Chapter 11
Observable Patterns of
Inheritance
Scientist
• Father of inheritance: Gregor Mendel
Mendel’s Insight Into Inheritance
Patterns
• Natural selection suggested that a
population could evolve if members
showed variation in heritable traits
• Variations that improved survival chances
would be more common in each
generation –in time, population would
change over time or evolve
Mendel’s Experimental Approach
• Gregor Mendel used experiments in plant
breeding and a knowledge of mathematics
to form his hypothesis
– He used green pea plants in his experiment
• This plant can self-fertlize itself
• True-breeding (white flowerwhite flower)
• Cross-fertilized by human manipulation of the
pollen
– Mendel crossed-fertilized true-breeding
garden pea plants having clearly constrasting
traits
Genetic terms (found on p. 179)
• Genes: units of information about specific
traits
• Locus: Each gene has a location of
chromosomes
• Homologous chromosome: diploid cells
have two genes (a gene pair) for each trait
• Alleles are various molecular forms of
gene for the same trait
Continue…
• True-breeding lineage occurs when
offspring inherited identical alleles,
generation after generation
• Hybrid Non-identical alleles
• Homozygous alleles: both alleles are the
same
– RR: Homozygous Dominant
– rr: Homozygous Recessive
Continue…
• Heterozygous: One allele is dominant and
other is recessive
– Rr: Heterozygous Dominant
• Dominant: more organisms in the
population with particular trait (represent
with a capital letter)
• Recessive: very few organisms in the
population with the trait (represent with a
lowercase letter)
Continue…
• Genotype: the genetic makeup of an
organisms
– How many are homozygous dominant,
heterozygous dominant, and homozygous
recessive
• Phenotype: Physical appearance
– Adjectives (describe the appearance)
– How many are dominant and recessive?
Continue…
• P- Parent Generation
• F1 First generation
• F2 Second generation
Law of Segregation
• States that 2n (diploid) organisms inherit
two genes per trait located on pairs of
homologous chromosomes
• During meiosis the genes segregate from
each other such that each gamete will
receive only one gene per trait.
Monohybrid Crosses
• Monohybrid crosses have two parents that
are true-breeding for contrasting forms of
a trait
• Self-fertilization: one form of the trait
disappears in the first generation offspring
(F1), only to show up in the second
generation
• We know now that all members in the
second generation are heterozygous
F2 Generation
• A genetic cross of a F2 generation would
show up a 3:1 phenotypic ratio
Steps to a Monohybrid Cross
• Look up on the board to see the steps to a
monohybrid cross
Testcross
• To support his concept of segregation,
Mendel crossed F1 plants with
homozygous recessive individuals
• A 1:1 ratio of recessive and dominant
phenotypes supported his hypothesis
Law of Independent Assortment
• States that during meiosis each gene of a
pair tends to assort into gametes
independently of other gene pairs located
on nonhomologous chromosomes
• Mendel showed F1 were all dominant for
purple flowers and tall
Dihybrid Crosses
• Steps on the board
Incomplete Dominance
• A dominant allele cannot completely mask
the expression of another
• Snapdragons
Codominance
• ABO blood types
• Both alleles are expressed in
heterozygotes
– Humans with both proteins are designated
with blood type AB
Multiple Allele System
• Whenever more than two forms of alleles
exist at a given locus
– Example: Four blood types: A, B, AB and O
Multiple Effects of Single Genes
• Pleiotropy: The expression of alleles at one
location can have effects on two or more
traits
• Example: Marfan Syndrome
• (Abe Lincoln)
– Gene for codes for a variant form of fibrillin1,
a protein in the extracellular matrix of
connective tissues
• Causes weakening connective tissues throughout
the body
• Effects: lanky skeleton, leaky heart valves, and
weakened blood vessels, lens displacement
Interactions Between Gene
Pairs
• Epistasis: One gene pair can influence
other gene pairs, with their combined
activities producing some effect on
phenotype
• Example: Coat Color in Mice
– Chicken Combs (Red portion on their heads)
Hair Color in Mammals
• In Labrador retrievers, one gene pair
codes for the quantity of melanin produced
while another codes for melanin deposition
• Another gene locus determines whether
melanin will be produced at all
Comb Shape in Poultry
• Sometimes interactions between two pairs
results in a phenotype that neither pair
can produce alone
• Comb shape in chickens is of at least four
types depending on the interactions of two
gene pairs
Rose
Pea
Single
Walnut
Regarding the Unexpected
Phenotypes
• Tracking even a single gene through
several generation may produce results that
are different
• Camptodactyly (Immobile, bent fingers) can
express itself on both or one hand because
a possible
gene product is missing
Polygenic Traits
• Traits expressed by more than one gene
• Eye Color (Three genes play a role of eye
color)
Environmental Effects
• Fur on extremities of certain
animals will be darker because
the enzyme for melanin production
will operate at cooler temperature but is
sensitive to
heat on the rest of the body.
• Hydrangea Plants: the color of floral
cluster based on the acidity of the soil