Chromosomes and Human Inheritance

1. Chromosomes and Human Inheritance Chapter 12

2. Impacts, Issues:Strange Genes, Tortured Minds • Exceptional creativity often accompanies neurobiological disorders such as schizophrenia, autism, chronic depression, and bipolar disorder • Examples: Lincoln, Woolf, and Picasso

3. 12.1 Human Chromosomes • In humans, two sex chromosomes are the basis of sex – human males have XY sex chromosomes, females have XX • All other human chromosomes are autosomes

4. Sex Determination in Humans • Sex of a child is determined by the father • Eggs have an X chromosome; sperm have X or Y

5. Sex Determination in Humans • The SRY gene on the Y chromosome is the master gene for male sex determination • Triggers formation of testes, which produce the male sex hormone (testosterone)

6. Karyotyping • Karyotype • A micrograph of all metaphase chromosomes in a cell, arranged in pairs by size, shape, and length • Detects abnormal chromosome numbers

7. 12.2 Autosomal Inheritance Patterns • Many human traits can be traced to autosomal dominant or recessive alleles that are inherited • Some of those alleles cause genetic disorders

8. Autosomal Dominant Inheritance • A dominant allele is expressed in homozygotes and heterozygotes • With one homozygous recessive and one heterozygous parent, children have a 50% chance of inheriting and displaying the trait

9. Autosomal Recessive Inheritance • Autosomal recessive alleles are expressed only in homozygotes; heterozygotes are carriers and do not have the trait • A child of two carriers has a 25% chance of expressing the trait

10. Fig. 12-4a, p. 188

11. Animation: Autosomal dominant inheritance

12. Animation: Autosomal recessive inheritance

13. 12.3 Too Young to be Old • Progeria • Genetic disorder that results in accelerated aging

14. 12.4 Examples of X-Linked Inheritance • Mutated alleles on the X chromosome cause or contribute to over 300 genetic disorders

15. X-Linked Inheritance Patterns • More males than females have X-linked recessive genetic disorders • Males have only one X chromosome and can express a single recessive allele

16. X-Linked Recessive Inheritance Patterns

17. Animation: X-linked inheritance

18. Some X-Linked Recessive Disorders • Hemophilia A • Bleeding caused by lack of blood-clotting protein • Duchenne muscular dystrophy • Degeneration of muscles caused by lack of the structural protein dystrophin

19. 12.5 Heritable Changes in Chromosome Structure • On rare occasions, a chromosome’s structure changes; such changes are usually harmful or lethal, rarely neutral or beneficial • A segment of a chromosome may be duplicated, deleted, inverted, or translocated

20. Duplication • DNA sequences are repeated two or more times; may be caused by unequal crossovers in prophase I

21. Deletion • Loss of some portion of a chromosome; usually causes serious or lethal disorders • Example: Cri-du-chat

22. Inversion • Part of the sequence of DNA becomes oriented in the reverse direction, with no molecular loss

23. Translocation • Typically, two broken chromosomes exchange parts (reciprocal translocation)

24. Nondisjunction • Changes in chromosome number can be caused by nondisjunction, when a pair of chromosomes fails to separate properly during mitosis or meiosis • Affects the chromosome number at fertilization • Monosomy (n-1) • Trisomy (n+1)

25. Nondisjunction

26. Autosomal Change and Down Syndrome • Only trisomy 21 (Down syndrome) allows survival to adulthood • Characteristics include physical appearance, mental impairment, and heart defects • Incidence of nondisjunction increases with maternal age

27. Change in Sex Chromosome Number • Changes in sex chromosome number may impair learning or motor skills, or be undetected • Female sex chromosome abnormalities • Turner syndrome (XO) • XXX syndrome (three or more X chromosomes) • Male sex chromosome abnormalities • Klinefelter syndrome (XXY) • XYY syndrome

28. Turner Syndrome • XO (one unpaired X chromosome) • Usually caused by nondisjunction in the father • Results in females with undeveloped ovaries

29. Defining Genetic Disorders and Abnormalities • Genetic disorder • An inherited condition that causes mild to severe medical problems, characterized by a specific set of symptoms (a syndrome)

30. 12.8 Prospects in Human Genetics • Genetic analysis can provide parents with information about their future children • Genetic counseling • Starts with parental genotypes, and genetic testing for known disorders • Information is used to predict the probability of having a child with a genetic disorder

31. Prenatal Diagnosis • Tests done on an embryo or fetus before birth to screen for sex or genetic problems • Three types of prenatal diagnosis • Amniocentesis • Chorionic villus sampling (CVS)

32. After Preimplantation Diagnosis • When a severe problem is diagnosed, some parents choose an induced abortion • In some cases; surgery, prescription drugs, hormone replacement therapy, or dietary controls can minimize or eliminate symptoms of a genetic disorder