Chapter 4 Lecture Concepts of Genetics Tenth Edition Extensions of Mendelian Genetics.

Chapter 4 Lecture

Concepts of GeneticsTenth Edition

Extensions ofMendelian Genetics

• Mendel’s work at start of 20th century well known

• Other biologists verify with crosses of other organisms

• Exceptions were observed• Extensions to Mendel’s principles of

heredity

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Cuenot’s Mice

• Cuenot worked with coat color in mice which come in many colors:– Black, gray, brown, white, yellow– But no true breeding yellow mice ever

obtained

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Genes and Alleles Interact to Produce Phenotypes

• Existing alleles subject to mutation• Give rise to new alleles• Single gene can have many alleles• A single gene many phenotypic effects• Single character may be controlled by

many genes• Expression of gene is affected by

interactions with other genes and with environment

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• Alternative forms of a gene are called alleles • Mutation is the source of alleles • The wild-type allele is the one that occurs

most frequently in nature• The other alleles of that gene are mutant• Genes with more than one allele are

polymorphic– New phenotypes result from changes in

functional activity of gene product• Eliminating enzyme function• Changing relative enzyme efficiency• Changing overall enzyme function

Multiple alleles for coat color in rabbits

• C gene for coat color• 4 alleles• C > cchd > ch > c

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Dominance not always complete

• Many genes have alleles that are neither dominant nor recessive to one another

• Heterozygotes have an intermediate phenotype

• Incomplete Dominance

• Phenotype ratio = Genotype ratio

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Incomplete Dominance

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© 2012 Pearson Education, Inc. Figure 4.1

Codominance

• Two alleles of a gene both produce their phenotypes when present in a heterozygote

• Ex: ABO blood group in humans• Gene I encodes an enzyme involved in

attaching sugars to the surface or RBC• Three alleles of the I gene: IA IB and i (IO )

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© 2012 Pearson Education, Inc.

Genes interact when they are expressed

• Many traits characterized by a distinct phenotype are affected by more than one gene

• In gene interaction, the cellular function of numerous gene products contributes to the development of a common phenotype

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Colorless precursor

Colorless intermediate

Purple pigment

Enzyme C Enzyme P

The recessive c allele encodes an inactive

enzyme

The recessive p allele encodes an inactive

enzyme

• The term epistasis describes the situation in which a gene can mask or modify the phenotypic effects of another gene

• Epistatic interactions often arise because two (or more) different proteins participate in a common cellular function– For example, an enzymatic pathway

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• If an individual is homozygous for either recessive allele, it will not make any functional enzyme C or enzyme P– Therefore, in the previous example, the

flowers remain white

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Labrador retrievers

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Allele B (black) is dominant to b (brown)Allele E (pigment deposition in hair) is dominant to e (no deposition so hair is yellow)

Labrador Retrievers

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The Expression of a Genotype May Be Influenced by Environmental Effects

• Temperature-sensitive allele:– An allele whose product is functional only at a

certain temperature.

Two parameters describe the effects of genes and environment on phenotype

1) Penetrance – proportion of individuals in a group with a given genotype that actually show the expected phenotype

– Ex: BRCA1 mutant allele, some people do not develop breast cancer

– The mutation is said to be incompletely penetrant

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2) Expressivity – degree or range to which a genotype is expressed in an individual

– Ex: BRCA1 mutant allele. A woman with the mutant allele may develop both breast and ovarian cancer as part of the phenotype, but another woman with same mutation may only get breast cancer.

– The mutation is said to have variable expressivity

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Genes are carried on chromosomes

• A gene is a sequence of DNA that resides at a particular site on a chromosome called a locus

• Genes that reside on a single chromosome are genetically linked together and this affects pattern of inheritance

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• X,Y system used for sex determination by many animal and plant species• X is a large chromosome

and encodes many genes• Y is a small chromosome

with few genes (not homologous to X in the traditional sense but has pairing region for synapsis)

Thomas Hunt Morgan and the Fly Room

• Early 20th century, Columbia University• Fruit fly – Drosophila melanogaster

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© 2012 Pearson Education, Inc. Figure 4.12

© 2012 Pearson Education, Inc. Figure 4.13

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• Thomas Hunt Morgan (1910) showed the X-linked inheritance in his studies of the white eyed mutation in Drosophila

• The inheritance pattern was clearly related to the sex of the parent carrying the mutant allele

• Reciprocal crosses between white eyed and red eyed flies did not yield identical results

• White locus is present on the X chromosome rather than one of the autosomes

Interesting video to watch on Morgan’s flies

• Calvin Bridges: http://library.cshl.edu/exhibits/bridges/_pages/page1_about.html

• The Fly Room movie: http://www.theflyroom.com

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© 2012 Pearson Education, Inc.

• Genes present on the X chromosome exhibit unique patterns of inheritance due to the presence of only one X chromosome in males and two in females

• Males cannot be either homozygous or heterozygous for X linked genes – they have only one copy of a gene in a diploid cell - hemizygous

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• Many traits controlled by X chromosome-linked traits– Red/green color blindness– Hemophilia– Only females are carriers of recessive alleles

© 2012 Pearson Education, Inc. Figure 4.14

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• Lethal X-linked recessive disorders are observed most often in males

• Only females can be heterozygous carriers that do not develop the disorders

• For example: Duchenne muscular dystrophy– Muscles slowly degenerate leading to

paralysis– Onset prior to the age of 6 and lethal around

30

Concept Check

Hemophilia (reduced blood clotting) is an X-linked recessive disease in humans. A woman with hemophilia mates with a man who exhibits normal blood clotting. What is the probability that their child will have hemophilia?

Concept Check

1/2

Hemophilia (reduced blood clotting) is an X-linked recessive disease in humans. A woman with hemophilia mates with a man who exhibits normal blood clotting. What is the probability that their child will have hemophilia?

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© 2012 Pearson Education, Inc.