Heredity – Chapter 4
Mendelian Genetics, Monohybrid and
Dihybrid Crosses and Beyond
Mendel’s Laws
Heredity
• Physical characteristics
– Plants and livestock
• Only certain characteristics passed on and this
remained a mystery
– Hybrid  offspring that differ from their parents in one
or more traits.
– Parents and children????
• An Austrian monk in the late nineteenth century
started to piece everything together
Heredity – Genetics of Inheritance
• Trait – distinguishing characteristic that will
make an individual unique
• Through selective breeding with animals and
plants it became clear that certain physical
traits could be transmitted (inherited) by each
generation from the preceding generation
• However, the specifics were completely
unknown
Heredity – Genetics of Inheritance
• Genetics – branch of biology dealing with the
principles of variation and inheritance in animals
and plants
• Mendel – The Grandfather of Genetics
– 1853 to 1861 in the Czech Republic
– Why did he succeed?
1. Organism – distinctive traits, reproductive processes (cross
pollination and self pollination)
2. Design
3. Analyze
Mendelian Genetics – An Introduction
• Purebred organism – descended from
ancestors of a distinct type or breed
• Through true breeding he was able to achieve
truly purebred organisms
• He completed this process for seven
distinctive phenotypic characteristics:
– Seed shape, seed colour, flower colour, pod
colour, pod shape, plant height, flower
position
Mendel & Peas: The grandfather of
genetics
Mendelian Genetics – Inheritance of
One Trait
• P generation – parent generation
– A true breeding tall plant is crossed with a
true breeding short plant
• F1 generation – first filial generation
– Refers to the offspring that are produced and
are often referred to as hybrid plants because
they are no longer purebreds
– This cross is called a monohybrid because
only one trait is being examined
Mendelian Genetics – Inheritance of
One Trait
• Blending theory stated that all offspring in the F1 generation
should have been medium height however all were tall!
• The allele for tall plants must have been dominant and the
allele for short plants must have been recessive
– Allele  two or more alternate forms of a gene. Located at the
same position on homologous chromosomes.
– Dominant trait  characteristic always expressed
– Recessive trait  characteristic that is latent (present but
inactive)
• Principle of Dominance = when individuals with
contrasting traits are crossed the offspring will express only
the dominant trait
The flower structure
Mendelian Genetics – Inheritance of
One Trait
• F1 generation was allowed to self pollinate
– F2 generation gave the Mendelian ratio
• 75% of offspring resembled dominant parent from the P
generation and 25% resembled recessive parent from the P
generation (fig. 5, p. 132)
• First Law of Heredity:
– Law of Segregation – inherited traits are determined by
pairs of alleles from each parent. These alleles separate
during gamete formation giving each offspring only one
allele from each parent
– Homozygous vs. Heterozygous
Mendelian Genetics – Probability
• When flipping a coin the probability of heads is
equal to tails for each flip of the coin.
• The chance therefore that two or more independent
events will occur together is the product of their
individual probabilities occurring alone.
• ½ x½ ¼
Mendelian Genetics – Punnett
Squares
• Punnett Square – a simple grid used to illustrate all
possible combinations of gametes from a given set
of parents.
• Allows you to calculate the probability of inheriting
a particular trait by creating all potential genotypes
from a mating between two parents.
• Genotype – genetic make up of an organism.
• Phenotype – is the appearance of the trait in an
organism determined by the genetic make up and the
environment to which the organism is exposed.
Mendelian Genetics – Test Cross
• Black sheep  brittle wool and difficult to dye.
– How would you avoid getting a sheep like this?
– Use a homozygous white ram
– But, how do you know if the ram is homozygous
or heterozygous?
– Complete a test cross
• Unknown crossed with a homozygous recessive
• If any black sheep born, you know the ram was
heterozygous
Mendelian Genetics – Selective
Breeding & Pedigrees
• Crosses can easily be done with plants and even
some animals in controlled situations
– Inbreeding  breeding is limited to a number of
desirable phenotypes.
– Hybridization  breeding of two different
parents to produce offspring with desirable
characteristics of both parents
Mendelian Genetics – Pedigrees
• Pedigrees - a diagram that illustrates the genetic
relationship among a group of individuals (fig. 4, p.
142)
• To look at human crosses family records need to be examined (medical,
historical, anecdotal).
• Records extending across several generations can be arranged in the form of
a family pedigree.
• Analysis of different pedigrees has shown that some traits are inherited as
simple dominant traits
– There are only two possible alleles for each trait
– Tongue rolling and widow’s peak are two examples of simple dominant
traits in humans
• Sex linked or Autosomal
• Dominant or Recessive
Beyond Mendel’s Laws
• Not all characteristics on earth can be explained
following Mendel’s concepts
• Multiple Alleles – many genes have more than two
alleles and this occurs in human blood typing and
the glycoproteins that are present on red blood cells
– Fly eye colour (p.143 & 144)
• Incomplete Dominance – a blending whereby the
heterozygote exhibits an intermediate phenotype
between two homologous phenotypes (fig. 1, p. 144)
• Co-dominance – when both alleles for a trait are
equally dominant (fig. 2, p. 145)
Mendelian Genetics – Inheritance of
Two Traits
• Mendel’s initial work involving investigating just
one trait. We are aware however that organisms are
composed of many different traits.
• Mendel wanted to know if the inheritance of one
characteristic influenced the inheritance of a
different characteristic.
• He needed to perform a dihybrid cross – crossing of
two pea plants that differed in two traits (pea shape
and pea colour)
Mendelian Genetics – Probability
• When flipping a coin the probability of heads is
equal to tails for each flip of the coin.
• The chance therefore that two or more independent
events will occur together is the product of their
individual probabilities occurring alone.
• ½ x½ ¼
• However dominant traits are ¾ and if dihybrid then
it is ¾ x ¾  9/16
Mendelian Genetics – Inheritance of
Two Traits
• He found that the results came out in a 9:3:3:1 ratio.
– 9 round yellow, 3 round green, 3 wrinkled yellow and 1
wrinkled green (fig. 4, p. 152)
• Second Law of Heredity:
– Law of Independent Assortment – inheritance of alleles
for one trait does not affect the inheritance of alleles for
another trait
• A test cross will also work to determine a genotype
of an unknown individual with two traits the same as
with one trait