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Unit 3 - Genetics
• Genetics – Deals with the principles of
variation and inheritance
• Heredity – the passing of genetic
characteristics from one generation to the
next (ex: Hair/eye colour)
Traits – distinguishing characteristics which
makes individuals unique
• Gene – a specific part of DNA, that makes a
certain trait (ex: eye color).
• Allele – two alleles makes one gene. You get
one allele from your mom and one from your
dad.
• Mendel studied seven pairs of contrasting
traits as follows:
Crash Course Biology Video - Heredity
https://www.youtube.com/watch?v=CBezq1fFUEA
• P generation – the parent generation
• F1 generation – the first filial generation;
offspring from a cross of the P generation
• F2 generation - the second filial generation;
offspring from a cross of the F1 generation
• Dominant – this allele is always expressed.
We use a capital letter to write a dominant
allele.
• Recessive – this allele is hidden, unless there is
no dominant allele present.
We use a lowercase letter to write a recessive
allele.
Video: Are Your Traits Dominant?
https://www.youtube.com/watch?v=mnSkz8s-b44
• Heterozygous – two different alleles. One is
dominant and the other is recessive.
Ex: Aa
• Homozygous – two alleles that are identical.
Either two dominant alleles or two recessive
alleles.
Ex: AA or aa
• Purebred – descended from ancestors of a
distinct type or breed. All purebreds share
similar traits.
• Hybrid – an organism heterozygous for a trait.
Ex: Aa
Lion + Tiger = Liger
• Phenotype – the physical traits of an organism
Ex: the color of your eyes
https://www.23andme.com/enca/gen101/phenotype/
• Genotype – the genetic make-up of an
organism. Indicated by letters in a Punnett
square.
Ex: AA
Aa
aa
• Punnett Square – a grid used to show all
combinations of gametes inherited from a set
of parents
• Monohybrid Cross – a cross of ONE trait
• Dihybrid cross – a cross of TWO traits
• Product Rule - the probability that two or
more independent events will occur together
is the product of their individual probabilities
of occurring alone
Who is Gregor Mendel?
• An Austrian Monk
• Studied Mathematics and Botany
• Pioneer in the study of Genetics and Heredity
Video: How Mendel's pea plants helped us
understand genetics
https://www.youtube.com/watch?v=Mehz7tCxjSE
Turn to page 4 in Booklet
Mendel’s First Cross (P1 Cross)
• Mendel crossed
(P): Pure tall (TT) x pure dwarf (tt)
• The result
(F1 ): All offspring were tall (Tt)
• But these were not pure tall, because each
parent had a different trait for stem height.
Instead, these offspring are referred to as
hybrids or hybrid tall.
– If there is at least ONE dominant allele, they
would have the dominant trait (A capital letter is
used to represent dominant. Ex: T
– In order to show the recessive trait, you must have
TWO recessive alleles. The dominant allele will
mask or prevent the expression of the recessive
allele. (A lowercase letter is used to represent
recessive. Ex: t)
Law of Segregation
• After his initial experiment, Mendel allowed
the hybrid F1 plants to self pollinate,
producing the second filial generation (F2
generation)
• Observed a 3:1 ratio of tall:short plants. Why
did this happen?
Law of Segregation
• Mendel made 4 conclusions:
• Each parent in the F1 generation starts with 2
hereditary factors, one dominant and one recessive
• Only one factor from each parent is contributed to the
offspring
• Each offspring inherits one factor from each parent. If
the dominant factor is present, it will be expressed
even if the recessive factor is also present
• The recessive factor will be expressed if only recessive
factors are present
Law of Segregation
• Law of Segregation – hereditary 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.
Law of Segregation
Law of Segregation
• What are these “factors” which Mendel described? We know them today
as Genes: the part of the chromosome that governs the expression of a
particular trait
• Each gene can occur in alternate forms called alleles. Remember the
principle of dominance: a dominant allele may prevent the expression of
the recessive allele.
Law of Segregation
•
•
•
•
Mendel’s “factors” are inherited as independent units. They are referred to as unit
characters
Unit Theory is a term describing Mendel’s “factors” of inheritance (genes), which
are inherited as independent units.
When two alleles at the same gene locus are identical, the individual is said to be
homozygous
When two alleles at the same gene locus differ, the individual is said to be
heterozygous
Genetics and Probability
• Heads or Tails?
• In groups of 3 or 4, complete the Probability
activity worksheet
Genetics and Probability
• Probability – the chance, or likelihood, of a particular outcome
• Product Rule – the probability that two or more independent events will
occur together is the product of their individual probabilities of occurring
alone
Punnett Squares
• Punnett squares can be used to predict and compare
the genetic variations that will result from a cross.
Punnett Squares
• Dominant Alleles are usually represented by an upper case
letter (eg. Purple plant (R))
• Recessive Alleles are usually represented by a lower case
letter (eg. White plant (r))
Punnett Squares
• Homozygous or Heterozygous?
– RR
– Rr
– Rr
– rr
Dominant or Recessive?
Punnett Squares
• What is the phenotypic ratio of this cross?
• In other words, what is the ratio of purple:white plants?
• Phenotype – The appearance of the trait in an organism
Punnett Squares
• What is the genotypic ratio of this cross?
• In other words, what is the ratio of homozygous dominant
:heterozygous : homozygous recessive (RR:Rr:rr)?
• Genotype – The genetic make-up of an organism
Punnett Squares
• Complete Dominance – a type of inheritance
where both heterozygotes and dominant
homozygotes share the same phenotype
Determining Genotypes
• Test Cross – Crossing an individual of unknown
genotype with a homozygous recessive individual.
Used to determine if an individual expressing a
dominant trait is homozygous or heterozygous
• Alaskan Malamute example
Complex Inheritance Patterns
• Mendel’s 2nd Experiment
– Does the inheritance of one trait influence the inheritance
of a different trait? (eg. Does pea shape influence pea
colour?)
– Mendel produced purebred pea plants: Round Yellow
plants (RY) and Wrinkled Green plants (ry)
– Performed a Dihybrid Cross  a cross of two individuals
that differ in two traits (pea shape/colour)
Dihybrid Cross
Law of Independent Assortment
• The inheritance of alleles for one trait does
not affect the inheritance of alleles for
another trait
• New combinations of alleles that are not
present in either parent
Incomplete Dominance
• Not all traits are completely dominant or
recessive
• Incomplete Dominance - The blending of the
traits of two different alleles at one gene locus
that occurs when neither allele is dominant
• Eg. White and Red snapdragons are
homozygous; Pink snapdragons are
heterozygous
Incomplete Dominance
Co-Dominance
• Similar to incomplete dominance  there is a
third phenotype, no definitive dominant or
recessive allele
• When both alleles of a gene locus are
expressed equally (co-dominate), this type of
inheritance is known as Co-Dominance
Co-Dominance