Extensions of Mendelian Genetics OutlineStudy Guide Broader course

Extensions of Mendelian Genetics Outline/Study Guide Broader course objective Explain more complex modes of inheritance and how this influences the inheritance and expression of genes; use this information in predicting genetic outcomes and the analysis of genetic data Necessary for Labs--Patterns of Inheritance in Maize, Blood typing. Lecture outline/study guide • Other factors that can change ideal Mendelian ratios – How can lethality affect the ratios of the remaining genotypes/phenotypes in Mendelian segregation? – What is the difference between “penetrance” and “expressivity”? – What is incomplete dominance? Co-dominance? What examples of each might you be able to give? • What is an “allelic series”? • Epistasis--two (or more) genes can interact to affect one phenotype (e. g. color of peppers). – When is a gene interaction considered “epistatic”? – What is the difference between the epistatic gene and the hypostatic gene? • What is the difference between “epistatic” vs. “dominant”?

2: 1 ratio from cross between two yellow mice results from a lethal allele.

Mm x (Manx) Mm (Manx) Sperm M M Lethality in Manx cats causes altered ratio Brooker, Figure 5. 13 b Egg m m MM (early embryonic death) Mm (Manx) mm (non-Manx) 1: 2 ratio of kittens that are born Copyright ©The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display

Phenotype affected by temperature Biology: Unity and Diversity of Life, Starr and Taggert

Penetrance—in this group of identical genotypes, how many actually show the phenotype? Fig from i. Genetics 1 st ed. , P. Russell

Expressivity—in this individual of particular genotype, how strongly does he express the phenotype? Fig from i. Genetics 1 st ed. , P. Russell

Variable expressivity of Neurofibromatosis (NF 1) Weak phenotype: cafe au lait spots

Strong phenotype--cutaneous neurofibromas

P 0 generation CWCW CRCR Gametes 1: 2: 1 phenotypic ratio NOT the 3: 1 ratio observed in simple Mendelian inheritance CR CW x Pink CRCW F 1 generation Gametes CR or CW Selffertilization F 2 generation In this case, 50% of the CR protein is not sufficient to produce the red phenotype CR CW CRCR CRCW Incomplete dominance as seen in plants CR CW CRCW Brooker Figure 5. 5 CWCW Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Allelic Series—being “dominant” or “recessive” depends upon which alleles are being expressed together e. g. flower color gene: C [enzyme necessary for making pigment] Allele CR Phenotype Red C 50 Dark pink C 20 Light pink C 0 white Enzyme activity 100% 50% 20% 0% Genotype Phenotype CR CR Red C 50 Dark pink C 50 C 20 Pink C 50 C 0 Light pink C 20 Light pink C 0 white

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Dominant (functional) allele: R (round) Recessive (defective) allele: r (wrinkled) Genotype RR Rr rr Amount of functional (starch-producing) protein 100% 50% 0% Phenotype Round Wrinkled With unaided eye (simple dominant/ recessive relationship) With microscope (incomplete dominance) Figure 5. 6

Why is this called a phenotype and not a genotype? Co-dominance— both alleles equally expressed A B A B B (example: bloodtypes— genotype and surface antigens A A A B B

ABO Blood Type • • Allele i is recessive to both IA and IB Alleles IA and IB are co-dominant (in cells with both alleles the trait is a mixture of both phenotypes seen in the homozygotes) Antigen O RBC Antigen A N-acetylgalactosa mine Antigen B RBC Antigen A Galactose RBC Blood type: O A B AB Genotype: ii IAIA or IAi IBIB or IBi IAIB B A and B Surface antigen: neither A nor B A Serum antibodies: against A and B against A Brooker Figure 5. 9 a Copyright ©The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display none Antigen B

Examples of sex limited inheritance? Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. © robert Maier/Animals, Animals (a) Hen © robert Maier/Animals, Animals (b) Rooster Brooker, fig 5. 12

Duplicate Recessive Epistasis white When the recessive allele of either gene masks the expression of the other gene. Precursor 1 Gene A Precursor 2 Gene B Purple Pigment white

x Duplicate Recessive Epistasis White variety #1 (CCpp) White variety #2 (cc. PP ) F 1 All purple (Cc. Pp) Complementation: Each recessive allele (c and p) is complemented by a wild-type allele (C and P). This phenomenon indicates that the recessive alleles are in different genes. Self-fertilization F 2 CP Epistasis: Homozygosity for the recessive allele of either gene results in a white phenotype, thereby masking the purple (wildtype) phenotype. Both gene products encoded by the wild-type alleles (C and P) are needed for a purple phenotype. CP Cp c. P cp Brooker, Figure 5. 14 Cp c. P cp CCPp Purple Cc. PP Purple Cc. Pp Purple CCpp White Cc. Pp Purple Ccpp White Cc. PP Purple Cc. Pp Purple cc. PP White cc. Pp White Cc. Pp Purple Ccpp White cc. Pp White ccpp White CCPP Purple CCPp Purple Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

“dominant” ≠ “epistatic” “dominant/recessive” • Alleles of same gene 7 q 21. 1 Gene 1—Hair color Black Brown Auburn Red Blond Epistatic—one gene masking another Gene 1—Hair color Black Brown Auburn Red Blond Gene 2 -Baldness Full hair partial hair bald

“dominant” ≠ “epistatic” “dominant/recessive” • Alleles of same gene 7 q 21. 1 Gene 1—Hair color Black Brown Auburn Red Blond Epistatic—one gene masking another Gene 1—Hair color Black Brown Auburn Red Blond Gene 2 -Baldness Full hair partial hair bald 14 p 1. 31 Hypostatic gene Epistatic gene

• A geneticists crossed a red eyed fly with another red eyed fly. In the next generation she observed phenotypic proportions of 263 red-eyed flies : 137 brown-eyed flies. What hypothesis would best explain the parental cross that gave rise to these flies? Use chi-square analysis to support your hypothesis. • How would you ‘prove’ this without doing the chi-square test?

Extensions of Mendelian Inheritance : practice questions The following comprehension questions (at end of each chapter section) in Brooker, Concepts of Genetics are recommended: • Comprehension Questions (at end of each section): 5. 2, 5. 4, 5. 6, 5. 8 Answers to Comprehension Questions are at the very end of every chapter. • Solved Problems at end of chapter (answers included): • Conceptual questions and Experimental/Application Questions at end of chapter (answers found by logging into publisher’s website, or find them in the book): – Concepts—C 1, C 2, C 3, C 4, C 5, C 6, C 8, C 9, C 10, C 11, C 15, C 20 – A little more challenging—C 18, E 1, E 2, E 7, E 10