Bio 342 Lect 3.pptx
Transcript
Because learning changes everything. Chapter 02 Extensions to Mendel’s laws Genetics: From Genes to Genomes EIGHTH EDITION Goldberg, Fischer © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. ® A gene can have more than...
Because learning changes everything. Chapter 02 Extensions to Mendel’s laws Genetics: From Genes to Genomes EIGHTH EDITION Goldberg, Fischer © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. ® A gene can have more than two alleles Mendel studied traits that were controlled by genes with two alleles, however, more than two alleles may exist • Multiple alleles of a gene can segregate in populations • Each individual can carry only two alleles • Dominance relations are always relative to a second allele and are unique to a pair of alleles © McGraw Hill 2 ABO blood types in humans are determined by three alleles of one gene allele → A type sugar allele → B type sugar (a) Genotypes i allele → no sugar Corresponding Phenotypes: Type(s) of Molecule on Cell Six genotypes produce four blood phenotypes A Dominance relations are relative to a second allele B • AB ii O • © McGraw Hill 3 Medical and legal implications of ABO blood group genetics Antibodies are made against type A and type B sugars • Successful blood transfusions occur only with matching blood types • Type AB are universal recipients, type O are universal donors © McGraw Hill 4 Human histocompatibility antigens are an extreme example of multiple alleles Three major genes (HLA-A, HLA-B, and HLA-C) encode histocompatibility antigens • Cell surface molecules present on all cells except RBCs and sperm • Facilitates proper immune response to foreign antigens (for example; virus or bacteria) Each gene has 400-to-1200 alleles each • Each allele is codominant to every other allele • Every genotype produces a distinct phenotype • Enormous phenotypic variation © McGraw Hill 5 Mutations are the source of new alleles Mutations are chance alterations of genetic material that arise spontaneously If mutations occur in gamete-producing cells, they can be transmitted to offspring • Frequency of gametes with mutations is © McGraw Hill 6 Nomenclature for alleles in populations Allele frequency is the percentage of the total number of gene copies for one allele in a population Most common allele is usually the wild-type (+) allele Rare allele is considered a mutant or derived allele Gene w/ only one common wild-type allele is monomorphic Gene with more than one common allele is polymorphic • High-frequency alleles of polymorphic genes are often referred to as common variants or polymorphisms © McGraw Hill 7 The mouse agouti gene controls hair color: One wildtype allele, many mutant alleles Wild-type agouti allele (A) produces yellow and black pigment in hair 14 different agouti alleles in lab mice, but only A allele in wild mice for example; mutant alleles a and • a recessive to A (aa has black only) dominant to a but recessive • to A ( mouse has black on back and yellow on belly) Access the text alternative for slide images. © McGraw Hill 8 One gene may contribute to several characteristics Pleiotropy is the phenomenon of a single gene determining several distinct and seemingly unrelated characteristics, for example: • Usually in regulatory proteins or expression factors With some pleiotropic genes • Heterozygotes can have a visible phenotype • Homozygotes can be lethal © McGraw Hill 9 Extensions to Mendel's analysis explain alterations of the 3:1 monohybrid ratio TABLE 2.1 For Traits Determined by One Gene: Alterations of the What Mendel Described Extension Extension’s Effect on Heterozygous Phenotype Complete dominance Incomplete Unlike either homozygote Table Summary: Indominance this table 1:2:1 should be read Codominance as 1; 3:1 as ratio of 3 to 1; 2:1 as ratio of 2 to 1. Extension’s Effect on Ratios Resulting from an Phenotypes coincide withratio genotypes in a2ratio the of 1 to to of 1:2:1 Two alleles Multiple alleles Multiplicity of phenotypes A series of 3:1 or 1:2:1 ratios All alleles are equally viable Recessive lethal alleles Heterozygotes survive but may have visible phenotypes If heterozygotes have a visible characteristic, 2:1 instead of 3:1 One gene determines one trait Pleiotropy: One gene influences several traits Several traits affected in different ways, depending on dominance relations Different ratios, depending on dominance relations for each affected trait © McGraw Hill 10 A comprehensive example: Sickle-cell disease Hemoglobin transports oxygen in RBCs • Two subunits – alpha (α) globin and beta (β) globin The β-globin gene has multiple alleles • Normal allele and 400 mutant alleles. • Most common mutation of β-globin cell disease causes sickle- Pleiotropic – affects >1 trait (deformed RBCs, anemia, heart failure, resistance to malaria) • Recessive lethality – heart failure Different dominance relations for different phenotypic aspects of sickle-cell disease (see Figure 2.8) © McGraw Hill 11 Pleiotropy of sickle-cell anemia: Dominance relations vary with the phenotype under consideration (a): (top): BSIP/Newscom; (a): (bottom): Janice Haney Carr/CDC Access the text alternative for slide images. © McGraw Hill 12 2.2 Extensions to Mendel for Two-Gene Inheritance Learning Objectives: • Conclude from the results of crosses whether a single gene or two genes control a trait • Infer from the results of crosses the existence of interactions between alleles of different genes including additivity, epistasis, redundancy, and complementation © McGraw Hill 13 Epistasis results from the effects of an allele at one gene masking the effects of another gene The allele that does the masking is epistatic to the other gene The gene that is masked is hypostatic to the other allele Epistasis can be recessive or dominant • Recessive – epistatic allele must be homozygous recessive • Dominant – One copy of an allele masks the other gene © McGraw Hill 14 Coat color in Labrador retrievers is determined by two genes • Gene B alleles determine black and brown • Recessive allele ee of gene E is epistatic to B and determines yellow (a): Vanessa Grossemy/Alamy Stock Photo © McGraw Hill 15 Recessive epistasis in Labrador retrievers • progeny of dihybrid crosses indicates recessive epistasis • 9 ∕ 16 black (B— E—) • 3 ∕ 16 brown (bb E—) • 4 ∕ 16 yellow (B— ee, bb ee) • Genotype ee masks the effect of all B genotypes Access the text alternative for slide images. © McGraw Hill 16 A biochemical explanation for coat color in Labrador retrievers Protein E helps generate eumelanin, protein B deposits it • B– deposits eumelanin densely (black) • bb deposits eumelanin less densely (brown) • ee animals cannot make eumelanin (yellow) Access the text alternative for slide images. © McGraw Hill 17 Dominant epistasis in summer squash progeny of dihybrid crosses indicates dominant epistasis I • 12 ∕ 16 white (A— B—, aa B—) • 3 ∕ 16 yellow (A— bb) • 1 ∕ 16 green (aa bb) The dominant allele of one gene masks both alleles of another gene Access the text alternative for slide images. © McGraw Hill 18 A biochemical explanation for dominant epistasis in summer squash • Protein B is a dominant allele that prevents pigment deposition. • It is epistatic to any A allele. Access the text alternative for slide images. © McGraw Hill 19 2.3 Extensions to Mendel for Complex Trait Inheritance Learning Objectives: • Discuss the factors that can cause different individuals with the same genotype to be phenotypically dissimilar • Explain how Mendelian genetics is compatible with the fact that many traits, such as human height and skin colors, exhibit continuous variation © McGraw Hill 20 The same genotype does not always produce the same phenotype In all of the traits discussed so far, the relationship between a specific genotype and its corresponding phenotype has been absolute Phenotypic variation for some traits can occur because of: • Effects of modifier genes • Effects of environment • Pure chance © McGraw Hill 21 Phenotype often depends on penetrance and/or expressivity Penetrance is the percentage of individuals with a particular genotype that show the expected phenotype • Can be complete (100%) or incomplete Expressivity is the degree or intensity with which a particular genotype is expressed in a phenotype • Can be variable or unvarying Access the text alternative for slide images. © McGraw Hill 22 Other effects of environment on phenotype Phenocopy - phenotype arising from an environmental agent that mimics the effect of a mutant gene • Not heritable © McGraw Hill 23
