Fundamentals of Genetics
Gregor Mendel
• Mendel is considered
the “father of
genetics”
– Genetics is the field of
biology that studies
how characteristics
are transmitted from
parents to offspring
Mendel’s Background
• In 1842, Mendel entered a monastery in
Austria & was given the task of tending to
the garden
• In 1851, he entered University of Vienna to
study math & science
• He then returned to the monastery—
taught H.S. & kept a garden plot
Mendel’s Work
• Mendel’s fascination with his garden & his
knowledge of statistics led him to the study
of heredity
– Heredity is the transmission of characteristics
from parents to offspring
• Mendel is most famous for his experiments
with garden peas
Mendel’s Garden Peas
• Mendel observed 7
characteristics of pea
plants
– Each characteristic
occurred in 2
contrasting traits
Pea Plant Characteristics
1.
2.
3.
4.
5.
6.
7.
Plant height (long or short stems)
Flower position along stem (axial or terminal)
Pod color (green or yellow)
Pod appearance (inflated or constricted)
Seed texture (smooth or wrinkled)
Seed color (yellow or green)
Flower color (purple or white)
Pea Plant Characteristics
Mendel’s Methods
• Mendel controlled how pea plants were
pollinated
– Pollination occurs when pollen grains produced in
anthers (male repro part) are transferred to stigma
(female repro part)
– Self-pollination occurs when pollen is transferred from
anthers of a flower to stigma of either same flower or
flower on the same plant
– Cross-pollination involves flowers of two separate
plants
– Most pea plants reproduce through self-pollination
Mendel’s Experiments
• Mendel began by growing plants that were
pure for each trait
– Pure for a trait means it would always produce
offspring with that trait
– Strain refers to a plant that is pure for a specific
trait
• Mendel allowed plants to self-pollinate for
several generations until he had a strain for
14 traits—called parental generation, P1
Mendel’s Experiment
• Mendel then cross-pollinated the strain
plants
• Mendel called offspring of P1 generation
the first filial generation, F1 generation
• Then allowed flowers of F1 generation to
self-pollinate
• Mendel called offspring of F1 generation
the second filial generation, F2 generation
Mendel’s Results
• Mendel crossed a plant pure for green
pods & one pure for yellow pods
• Result produced a F1 generation of only
green-podded plants
Mendel’s Results
• Then allowed F1 generation to selfpollinate
• F2 generation was 3/4ths green pods &
1/4th yellow pod
Mendel’s Conclusions
• Mendel’s observations led him to
hypothesize that something within pea
plants controlled the characteristics he
observed—called them factors
• Mendel also hypothesized that each trait
was inherited by means of a separate
factor
• Since the characteristics had two forms,
he reasoned that there must be a pair of
factors controlling each trait
Recessive & Dominant Traits
• Whenever Mendel crossed strains, one of the P1
traits failed to appear in F1 plants
• In every case, trait reappeared in ratio of about
3:1 in F2 generation
• Led Mendel to conclude that one factor in a pair
may prevent the other from having an effect
– Dominant factor masks or dominates the other factor
for a specific characteristic
– Recessive factor is controlled by dominant factor
Law of Segregation
• States that a pair of factors is segregated,
or separated, during the formation of
gametes
Law of Independent Assortment
• Factors for different characteristics are
distributed to gametes independently
– Factors for different characteristics are not
connected
Chromosomes & Genes
• Molecular genetics is study of structure &
function of chromosomes & genes
– Chromosome is threadlike structure made up
of DNA
• Chromosomes occur in pairs
– Gene is segment of DNA on a chromosome
that controls a particular hereditary trait
• Genes occur in pairs
Alleles
• Allele refers to each of several alternative forms
of a gene
– Term replaces Mendel’s factors
– Letters are used to represent alleles
• Capital letters for dominant alleles
– Example: G for green pod color
• Lower case letters for recessive alleles
– Example: g for yellow pod color
– Two alleles for each trait
• Dominant—Both capital or one capital, one lower-case
– AA or Aa
• Recessive—Both lower case
– aa
Homozygous vs Heterozygous
• Homozygous for a characteristic occurs
when both alleles of a pair are alike
– Homozygous dominant—AA
– Homozygous recessive—aa
• Heterozygous for a characteristic occurs
when the two alleles in the pair are
different
– Heterozygous—Aa
Genotype vs Phenotype
• Genotype refers to the genetic makeup of
an organism
– Consists of alleles that organism inherits from
its parents
• Phenotype refers to appearance of an
organism as a result of its genotype
Probability
• Probability is the likelihood that a specific
event will occur
– May be expressed as decimal, %, or fraction
• Probability = # of times an event is expected to happen
# of opportunities for an event to happen
• Punnett squares aid in predicting the
probability that certain traits will be
inherited by offspring
Predicting Results of Monohybrid
Crosses
• Monohybrid cross is between individuals
that involves one pair of contrasting traits
• Punnett squares are used to predict the
outcome of different types of crosses
Genotypic vs Phenotypic
• Genotypic ratio refers to the ratio of
genotypes that appear in offspring
• Phenotypic ratio refers to the ratio of
phenotypes that appear in offspring
Testcross
• Recall that in humans, both RR and Rr result in
right hand dominance. How might you
determine whether the person is homozygous
(RR) or heterozygous (Rr)?
• Can perform a testcross
• Testcross is when an individual of unknown
genotype if crossed with a homozygous
recessive individual
– Used to determine the genotype of any individual
whose phenotype is dominant
Complete Dominance
• In Mendel’s pea plant experiments, one
allele was completely dominant over
another—referred to as complete
dominance
• In complete dominance, heterozygous
individuals are indistinguishable from
homozygous individuals
– For example, PP and Pp would produce
purple pea plant flowers
Incomplete Dominance
• Incomplete dominance occurs when two or more
alleles influence the phenotype, resulting in a
phenotype intermediate between the dominant
and recessive trait
• Example: In certain flowers, 4 o’clocks, both
alleles for red (R) flowers and allele for white
flowers (r) influences the phenotype—neither
allele is completely dominant over the other
allele. If a red and white 4 o’clock crosspollinates, the resulting offspring will be pink (Rr)
• What if two pinks cross?
Co-dominance
• Co-dominance occurs when both alleles
for a gene are expressed in a
heterozygous offspring
• Neither allele is dominant nor recessive,
nor do the alleles blend in phenotype
– Example: A red coat horse (R) is crossed with
a white coat horse (R’), result is a horse with
roan coat color (RR’)
Roan Horse
Dihybrid Cross
• Dihybrid cross is a cross between
individuals that involves two pairs of
contrasting traits
o Punnett squares now have 16 blocks