

Chromosome Inversions Lead to Unbalanced Meiotic Products A paracentric inversion does not include the centromere A pericentric inversion includes the centromere