! Augustinian monk (1822-1884) ! First to use scientific method to study inheritance
Garden Pea
Monohybrid Cross Removed stamens from purple flower Transferred spermbearing pollen from stamens of white flower to eggbearing carpel of purple flower
Parental generation (P) Carpel Stamens Pollinated carpel matured into pod
P Generation (true-breeding parents)
Examined offspring: all purple flowers
Purple flowers
White flowers
F1 Generation (hybrids)
Planted seeds from pod
First generation offspring (F1)
Mendel used traits with only 2 variants (alleles)
All plants had purple flowers
Monohybrid Cross
Punnett Square
Appearance: Genetic makeup:
Purple flowers PP
White flowers pp
P
p
Gametes
P Generation (true-breeding parents)
P Generation
Purple flowers
White flowers
F1 Generation Appearance: Genetic makeup:
Purple flowers Pp 1
Gametes:
2
1
P
p
2
F1 Generation (hybrids)
All plants had purple flowers
F1 sperm P
p
PP
Pp
Pp
pp
F2 Generation P F1 eggs
F2 Generation
p
3
A 3:1 ratio (Phenotype) is really a 1:2:1 ratio (Genotype)
3
Phenotype
Genotype
Purple
PP (homozygous
Purple
Pp (heterozygous
Purple
Pp (heterozygous
White
pp (homozygous
Ratio 3:1
Ratio 1:2:1
Pea Characteristics
1
2
1
1
Mendel produced truebreeding pea plants with 7 different traits
:1
Pea Characters
Mendel’s First Hypothesis
! Genes for genetic characters occur in pairs • One gene inherited from each parent • Alleles are different versions of a gene
! Diploid: two copies of each gene
Mendel’s Second Hypothesis
Mendel’s Third Hypothesis
! If two alleles of a gene are different, one allele is dominant over the other
! Two alleles of a gene segregate (separate) and enter gametes singly
• Dominant allele is expressed • Recessive allele is masked
! Recessive alleles only expressed when two copies of the allele present
• Half the gametes carry one allele, half carry the other allele (haploid) • Principle of Segregation
! Two gametes fuse to produce a zygote that contains two alleles (diploid)
Rules of Probability
Probability in Mendel’s Crosses (1) ! Purple-flowered ! white-flowered (PP ! pp) • Probability of PP zygote = " ! " = # • Probability of pp zygote = " ! " = #
Probability in Mendel’s Crosses (2) ! Purple-flowered ! white-flowered (PP ! pp) • Probability of Pp zygote = " ! " = # • Probability of pP zygote = " ! " = # • Total probability of heterozygote = # + # = "
Testcross ! Determines if an individual with a dominant phenotype is a heterozygote or homozygote
Dominant phenotype, unknown genotype: PP or Pp?
Recessive phenotype, known genotype: pp
If Pp, then 1 2 offspring purple and 1 2 offspring white:
If PP, then all offspring purple: p
p
P
p
p
Pp
Pp
pp
pp
P Pp
Pp
P
P Pp
Pp
Dihybrid Cross
Mendel’s Fourth Hypothesis
P Generation
YYRR Gametes YR
F1 Generation
Hypothesis of dependent assortment
yyrr yr
! Alleles of genes that govern two different characters segregate independently during formation of gametes
YyRr Hypothesis of independent assortment Sperm
1
F2 Generation (predicted offspring)
2
1
yr 3
YYRR YyRr
1
YyRr
1
4
yyrr 1
4
YR
1
4
Yr
1
4
yR
1
4
• Principle of Independent Assortment
yr
Eggs 1
Eggs 1 YR 2 1
2
Sperm YR 1 2 yr
4
YR
4
Yr
4
yR
4
yr
YYRR YYRr YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
! Due to independent assortment during meiosis
4 1
Phenotypic ratio 3:1
9
16
3
16
yyRr 3
16
yyrr 1
16
Phenotypic ratio 9:3:3:1
Chromosome Theory of Inheritance
Chromosome Theory of Inheritance
! Walter Sutton (1903) noted similarities between inheritance of genes and behavior of chromosomes in meiosis and fertilization
Diploid nucleus before replication
• Chromosomes occur in pairs in diploid organisms • Chromosomes of each pair are separated and delivered singly to gametes • Independent assortment of chromosomes • One chromosome of each pair is derived from the male parent; one from the female parent
Metaphase I of meiosis First meiotic division
Second meiotic division Gametes
1/4 R Y
1/4 r y
1/4 R y
1/4 r Y
Homologous Chromosomes
Human Traits
! Locus
! Follow Mendelian principles
• Site occupied by a gene on a chromosome • Alleles on different homologous chromosomes have same loci
• Albinism, webbed fingers, short-limbed dwarfism
Later Modifications and Additions to Mendel’s Hypotheses
Later Modifications and Additions to Mendel’s Hypotheses (cont.)
! Incomplete Dominance: dominant alleles do not completely mask recessive alleles
! Polygenic Inheritance: a character is controlled by the effects of several genes
! Codominance: the effects of different alleles are equally detectable in heterozygotes
! Pleiotropy: two or more characters are affected by a single gene
! Epistasis: the activity of one gene influences the activity of another gene
Incomplete Dominance
Incomplete Dominance: Snapdragons (1) ! P Generation
! Some or all alleles of gene are neither completely dominant nor recessive ! Heterozygote phenotype • Different from either homozygote phenotype
• Red flowered ! White flowered
! F1 generation • All pink flowered
Snapdragons (2)
Incomplete Dominance in Human Traits
! From F1 generation
! Sickle-cell disease
• Both pink flowered
! F2 generation • # red flowered • # white flowered • " pink flowered
• Homozygote recessive has sickle-cell disease • Heterozygote has milder sickle-cell trait
! Familial hypercholesterolemia • Homozygote has severe form of disease • Heterozygote has mild form of disease
! Tay-Sachs disease • Homozygote has serious symptoms • Heterozygote has no symptoms but has detectable biochemical effects
ABO Blood System: Codominance / Multiple Alleles ! The system demonstrates both • Multiple alleles • 3 alleles of the I gene (IA, IB, and i) • Codominance • IA and IB are dominant to i but codominant to each other
Epistasis
Environmental Influences ! Coat color in Himalayan rabbits and Siamese cats • Allele produces an enzyme that allows pigment production only at temperatures below 33oC
Epistasis
! Genes interact • Allele of one locus inhibits or masks effects of allele at a different locus • Some expected phenotypes do not appear among offspring
BbCc
BbCc
Sperm 1
1
1
1
1
BC
4
1
4
bC
1
4
1
Bc
4
bc
4
BC
BBCC
BbCC
BBCc
BbCc
4
bC
BbCC
bbCC
BbCc
bbCc
4
Bc
BBCc
BbCc
BBcc
Bbcc
4
bc
BbCc
bbCc
Bbcc
bbcc
9
16
3
16
4
16
A Polygenic Trait: Eye Color
Polygenic and Multifactorial Traits: Skin Color
Polygenic Inheritance follows a bell-shaped curve
Polygenic Inheritance: Continuous Variation in Human Height
AaBbCc
aabbcc 20/64
Fraction of progeny
15/64
6/64
1/64
AaBbCc
Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC
Pleiotropy ! One gene affects more than one character ! Sickle-cell disease • Recessive allele affects hemoglobin structure and function • Leads to blood vessel damage • Damages many tissues, organs, and functions • Many different symptoms result
