Post on 27-Dec-2015
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