Chapter 14: Mendel & The Gene Idea

Mendelian genetics Character = heritable feature (ex. fur color) Trait = character variant (ex. brown) True-bred = all offspring same variety Hybridization = crossing 2 different true-breds P generation = parents F1 generation = first filial generation

Law of Segregation Alternative versions of genes: alleles account for variation Per character: Offspring gets 2 alleles (1/parent) If the two alleles differ: Dominant fully expressed/detectable Recessive not expressed/detectable Alleles for each character segregate (separate) during gamete production (meiosis). Mendel’s Law of Segregation

Genetic vocabulary……. Punnett square: Homozygous: Heterozygous: Predicts results of crossing (if parents genotypes known) Homozygous: pair of identical alleles for a character Heterozygous: 2 different alleles for a character Phenotype: organism’s expressed traits Genotype: organism’s genetic makeup Testcross: breeding rec homo X dom phenotype (unknown genotype)

The Law of Independent Assortment Law of Segregation involves 1 character. What about 2+ characters? Monohybrid cross vs. dihybrid cross The two pairs of alleles segregate independently of each other. Mendel’s Law of Independent Assortment

EXPERIMENT P Generation F1 Generation Hypothesis of Hypothesis of Fig. 14-8a EXPERIMENT P Generation YYRR yyrr Gametes YR  yr 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 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 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

Non-single gene genetics, I Incomplete dominance: appearance between phenotypes of 2 parents Ex: snapdragons (red x white = pink) Codominance: 2 alleles affect the phenotype in separate, distinguishable ways (red + white = red & white) Ex: sickle cell Multiple alleles: more than 2 possible alleles for a gene Ex: human blood types: A B O

Non-single gene genetics, II Pleiotropy: genes w/ multiple phenotypic effects Ex: sickle-cell anemia Epistasis: gene at 1 locus affects expression of gene at 2nd locus Ex: mice coat color Polygenic Inheritance: additive effect of 2+ genes on a single character Ex: human skin pigmentation and height

Pleiotrophy

Epistasis

Polygenic traits

Human disorders The family pedigree Recessive disorders: Cystic fibrosis Tay-Sachs Sickle-cell * Dominant disorders: rare Huntington’s Testing: Amniocentesis Chorionic villus sampling (CVS)

Pedigrees

Video: Ultrasound of Human Fetus I Genetic testing Video: Ultrasound of Human Fetus I

Heterozygous phenotype same as that of homo- zygous dominant PP Pp Fig. 14-UN2 Degree of dominance Description Example Complete dominance of one allele Heterozygous phenotype same as that of homo- zygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes CRCR CRCW CWCW Codominance Heterozygotes: Both phenotypes expressed IAIB Multiple alleles In the whole population, some genes have more than two alleles ABO blood group alleles IA , IB , i Pleiotropy One gene is able to affect multiple phenotypic characters Sickle-cell disease

Relationship among genes Description Example Epistasis Fig. 14-UN3 Relationship among genes Description Example Epistasis One gene affects the expression of another BbCc BbCc BC bC Bc bc BC bC Bc bc 9 : 3 : 4 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc AaBbCc

You should now be able to: Define the following terms: true breeding, hybridization, monohybrid cross, P generation, F1 generation, F2 generation Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F2 generation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Define and give examples of pleiotropy and epistasis Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance Define and give examples of pleiotropy and epistasis Explain why lethal dominant genes are much rarer than lethal recessive genes Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings