Fig. 07.0 Mendelian Genetics Mendelian Genetics Outline I. Mendel s Ideas About Genetics. Experimental Design with garden peas 2. Monohybrid Crosses. Principle of Segregation 2. Principle of Dominance 3. Dihybrid cross. Principle of Independent Assortment II. Extensions of Mendelian Genetics: Gene Interactions. Test Cross 2. Incomplete Dominance 3. Multiple Alleles 4. Epistasis 5. Polygenic Inheritance III. Human Genetics Fig. 07.05 Why peas?. Many pea varieties were available. 2. Small plants were easy to grow. 3. Peas self-fertilize. 4. Peas cross-fertilize. Traits used by Mendel had 2 Contrasting Forms
Pea characteristics studied by Mendel Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Monohybrid Cross Flower color Flower position White Parental Pollen transferred Parental Seed color Seed shape Pod shape Axial Terminal Yellow Green Round Wrinkled Inflated Constricted All purple flowers result Anthers removed Pod color Green Yellow F Stem length Tall Dwarf Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Monohybrid Cross Results of Mendel s Crosses Parental White F X F 2 3 White 2
Flower Parent (PP) Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Monohybrid Cross & Punnett Square PP x pp Pp White Flower Parent (pp) P Gametes p Pp p Pp P Pp Pp F Gametes Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Monohybrid Cross & Punnett Square Second Filial Generation (F2) Flower Parent (Pp) Phenotypic Ratio = 3: Genotypic Ratio = :2: Flower Parent (Pp) P Gametes p Gametes P PP Pp p Pp pp F 2 Monohybrid Crosses Genotype: Alleles of an individual PP = homozygous dominant Pp = heterozygous pp = homozygous recessive Phenotype: outward appearance or white pea flowers Summa of Mendel s Model of Inheritance. Parents transmit information about traits. Each individual receives two factors 2. Mendel s Principle of Segregation Gametes can only receive one of two alleles. 3. Mendel s Principle of Dominance One factor can be preferentially expressed 4. Not all factors are identical for a given trait. Alleles can be different Homozygous or Heterozygous combinations 5. Alleles do not influence each other. They remain discrete. They do not blend. 3
Examples of inherited traits in humans Dominant Traits Recessive Traits Recessive Traits. Cystic fibrosis 2. Sickle cell anemia Fig. 07.09 Test Cross: Confirmation of Segregation Freckles No freckles Dominant Traits. Huntington Disease Widow s peak Straight hairline Free earlobe Attached earlobe Dihybrid Cross Hypothesis: Dependent assortment? Parental RY x ry Hypothesis: Dependent assortment RY ry P Dihybrid Cross Hypothesis: Independent assortment RY Gametes X X ry F F 2 x ry F 2 Eggs F 2 2 Sperm 2 2 Actual results contradict hypothesis 4 4 Eggs 4 4 ry Ry Sperm ry Ry 4 4 4 4 RY RrYY Ry RyYY rryy Ry RRyy rryy Ry rryy Ry ry Actual results support hypothesis 9 6 3 6 3 6 6 Yellow round Green round Yellow wrinkled Green wrinkled 4
Dihybrid Cross Hypothesis: Independent assortment F cross X ry Ry ry Ry RY x ry Parents F Yellow round Green Round Yellow wrinkled Green wrinkled Mendel s Second Law of Heredity: Principle of Independent Assortment. In a dihybrid cross, alleles of each gene assort independently. 2. Fate of one pair of alleles associated with one trait does not influence the fate of another pair of alleles associated with a different trait. 3. Genes located on different chromosomes assort independently. Incomplete Dominance in Japanese Four O Clock Parental F F2 Incomplete Dominance In Japanese Four O Clock heterozygote is intermediate in phenotype between the 2 homozygotes 5
Incomplete Dominance in Humans - Hypercholesterolemia HH Homozygous for ability to make LDL receptors LDL LDL receptor Genotypes Hh Heterozygous Phenotypes hh Homozygous for inability to make LDL receptors Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ee No dark pigment in fur Yellow Lab eebb eeb_ Yellow fur Yellow fur Epistasis alleles E = express pigment in fur e = pigment not expressed Epistasis E_ Dark pigment in fur E_bb E_B_ Chocolate Lab Black Lab Brown fur Black fur Pigment alleles B = Black fur e = chocolate/brown Cell Normal Mild disease Severe disease Antigens, Blood Type & Multiple Alleles O Type Blood B Type Blood Multiple Alleles I A = galactosamine antigen on RBC surface I B = galactose antigen on RBC surface i = no antigens on RBC surface A Type Blood Glycolipid AB Type Blood Phenotype A B AB O Genotype I A I A or I A i I B I B or I B i I A I B i i Sugar Exhibited Galactosamine Galactose Galactosamine and galactose None 6
Blood Group (Phenotype) Genotypes O Multiple alleles for ABO blood groups i i Antibodies Present in Blood Anti-A Anti-B Reaction When Blood type Below Is Mixed with blood type on far left column O A B AB ABO blood groups, Antigens and Antibodies Galactosamine Galactose A B AB I A I A or I A i I B I B or I B i I A I B Anti-B Anti-A = agglutination = no agglutination Rh factor Polygenic Inheritance Rh factor = protein Genotypes Rh + / Rh + Rh + / Rh - Rh - / Rh - Phenotypes Rh positive Rh positive Rh negative Rhesus monkeys 7
Fraction of population A model for polygenic inheritance of skin color P aabbcc (ve light) F AaBbCc AABBCC (ve dark) AaBbCc Continuous Variation Skin Color & Polygenic Inheritance Environmental Influences Sperm 20 6 5 20 5 6 F 2 Eggs 5 6 Skin color Genetic Counseling Human Genetics Cell cultures can reveal genetic disorders based on: alterations in chromosome number proper enzyme functioning association with known genetic markers When? Before birth After birth Adult
5,555 Some Important Genetic Disorders 00+ Recessive disorders 400+ Dominant disorders Fig. 07.24 Sickle Cell Anemia Phenotypes: Carrier X Carrier Alleles: S = normal s = Sickle cell Genotypes: Ss X Ss Sickle-cell disease Pleiotropic (multiple) effects of a single human gene Breakdown of red blood cells Physical weakness Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Red blood cells to become sickle-shaped Anemia Sickle cells Heart failure Clumping of cells and clogging of small blood vessels Pain and fever Brain damage Accumulation of sickled cells in spleen Damage to other organs Spleen damage Testing a fetus for genetic disorders Ultrasound monitor Fetus Placenta Uterus Amniocentesis Cervix Amniotic fluid Fetal cells Needle inserted through abdomen to extract amniotic fluid Ultrasound monitor Fetus Placenta Several weeks Uterus Centrifugation Tests Chorionic villus sampling Cervix Several hours Extract tissue from chorionic villi Fetal cells Chorionic villi Impaired mental function Paralysis Pneumonia and other infections Rheumatism Kidney failure Kaotyping 9
Fig. 3.35 Prenatal Diagnosis Autosomal Nondisjunction or Aneuploidy Adult Screening Hexoseaminidase and Tay-Sachs Disease Pedigree Analysis Autosomal recessive aa = affected Aa = carrier (normal) AA = normal Pedigree Analysis Autosomal Dominant. Affected children can have parents with a normal phenotype 2. Heterozygotes have a normal phenotype 3. Two affected parents will always have affected children 4. Affected individuals who have non-carrier spouses will have normal children 5. Close relatives who have children are more likely to have affected children. 6. Equal frequency of both males and females 0
Pedigree showing inheritance of deafness in a family from Martha s Vineyard A test for red-green color blindness Female Male Fig. 07.23 Pedigree Analysis Sex or X-linked
END Mendelian & Human Genetics 2