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CHAPTER 7GENETICS(Part 3)

Resilience expense 1CONTENT DNA ReplicationModels of DNA ReplicationDNA Replication Process

Genetic code and its features

Protein synthesis: Transcription and TranslationTranslation in prokaryotes and eukaryotesTranslation Initiation, Elongation and Termination steps.BIO095 (2013/2014)CONTENT Gene Regulation and expression

Mendelian geneticsMonohybrid inheritanceDihybrid inheritanceExtension of Mendelian Genetics

Population GeneticsGene populationHardy-Weinbergs PrincipleFactor that change the frequency in population BIO095 (2013/2014)PART 3(1) LEARNING OBJECTIVES Explain about the DNA replication process.

Summarize how DNA replicates (semiconservative) by citing the Meselson and Stahls experiment.

Outline the flow of genetic information process in cells, from DNA RNA Proteins via looking at the transcription and translation process.

BIO095 (2013/2014)(2) LEARNING OBJECTIVES Explain the concept of operon and gene regulation.

Describe the components of lac operon and their functions in E. coli.

Describe the mechanism of the operon in the absence and presence of lactose.

BIO095 (2013/2014)(3) LEARNING OBJECTIVES Define important terms in genetics.

Describe Mendel Principles of Segregation and Independent Assortment.

Explain population genetics, gene pool, allele frequencies and genetic equilibrium.

State and explain five assumptions of Hardy-Weinberg Law fro genetic equilibrium.

Calculate allele and genotype frequencies. BIO095 (2013/2014)GLOSSARY TERMSGENETICSThe study of inheritance, the transmission of particular characteristics from generation to generation by means of the genetic code which is transferred to the offspring.

Genetics is the science of heredity and variation in living organisms.

Knowledge of the inheritance of characteristics has been implicitly used since prehistoric times for improving crop plants and animals through selective breeding.

However, the modern science of genetics seeks to understand the mechanisms of inheritance.

BIO095 (2013/2014)Genetics is the science of heredity and variation in living organisms.

7GLOSSARY TERMSGENESA basic heredity unit located on a specific locus of a chromosome, that determines a specific characteristic in the organism.

A gene, consists of a DNA segment (or RNA in some viruses) containing a particular sequence of nucleotides, that codes for the synthesis of a specific polypeptide chain or protein in an organism.

BIO095 (2013/2014)

Genes are units of information about specific traits or characteristics that are located on chromosomes. These traits are heritable Each gene can be found distributed among the chromosome. Each gene has a particular location called the gene locus (pl-loci).

8GLOSSARY TERMSALLELESare alternative forms of the same gene & occupy the same locus of a pair of homologous chromosomes.

Alleles are responsible for determining the contrasting characteristics of a gene.

Example:Height of pea plant.T dominant allele for tall stemt recessive allele for short stem.

BIO095 (2013/2014)For example, one allele in a gene specifies for black hair where else the other allele specifies for blonde hair9LOCUS Position of an allele on a chromosome or within a DNA molecule. The 2 alleles of a gene occupy the same locus on a pair of homologous chromosomes.

DOMINANT ALLELEThe allele that produces the phenotypic characteristic in the presence of homozygous or heterozygous condition. i.e. when a dominant allele is paired with a recessive allele, it masks (cover) the effect of the recessive allele on a particular trait. BIO095 (2013/2014)GLOSSARY TERMS10RECESSIVE ALLELEThe phenotypic characteristic of a recessive allele is not expressed in the heterozygous condition.

The phenotypic characteristic is only expressed in the presence o another identical recessive allele, that is in the homozygous condition.

A capital letter represents a dominant allele, a small letter represents a recessive allele

TRAITEach variant for a characteristic. Example: Tall stem or short stem for pea plant height is called a trait. BIO095 (2013/2014)GLOSSARY TERMS11Another example:black hair is the dominant trait and can be marked by the capital letter B; wherelse blonde hair is a recessive trait that can be marked by the small letter b.

BIO095 (2013/2014)GLOSSARY TERMS

12GENOTYPEThe genetic makeup or set of alleles, of an organism. Usually refer to the allelic components of a particular gene or set of genes, and may also refer to the entire genome of an organism.

PHENOTYPE The physical or physiological traits of an organism,which are determined by its genotype (genetic makeup).Example: height, eye color, human blood type.

BIO095 (2013/2014)GLOSSARY TERMSPhenotype organisms with the same genotype, sometimes may have different phenotypes because of interaction of genes or the effects of environmental factor.

Genotype refers to the alleles present in an individual. Phenotype refers to that particular individuals observable trait

13HOMOZYGOTEDescribing an organism or cell where both alleles of a gene at a given locus on a pair of homologous chromosomes are IDENTICAL. They may be either:Homozygous dominantindividual has a pair of dominant alleles.Example: (BB) for the hair color trait (black). Homozygous recessiveindividual has a pair of recessive alleles .Example: (bb) for the hair color trait (blonde).

BIO095 (2013/2014)GLOSSARY TERMSHomozygous organism are referred to as homozygotes.They breed true when crossed with identical homozygous organisms. 14HETEROZYGOTEDescribing an organism or cell where the alleles of a gene at a given locus on homologous chromosomes are DIFFERENT.

The observable trait is determined by the dominant allele.

A heterozygous individual has a pair of nonidentical allele .Example: Bb for the hair color trait. (Black)

BIO095 (2013/2014)GLOSSARY TERMSHeterozygous organisms are referred to as heterozygotes. they do not breed true. 15.: In order to see which gene will be expressed in a particular individual, we have to see whether it is:a homozygous dominant, Heterozygous, or homozygous recessive condition

In human, a diploid cell (2n) has: 22 pairs of homologous chromosomes called the autosomes, and1 pair of sex chromosomes.a human somatic cell has 23 pair of chromosomes (2n = 46)

BIO095 (2013/2014)GLOSSARY TERMS16PURE LINEA population of organisms all having a same particular trait, that has been genetically unchanged through may generations. The organisms are homozygous & shows pure breeding for the trait concerned.

P1 GENERATIONFirst parental generation (i.e. the immediate parents of the F1 generation)They should be homozygous.

F1 GENERATION (first filial generation)The first generation of offspring from a cross of first parental (P1) generation.

F2 GENERATION (second filial generation)The second generation produced by crossing 2 F1 individuals.

BIO095 (2013/2014)GLOSSARY TERMSFilial = latin words for sons & daughters.

CROSS The mating of 2 individuals in a genetic analysis. 17BIO095 (2013/2014)GLOSSARY TERMS

GenotypeDescribed asPhenotypeBBHomozygous dominantBlack hairBbHeterozygousBlack hairbbHomozygous recessiveBlonde hair18MENDELIAN GENETICS Monohybrid Inheritance Dihybrid InheritanceBIO095 (2013/2014)19He was an Austrian priest and scientist .

Remembered as The Father of Modern Genetics. for his study on the inheritance of traits in pea plants.

Was inspired to study variation in plants.

He conducted his study in the monastery's garden.

Between 1856 and 1863, Mendel cultivated and tested some 29,000 pea plants (Pisum sativum)

BIO095 (2013/2014)Gregor Johann Mendel (1822 1884)

(July 22, 1822 January 6,1884)

He entered the abbey in 1843. In 1851 he was sent to the University of Vienna to study, returning to his abbey in 1853 as a teacher, principally of physics.

20 At Mendels time, people taught that traits of the parents are blended together in the offspring.

From his observation, he realized that this is not true.

If offspring is a results of blending the parents trait, the result of the cross between a purple flower and a white flowerthe result would be pale purple

However, this is not evidenced

BIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIES21Mendel were genius enough to choose pea plants because:Pea plants is easy to growPea plants come in many varietiesPea plants carry out controlled pollination (self fertilization)Pea plants have petals that completely closed its reproductive structures, therefore it is easier to control pollination

BIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIES22Pea plantThe sex organs of a pea plant are in its flower.Each pea flower has both sex organs.Male StamensFemale Carpel

In nature, pea plants usually self-fertilize.

Self-fertilize Pollen grains released from the stamens lands on the carpel of the same flower.Sperms from the fertilize ova in the carpel.

BIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIES23CROSS-POLLINATIONFertilization between different plant.The transfer of pollen from flowers of one plant to flowers of another plant of the same species.

To carry out cross-pollination in pea plant, Mendel:removed the immature stamens of a plant before they produce pollen,then dusted pollen from another plant into the emasculated flowers.Each resulting zygote then developed into embryo encased in a seed (pea)

BIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIESEmasculated weakened / the plant that had been removed its stamen. 2425

Remove stamens from purple flower.Transferred pollen from stamens of white flower to carpel of purple flower. Pollinated carpel matured into pod. Seeds from pod are planted. A GENETIC CROSS (Hybridization) 25BIO095 (2013/2014)

Offspring: All purple flowers. The character of interest FLOWER COLORPollen from a white flower fertilizes ova of a purple flower.The first generation (F1) hybrids all have purple flowers.The result is the same for the RECIPROCAL CROSS i.e. the transfer of pollen from a purple flowers to white flowers. 26CHARACTERIs a heritable feature,Example: flower color, that varies among individuals.

TRAITA trait is the different variant for a character,Example: Purple or white color for flowers.

BIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIES27True-breedingPlant variety that when the plant self-pollinate, all their offspring are of the same variety (have the same trait(s))

Example:A plant with purple flowers is true-breeding if seeds produced by self-pollination, all give rise to plants (offspring) with purple flowers too. BIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIESTrue breeding homozygous28Mendel cross pollinated 2 contrasting true-breeding pea varieties:Purple-flowered plants & white-flowered plants.(Monohybrid cross)

Hybridization,Mating or crossing of 2 true-breeding varieties.

P generation (Parental generation)True-breeding parents.

F1 generation (First filial generation)Hybrid offspring of P generation. filial = sonBIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIES29F1 hybrids to self-pollinated & produces F2 generation

F2 generation Second filial generation

Mendel usually followed traits for at least these 3 generations.The P, F1 & F2 generations.

BIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIES30Based on Mendels quantitative analysis of F2 plants, have revealed the 2 fundamental principles of heredity:

The Law of SegregationThe Law of Independent Assortment

BIO095 (2013/2014)MENDELS EXPERIMENTS & DISCOVERIES31BIO095 (2013/2014)MONOHYBRID INHERITANCE & THE PRINCIPLE OF SEGREGATION32The cross between purple-flowered & white-flowered pea plants, produces F1 hybrids (offspring) with all purple flowers.

Mendel allowed the F1 plant to self-pollinate & planted their seed.

In the F2 generation, the white-flowered trait reappearedproduces pea plants with the ratio of 3 purple to 1 white flowers.

BIO095 (2013/2014)MONOHYBRID INHERITANCE33BIO095 (2013/2014)MONOHYBRID INHERITANCE

Phenotypic ratio:3 Purple : 1 White34Purple flower is a dominant trait

White flower is a recessive trait 34Mendel observed the same pattern of inheritance in 6 other pea plant characters, each represented by two traits

BIO095 (2013/2014)MONOHYBRID INHERITANCE35

36Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring.

4 related concepts make up this model.

These concepts can be related to genes and chromosomes

BIO095 (2013/2014)THE LAW OF SEGREGATION37The first concept:Alternative versions of genes account for variations in inherited characters.

Alleles are alternative versions of a gene / variants of a gene.

Each gene resides at a specific locus on a specific chromosome.

At that locus, the DNA: vary in its sequence of nucleotides..: differ in its information content. BIO095 (2013/2014)THE LAW OF SEGREGATION38.

BIO095 (2013/2014)THE LAW OF SEGREGATIONThe homologous pair of chromosomes represent an F1 hybrid (with the gene for flower color) which inherited:The purple flower allele from 1 parent, &The white flower allele from the other parent.

39The second concept:For each character, an organism inherits 2 alleles, one from each parent.

A diploid organism has homologous pairs of chromosomes,one chromosome of each pair is inherited from each parent.

A genetic locus is represented twice in a diploid cell.

Homologous loci (on a pair of homologous chromosomes) may have:2 identical alleles - as in the true-breeding plants of Mendels P generation2 different alleles - as in the F1 hybrids (Purple & white flower alleles)

BIO095 (2013/2014)THE LAW OF SEGREGATION40The third concept:If the 2 alleles at a locus differ, then one (the dominant allele) determines the organisms appearance, and the other (the recessive allele) has no noticeable effect on the organisms appearance.

In the flower-color example, the F1 plants had purple flowers because:The purple-flower allele is dominant &The white-flower allele is recessive.

BIO095 (2013/2014)THE LAW OF SEGREGATION41The fourth concept now known as the LAW OF SEGREGATION, states that the 2 alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.

This segregation of alleles corresponds to:the distribution of homologous chromosomes to different gametes in meiosis.

Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism.

BIO095 (2013/2014)THE LAW OF SEGREGATION42Mendels segregation model accounts for, the 3:1 phenotypic ratio observed in the F2 generation.

Punnett square,shows the possible combinations of sperm and egg,predicts the results of a genetic cross between individuals of known genotype (genetic makeup).

A capital letter represents a dominant allele.A lowercase letter represents a recessive allele.

BIO095 (2013/2014)Monohybrid Inheritance: PUNNETT SQUARE43

44Each gamete contains 1 allele for the flower color gene.Parental gametes unite,Produces F1 hybrid with genotype Pp.All have purple flower. F1 hybrid produces gametes: of the gametes receive P allele of the gametes receive p allele(the allele segregate)Punnett square shows all possible combinations of alleles in offspring. Genotypic ratio: PP : Pp : ppPhenotypic ratio: 3 Purple : 1 White

True-breeding plant of the parental generation.

44Homozygous

An organism having a pair of identical alleles for the gene controlling a character.

Example: True-breeding pea plant with genotype:PP Homozygote dominant have purple flowerpp Homozygote recessive have white flowerBIO095 (2013/2014)Genetic Vocabulary45Heterozygous

An organism having 2 different alleles for a gene controlling a character are heterozygous for that gene.

Heterozygotes are not true-breeding.

Example: The F1 generation with genotype Pp,self-pollinate & produces both:Purple-flowered & white-flowered offspring. BIO095 (2013/2014)Genetic Vocabulary46MONOHYBRID

An organism that is heterozygous with respect to a single gene of interest.Monohybrid results from a cross between parents (true-breed) for different alleles.

Mendel performs breeding experiments which involves only a single (mono) character (e.g. flower color),to derive The Law of Segregationthe hybrid produced are monohybrids (heterozygous for one character).

Example:Parents of genotype AA & aa produces a monohybrid with genotype Aa.BIO095 (2013/2014)Genetic VocabularyThe parents are true breed for different alleles of a gene produces offspring that monohybrid (heterozygous) 47BIO095 (2013/2014)Genetic VocabularyPhenotypeThe physical (appearance) & physiological traits of an organism.

GenotypeThe genetic make up of an organism. 48

48BIO095 (2013/2014)DIHYBRID INHERITANCE& THE PRINCIPLE OF INDEPENDENT ASSORTMENT49DIHYBRIDAn organism that is heterozygous with respect to 2 genes of interest (heterozygous for 2 characters).

Dihybrid results from a cross between2 true-breeding parents (homozygous for different alleles) differing in 2 characters. (Involves 2 characters 2 genes)

Example: Parents of genotpye AABB & aabb produces a dihybrid of genotype AaBb. BIO095 (2013/2014)DIHYBRID INHERITANCEParents :AABBXaabbGametes : Offspring :AaBbABab50BIO095 (2013/2014)DIHYBRID INHERITANCEExample:Crossing 2 true-breeding pea varieties (which differ in 2 characters), i.e. a parental cross between: a plant with yellow-round seeds (YYRR) & a plant with green-wrinkled seed (yyrr).

Y=Yellow seeds (dominant allele)y=Green seeds (recessive allele)R=Round seeds (dominant allele)r=Wrinkled seeds (recessive allele)Alleles for seed colorAlleles for seed shape51BIO095 (2013/2014)DIHYBRID INHERITANCEThe crossing produce the F1 plants, that:are dihybrids,with genotype YyRr (heterozygous for both characters)exhibit both dominant phenotypes : Yellow-round seed.

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52BIO095 (2013/2014)DIHYBRID INHERITANCEThe F1 hybrids are allow to self-pollinate to produce F2 offspring.

In the production of gametes of F1 dihybrid (YyRr),the alleles for seed color (Y & y) segregate independently of the alleles for seed shape (R & r).: each gamete has 1 allele for each gene.F1 gametes produced

There will be 16 (4 X 4) probable ways of the alleles can combine in the F2 generation.

These combinations produces F2 offspring with phenotypic ratio of: 9 : 3 : 3 : 1

53YRyRYryr53BIO095 (2013/2014)DIHYBRID INHERITANCEPhenotypic ratio:9 yellow-round :3 green-round :3 yellow-wrinkled : 1 green-wrinkled54

54Genotypic ratio:9/16 Y_R_ : 3/16 yyR_ : 3/16 Y_rr : 1/ 16 yyrr

Phenotypic ratio:9/16 yellow-round : 3/16 green-round : 3/16 yellow-wrinkled : 1/16 green-wrinkled

From the dihybrid cross, Mendel discovered that:9 have at least one copy of the dominant alleles R and Y.3 have at least one copy of the dominant allele R and are homozygous for yy.3 have at least one copy of the dominant allele Y and are homozygous for rr.1 combination is homozygous for rr and yy.

BIO095 (2013/2014)DIHYBRID INHERITANCE55Using a dihybrid cross, Mendel developed: The Law Of Independent Assortment

The law of independent assortment states that:each pair of alleles (from 1 gene) segregates independently of each other pair of alleles (from another gene) during gamete formation.

This law applies only to genes on different, nonhomologous chromosomes.

Genes located near each other on the same chromosome tend to be inherited together.BIO095 (2013/2014)THE LAW OF INDEPENDENT ASSORTMENT56BIO095 (2013/2014)CHROMOSOMAL EXPLANATION OF INDEPENDENT ASSORTMENT57Mitosis and meiosis were first described in the late 1800s.

The chromosome theory of inheritance states:Mendelian genes have specific loci (positions) on chromosomes.Chromosomes undergo segregation and independent assortment.

The behavior of chromosomes during meiosis was said to account for: Mendels laws of segregation & independent assortment.BIO095 (2013/2014)Relating Mendelism To Chromosomes58Correlating the results of Mendels dihybrid cross with the behavior of chromosomes. 59

Crossing between 2 parents that are true-breeding. Involves 2 genes:The gene for seed shape (alleles R & r)The gene for seed color (Y & y) These genes are on different chromosomes. Each gametes gets 1 allele from each gene. 59

60Us61- Fertilization recombines alleles (from each gene) at randoms.

- Each individual gets random recombination of alleles for each gene.

Fertilization among the F1 plantsFertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. 61BIO095 (2013/2014)EXTENDING MENDELIAN GENETICS Codominance Incomplete Dominance Multiple Alleles Epistasis Polygenic Inheritance

62Mendel was lucky to choose pea plants because, they most of the characters of pea plants are determined by one gene with two alleles dominant and recessive.

However, this condition cannot always be met in nature

There are also other hereditary patterns that was not reported by Mendel.BIO095 (2013/2014)Gene Interaction63Morphological features are all affected by the expression of many different genes in combination with environmental factors.Example of morphological features height, weight, growth rate & pigmentation

GENE INTERACTION:2 or more different genes influence the outcome of a single trait.

well look at:How 2 genes will affect the same trait.There are many different ways that the alleles of 2 genes may interact to affect a trait.

BIO095 (2013/2014)Gene Interaction64In cases where there is an absence in complete dominance,codominance & incomplete dominance can be one of the cause.

CODOMINANCEThe phenomenon in which 2 alleles are both expressed in the heterozygous.

With codominance,a cross between organisms with 2 different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.

BIO095 (2013/2014)CODOMINANCE65Examples:Roan fur in cattleThe ABO blood

Roan fur in cattleCattle can be:Red (RR = all red hairs), White (WW = all white hairs), orRoan (RW = red & white hairs together).

BIO095 (2013/2014)CODOMINANCE

66ABO Blood GroupThe ABO group of antigen,determine blood type in human. is also an example of multiple alleles inheritance. The plasma membrane of RBC contains surface antigens.

3 types of RBC surface antigens: A, B & O.

The synthesis of these antigens is controlled by 3 alleles: IA, IB & i.Allele i is recessive to both IA & IB alleles.

BIO095 (2013/2014)CODOMINANCE67Codominance : ABO Blood Group68Blood type:OABABGenotype:iiIAIA or IAiIBIB or IBi

IAIBSurface antigen: OABA & BSerum antibodies Against A & BAgainst BAgainst ANone

The ABO Blood Group68The A blood individual,Genotype: Homozygous (IAIA) or Heterozygous (IAi)RBC contains the surface antigen A.

The B blood individual,Genotype: Homozygous (IBIB) or Heterozygous (IBi)RBC contain the surface antigen B.

BIO095 (2013/2014)CODOMINANCESurface antigen A & B have significantly different molecular structure.

69The blood type AB individual,Genotype: Heterozygous (IAIB)Express both surface antigen A & B.

The IA & IB alleles are codominant to each other.

In this case, neither allele is dominant to the other. Therefore, because each allele is fully expressed, they are said to be codominant.

BIO095 (2013/2014)CODOMINANCESurface antigen A & B have significantly different molecular structure.

70Incomplete dominanceThe pattern of inheritance where,the heterozygote (that carries 2 different alleles) has a phenotype that is intermediate between either corresponding homozygous parents.

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.

BIO095 (2013/2014)Incomplete Dominance Intermediate Phenotype71Examples:The four-oclock plant (Mirabilis jalapa) is observed for flower color alleles.

BIO095 (2013/2014)Incomplete Dominance Intermediate Phenotype72A homozygous red-flowered (CRCR) four-oclock plant was crossed to a homozygous white-flowered (CWCW) plant.

Alleles involved:CR = Wild-type allele for red flower.CW = Wild-type allele for white flower. F1 offspring All had pink flower (100%).

F1 generation were allowed to self-fertilized.BIO095 (2013/2014)Incomplete Dominance Intermediate Phenotype

73SBIO095 (2013/2014)Incomplete Dominance Intermediate Phenotype74F2 generation produced, with phenotypic ratio of: red-flowered plants ; pink-flowered plants ; white-flowered plants.

The pink-flowered plants:are heterozygotes (CRCW)has intermediate phenotype.

F2 generation displays a 1:2:1 ratio (for both genotypic & phenotypic ratio)

self-fertilized74SBIO095 (2013/2014)Incomplete Dominance Intermediate Phenotype75F2 generation:Genotypic ratio:1 CRCR : 2 CRCW : 1 CwCW

Phenotypic ratio1 red-flowered : 2 pink-flowered : 1 white-flowered

self-fertilizedTwo copies of the allele R are needed to produce red flowers. One copy results in pink flowers. Therefore, the R allele is incompletely dominant over the r allele. Because the r allele does not produce any color, the absence of R alleles produces white flower.

75A gene can exist in multiple alleles that are different from each other

Because alleles represent different forms of a gene,.: a gene can have more than 2 alleles.

Any individual can carry only 2 alleles of a gene, but in the population, many different alleles of a gene can be present.BIO095 (2013/2014)MULTIPLE ALLELES76Example 1: The gene that determines ABO blood group in human.

ABO blood types are determined by antigen molecules on the surface of RBCs.

There is one gene (I) for ABO blood types, with 3 alleles IA, IB and iO

BIO095 (2013/2014)MULTIPLE ALLELES

77The A & B alleles,control the formation of slightly different forms of the molecule (antigens) on the surface of the RBCs, & determine whether the person has an A, B or AB blood group.

The third allele O,does not make any antigen, &individuals homozygous for this allele (io), carry neither the A nor the B antigen on their cells.

Because of the 3 alleles, there are 6 possible genotypes.BIO095 (2013/2014)MULTIPLE ALLELES78BIO095 (2013/2014)MULTIPLE ALLELESBlood Type(Phenotype)GenotypeAIAIA, IAiOBIBIB, IBiOABIAIBOioio79Example 2:Coat color in rabbits is controlled by multiple alleles. There are 4 different alleles:

BIO095 (2013/2014)MULTIPLE ALLELESAlleles Contribution to phenotype at molecular levelPhenotype (Coat color traits)Wild type allele, CProvides full coat colorFull coat colorChinchilla allele, cchProduces a partial defect in pigmentation.Chinchilla pattern coat colorHimalayan allele, chResult in pigmentation in certain body parts. Himalayan pattern coat colorAlbino allele, cDefective allele.Cannot produce a protein required for pigment synthesis. Albino 80BIO095 (2013/2014)MULTIPLE ALLELESThe relationship between alleles:

A rabbit can inherit only 2 copies of these alleles.

The phenotype depends on the dominant / recessive relationships among these combinations.

C- is dominant to cch, ch & c.cch- is recessive to C- is dominant to ch, & cch- is recessive to C & cch - is dominant to cc- is recessive to C, cch, ch 8182PhenotypeGenotypeFull coat color

CCCcchCchCc

b) Chincilla

- cchcch- cchch- cchcc

c) Himalayan

- chch- chc

d) Albino

- cc

82EPISTASISA phenomenon in which a gene at one locus can mask (suppresses) the phenotypic expression of a different gene at a second locus.These genes occupy different loci on separate chromosome & are independently inherited. An example of gene interaction.

BIO095 (2013/2014)EPISTASISEpistasis takes place when the action of one gene is modified by one or several genes, sometimes called modifier genes. For example, the gene causing albinism would hide the gene controlling color of a person's hair. In another example, a gene coding for a widow's peak would be hidden by a gene causing baldness.

83Example: Inheritance of coat color in miceIn mice, black coat color is dominant to brown

The 1st gene: code for black or brown pigment control coat color.The 2 alleles for this character: B & b

The genotype for mouse with brown fur bbBIO095 (2013/2014)EPISTASIS84The 2nd gene: Determines whether or not pigment will be deposited in the hair. The 2 alleles for this character: C& c

The dominant allele, C (for color),results in the deposition of black or brown pigment,(depending on the genotype at the 1st locus)

But if the mouse is homozygous recessive, cc (for the 2nd locus), the coat is white (albino), (no pigment deposition)(regardless of the genotype at the black/brown locus) BIO095 (2013/2014)EPISTASIS85.: the gene for pigment deposition (C/c) is epistatic to the gene that codes for black or brown pigment (B/b)

Crossing 2 black mice that are heterozygous for both genes (BbCc).The phenotypic ratio of the offspring produced are:9 black : 3 brown : 4 whiteBIO095 (2013/2014)EPISTASISOther types of epistatic interaction produce different ratios,But all are modified versions of 9:3:3:186

87Many character, such as human skin color & height,vary in the population along a continuum (in gradations).These are called quantitative characters.

Quantitative charactersA heritable feature in a population that varies continuously as a result of environmental influences & the additive effect of 2 or more genes (polygenic inheritance).

Quantitative variation usually indicates polygenic inheritance.

BIO095 (2013/2014)POLYGENIC INHERITANCE88POLYGENIC INHERITANCE an additive effect of 2 or more genes on a single phenotypic character.

These genes may be located on many different pairs of chromosomes.

Each dominant allele has a quantitative and additive effect on the phenotype.

This result in a continuous variation in phenotypes, resulting in a distribution of these phenotypes that resembles a bell-shaped curve called normal distribution.

BIO095 (2013/2014)POLYGENIC INHERITANCE89Example:Skin pigmentation in humans,is controlled by at least 3 separately inherited genes (probably more).Lets consider 3 genes, A, B, CEach gene has a dark-skin allele which contribute 1 unit of darkness to the phenotype. The dark-skin allele (A, B & C) is incompletely dominant to a / b/ c alleles. BIO095 (2013/2014)POLYGENIC INHERITANCE90Person with genotype:AABBCC very darkaabbcc very lightAaBbCc have intermediate skin shade

The alleles have a cumulative effects..: the genotype AaBbCc & AABbcc would make the same genetic contribution (3 units) to skin darkness.

Environmental factors, e.g. exposure to sun, also affect the skin color phenotype & help make the graph a smooth curve (normal distribution) rather than a stairs like histogram.

BIO095 (2013/2014)POLYGENIC INHERITANCE91

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93In the case of height, differences in nutrition ensure a bell-shaped curveBIO095 (2013/2014)

94Eye color & skin color, are the cumulative result of many genes involved in the production and distribution of melanin.

Many human disorders such asCleft lip/palate, Diabetes, Allergies, &cancers are controlled by polygenes and are subjected to environmental influences.BIO095 (2013/2014)POLYGENIC INHERITANCE95The end of Part 3!!