1. Human Genes & Chromosomes

2. Human Genetic Disorders Nondisjunction is a cause of some human genetic disorders –In nondisjunction, the members of a chromosome pair fail to separate at anaphase, producing gametes with an incorrect number of chromosomes. –Nondisjunction can occur during meiosis I or II.

3. Meiosis I Abnormal Gametes Homologous chromosomes fail to separate. Meiosis II Sister chromatids fail to separate. Abnormal Normal n n n  1 n – 1 n  1 n – 1 NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II

4. Nondisjunction Homologous chromosomes fail to separate Meiosis I: Nondisjunction Meiosis II

5. Nondisjunction

6. Human Genetic Disorders Many different human genetic disorders, all with certain causes Can be a result of autosomal chromosomes or sex chromosomes being affected.

7. Chromosomal Disorders Error during meiosis – homologous chromosomes fail to separate = nondisjunction If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosomes may result –Ex. Down Syndrome – 3 copies of chromosomes on pair 21 –Ex. Klinefelter’s – XXY – too many copies of X chromosome in males = infertility Klinefelter’s

8. Human Autosomal Genetic Disorders Recessive Alleles –Genetic disorders can be carried on recessive alleles, and need both recessive alleles to have the disease –Ex. Cystic Fibrosis – recessive – excessive mucus in lungs, pancreas, reduced life span –Ex. Tay Sachs Disease – recessive – nervous system breakdown and death in first few years of life

9. Human Autosomal Genetic Disorders Dominant Alleles –Effects of dominant allele expressed even when recessive allele present –Ex. Dwarfism (not all types of dwarfism are a result of this dominant gene) Dd x Dd Dd D d D D D d D d dd D = dwarfism d = normal height

10. Human Autosomal Genetic Disorders –Ex. Huntingon’s Disease – dominant - nervous system breaks down, but not until middle age

11. Sex-Linked Genes Special pattern of inheritance for genes located on X and Y chromosomes Many sex-linked genes are found on the X chromosome because many genes are located on it

12. Colorblindness 3 genes associated with color vision are located on the X chromosome In males, a defective version of any one of these genes results in colorblindness Common in males – 1 in 10 have it Rare in females – why? –Males have just one X chromosome and all X-linked alleles are expressed in males, even if they are recessive –Females need both recessive alleles to have colorblindness = rarer Can be passed from daughters to their children

13. Colorblindness Father (normal vision) Colorblind Normal vision Mother (carrier) Daughter (normal vision) Son (normal vision) Daughter (carrier) Son (colorblind) Male Female

14. Colorblindness Father (normal vision) Colorblind Normal vision Mother (carrier) Daughter (normal vision) Son (normal vision) Daughter (carrier) Son (colorblind) Male Female

15. Colorblindness

16. Other Sex-Linked Disorders Hemophilia – blood doesn’t clot correctly Duchenne Muscular Dystrophy – weakening and loss of skeletal muscle