1. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 1 Chromosomes and Human Genetics Mendel was unaware of chromosomes  The physical structure of genes was unknown Chromosomal Theory of Inheritance  Genes are on the chromosomes

2. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 2 Homologous Chromosomes Chromosomes that pair up during meiosis  Contain the same genes  May have different alleles of these genes  One came from each parent Each is one long DNA molecule  A gene is a short region of the molecule  Each chromosome can have > 1,000 genes

3. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 3 Homologous Chromosomes

4. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 4 Sex Determination 22 pairs of chromosomes are homologous  Called autosomes Females have 2 X chromosomes (XX) Males have X and Y chromosomes (XY)  Y is very small  Has few genes (~80) mostly dealing with maleness

5. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 5 Sex Determination During reproduction, males determine sex  If the sperm has an X, it’s a girl  If the sperm has a Y, boy Y chromosome has the SRY gene SRY is a master switch that controls other genes at other loci to develop the embryo into a boy Males only have one X chromosome

6. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 6 Linkage and Crossing Over Linkage: 2 genes on the same chromosome  Do not sort Independently!  Segregate together on the same chromosome Exception: Crossing Over  Linked genes should segregate together  Crossing over causes them to separate

7. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 7 Linkage and Crossing Over Test cross of Linkage Independent Assortment predicts ¼ of each phenotype Results are much different These genes are linked

8. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 8 Linkage and Crossing Over One would think that linked genes should always sort together One expect to see only two phenotypes  Both recessive or both dominant traits  50% of each as there are 2 chromosomes Crossing over changes this result  Results are usually in between total linkage and independent assortment.

9. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 9 Crossing Over Homologous chromosomes line up during meiosis Parts of maternal and paternal chromosomes exchange The chance that this crossing over occurs increases with the distance between the two linked loci. Before Meiosis Meiosis Gametes produced

10. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 10 Human Genetic Variation F 1 aren’t identical to sexually reproducing P They differ with respect to their genes Variation is important for populations Variation is produced in 4 ways:  Mutation (creates new genetic variation)  Independent assortment (sorts preexisting genes)  Crossing over (increases the sorting)  Fertilization (sexual reproduction shuffles the genes in eggs & sperm & then 2 parents join to form an offspring)

11. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 11 Meiosis and Independent Assortment Generate Variation Each homologous chromosome independently sorts 23 pairs of chromosomes 2 23 or 8,388,608 possible gametes Fertilization: 2 23 X 2 23 possible combinations  Over 64 trillion possible offspring  Crossing over increases this!

12. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 12 Human Genetic Disorders Many diseases are genetically inherited Studied through Pedigree analyses Show disease patterns in families

13. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 13 Autosomal Recessive Diseases Represents several thousand disorders  Can be mild or lethal Usually both parents are Heterozygous (Aa) and don’t show the phenotype Two carriers have ¼ chance of producing affected offspring

14. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 14 Autosomal Dominant Diseases One copy of this gene causes the disease More rare than recessive diseases  Affected individuals may not survive to mate  Recur due to new mutations Some diseases express late in life  More common; individuals can reproduce

15. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 15 Sex-linked Diseases Genes on the X chromosome Mothers who have a recessive gene pass it on 50% of sons are affected (only one X) Females rarely get these recessive diseases as they need to copies Hemophilia, color blind

16. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 16 Chromosomal Abnormalities Some disorders are caused by changes in chromosomes  Less common – usually lethal Two main types:  Physical changes in chromosome structure  Changes in chromosome number

17. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 17 Changes in Structure There are four ways the structure can change

18. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 18 Changes in Chromosomal Number Errors in meiosis result in extra chromosomes Most zygotes are not viable Down’s syndrome – trisomy 21  An extra copy of chromosome 21  Live to adulthood, but with developmental problems

19. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 19 Sex Chromosomal Number Extra X and Y chromosomes are possible  XXY or XYY = males  XXX, XXXX, or X0 (have one X) = females Viable but usually sterile  Can have developmental difficulties

20. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 20 Concept Quiz Which of the following is not true of homologous chromosomes? A. They contain the same alleles. B. They contain the same genes. C. One came from each parent. D. Each is duplicated during replication.

21. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 21 Genes that are linked on the same chromosome always segregate together. True or False? Concept Quiz

22. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 22 Concept Quiz Autosomal dominant disorders A. Are carried on the X chromosome. B. Often express late in life. C. Are spread by carriers (heterozygotes) mating. D. Are more common than recessive disorders.

23. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 23 Cri du Chat The results of a chromosomal deletion Affected individuals have severe mental retardation