2. AP Biology Lecture #29 Chromosomal Errors

3. 2006-2007 Errors of Meiosis Chromosomal Abnormalities

4. Chromosomal abnormalities Incorrect number of chromosomes – nondisjunction chromosomes don’t separate properly during meiosis – breakage of chromosomes deletion duplication inversion translocation

5. Nondisjunction Problems with meiotic spindle cause errors in daughter cells – homologous chromosomes do not separate properly during Meiosis 1 – sister chromatids fail to separate during Meiosis 2 – too many or too few chromosomes 2n n n n-1 n+1

6. Alteration of chromosome number all with incorrect number1/2 with incorrect number error in Meiosis 1 error in Meiosis 2

7. trisomy 2n+1 Nondisjunction Baby has wrong chromosome number~ aneuploidy – trisomy cells have 3 copies of a chromosome – monosomy cells have only 1 copy of a chromosome n+1n monosomy 2n-1 n-1n

8. Human chromosome disorders High frequency in humans – most embryos are spontaneously aborted – alterations are too disastrous – developmental problems result from biochemical imbalance imbalance in regulatory molecules? – hormones? – transcription factors? Certain conditions are tolerated – upset the balance less = survivable – but characteristic set of symptoms = syndrome

9. Down syndrome Trisomy 21 – 3 copies of chromosome 21 – 1 in 700 children born in U.S. Chromosome 21 is the smallest human chromosome – but still severe effects Frequency of Down syndrome correlates with the age of the mother

10. Sex chromosomes abnormalities Human development more tolerant of wrong numbers in sex chromosome But produces a variety of distinct syndromes in humans – XXY = Klinefelter’s syndrome male – XXX = Trisomy X female – XYY = Jacob’s syndrome male – XO = Turner syndrome female

11. XXY male – one in every 2000 live births – have male sex organs, but are sterile – feminine characteristics some breast development lack of facial hair – tall – normal intelligence Klinefelter’s syndrome

13. Jacob’s syndrome male XYY Males – 1 in 1000 live male births – extra Y chromosome – slightly taller than average – more active – normal intelligence, slight learning disabilities – delayed emotional maturity – normal sexual development

14. Trisomy X XXX – 1 in every 2000 live births – produces healthy females Why? Barr bodies – all but one X chromosome is inactivated

15. Turner syndrome Monosomy X or X0 – 1 in every 5000 births – varied degree of effects – webbed neck – short stature – sterile

16. 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

17. Chromosomal errors VI Deletion Duplication Inversion Reciprocal translocation Nonhomologous chromosomes Homologous chromosomes

18. Genomic imprinting Def: a parental effect on gene expression Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. Fragile X syndrome: higher prevalence of disorder and retardation in males

19. Human disorders The family pedigree Recessive disorders: Cystic fibrosis Tay-Sachs Sickle-cell Dominant disorders: Huntington’s achondroplasia

20. Recessive diseases The diseases are recessive because the allele codes for either a malfunctioning protein or no protein at all – Heterozygotes (Aa) carriers have a normal phenotype because one “normal” allele produces enough of the required protein

21. Heterozygote crosses Aa x Aa Aa male / sperm A a female / eggs AA Aaaa Aa A a A a AAAaaaAa Heterozygotes as carriers of recessive alleles carrier disease

22. Fig. 14-16 Parents Normal Sperm Eggs Normal (carrier) Normal (carrier) Albino Aa A A AA Aa a aa a 

23. Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

24. Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left)

25. Pedigree analysis Pedigree analysis reveals Mendelian patterns in human inheritance – data mapped on a family tree = male= female= male w/ trait = female w/ trait

26. Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Widow’s peakNo widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Wwww Ww ww Ww wwWW Ww or

27. Human disorders Testing: amniocentesis chorionic villus sampling (CVS) Examination of the fetus with ultrasound is another helpful technique

28. Fig. 14-18 Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation Fluid Fetal cells Several hours Several weeks Several weeks (a) Amniocentesis (b) Chorionic villus sampling (CVS) Several hours Several hours Fetal cells Bio- chemical tests Karyotyping Placenta Chorionic villi Fetus Suction tube inserted through cervix

29. Genetic counseling Pedigree can help us understand the past & predict the future Thousands of genetic disorders are inherited as simple recessive traits – from benign conditions to deadly diseases albinism cystic fibrosis Tay sachs sickle cell anemia PKU