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Mendel, Genes, and Inheritance
Chapter 12
Why It Matters
 Red blood cells in sickle-cell disease
12.1 The Beginnings of Genetics:
Mendel’s Garden Peas
 Mendel chose true-breeding garden peas for his
experiments
 Mendel first worked with single-character crosses
 Mendel’s single-character crosses led him to
propose the principle of segregation
 Mendel could predict both classes and
proportions of offspring from his hypotheses
12.1 (cont.)
 Mendel used a testcross to check the validity of
his hypotheses
 Mendel tested the independence of different
genes in crosses
 Mendel’s research founded the field of genetics
 Sutton’s chromosome theory of inheritance
related Mendel’s genes to chromosomes
Blending Theory of Inheritance
 Popular belief until about 1900
• Hereditary traits blend evenly in offspring through
mixing of parents’ blood
 Does not explain some observations:
• Extremes do not gradually disappear
• Offspring sometimes have different traits than
either parent
Gregor Mendel
 Founder of genetics
 Augustinian monk
(1822-1884)
 First to use scientific
method to study
inheritance
Pea Experiments
 Garden pea (Pisum sativum)
• Easy to grow
• Clearly defined characters or traits
• True-breeding varieties
• Self-fertilized plants (same trait each generation)
• Easy to cross
• Cross-pollination between parents
Garden Pea
Pea Characters
Single-Character Crosses
 P generation (Parents)
• Each pea produced contains an embryo
 F1 generation (Filial)
• First generation
 F2 generation
• Second generation
Flower Color Cross
 P generation
• Purple flowers crossed with white flowers
 F1 generation
• All F1 seeds formed purple flowers
• Purple flower offspring crossed
 F2 generation
• Purple flowers (75%)
• White flowers reappeared (25%)
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
 If two alleles of a gene are different, one allele is
dominant over the other
• Dominant allele is expressed
• Recessive allele is masked
 Recessive alleles only expressed when two
copies of the allele present
Mendel’s Third Hypothesis
 Two alleles of a gene segregate (separate) and
enter gametes singly
• 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)
Monohybrid Cross
Terminology
 Homozygous
• Both alleles the same
• PP (dominant)
• pp (recessive)
 Heterozygous
• Two different alleles
• Pp
Terminology
 Genotype
• Genetic constitution of an organism
• PP, Pp, pp
 Phenotype
• Outward appearance
• Purple flowers, white flowers
Product Rule in Probability
 Probability of two independent events occurring
in succession
• Individual probabilities multiplied
 Coin flip probabilities
•
•
•
•
Heads = ½
Tails = ½
Two heads = ½ × ½ = ¼
Two tails = ½ × ½ = ¼
Rules of Probability
Sum Rule in Probability
 Probability of two different events producing the
same outcome
• Individual probabilities added
 Probability of a heads and a tails in two tosses:
• First possibility: heads then tails
• Heads = ½, Tails = ½ (½ × ½ = ¼)
• Second possibility: tails then heads
• Tails = ½, Heads = ½ (½ × ½ = ¼)
• Total probability: ¼ + ¼ = ½
Probability in Mendel’s Crosses (1)
 Purple-flowered × white-flowered (PP × pp)
• Probability of PP zygote = ½ × ½ = ¼
• Probability of pp zygote = ½ × ½ = ¼
Probability in Mendel’s Crosses (2)
 Purple-flowered × white-flowered (PP × pp)
• Probability of Pp zygote = ½ × ½ = ¼
• Probability of pP zygote = ½ × ½ = ¼
• Total probability of heterozygote = ¼ + ¼ = ½
Probability in Mendel’s Crosses (3)
 Heterozygous cross (Pp × Pp)
• Genotype probabilities
• PP zygote = ½ × ½ = ¼
• pp zygote = ½ × ½ = ¼
• Pp zygote = ¼ + ¼ = ½
• Phenotype probabilities
• Purple flowers = PP + Pp = ¼ + ½ = ¾
• White flowers = pp = ¼
Testcross
 Determines if an individual with a dominant
phenotype is a heterozygote or homozygote
Mendel’s Fourth Hypothesis
 Alleles of genes that govern two different
characters segregate independently during
formation of gametes
• Principle of Independent Assortment
 Due to independent assortment during meiosis
Dihybrid Cross (1)
 Pea shape
Pea color
• R = round
• r = wrinkled
Y = yellow
y = green
 P generation:
RR YY × rr yy
• RR YY parent produces R y gametes
• rr yy parent produces r y gametes
 F1 generation
• All offspring Rr Yy genotype
• All offspring round smooth phenotype
Dihybrid Cross (2)
 Two heterozygotes crossed
 P generation:
Rr Yy × Rr Yy
• Rr Yy parents produce 4 kinds of gametes
• ¼ R Y, ¼ R y, ¼ r Y, ¼ r y
 F1 generation
• Offspring have four phenotypes
•
•
•
•
9/16 = round yellow
3/16 = wrinkled yellow
3/16 = round green
1/16 = wrinkled green
}
9:3:3:1 ratio
Dihybrid Cross
Cross: Rr Yy x Rr Yy
Gametes (pollen)
Gametes
(eggs)
Phenotypic ratio: 9 round yellow : 3 round green :
3 wrinkled yellow : 1 wrinkled green
Fig. 12-9, p. 242
Dihybrid Testcross
 P Generation
• Rr Yy × rr yy
 F1 Generation
•
•
•
•
¼ = round yellow
¼ = round green
¼ = wrinkled yellow
¼ = wrinkled green
}
1:1:1:1 ratio
Mendel’s Legacy
 Mendel’s results presented in 1866
• Only known locally
 Mendel died in 1884
 Work was rediscovered in early 1900s
 Mendel is considered the founder of genetics
Chromosome Theory of Inheritance
 Walter Sutton (1903) noted similarities between
inheritance of genes and behavior of
chromosomes in meiosis and fertilization
• 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
Chromosome Theory of Inheritance
Homologous Chromosomes
 Locus
• Site occupied
by a gene on a
chromosome
• Alleles on
different
homologous
chromosomes
have same
loci
Animation: Genetic terms
Human Traits
 Follow Mendelian principles
• Albinism, webbed fingers, short-limbed dwarfism
12.2 Later Modifications and Additions to
Mendel’s Hypotheses
 In incomplete dominance, dominant alleles do
not completely mask recessive alleles
 In codominance, the effects of different alleles
are equally detectable in heterozygotes
 In epistasis, genes interact, with the activity of
one gene influencing the activity of another gene
12.2 (cont.)
 In polygenic inheritance, a character is
controlled by the common effects of several
genes
 In pleiotropy, two or more characters are
affected by a single gene
Incomplete Dominance
 Some or all alleles of gene are neither
completely dominant nor recessive
 Heterozygote phenotype
• Different from either homozygote phenotype
Snapdragons (1)
 P Generation
• Red flowered
×
White flowered
• C RC R × C W C W
 F1 generation
• All offspring CRCW
• All pink flowered
Snapdragons (2)
 From F1 generation
• CRCW × CRCW
• Both pink flowered
 F2 generation
• ¼ red flowered
• ¼ white flowered
• ½ pink flowered
Incomplete Dominance in Human Traits
 Sickle-cell disease
• 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
Codominance
 Different alleles of gene have equal effects in
heterozygotes
• Both alleles expressed
 Human M, MN, and N blood types
• LMLM = M glycoprotein present; blood type M
• LNLN = N glycoprotein present; blood type N
• LMLN = both glycoproteins present; blood type MN
 Similar inheritance to incomplete dominance
Multiple Alleles
 More than three alleles for a gene
• Found among all individuals in a population
• Diploid individuals only have two of the alleles
 Phenotype depends on relationship between
different pairs of alleles
• Still follows Mendel’s principles
Multiple Alleles
 Small differences in DNA sequences result in
multiple alleles
Human ABO Blood Group
 Antigens
•
•
•
•
Glycoproteins on surface of red blood cells
IA allele produces A antigen (dominant)
IB allele produces B antigen (dominant)
i allele produces neither A nor B (recessive)
 Blood types (phenotypes)
•
•
•
•
IAIA or IAi = type A blood
IBIB or IBi = type B blood
ii = type O blood
IAIB = type AB blood
Human ABO Blood Group
 Immune system produces antibodies against
antigens not found on its own red blood cells
Human ABO Blood Group
 Inheritance of human blood types
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
Labrador Retrievers
 Melanin pigment gene
• B allele: black fur color (dominant)
• b allele: brown fur color (recessive)
 Pigment deposition gene
• E allele: pigment deposition normal (dominant)
• e allele: pigment deposition blocked (recessive)
 Phenotypes
• Black fur: BB EE, BB Ee, Bb EE, Bb Ee
• Brown fur: bb EE, bb Ee
• Yellow fur: BB ee, Bb ee, bb ee
Labrador Retrievers
Polygenic Inheritance
 Several genes at different loci interact to control
the same character
• Produces continuous variation
 Phenotypic distribution: Bell-shaped curve
 Often modified by environmental effects
Continuous Variation in Human Height
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
Pleiotropy and Sickle-Cell Disease