February 23, 2009February 23, 2009

ObjectiveObjective: Discuss the effects of : Discuss the effects of nondisjunctionnondisjunction

Drill:Drill:

What is crossing over? What phase of What is crossing over? What phase of meiosis does it occur?meiosis does it occur?

NondisjunctionNondisjunction

Review: What is a mutation?Review: What is a mutation?

Mutation:Mutation: A change in the nucleotide A change in the nucleotide sequence of DNAsequence of DNA

– A single nucleotide change can have A single nucleotide change can have serious effectsserious effects

Chromosomal mutationChromosomal mutation

A A chromosomal mutationchromosomal mutation involves a change involves a change in the structure or number of chromosomesin the structure or number of chromosomes

4 types of chromosomal mutations:4 types of chromosomal mutations:

– Deletion:Deletion: loss of all or part of a loss of all or part of a chromosomechromosome

– Duplication:Duplication: extra copy of all or part of a extra copy of all or part of a chromosomechromosome

– Inversion:Inversion: reverses the direction of parts reverses the direction of parts of a chromosomeof a chromosome

– Translocation:Translocation: part of one chromosome part of one chromosome breaks off and attaches to another breaks off and attaches to another chromosomechromosome

NondisjunctionNondisjunction

Other types of chromosomal Other types of chromosomal mutations alter the number of mutations alter the number of chromosomes found in the cell.chromosomes found in the cell.

NondisjunctionNondisjunction: The failure of : The failure of homologous chromosomes or homologous chromosomes or sister chromatids to separate sister chromatids to separate during meiosis.during meiosis.

– Produces gametes that Produces gametes that have too many or too few have too many or too few chromosomeschromosomes

Normal meiosis

NondisjunctionNondisjunction

Disorders due to NondisjunctionDisorders due to Nondisjunction

In humans, a zygote with In humans, a zygote with 45 chromosomes has only 45 chromosomes has only one copy of a particular one copy of a particular chromosome chromosome monosomymonosomy

In humans, a zygote with In humans, a zygote with 47 chromosomes has three 47 chromosomes has three copies of a particular copies of a particular chromosome chromosome trisomytrisomy

– Example Down syndrome Example Down syndrome (trisomy 21)(trisomy 21)

Nondisjunction in the Sex Nondisjunction in the Sex ChromosomesChromosomes

Nondisjunction can also affect the sex chromosomes, Nondisjunction can also affect the sex chromosomes, where there can be too many or too few X or Y where there can be too many or too few X or Y chromosomes (XX- normal female, XY-normal male)chromosomes (XX- normal female, XY-normal male)

Examples of disorders:Examples of disorders:

– XXY (Klinefelter’s syndrome)XXY (Klinefelter’s syndrome)

– XO (Turner’s syndrome)XO (Turner’s syndrome)

KaryotypeKaryotype

Karyotype:Karyotype: a picture of chromosomes in a dividing cell that a picture of chromosomes in a dividing cell that are arranged in pairs by size.are arranged in pairs by size.

– First 22 pairs are autosomesFirst 22 pairs are autosomes

– Last pair are the sex chromosomesLast pair are the sex chromosomes

Karyotype AKaryotype A

* Normal male

Karyotype BKaryotype B

* Turner’s syndrome

* Only 1 sex chromosome (X)- XO

* Webbed neck, short stature, and infertility

* Female

Karyotype CKaryotype C

* Cri du chat syndrome

* Deletion of short arm of chromosome 5

X Y

* Distinctive cry, low birth weight, respiratory problems, may have a shortened lifespan

* Female

Karyotype DKaryotype D

* Klinefelter’s syndrome

* 2 X chromosomes and 1 Y (XXY)

* Tall, sterile, feminine characteristics, sometimes mentally retarded

* Male

Karyotype EKaryotype E

* Normal female

Karyotype FKaryotype F* Down syndrome

* 3 copies of chromosome 21 (Trisomy 21)

* Mental retardation, characteristic facial features, short stature, and heart defects

* Male