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Mendel and Heredity
Standard 3
 Students know and understand the characteristics and
structure of living things, the processes of life, and
how living things interact with each other and their
environment.
 Benchmark 3.13: some traits can be inherited while
others are due to the interaction of genes and the
environment.
 Assessment objective 3.13a: Classify well-known
conditions as being purely genetic or the result of the
interaction of genes and the environment.
Vocabulary
 Character
 Trait
 Hybrid
 Generation
 Allele
 Dominant
 Recessive
 Genotype
 Phenotype
Homozygous
Heterozygous
Punnett square
Probability
Pedigree
Genetic disorder
Polygenic character
Codominance
Linked
Origins of Hereditary Science
 Modern genetics is based on
Mendel’s explanations for the
patterns of heredity in garden
pea plants.
 Seven characters Mendel
studied in his plants
included:
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Flower color
Seed color
Seed shape
Pod color
Pod shape
Flower position
Plant height
Gregor Mendel Introduction (1:30)
Contrasting Traits
 In the study of heredity,
physical features that are
inherited are called characters.
 A trait is one of several
possible forms of a character.
 For example, the color of a
flower is inherited and
therefore a character.
 A variation of the color is a
possible form of a character
and therefore a trait.
 In garden pea plants there
are both male and female
reproductive parts.
 Plants can self-pollinate
(fertilize itself) or crosspollinate with other pea
plants.
 Plants with different traits
that cross-pollinate
produce offspring called
hybrids.
Gregor Mendel Research and Principles (4:30)
A generation is a group of offspring from a
given group of parents.
 Mendel first ensured
that his plants were truebreeding for a certain
trait by letting the plants
self-pollinate for several
generations.
 The first group of
parents that are crossed
are called the parental
generation or P
generation.
 Mendel then crossed two
P generation plants that
had contrasting traits
like white and purple
flowers.
 He called the offspring of
the P generation the first
filial generation or F1
generation.
 Mendel then let the F1
generation plants selfpollinate and produce
new plants. He called
this new generation of
offspring the second
filial generation or F2
generation.
 Notice how many of each
trait was produced—
what’s the deal?
Mendel’s Theories
(2:51)
 Mendelian theory explains
 Traits can come from
simple patterns of
inheritance. In these
patterns, tow of several
versions of a gene combine
and result in one of several
possible traits.
 An organism’s traits come
from different versions of
genes. Each version is
called an allele.
either parent—because of
meiosis.
 For every pair of traits, only
one expresses itself.
 The allele that expresses
itself is said to be
dominant.
 The other allele is a
recessive allele and its trait
is not expressed.
Random Segregation of Alleles
 Even though a gamete
receives an allele from each
parent, only chance (or
fate) determines which one
it will receive.
 The law of segregation
holds that when an
organism produces
gametes, each pair of
alleles is separated and
each gamete has an equal
chance of receiving either
one of the alleles.
 Dominant alleles are
(4:41)
indicated with a capital
letter and recessive alleles
have a lower case letter.
 Given “a” and “A”, which is
dominant and which is
recessive?
Mendel’s Findings in Modern Terms
 The set of alleles that an
individual has for a
character is called the
genotype.
 The trait that results
from a set of alleles is the
phenotype.
 Genotype determines
phenotype.
Homozygous & Heterozygous
 If an individual has two of
the same alleles of a certain
gene, the individual is
homozygous for the related
character.
 If an individual has two
different alleles of a certain
gene, then they are
heterozygous.
 Between pp and Pp, which
is homozygous and which
is heterozygous?
Significance of test cross as genetic tool (1:30)
Mendel’s Second Experiment
 Mendel used dihybrid
crosses which involve two
characters like color and
shape to further test how
patterns appear.
 The Law of Independent
Assortment holds that
during gamete formations,
the alleles of each gene
segregate independently.
 Alleles can mix and match.
Introduction dihybrid crosses (3:15)
Mendel’s Second Law
 Genes are linked to each
other as parts of
chromosomes.
 Genes that are located
close together on the
same chromosome will
rarely separate
independently, thus they
are said to be linked
because they’re so close
together.
Simulating a dihybrid cross (2:57)
Punnet Squares
 Punnet squares show all
the genotypes that could
result from a given cross.
 As shown, the simplest
box is 4 squares with the
parent’s traits on the top
and side of the square.
 Watch the video
 Do the Test Cross
QuickLab on page 277
Punnet Square (4:49)
Using Probability
(7:04)
 Probability is the likelihood that a specific event could
occur.
 Punnet squares predict probability
 Probability can be calculated using the formula:
Number of one kind of possible outcome
 Probability = Total number of all possible outcomes
 Probability formulas can be used to predict the
probabilities that specific alleles will be passed on to
offspring.
Using a Pedigree
 A pedigree is a family
history that shows how a
trait is inherited over many
generations.
 Genetic disorders or
diseases can be inherited.
 Pedigrees can answer
questions associated with
sex linkage, dominance,
and heterozygosity.
Sex linked inheritance (29:06)
Dominant/Recessive
Heterozygous/Homozygous
 If a person has a trait that is
autosomal and dominant and
has even one dominant allele,
they will show the trait.
 If a person has a recessive trait
and only one recessive allele,
they will not show the trait but
could pass it on.
 If a person is either
heterozygous or
homozygous dominant for
an autosomal gene, their
phentoype will show the
dominant trait.
 Homozygous recessive
shows the recessive trait.
 If a child shows the
recessive trait, that means
that both parents are
heterozygous carriers of
the recessive allele.
 When several genes affect a
character it is called a
polygenic character.
 Examples are eye color, skin
color and height.
 Most characters are
polygenic.
 Incomplete dominance
occurs when an offspring has
a phenotype that is inbetween the traits of its two
parents.
 An example is when a red and
white snapdragon flower is
crossed and a pink offspring
results. It shows that neither
the red or the white is
completely dominant.
 Incomplete dominance in
snapdragons (2:07)
 Incomplete dominance in
humans and plants (2:01)
Multiple alleles and Codominance
 Multiple alleles are genes that
have three or more possible
alleles.
 In humans, blood types are
determined by 3 alleles: IA, IB,
and i. These can produce
blood types of A, B, AB and O
 Codominance is a condition
in which both alleles for the
same gene are fully expressed.
 Human blood groups are
examples of codominance.
Multiple Alleles and
Codominance in Blood (11:09)
Genes affected by environment
and genes linked within chromosomes
 Phenotype can be affected by
conditions in the
environment such as
nutrients and temperature.
 Many arctic animals have
genes that cause their fur to
be dark in the summer and
light in the winter in order to
increase chances of survival
through camoflauge.
 Environmental Factors
Which Influence the
Expression of Traits (02:02)
 During meiosis, genes that
are close together on the
same chromosome are less
likely to be separated than
genes that are far apart.
 Genes that are close together
as well as the traits they
determine are said to be
linked.
 QuickLab pg 281