GENETICS
Chapter 17
Wonder about your genes?
With a partner fill out
the human traits
worksheet, decide if
you have recessive or
dominant traits.
Definitions of Genetics and
Heredity
Genetics: The field of science that deals with
heredity.
Heredity: Passing on of characteristics from
parents to offspring.
The field of genetics is fairly recent; however
people have been manipulating genes for
centuries. Examples??
Selective Breeding (Pets, Livestock, Crops)
Who was Gregor Mendel?
Born in 1822
Studied science,
mathematics and
statistics
Studied heredity in peas
for 10+ years
Discovered the
principles of heredity
Died before anyone
took his study seriously
Why peas?
1.
2.
3.
4.
Several generations grown per season
Pea plants can self and cross pollinate
Easy to control whether a plant self
pollinates or cross pollinates
Peas have several inheritable traits
that are “either/or” (only come in 2
forms)
-either tall or short, purple or white, etc.
These are the seven characteristics, each
having two contrasting traits, that Mendel
identified.
Terminology
Characteristic: observable feature
that can appear in more than one form
(flower color)
Trait: a variation that can exist for a
characteristic (purple or white flowers)
Purebred: all offspring and their
descendents have the same trait for a
particular characteristic when they are
cross pollinated (used in Mendel’s experiments)
Terminology
Hybrid: results from the crossing of 2
purebred plants that have different
traits (purple x white)
Monohybrid cross: crossbreeding
experiment that follows the inheritance
of a single characteristic over several
generations
Hypotheses
A) There are elements that are passed
on by a parent to its offspring.
B) Different elements control different
traits.
C) These elements remain unchanged
during the life of any organism.
D) Elements are passed on in the
gametes (reproductive cells).
Mendel’s experiments
Mendel found that you must follow at least 3
generations to determine inheritance patterns
P generation: parent generation, often
purebred for a trait
F1 generation: first filial generation,
offspring of the P generation, often hybrids
F2 generation: 2nd filial generation, F1
generation is self-pollinated or crossed
Mendel’s Monohybrid Cross
Mendel self- fertilized the pea plants until he
got true-breeding traits - the offspring
produced resembled the parent in all cases.
(This took many generations)
He then crossed purebreds, one with purple
flowers and one with white (produced a
hybrid)
The F1 generation was all purple
Mendel called the purple trait “dominant”
and the white trait “recessive”
Mendel’s Monohybrid Cross
Dominant trait: fully expressed in the
offspring
Recessive trait: not visible in a hybrid
Mendel then took the purple F1 offspring
and self-pollinated them or crossed them
with another F1 plant
The results…
In the F2 generation:
¾ were purple
¼ were white
The recessive trait had reappeared!
Magic?
The famous 3:1 ratio
The recessive trait must have been
masked, or hidden
The F2 generation had a 3:1 ratio of
dominant (purple) to recessive trait
(white)
Mendel did this for all the traits and got
similar results in the second generation
(75% of one trait, 25% of another trait)
Quick Check – Answer T or F
1.
2.
3.
4.
5.
It is possible for the sperm and egg from the same
pea plant to combine to produce healthy offspring
The F2 generation are the parents of the F1
generation
When 2 purebred plants with different traits for the
same characteristic are crossed, the offspring is a
hybrid
If two possible traits are yellow peas and green peas,
the characteristic for the trait could be “pea color”
The either/or nature of stem length means that the
stem can be long, short, or some length in between
End.
Probability
Mendel did not get exact 3:1 ratios
Eg: F2 = 705 purple to 224 white, which
is a 3.15:1 ratio
Because the outcomes of breeding are
due to chance!
Probability: a measure of the
likelihood that an event can happen,
governed by the rules of chance
Mendel’s Gene Hypothesis
1.
2.
Based on his data, Mendel formed the
following ideas:
A hereditary unit of info, a gene, is
passed from parent to offspring, eg:
the gene for the characteristic “flower
color”
An allele is one of the possible
versions of the gene, eg: the allele for
the purple trait or the allele for the
white trait
Mendel’s Hypotheses
1.
2.
3.
The presence of different alleles is
responsible for the variation in the
appearance of an organism
An organism always has 2 genes for each
characteristic, one inherited from each
parent
If the alleles on both of the genes are the
same, then the organism is purebred
Mendel’s Hypotheses
4.
If the 2 alleles are different, then the
organism is a hybrid for the characteristic.
One trait will be dominant (purple) and
one (white) will be recessive.
*Only the dominant trait will be
expressed. When both genes exist, the
dominant trait hides the recessive trait*
Most traits are independently sorted
Mendel found that of the 7 characteristics
he studied, none of the traits had anything
to do with the others
Eg: the shape of a pea pod has NO
influence on whether the plant is tall or
short
This is the law of independent
assortment
Punnett Squares
PP x pp
↓
Pp x Pp
↓
PP
Pp
Pp
pp
Quick Check - Answer T or F
1.
2.
Mendel studied the color of pea pods, which can
be green or yellow. He found that when greenpod plants self-pollinated, 508 offspring
resulted 428 had green pods and 152 had
yellow pods
For the characteristic “pod color”, green is
dominant and yellow is recessive
The hybrid plants had 2 alleles, one for green
and one for yellow
Answer T or F
3.
4.
5.
The yellow-pod plants had only 1 type of
allele, the one for yellow
The ratio of 428:152 reduces to 2.82:1.
The ratio should have been 3:1, but some
mistakes were probably made when
collecting data
The 152 plants with yellow pods were
purebred for the yellow trait
Dominance and Recessiveness
One gene is indicated by a single letter.
Capital = Dominant; Lower Case = Recessive
Organisms with matching alleles are
homozygous for that trait
All purebred traits are homozygous
Homozygous dominant = purebred for
dominant trait = PP
Dominance and Recessiveness
Homozygous recessive = purebred for
recessive trait = pp
Organisms with mismatched alleles are
hybrids = Pp = heterozygous for that trait
(hetero means different)
Phenotype and Genotype
Phenotype: the outward appearance of a
characteristic in an organism
Eg: color = purple phenotype or white
phenotype
Phenotypic ratio: the number of each
phenotype expressed in the offspring
Eg: if 30 plants are tall and 10 are short, the
phenotypic ratio is 3:1
Phenotype and Genotype
Genotype: the genetic makeup of an
organism
The two genes for each trait may have
matching alleles (PP and pp) or nonmatching alleles (Pp)
The alleles may be dominant (PP) or
recessive (pp)
Genotype Summary
Genotype
Gene Combination
Description
Homozygous
PP or pp
Organisms with matching
alleles
Homozygous
dominant
PP
Organisms with matching
dominant alleles
Homozygous
recessive
pp
Organisms with matching
recessive alleles
Heterozygous
Pp
Organisms whose alleles
are not the same (hybrids)
Genotypic ratio: the number of each
genotype expressed in the offspring
Eg: If the results in a pea height
example were 10 PP, 20 Pp, and 10 pp,
the genotypic ratio would be 1:2:1
This is the genotypic ratio in monohybrid
crosses, while the phenotypic ration is 3:1
Monohybrid Cross- follows the
inheritance of a single characteristic
PP x pp
↓
Pp x Pp
↓
•The
phenotypic
ratio is 3:1
PP
Pp
Pp
pp
•The
genotypic
ratio is
1:2:1
Definitions 2
Yellow (Y) is dominant
Green (y) is recessive
Phenotype
Seed Colour
Yellow
Seed Colour
Green
Genotype
Definitions 2
Yellow (Y) is dominant
Green (y) is recessive
Phenotype
Genotype
Seed Colour
Yellow
YY or Yy
Seed Colour
Green
yy
end
Definitions
One gene is indicated by a single letter.
Capital = Dominant; Lower Case = Recessive
For example, Y = dominant (yellow) y =
recessive (green)
Since you receive one gene from each parent,
you have two genes for each trait.
Homozygous (YY or yy) both alleles are the
same
Heterozygous (Yy) two different alleles
Results
First Generation (also called the F1
generation)
All yellow seeds
Second Generation ( F2 generation) –
crossed two plants from the F1
generation
¾ yellow seeds, ¼ green seeds
Results (Con’t)
Mendel did this for all the traits and got
similar results in the second generation
(75% of one trait, 25% of another
trait).
Second generation results:
75% tall plants; 25% short plants
75% round seeds; 25% wrinkled seeds
Mendel’s Conclusions
1) There are elements (genes). As predicted
these elements must remain unchanged. F2
green seed!!
2) Gametes produced by parents with
contrasting forms of a trait must contain
different forms of the element. (different
gene for green and yellow seeds!!)
3) Receive one element from each parent;
therefore offspring contains a pair of these
of elements.
Mendel’s Conclusions (Con’t)
4) One form of each trait is dominant over the
other trait, which is recessive.
Which trait was dominant, which trait was
recessive in the experiment?
The first generation was all yellow. When
both genes exist, the dominant trait hides
the recessive trait.
Assignment
Language of Genetics
Use your textbook to find the definitions
for all terms on the worksheet
Bonus – word search on back of sheet
Review 17.1, p. 392, # 1 - 5
Definitions
Contrasting forms of the trait must contain different forms of
the element. Different forms of genes are called Alleles.
One gene is indicated by a single letter.
Capital = Dominant; Lower Case = Recessive
For example, Y = dominant (yellow) y = recessive (green)
Since you receive one gene from each parent, you have two
genes for each trait.
Homozygous (YY or yy) both alleles are the same
Heterozygous (Yy) two different alleles
Definitions 2
Genotype:
Genetic makeup;
for example Yy
Phenotype:
Outward (visible)
appearance;
for example a yellow
seed.
Phenotype Genotype
Seed
Colour
Seed
Colour
Yellow
Green
Yy or YY
yy
Probability of Genetic
Events: The Punnett Square
In three steps, it’s an easy way to
determine the probability of offspring.
Step 1: Make a 2 X 2 Square.
Step 2: Put the alleles of each parent
on the outside of the square.
Step 3: Combine alleles to make
potential offspring in the middle of the
square.
Punnett Square Problems
First Generation
Second Generation
Coin Genetics