Download Mendelian Genetics

Genetics
Unit 3
Mendelian Genetics
• Genetics – An aspect of science that deals
with the principles of variation and
inheritance.
• Heredity – The passing of genetic
characteristics from one generation to the
next (eg. Hair/eye colour)
Genetic Terms
Traits – Distinguishing characteristics which makes
individuals unique (physical, personality, etc.)
“You get all of your best traits from your mother.”
Purebred – An organism descended from ancestors
of a distinct type or breed. They all share similar traits.
Gregor Mendel
• An Austrian Monk
• Studied Mathematics and Botany
• Pioneer in the study of Genetics and Heredity
One of Mendel’s Experiments
Draw Mendel’s Experiment Here
Mendelian Genetics
• P Generation – Parent generation.
• F Generation – First filial generation or first
set of offspring produced
1
• Hybrid – The result of a cross between two
different purebred plants
• Dominant – A trait which is always expressed,
or always appears in the individual
• Recessive – A trait which is present but
inactive and is therefore not in usually
expressed in an individual.
• Principle of Dominance – Mendel concluded
that when 2 different traits are crossed, one
trait is always dominate over the other.
Mendel’s Experiment AGAIN!
Draw Mendel’s Experiment – focusing
on F generation
2
• F Generation – The second group of offspring
produced
2
• Mendelian Ratio – 3:1 ratio of the occurrence of
specific traits in offspring when the parents only
differ in one trait.
• Monohybrid Cross – A cross where only one trait
(ex. Height) is crossed.
Monohybrid Cross
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
•
•
•
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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