Patterns of InheritancePatterns of Inheritance

By observing how traits are passed to By observing how traits are passed to the next generation, how can the the next generation, how can the inheritance patterns be used to inheritance patterns be used to understand the principles of heredity? understand the principles of heredity?

Use of Garden Pea for Use of Garden Pea for Genetics ExperimentsGenetics Experiments

Intact pea flowerIntact pea flower Flower dissected to showFlower dissected to showreproductive structuresreproductive structures

StamensStamens(male)(male)

produceproducepollenpollen

StamensStamens(male)(male)

produceproducepollenpollen

CarpelCarpel(female)(female)

producesproduceseggseggs

CarpelCarpel(female)(female)

producesproduceseggseggs

Mendel’s Experiment With Mendel’s Experiment With Peas Differing in a Single TraitPeas Differing in a Single Trait

F1 smooth plants x F1 smooth plantsF1 smooth plants x F1 smooth plants

Parental: Smooth seed x Wrinkled seedParental: Smooth seed x Wrinkled seed

F1: All smooth seed coatsF1: All smooth seed coats

F2: 5474 smooth: 1850 wrinkledF2: 5474 smooth: 1850 wrinkled (3/4 smooth to 1/4 wrinkled) (3/4 smooth to 1/4 wrinkled)

Patterns of InheritancePatterns of Inheritance

Mendel needed to explainMendel needed to explain

1.1. Why one trait seemed to disappear Why one trait seemed to disappear in the first generation.in the first generation.

2. Why the same trait reappeared in 2. Why the same trait reappeared in the second generation in one-fourth the second generation in one-fourth of the offspring. of the offspring.

Mendel’s Proposal Mendel’s Proposal

1.1. Each trait is governed by two Each trait is governed by two factors – now called genes.factors – now called genes.

2. Genes are found in alternative 2. Genes are found in alternative forms called alleles. forms called alleles.

3. Some alleles are dominant and 3. Some alleles are dominant and mask alleles that are recessive. mask alleles that are recessive.

Mendel’s Experiment With Peas Mendel’s Experiment With Peas Differing in a Single TraitDiffering in a Single Trait

Parental: Smooth seed x Wrinkled seedParental: Smooth seed x Wrinkled seed

F1: All smooth seed coatsF1: All smooth seed coats

SSSS ssss

HomozygousHomozygousRecessiveRecessive

HomozygousHomozygousDominantDominant

SsSs HeterozygousHeterozygous

F1 smooth plants x F1 smooth plantsF1 smooth plants x F1 smooth plants

SsSs

HeterozygousHeterozygous

SsSs

HeterozygousHeterozygousF2F2

S S S S

Homozygous parents can only pass Homozygous parents can only pass one form of an allele to their offspring. one form of an allele to their offspring.

S s S s

Heterozygous parents can pass either of Heterozygous parents can pass either of two forms of an allele to their offspring. two forms of an allele to their offspring.

Locus: Area on the chromosome where a gene is located.For a heterozygote, homologous chromosomes will have different alleles at the same locus.

Additional Genetic TermsAdditional Genetic Terms

TermTerm DefinitionDefinition ExampleExample

GenotypeGenotype Alleles carried by an Alleles carried by an individualindividual

SS, Ss, ssSS, Ss, ss

PhenotypePhenotype Physical characteristic Physical characteristic or appearance of an or appearance of an individualindividual

smooth or smooth or wrinkledwrinkled

Mendel’s Principle of Mendel’s Principle of

Genetic SegregationGenetic Segregation

In the formation of gametes, theIn the formation of gametes, the members of a pair of alleles separatemembers of a pair of alleles separate

(or segregate) cleanly from each other (or segregate) cleanly from each other so that only one member is included in so that only one member is included in each gamete.each gamete.

Each gamete has an equal probability of Each gamete has an equal probability of containing either member of the allele containing either member of the allele pair.pair.

Genetic SegregationGenetic Segregation Parentals: Parentals: SS x ssSS x ss

F1 x F1: F1 x F1: Ss x SsSs x Ss

Traits Studied by MendelTraits Studied by Mendel

Plant size

Flower location

Flower color

Pod color

Pod shape

Seed shape

Seed color

Mendel’s Experiment With Peas Differing in Two TraitsMendel’s Experiment With Peas Differing in Two Traits

F1: All smooth yellow seed coatsF1: All smooth yellow seed coats

Parental: Smooth Yellow x Wrinkled GreenParental: Smooth Yellow x Wrinkled Green

F1 plants x F1 plantsF1 plants x F1 plants

1/161/16 32 wrinkled, green32 wrinkled, green

3/163/16101 wrinkled, yellow101 wrinkled, yellow

3/163/16108 smooth, green108 smooth, green

9/169/16315 smooth, yellow315 smooth, yellow

F2F2

Patterns of InheritancePatterns of Inheritance

Mendel needed to explainMendel needed to explain

1.1. Why non-parental combinations Why non-parental combinations appeared in the F2 offspring.appeared in the F2 offspring.

2. Why the ratio of phenotypes in the 2. Why the ratio of phenotypes in the F2 generation was 9:3:3:1.F2 generation was 9:3:3:1.

Mendel’s Principle of Mendel’s Principle of Independent Assortment Independent Assortment

When gametes are formed, the When gametes are formed, the alleles of one gene segregate alleles of one gene segregate independently of the alleles of independently of the alleles of another geneanother gene producing equal producing equal proportions of all possible gamete proportions of all possible gamete types. types.

Genetic Segregation + Independent AssortmentGenetic Segregation + Independent Assortment

Parentals: SSYY x s s y yParentals: SSYY x s s y y

F1:F1:

SY SY SY SY SY SY SY SY sy sy sy sysy sy sy sy

Genetic Segregation + Independent AssortmentGenetic Segregation + Independent Assortment

F1 x F1 : S s Y y x S s Y yF1 x F1 : S s Y y x S s Y y

SY Sy sY sy SY Sy sY sy

Four different types of gametes Four different types of gametes are formed in equal proportions. are formed in equal proportions.

SY Sy sY sySY Sy sY sy

F1 x F1 F1 x F1 SsYy X SsYySsYy X SsYy

SYSY

SSyy

ssYY

sysy

14

14

14

14

SYSY SSyy ssYY sysy14

14

14

14

Eggs

Pollen

F2 Genotypes and PhenotypesF2 Genotypes and Phenotypes

PhenotypesPhenotypes GenotypesGenotypes

SmoothSmooth

YellowYellow

Smooth Smooth

GreenGreen

Wrinkled Wrinkled

YellowYellow

Wrinkled Wrinkled

GreenGreen

Meiotic Segregation Explains Independent AssortmentMeiotic Segregation Explains Independent Assortment

Two possibleorientations

Additional Genetic Additional Genetic Patterns Patterns

Mendel’s peasMendel’s peas Alternative PatternAlternative Pattern

Complete DominanceComplete Dominance Incomplete DominanceIncomplete Dominance

Incomplete dominance: neither allele Incomplete dominance: neither allele masks the other and both are observed as masks the other and both are observed as a blending in the heterozygotea blending in the heterozygote

Incomplete Dominance Incomplete Dominance

Four o’clock flowersFour o’clock flowersR = red, R’ = whiteR = red, R’ = white

Red x White Red x White RR R’R’ RR R’R’

PinkPink RR’ RR’

Incomplete DominanceIncomplete Dominance

F1 x F1F1 x F1Pink x PinkPink x PinkRR’ x RR’RR’ x RR’

Genotypic Ratio:Genotypic Ratio:Phenotypic Ratio:Phenotypic Ratio:

Additional Genetic Patterns Additional Genetic Patterns Mendel’s peasMendel’s peas Alternative PatternsAlternative Patterns

Complete DominanceComplete Dominance CodominanceCodominance

Two alleles per geneTwo alleles per gene Multiple AllelesMultiple Alleles

Codominance: Neither allele masks the other Codominance: Neither allele masks the other

so that effects of both alleles are observed in so that effects of both alleles are observed in heterozygotes without blendingheterozygotes without blending

Multiple Alleles: Three or more alleles exist for Multiple Alleles: Three or more alleles exist for one trait one trait

Note: A diploid individual can only carry any twoNote: A diploid individual can only carry any two of these alleles at once. of these alleles at once.

Multiple Alleles and CodominanceMultiple Alleles and Codominance

ABO Blood Type in HumansABO Blood Type in Humans

Blood TypeBlood Type AlleleAllele

Type AType A AA

Type BType B BB

Type OType O oo

AA == BB > o> o

A and B are codominant.A and B are codominant.AA and B are completely dominant over o.and B are completely dominant over o.

•10%10%

•40%40%

•46%46%

•4%4%

•B or ABB or AB

•A or ABA or AB

•O,AB,O,AB,A,BA,B

(universal)(universal)

•ABAB(universal)(universal)

•B or OB or O

•A or OA or O

•OO

•AB, A,AB, A,B, OB, O

(universal)(universal)

•AA

•BB

•BothBoth

•NeitherNeither

•BB or BoBB or Bo

•AA or AoAA or Ao

•oooo

•ABAB

•OO

•ABAB

•BB

•AA

•FreqFreq•DonatesDonates•Re-Re-

ceivesceives•Anti-Anti-bodiesbodies

•Antigen on RBCsAntigen on RBCs•GenotypeGenotype•TypeType

Human ABO Blood TypesHuman ABO Blood Types

Type A

Type B

A and B

Neither

Codominance is observed for Type AB Blood since the Codominance is observed for Type AB Blood since the

products of both the A and B allelesproducts of both the A and B alleles are found on the cells.are found on the cells.

Inheritance of Rh Factor Inheritance of Rh Factor PhenotypePhenotype Genotype*Genotype* Gene Gene

ProductProduct

Antibodies Antibodies PresentPresent

Rh PositiveRh Positive RR or RrRR or Rr Rhesus ProteinRhesus Protein None None

Rh NegativeRh Negative rrrr NoneNone None None unless unless exposedexposed

*Although there are multiple R alleles, R*Although there are multiple R alleles, R11, R, R22, R, R33, etc. all are, etc. all are

completely dominant over all of the r alleles, rcompletely dominant over all of the r alleles, r11, r, r22, r, r33, etc. , etc.

ABO Blood Type and Rh Factor are controlled by ABO Blood Type and Rh Factor are controlled by separate genes. They show independent assortment. separate genes. They show independent assortment.

Multiple Alleles and CodominanceMultiple Alleles and Codominance Type A, Rh positive x Type B, Rh negativeType A, Rh positive x Type B, Rh negative

Phenotypic Ratio of OffspringPhenotypic Ratio of Offspring

Additional Genetic PatternsAdditional Genetic Patterns

Mendel’s peasMendel’s peas Alternative PatternsAlternative Patterns

One gene affects One gene affects one traitone trait

Polygenic InheritancePolygenic Inheritance

Polygenic InheritancePolygenic Inheritance: : Many genes affect Many genes affect one trait one trait

Example of Polygenic InheritanceExample of Polygenic Inheritance Two genes affecting one trait Two genes affecting one trait

Number of Number of Dominant Dominant

AllelesAlleles

Skin Color*Skin Color*

(Phenotype)(Phenotype)

GenotypesGenotypes % Pigmentation*% Pigmentation*

00 WhiteWhite aabbaabb 0-11%0-11%

11 Light BlackLight Black Aabb or aaBbAabb or aaBb 12-25%12-25%

22 Medium BlackMedium Black AAbb or AaBb or AAbb or AaBb or aaBBaaBB

26-40%26-40%

33 Dark BlackDark Black AABb or AaBBAABb or AaBB 41-55%41-55%

44 Darkest BlackDarkest Black AABBAABB 56-78%56-78%

*Based on a study conducted in Jamaica. *Based on a study conducted in Jamaica.

Example of Polygenic InheritanceExample of Polygenic Inheritance

Medium Black Woman X Darkest Black ManMedium Black Woman X Darkest Black Man (her mother is white)(her mother is white)

Additional Genetic PatternsAdditional Genetic Patterns

Mendel’s peasMendel’s peas Alternative PatternsAlternative Patterns

One gene affects One gene affects one traitone trait

PleiotropyPleiotropy

Pleiotropy: One gene affects many traitsPleiotropy: One gene affects many traits

Sickle-Cell AnemiaSickle-Cell Anemia One gene affects many One gene affects many

phenotypic characteristicsphenotypic characteristics

Gene Gene ProductProduct

Cell ShapeCell Shape Disease Disease ConditionsConditions

SSSS Hemoglobin AHemoglobin A Spherical, slightlySpherical, slightly

concaveconcave

No anemiaNo anemia

SS’SS’ Hemoglobin A Hemoglobin A

Hemoglobin SHemoglobin S

Some sickling under Some sickling under extreme conditionsextreme conditions

Sickle Cell TraitSickle Cell Trait

Resistance to Resistance to MalariaMalaria

S’S’S’S’ Hemoglobin SHemoglobin S Sickled under low OSickled under low O22

tensiontensionSickle Cell Sickle Cell AnemiaAnemia