Human Genetics: Sex Chromosomes and Genetic Disorders

1. Human Genetics Chapter 14

2. Sex Chromosomes Determine the sex of the individual Humans have 2 Females – XX Males – XY Autosomal Chromosomes - (Autosomes) • the remaining 44 chromosomes that are not sex chromosomes..

3. Sex Chromosomes and Sex-linked genes X – Y system : Humans X – 0 system : arachnids and many insects Z – W system : Birds, Fish, some insects like butterflies

4. Sex – linked trait Alleles - • alternate forms of the same gene. • Must have 2 alleles to express a recessive trait Females - • 1 allele of recessive trait = • carriers of the gene • don’t express the trait Males - • one allele express that trait.

5. XH Y XHXh XH XH XH Y XH Xh XhY Punnett Squares with Sex Chromosomes • % of children with the disorder: H = normal h = hemophelia • 25% • % of son’s with the disorder: • 50% • % of daughters that would be a carrier of the disorder? • 50%

6. Pedigree Chart A chart that shows: • The genetic relationships within a family • How traits are passed from generation to generation.

7. Blood Types in Humans • ABO Blood Types • A (IA) and B (IB) are codominant • O (i) is recessive

8. Karyotype • a picture of chromosomes arranged in pairs.

9. CHROMOSOMAL DISORDERS Down Syndrome Turners Syndrome Klinefelter’s Syndrome Swyer Syndrome

10. Chromosomal Disorders • Nondisjunction • An error in meiosis when homologous chromosomes fail to separate • Abnormal numbers of chromosomes in gametes

11. Down Syndrome • Down Syndrome • 3 copies of chromosome 21 • Mental disability and wide set eyes • 1/800 babies born with Down Syndrome in the US • Trisomy – end up with 3 copies of a chromosome • How Down Syndrome Occurs

12. Turners Syndrome • Turners Syndrome • Only one X chromosome (only 45 chromosomes) • Sterile females, short in stature, limited development at puberty, limited to no secondary sex features (breasts etc.) • Hard to detect. • Monosomy - end up with only 1 chromosome. • How Turners Syndrome Occurs

13. Klinefelter’s Syndrome • Klinefelter’s Syndrome • Develop as males. • Males with this disease inherit an extra X chromosome (anywhere from 1 to 4 extra X’s). • Males with this disease are sterile and have female characteristics such as breasts. • How Klinefelters Syndrome Occurs

14. Swyer Syndrome & the SRY Gene • A gene located on the Y chromosome. • If this gene is not present, the child will develop as a female even if she has the Genotype XY. • Female will have masculine features and will be infertile • Normal male develompment: • The SRY gene activates the testes, which in turn produce testosterone. • Without SRY Gene: • the testes never develop, and testosterone is not released causing the genotypical male to be a female.

15. AUTOSOMAL DISORDERS Albinism Cystic Fibrosis PKU Tay-Sachs Disease Huntington’s Disease Sickle Cell Anemia

16. Autosomal Genetic Disorders • Autosomal Dominant • Huntingtons Disease and others • Autosomal Recessive • Cystic Fibrosis, Sickle Cell Anemia, Tay-Sachs, Spinal Muscular Atrophy • Autosomal Codominant • Sickle Cell Anemia

17. Autosomal Recessive • Recessive Alleles • Both parents carry one normal gene and one altered gene. • What’s the chance that a child will end up with the disorder? • What’s the chance that a child will end up being normal? • What’s the chance that a child will end up being a carrier of the disorder? • 25% - 25 % • 50%

18. Tay Sachs Disease • Tay Sachs Disease: • Lipid accumulation in brain cells results in nervous system breakdown & death within a few years of life. • Death within 1-3 years of age. • 1/27 born (linked to Jewish decent)

19. Cystic Fibrosis • Cystic Fibrosis: • Increased infections due to an excess of liquid in the lungs and digestive tract • Early death is common. • 1 in 25 people born w/c.f.

20. PKU • PKU – phenylketonuria • Lack enzyme to break down phenylalanine which is found in milk. • Can cause mental disabilities, organ damage and unusual posture. • Can test at Birth • With careful dietary supervision, children born with PKU can lead normal lives, and mothers who have the disease can produce healthy children. • Caused by a recessive allele on chromosome 12. • 1 in 15,000 born with PKU

21. Albinism • Albinism • Little to no pigment in eyes, skin, or hair due to lack of melanin • 1 in 17,000 in the USA have some type of albinism.

22. Autosomal Dominant • Dominant Alleles • These genes can be inherited from either parent. • How many dominant alleles must be present for the individual to express that gene in their phenotype? • Only one • Always expressed if that allele is present

23. Dwarfism • Achondroplasia (Dwarfism)

24. Huntington’s Disease • Huntington’s Disease • progressive loss of muscle control and mental retardation • Symptoms occur when people reach their 30’s

25. Codominant Autosomal • Codominant Alleles • Both dominant alleles are fully expressed.

26. Sickle Cell Anemia • Disorder common in African Americans • Abnormal (sickle) shape of cells • This causes them to get stuck in capillaries blocking blood. • Just a single DNA base change results in this disease. • Offers some resistance to malaria parasite. • Lack of oxygen to tissues damages tissues.

27. SEX LINKED DISORDERS

28. Sex-Linked Genes • Sex-Linked Genes • Caused by genes located on x & y chromosomes • Males • Males have just one X chromosome, thus all X-linked alleles are expressed in males, even if they are recessive. • Females • For a recessive allele to be expressed in females, how many copies of the allele must be present? • 2

29. Sex LinkedDominant Recessive

30. X-Linked Genetic Disorders • X-Linked Dominant • X-Linked Recessive • Hemophelia, Muscular Dystrophy, Color Blindness, Duchenne Muscular Dystrophy

31. Colorblindness Colorblindness • inability to see color or certain colors • Found on the X chromosome. • Red-green most common form. • 1/10 females & 1/100 males in US Everyone Should See This One

32. Hemophilia • Hemophilia • Inability to clot blood normally due to lack of necessary protein . • Individuals could bleed to death from even a minor cut. • Found on the X chromosome. • 1/10,000 males

33. Muscular Dystrophy • Duchene Muscular Dystrophy • the progressive weakening and loss of skeletal muscle leading to death in early adulthood. • Found on the X chromosome.

34. Y-Linked Genetic Disorders • Y-Linked • Male Infertility

35. CHROMOSOME INACTIVATION

36. X-Chromosome Inactivation • In X-chromosome inactivation in females, one of the X chromosomes is inactivated. • It condenses in the nucleus and is called a Barr body. • Calico cats are female only. Gene for orange fur is on one X and the gene for black is on the other X. Some regions of the cat’s body have the X chromosome with the gene for black fur inactivated while other regions have the orange color inactivated.

37. Genetic Testing • Testing for alleles • parents can have this done to identify genes that may cause disorders in their children.

38. Gene Therapy • Gene Therapy • An absent or faulty gene is replaced by a normal, working gene. • This is an option for parents who go through allele testing so that they can avoid having children with disorders. • At this point in time, gene therapy is a high-risk experimental disorder, but could be very useful in the future.

39. Human Genome Project • An attempt to sequence all human DNA (began in 1990 and was complete by 2000 • Rapid Sequencing – used computers to piece together fragments of DNA until the entire genome was mapped out. • Humans – 31,000 genes • A Breakthrough for Everyone – all data is available on the Internet.

40. Ethical Issues • It is important for us as a society to decide what is ethical and what is not. • Is it ethical to allow parents to change a gene if they know that their child is going to be born with a disorder such as Tay Sachs and will die by the age of 3? • Is it ethical to allow parents to change characteristics of an individual such as eye color and hair color?