Concept 14.2: The laws of probability govern Mendelian inheritance Mendel’s laws of segregation and independent assortment reflect the rules of probability When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Probability Line

Probability Line

1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 Rr Rr  Segregation of Segregation of Fig. 14-9 Rr Rr  Segregation of alleles into eggs Segregation of alleles into sperm Sperm 1/2 R 1/2 r Figure 14.9 Segregation of alleles and fertilization as chance events R R 1/2 R R r 1/4 1/4 Eggs r r 1/2 R r r 1/4 1/4

Parents Normal Normal Aa Sperm A a Eggs Aa AA A Normal (carrier) Fig. 14-16 Parents Normal Normal Aa  Aa Sperm A a Figure 14.16 Albinism: a recessive trait Eggs Aa AA A Normal (carrier) Normal Aa aa a Normal (carrier) Albino

Parents Dwarf Normal Sperm Eggs Dwarf Normal Dwarf Normal Dd dd D d Dd Fig. 14-17 Parents Dwarf Normal Dd  dd Sperm Figure 14.17 Achondroplasia: a dominant trait D d Eggs Dd dd d Dwarf Normal Dd dd d Dwarf Normal

Huntington’s Disease

Huntington’s Disease Click image to view video

Effects of HD

Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Gene Linkage – Genes found on same chromosome and stay together during meiosis (unless cross-over occurs) In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Dihybrid Cross EXPERIMENT RESULTS Fig. 14-8 P Generation F1 Generation YYRR yyrr Gametes YR  yr Dihybrid Cross F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Sperm or Predicted offspring of F2 generation 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1

Gene Linkage: Drosophilia melanogaster –has only 4 pairs of chromosomes. Therefore must have large # of genes on each. Dr. Thomas Hunt Morgan (1910) mapped these genes. Genes stay together during process of meiosis unless crossing over occurs. This does not support Mendel’s Principle of Independent Assortment Chromosome #2 has genes controlling Eye color, wing type, body color, etc. Crosses involving linked genes do not give The same phenotypic ratio as unlinked.

Figure 15.3 Morgan’s first mutant

Morgan’s Choice of Experimental Organism Several characteristics make fruit flies a convenient organism for genetic studies: They breed at a high rate A generation can be bred every two weeks They have only four pairs of chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Gene map of a fruit fly (Drosophila melanogaster)

Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Gene Linkage – Genes found on same chromosome and stay together during meiosis (unless cross-over occurs) In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Gene Linkage – Genes found on same chromosome and stay together during meiosis (unless cross-over occurs) In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Most genes exist in populations in more than two allelic forms Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Gene Linkage – Genes found on same chromosome and stay together during meiosis (unless cross-over occurs) In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The approximate distribution of blood types in the U. S The approximate distribution of blood types in the U.S. population is as follows: O-positive: 38 percent. O-negative: 7 percent. A-positive: 34 percent. A-negative: 6 percent. B-positive: 9 percent. B-negative: 2 percent. AB-positive: 3 percent. AB-negative: 1 percent.

Red blood cell appearance Phenotype (blood group) Fig. 14-11 Allele Carbohydrate IA A IB B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Red blood cell appearance Phenotype (blood group) Genotype Figure 14.11 Multiple alleles for the ABO blood groups IAIA or IA i A IBIB or IB i B IAIB AB ii O (b) Blood group genotypes and phenotypes