Nondisjunction Objective Discuss the effects of nondisjunction Answer

Nondisjunction

Objective: Discuss the effects of nondisjunction Answer: What is crossing over? What phase of meiosis does it occur?

Review: What is a mutation? Mutation: A change in the nucleotide sequence of DNA – A single nucleotide change can have serious effects

Chromosomal mutation A chromosomal mutation involves a change in the structure or number of chromosomes 4 types of chromosomal mutations: – Deletion: loss of all or part of a chromosome – Duplication: extra copy of all or part of a chromosome – Inversion: reverses the direction of parts of a chromosome – Translocation: part of one chromosome breaks off and attaches to another chromosome

Nondisjunction Other types of chromosomal mutations alter the number of chromosomes found in the cell. Nondisjunction: The failure of homologous chromosomes or sister chromatids to separate during meiosis. – Produces gametes that have too many or too few chromosomes Normal meiosis

Nondisjunction

Disorders due to Nondisjunction In humans, a zygote with 45 chromosomes has only one copy of a particular chromosome monosomy In humans, a zygote with 47 chromosomes has three copies of a particular chromosome trisomy – Example Down syndrome (trisomy 21)

Nondisjunction in the Sex Chromosomes Nondisjunction can also affect the sex chromosomes, where there can be too many or too few X or Y chromosomes (XX- normal female, XY-normal male) Examples of disorders: – XXY (Klinefelter’s syndrome) – XO (Turner’s syndrome)

Karyotype: a picture of chromosomes in a dividing cell that are arranged in pairs by size. – First 22 pairs are autosomes – Last pair are the sex chromosomes

Karyotype A * Normal male

Karyotype B * Normal female

Karyotype C * Turner’s syndrome * Only 1 sex chromosome (X)XO * Webbed neck, short stature, and infertility * Female

Karyotype D * Cri du chat syndrome * Deletion of short arm of chromosome 5 * Distinctive cry, low birth weight, respiratory problems, may have a shortened lifespan X Y * Female

Karyotype E * Klinefelter’s syndrome * 2 X chromosomes and 1 Y (XXY) * Tall, sterile, feminine characteristics, sometimes mentally retarded * Male

Karyotype F * Down syndrome * 3 copies of chromosome 21 (Trisomy 21) * Mental retardation, characteristic facial features, short stature, and heart defects * Male