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Complex Inheritance and Human Heredity Chapter 11 Basic Patterns of Human Inheritance Chapter 11.1 Recessive Genetic Disorders • A recessive trait is expressed when the individual is homozygous recessive for that trait. • An individual who is heterozygous for a recessive disorder is called a carrier. • Ex) Cystic Fibrosis, Albinism, TaySachs Disease, Galactosemia Dominant Genetic Disorders • Individuals with a dominant disorder will have at least one dominant allele. • Ex) Huntington’s Disease, Achondroplasia Pedigrees • A diagram that traces the inheritance of a particular trait through several generations. • A pedigree uses symbols to illustrate inheritance of the trait. Pedigrees http://www.sciencecases.org/sickness_and_health/sickness_and_health_notes.asp Pedigrees Affected Male Female Carrier Female Affected Male http://www.214bio.com/TOPICS/DNA/new214bio/simple_pedigree.gif Analyzing Pedigrees • Pedigrees are used to infer genotypes from the observation of phenotypes. • If good records are kept within families, disorders in future offspring can be predicted. Complex Patterns of Inheritance Chapter 11.2 Incomplete Dominance • The heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes. • Ex) Snapdragons (Flower) • RR (red) x rr (white) • 100% Rr (pink) Codominance • Both alleles are expressed in the heterozygous condition. • Ex) Sickle-cell disease • Individuals will have normal and sickleshaped cells. AA = normal Aa = sickle cell trait (few symptoms) aa = sickle cell anemia Multiple Alleles • Some forms of inheritance are determined by multiple alleles. • Ex) Blood Types • ABO blood groups have 3 forms of alleles. • IA is blood type A, IB is blood type B, and i is blood type O. • i is recessive to IA and IB. • IA and IB are codominant resulting in the AB blood type. • Rh factors are either positive or negative. Rh+ is dominant. Multiple Alleles Epistasis • Epistasis is an interaction where one allele hides the effects of another allele. • Ex) Coat color in Labrador retrievers http://faculty.southwest.tn.edu/rburkett/GB%20Genetics.htm Sex Determination • Sex chromosomes: determine an individual’s gender • Autosomes: the other 22 pairs of chromosomes • Females have 22 pairs of autosomes and one pair of X chromosomes (XX). • Males have 22 pairs of autosomes and one X and one Y chromosome (XY). Sex Determination • The X chromosomes is larger than the Y chromosome because it carriers genes that are necessary for the development of both male and females. • To balance the “dose” of X-related genes, one of the X chromosomes in females is inactivated (X-inactivation). Sex-Linked Traits • Traits controlled by genes located on the X chromosome are called sex-linked or X-linked traits. • Because males only have one X chromosome they are affected by recessive X-linked traits more than females. • In females the other X-chromosome will often mask the effect of the recessive trait. Sex-Linked Traits • Red-green color blindness is a recessive X-linked trait. • 8% of males in the US are red-green color blind. http://www.colblindor.com/2010/03/02/what-is-color-blindness/ Color Blindness Hemophilia The Royal Family Hemophilia Polygenic Traits • Many phenotypic traits arise from the interaction of multiple pairs of genes and are called polygenic traits. • Ex) Skin color, height, eye color, and fingerprint pattern. Environmental Influences • The environment also has an effect on phenotype. • Ex) Heart disease is inherited but diet and exercise play an important role in the occurrence and seriousness of the disease. Twin Studies • By studying identical twins, geneticists can separate genetic contributions from environmental conditions. Chromosomes and Human Heredity Chapter 11.3 Karyotypes • Scientists study whole chromosomes by using images of chromosomes stained during metaphase. • The staining bands identify or mark identical places on homologous chromosomes. • The homologous chromosomes are arranged in decreasing size to produce a micrograph called a karyotype. Karyotype http://www.cancerquest.org/cell-cycle-mitosis.html Telomeres • Chromosomes end in protective caps called telomeres. • The cap serves as a protective structure for the chromosome. • Scientists believe that telomeres may play a role in both aging and cancer. Telomeres http://www.immortalhumans.com/telomeres-is-it-an-accurate-biological-measurement-for-aging/ Nondisjunction • When sister chromatids fail to separate properly during cell division it is called nondisjunction. • If this occurs during meiosis I or meiosis II the resulting gametes will not have the correct number of chromosomes. Nondisjunction • Nondisjunction can result in extra copies of a chromosome or only one copy of a chromosome. • Trisomy: having a set of three chromosomes of one kind. • Monosomy: having only one of a particular type of chromosome. Down Syndrome • Disorder resulting from an extra chromosome 21. • Often called trisomy 21. • Characteristics include distinctive facial features, short stature, heart defects, and mental disability. Down Syndrome http://geneticdisorderinfo.wikispaces.com/Down+Syndrome Fetal Testing • Amniocentesis: diagnosis of chromosome abnormalities and other defects • Chorionic villus sampling: diagnosis of chromosome abnormalities and certain genetic defects • Fetal blood sampling: diagnosis of genetic or chromosome abnormalities, checks for fell blood problems and oxygen levels