Introduction to Mendelian Genetics
Monohybrid Crosses
Lesson 4.
Learning goals
• Understand the basic terminology of
mendelian genetics, not limited to
(phenotype, genotype, homozygous,
heterozygous, P,F1,F2 generation(s),
dominant, recessive, alleles, gene
• Solve monohybrid problems using Punnett
square method
Gregor Mendel (1822-1884)
• Used pea plants to study
genetics, heredity and
variation.
• Looked at 7 different traits
that only had two variations.
• From his meticulous work he
came up with many “key
terms” and, more importantly,
two generalizations that later
became known as Mendel’s
Laws.
1677 Anton van Leewenhoek
• First discovered living sperm (also
designed first microscope)
• Believed he saw a complete miniature
person in the head of a sperm;
homunculi
19th Century British
• From breeding ornamental plants,
scientists believed in a blending theory
of inheritance.
• Charles Darwin – theorized that offspring
have variations of their parents'
characteristics, but he was unable to
explain why (1870s).
Gregor Mendel (1822-1884)
• Austrian monk
• Considered the father of
genetics (now aka Mendelian
Genetics)
• Why the pea plant?
– grows easily and quickly
– has seven traits that are easily
identified
– it is easy to manipulate
7 traits Mendel looked at
Mendel’s true breeding experiments
• Pea plants reproduce
sexually, but usually
self fertilize
• True breeding:
organisms that exhibit
the same traits
generation after
generation
Mendel's Pea Plant Experiment
• Mendel selected true bred plants for different traits and
cross-fertilized them to see what would happen.
• True breeding: Individuals that only contain one variation
of a trait and therefore can only pass this one variation on
to future generations. We now call these individuals
homozygous, or having two alleles that are the same.
• For example:
– Tall x dwarf
• What do you think the offspring looked like?
X
• 100% of the offspring plants were tall!
(Instead of a blending – where all
plants would be medium sized...)
X
• Mendel then let the F1 generation of offspring selfpollinate...
• F1 generation – (filial (son or daughter) generation)
offspring from the cross (reproduction) of the Parent or P
generation.
x
• this time most were tall, but some were short and ALWAYS
3:1 ratio
Why?
• Each plant in the F1 generation carried an allele from the P
generation; a tall allele and a dwarf allele. They are what
are called hybrids.
• Hybrids: Individuals that contain more than one variation
of a trait and therefore can pass on more than one
variation to future generations. We now call these
individuals heterozygous, or having two alleles that are
different from each other.
• Traits are dominant or recessive. If a dominant trait is
inherited, it will be expressed.
• What is the dominant trait for height in pea plants?
Mendel’s “Key Terms”
• Phenotype: the appearance of traits in an organism (eg.
short or tall)
• Genotype: the specific genes that an organism has
(which alleles, dominant or recessive)
• Every individual has a phenotype and genotype for every
trait.
• The phenotype is easy to figure out, but occasionally the
genotype is hard to determine, especially for dominant
traits.
Alleles: Alternate forms of a gene
Genotype vs. Phenotype
Which Allele is expressed?
Can you tell the genotype of an
individual by their phenotype?
Mendel's Laws and Principles
• Law of Segregation: when any individual produces
gametes, the copies of a gene separate so that
each gamete receives only one copy of a gene and
therefore only one allele for all possible traits.
Principle of Segregation Demystified
Law of Segregation
Segregation
The principle of segregation is explained by the behavior of homologous
chromosomes at meiosis.
• Law of Independent Assortment: alleles of different
genes assort independently of one another during
gamete formation, so different traits are inherited
independently of each other
• In plain English, this means that if you are able to be
dominant for one trait, it will have no influence on
whether you will be dominant for another trait.
http://www.youtube.com/watch?v=mx6maAOOfxQ
Punnett Square
Punnett Square
Dominant traits
This phenomenon can be explained by the
fact that some ________ are dominant. A
dominant allele is an allele that __________
its phenotypic effect when present in the
individual.
Recessive traits
9. A recessive allele is expressed only when
_____ alleles are of the recessive form.
The genotype of an individual showing a
recessive phenotype must be
____________ for the recessive allele.
Punnett Square
Punnett Square
Genotypic proportions
11. The genotypic proportions of the
offspring from the cross of two
heterozygous parents (Bb) are:
• BB __/4
• Bb __/4
• bb __/4
Phenotypic proportions
12. The phenotypic proportions of the
offspring are:
• Purple __/4 or __%
• White __/4 or __%
Monohybrid cross problem
Curly hair is dominant over straight hair. If a
mother is homozygous dominant for curly
hair and the father has straight hair, what
type of hair will the children have?
Test Crosses
• When geneticists want to know if an individual is
heterozygous or homozygous
– Why? For breeding purposes
• Cross between unknown phenotype and homozygous
recessive individual
– (Pp or PP) x pp
• Pp x pp = 50% purple, 50% white
• PP x pp = 100% purple