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Chapter 11

Chapter 11. Complex Inheritance & Human Heredity. Chapter 11.1. Basic Patterns of Human Inheritance. Recessive Genetic Disorders. A recessive trait is expressed when an individual holds TWO recessive alleles ( homozygous recessive ) Example s recessive genetic disorders include:

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Chapter 11

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  1. Chapter 11 Complex Inheritance & Human Heredity

  2. Chapter 11.1 Basic Patterns of Human Inheritance

  3. Recessive Genetic Disorders • A recessive trait is expressed when an individual holds TWO recessive alleles (homozygous recessive) • Example s recessive genetic disorders include: • Cystic fibrosis • Albinism • Tay-Sachs disease • Galactosemia • Individuals heterozygous for a recessive disorder will express the dominant trait (no disorder) but will be a carrierof the disorder

  4. Dominant Genetic Disorders • Some disorders are caused by dominant genes • Those people without the disorder are homozygous recessive • Examples of Dominant Genetic Disorders: • Huntington’s Disease • Achondroplasia • Huntington’s Disease - http://www.youtube.com/watch?v=65xf1olEpQM

  5. Pedigrees • A diagram that traces the inheritance of a particular trait through several generations • Read about Pedigrees on pp. 299-301 • Complete the Mini-Lab on p. 300

  6. Complex Inheritance Patterns Chapter 11.2 Sex Determination, Sex-Linked Traits, & Sex Influenced Traits

  7. INCOMPLETE DOMINANCE  With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

  8. Problem: • There are foxes that roam the forests in northern Baltimore County. They come in three colors, blue, gold, and green. This trait is controlled by a single gene with incomplete dominance. • A homozygous (BB) individual is blue, a homozygous (bb) individual is gold, and a heterozygous (Bb) individual is green. • What would be the genotypes and phenotypes of the offspring if a blue fox were crossed with a gold one?

  9. CODOMINANCE

  10. Longhorns may be white (CWCW), red (CRCR) or roan (CRCW). Roan longhorns have a mixture of both white hairs and red hairs due to a codominant gene. • "A single copy can be expressed by just a few white hairs on the face or extremities, unevenly roaned patches, or an even mix of white and colored hairs all over the body. Two copies produce an almost white animal, with some pigment around the ears.” • What would the phenotypes and genotypes of the offspring if a red male were crossed with a white female?

  11. Sickle Cell Disease • Sickle cell disease is caused by the allele that controls the formation of the protein hemoglobin. • Hemoglobin is the part of the red blood cell that carries oxygen. • The allele for normal hemoglobin (A) results in red blood cells that are disc-shaped. • The sickle cell allele (S) changes the hemoglobin and results in red blood cells that are sickle -shaped. • People who are heterozygous for these two alleles (AS) have both normal and sickle-shaped red blood cells. PROBLEM: What is the risk that two people heterozygous for the sickle-shaped allele will have a child with sickle cell disease?

  12. MULTIPLE ALLELES • A single gene that has more than two possible alleles. Notice that that the allele for "O" (i) is recessive to the alleles for "A" & "B". With three alleles we have a higher number of possible combinations in creating a genotype.

  13. Problem: • A woman with Type O blood (ii) and a man who is Type AB (IAIB) have are expecting a child.  • What are the possible blood types of the child?

  14. Epistasis • One allele hides the effects of another allele Primula flowers produce a blue pigment with the dominant Gene K However, the presence of a dominant Gene D will mask effect of Gene K (KKDD, KKDd, KkDD, KkDd will all appear another color)

  15. Sex Determination • Gender is determined by a pair of chromosomes called the sex chromosomes. • two types: X and Y • females have two X chromosomes • males have one X and one Y chromosome • In humans, the remaining 22 pairs of chromosomes are homologous and are referred to as autosomes.

  16. Dosage Compensation • X chromosome is larger than Y • ‘X’ contains 900-1400 genes • ‘Y’ contains fewer than 100 genes – mostly related to “maleness” • In females, one of the X chromosomes is inactivated. • X-inactivation occurs randomly in each body cell. • Form of dosage compensation. • Inactivated X-chromosome condenses into a darkly staining region called a Barr body. • Example: • Calico color in cats Calico cat

  17. Sex-Linked Traits • Traits controlled by genes on the sex chromosomes are called sex-linked traits. • Mostly X-linked traits; very few Y-linked traits. • Males disproportionately affected by recessive alleles on the X chromosome. • Examples: • Red-green color blindness • Hemophilia

  18. Red-Green Colorblindness • A recessive X-linked trait • Affects 8-12% of males in the United States; less than 0.5% of females. • Results when the color-detecting cones in the retina of the eyes function ‘poorly’ in discriminating between red and green colors. Red-green colorblind vision Normal vision

  19. Color blindness Problem: PROBLEM: • A man with normal vision and a woman who is heterozygous for the colorblind allele want to have a child. • ‘B’ is the allele for normal vision • ‘b’ is the allele for color blindness • What is the probability that their child will be colorblind? • Could they have a colorblind daughter? SOLUTION: • 25% • No, they cannot have a colorblind daughter. The only child that could be affected would be a boy.

  20. Red-Green Color Blindness Other Colorblindness Tests

  21. Variations in Red-Green Color Blindness

  22. Sex-Linked Practice Problem: Hemophilia • A recessive X-linked disorder • Results in a delayed clotting of blood • Will it be more prevalent in males or females? Why? • A man with hemophilia has children with a woman who is a carrier for hemophilia. What is the chance that their next child will have hemophilia? Their next son? Their next daughter?

  23. Sex Influenced Traits • Traits located on the autosomes can sometimes be affected by the proportion of sex hormones produced by the body. • Ultimately sex hormone production is prescribed by the sex chromosomes. • These are called sex-influenced traits • Example: • Male pattern baldness – gene expressed in the presence of testosterone

  24. Polygenic Traits • Traits affected by more than one gene • Examples: • Skin color • Eye color • Height

  25. References ADAM Inc., Male Pattern Baldness. 14 Apr 2008. U.S. National Library of Medicine 16 Apr 2008 <http://www.nlm.nih.gov/medlineplus/ency/imagepages/17083.htm> ADAM Inc., Various Tests for Color Blindness. 2007. New York Times Company. 16 Apr 2008 <http://www.nytimes.com/imagepages/2007/08/01/health/adam/9962Colorblindnesstests.html>. Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007. Color in Computer Graphics. 25 Feb 1998. Cornell University Program in Computer Graphics. 16 Apr 2008 <www.graphics.cornell.edu/online/tutorial/color/>. Waggoner, Terrace L.. "About Color Blindness (Color Vision Deficiency)." Colors for the Color Blind. U.S. Naval Hospital, Pensacola, FL. 16 Apr 2008 <http://www.toledo-bend.com/colorblind/aboutCB.html>. "X chromosome." Genetics Home Reference: Your Guide to Understanding Genetic Conditions. 14 Apr 2008. U.S. National Library of Medicine. 16 Apr 2008 <http://ghr.nlm.nih.gov/chromosome=X>. “X chromosome.” Photo Researchers, Inc. 16 Apr 2008.

  26. Chromosomes and Human Heredity Chapter 11.3 Karyotypes and Nondisjunction

  27. Karyotypes • Some inherited traits can be identified at the chromosome level. • Geneticists use karyotypes. • Chromosomes are stained. • A photomicrograph is taken of a cell’s chromosomes during metaphase. • Chromosomes pairs are arranged in order of decreasing size.

  28. Example Karyotype

  29. Fetal Tests Amniocentesis procedure

  30. Nondisjunction • Cell division in which either the homologous pairs or sister chromatids do not separate correctly, resulting in gametes with an abnormal number of chromosomes.

  31. Types of Nondisjunction • Monosomy – having only one of a particular type of chromosome. • Trisomy – having a set of three chromosomes of one kind • e.g. Down syndrome (Trisomy 21) • Results in distinctive facial features, short stature, heart defects, and mental disability

  32. Nondisjunction in Sex Chromosomes • Females can survive with only one X chromosome or with three. Males can survive with either an extra X or an extra Y. Males cannot survive with only a Y chromosome.

  33. References Biggs, Haggins, Holliday. Biology (Glencoe Science)(Teacher Wraparound Edition). New York: GLENCOE MCGRAW HILL, 2007. Farabee, M. J. Human Genetics. 2001. 01 Apr. 2009 <http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookhumgen.html>. Downschild. Digital image. About Downs Syndrome. 2008. The Coventry and Rugby Down's Syndrome Support Group. 01 Apr. 2009 <http://www.downssupport.org.uk/aboutdowns.html>. "Genetics and pregnancy loss." Miscarriage Management™ -. 2009. Sydney IVF Limited. 01 Apr. 2009 <http://www.miscarriage.com.au/basepage.cfm?id=16>. Karyotype. Digital image. AP Biology. 01 Apr. 2009 <http://www.nkellogg.com/apbiology.htm>. Nondisjunction. Digital image. 01 Apr. 2009 <http://porpax.bio.miami.edu/~cmallery/150/mendel/heredity.htm>.

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