Errors in Meiosis Karyotypes & Chromosomal AbnormalitiesKaryotypes & Chromosomal Abnormalities

Karyotype A photograph of an organism’s chromosomes Homologous chromosomes are grouped together Based on size, banding pattern, and location of centromere Arranged from largest  smallest, sex chromosomes at the end

Karyotype Helpful in finding chromosomal abnormalities Will NOT show genetic traits or disorders Can NOT tell if baby has blue eyes Can NOT tell if baby has cystic fibrosis Can tell if baby has too many chromosomes or pieces missing

Chromosomal Abnormalities Nondisjunction Chromosome (or chromosome pair) fails to separate during meiosis Results in an incorrect number of chromosomes for daughter cells

Chromosomal Abnormalities Problems with meiotic spindle causes errors Homologous chromosomes do not separate properly during Meiosis I Sister chromatids do not separate properly during Meiosis II Results in too many or too few chromosomes 2n n n n-1 n+1

all with incorrect number1/2 with incorrect number error in Meiosis 1 error in Meiosis 2

Nondisjunction Trisomy Cells have three copies of a chromosome Monosomy Cells have one copy of a chromosome trisomy 2n+1 n+1 n monosomy 2n-1 n-1 n

Nondisjunction in Humans High frequency in humans Most embryos spontaneously abort Alterations are too disasterous Developmental problems result from biochemical imbalance Regulatory molecules? Hormones? Transcription factors? Certain conditions are tolerated Smaller chromosomes or sex chromosomes = survivable Have a characteristic set of symptoms = syndrome Polyploidy = nondisjuction of ALL chromsome pairs -occurs in plants -fatal in animals

Down Syndrome Trisomy 21 3 copies of chromosome 21 1 in 700 children born in the U.S. Chromosome 21 is the smallest human chromosome But still has severe effects Frequency of Down’s Syndrome correlates with mother’s age

Mother’s age Incidence of Down Syndrome Under 30<1 in in in in in in in in in in in in in 12 Rate of miscarriage due to amniocentesis:  1970s data 0.5%, or 1 in 200 pregnancies  2006 data <0.1%, or 1 in 1600 pregnancies

Genetic Testing Amniocentesis in 2 nd trimester Sample of embryo cells Stain & photograph chromosomes Usually in metaphase Analysis of karyotype

Sex Chromosome Abnormalities Human development more tolerant of wrong numbers in sex chromosomes Still produces a variety of distinct syndromes in humans XXY = Klinefelter’s Syndrome (male) XXX = Trisomy X Syndrome (female) XYY = Jacob’s Syndrome (male) XO = Turner’s Syndrome (female)

Klinefelter’s Syndrome XXY male 1 in every 2000 live births Have male sex organs, but are sterile Feminine characteristics Some breast development Lack of facial hair Tall Normal intelligence

Jacob’s Syndrome XYY male 1 in every 1000 live male births Slightly taller than average More active Normal intelligence, possible reading disabilities Delayed emotional maturity Normal sexual development

Trisomy X XXX female 1 in every 2000 live births Produces healthy females Why? Barr bodies All but 1 X chromosome becomes inactivated

Turner’s Syndrome Monosomy X or XO female 1 in every 5000 live births Varied degree of effect Webbed neck Short Sterile

Changes in Chromosome Structure deletion loss of a chromosomal segment duplication repeat a segment inversion reverses a segment translocation move segment from one chromosome to another error of replication error of crossing over

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