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Objectives
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1. Be able to explain Gregor Mendel’s
Law of Dominance.
2. Be able to do a monohybrid cross and
calculate the ratios.
3. Be able to apply the use of a test cross.
Gregor Mendel –
Father of Modern Genetics
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True-breeding term used to
describe
organisms that
produce offspring
identical to
themselves if
allowed to selfpollinate.
The work of Mendel
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Mendel had truebreeding pea
plants.
He asked the
question: What
would happen if
the pea plants
were breed with
pea plants with
different traits?
Vocabulary Words
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Trait - specific characteristic that varies from one
individual to another
Gene - sequence of DNA that codes for a protein
and thus determines a trait
Allele - one of a number of different forms of a
gene
Hybrid - offspring of crosses between parents
with different traits
The principle of dominance states that some
alleles are dominant and others are recessive.
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P – Parent generation
F1 – first generation of offspring (f – filial
from latin filius “son”)
Mendel’s Law of Dominance
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Law states that the dominant trait is the only
trait to appear in F1 generation in a cross
between two pure lines (BB x bb)
Albinism, Rh blood factor, Cystic fibrosis
Monohybrid cross- a cross that involves only
one trait with two phenotypes
Punnett Square-device for predicting possible
offspring of crosses between different
genotypes
Punnett Square
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diagram showing the gene combinations that
might result from a genetic cross
Cross BB
and bb
B
B
b
Bb
Bb
b
Bb
Bb
Activity #1
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People with red hair have the recessive
genotype, nn. Those with any other color hair
have at least one dominant allele, N.
1. What is your phenotype?
2. What is your genotype?
3. Will a person with red
hair have children with
red hair?
4. Do three Punnett Squares:
Nn x nn, NN x nn, nn x nn
Activity #2
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Long eyelashes are the result of a dominant allele,
S. Short eyelashes are the result of the recessive
genotype, ss.
1. What is the phenotype
of the person next to you?
2. What is the genotype of the
person next to you?
3. Do the Punnett Square: Ss x Ss
4. Calculate the ratio: homozygous dominant to
heterozygous to homozygous recessive
Problem Solving
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Black coat color in guinea pigs is dominant
over white coat color.
How can you determine if a black guinea pig is
pure or a hybrid?
Use B – black
and
b - white
Test cross
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Is a black guinea pig pure or hybrid?
Do a test cross by mating the animal with
pure recessive (white)
Homozygous recessive x homozygous
dominant = all offspring 0% recessive
Homozygous recessive x heterozygous =
50% recessive (white)
Activity
Using a Punnett Square,
estimate the ratio of offspring
in the F1 and F2 generation
from the cross between a
homozygous dominant rabbit
with floppy ears (EE) and a
homozygous recessive rabbit
with straight ears (ee) .
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Today’s Objectives
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1. Be able to explain Gregor Mendel’s
explanation of segregation.
2. Be able to explain Gregor Mendel’s
principle of independent assortment.
3. Be able to do a two-factor cross and
calculate the ratios.
4. Be able to write a summary of Mendel’s
Principles.
Crossing the F1 generation
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Segregation - separation of alleles during
gamete formation
When each F1 plant flowers, the two
alleles are segregated from each other so
that each gamete carries only a single copy
of each gene. Therefore, each F1 plant
produces two types of gametes—those with
the allele for tallness and those with the
allele for shortness.
Cross of heterozygous yellow and
round peas.
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How many
different
phenotypes do
we have?
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4
What is the ratio
for the
phenotypes?
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9:3:3:1
Using ratios from Punnett
Squares
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Ratio of tall to short pea plants is 3:1
If we breed heterozygous tall pea plants
with each other and in one generation we
made 1000 plants, how many tall pea
plants and short pea plants should we have?
Summary of Mendel’s Principles
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The inheritance of biological characteristics is determined
by individual units known as genes. In organisms that
reproduce sexually, genes are passed from parents to their
offspring.
In cases in which two or more forms of the gene for a
single trait exist, some forms of the gene may be
dominant and others may be recessive.
In most sexually reproducing organisms, each adult has
two copies of each gene—one from each parent. These
genes are segregated from each other when gametes are
formed.
The alleles for different genes usually segregate
independently of one another.
Objectives
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1. Be able to explain incomplete
dominance.
2. Be able to explain codominance.
3. Be able to explain multiple alleles.
4. Be able to explain polygenic traits.
Beyond Dominant and Recessive
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Incomplete
dominance – (in
between) situation
in which one allele
is not completely
dominant over
another
There is no white.
There is no red. new phenotype pink
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Codominance situation in which both
alleles of a gene
contribute to the
phenotype of the
organism
Red and white hair
color in roan cattle,
black and white feather
color in certain
varieties of chickens
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Multiple alleles - three or more alleles of the same gene
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Coat color in rabbits is determined by a single gene that has at
least four different alleles
Blood type in humans
Eye color in humans
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Polygenic Traits
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Means “having many genes”
Two or more genes control a trait
Eye color in fruit flies (at least three genes)
Skin color in humans (probably more than four genes)
Objectives
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Be able to describe Thomas Hunt Morgan’s
discovery that seemed to violate Mendel’s
principles
Be able to define the structures that actually
assort independently
Be able to explain how Alfred Sturtevant used
gene linkage to create gene maps
Be able to explain how gene maps are produced
Linkage and Gene Maps
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It’s easy to see how genes located on
different chromosomes assort
independently, but what about genes
located on the same chromosome?
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Wouldn’t they generally be inherited
together?
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Thomas Hunt Morgan’s studies back in
1910 helped us to answer this question.
Thomas Hunt Morgan
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Did research on fruit flies
Identified more than 50
Drosophila genes
Discovered many
appeared to be “linked”
together
Example: reddish-orange
eyes and miniature wings
Did this violate Mendel’s
principle of independent
assortment?
Morgan’s Conclusions
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Each chromosome is actually a group of linked
genes.
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Mendel’s principle of independent assortment still
holds true.
http://www.zerobio.com/independent_assortm
ent.htm
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It is the chromosomes, however, that assort
independently, not individual genes
Linkage and Gene Maps
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Just because two genes are
located on the same
chromosome does not mean
that they are linked together
forever.
Crossing-over in meiosis
http://www.zerobio.com/crossin
g_over.htm
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Also, the further apart they
are the more likely they are to
separate. The closer they are
the less likely they are to
separate.
Alfred Sturtevant
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A Columbia University
student who worked in
Mendel’s lab
Used gene linkage to create
gene maps
The rate at which linked
genes were separated and
recombined could be used to
create a map of distances
between the genes
Drosophila melanogaster Gene Maps
Chromosome 2:
http://www.pearsonsuccessnet.com/ebook/products/0-13036843-1/sb04069.pdf