 Austrian monk, born 1822, who
carried out work with various plants
that contributed to the
understanding of genetics
 Most notable of the plants was his
work with pea plants


Fertilization: production of a new cell
True-breeding: plants that, if allowed
to self-pollinate, would produce
offspring identical to the parents
 Mendel cross-pollinated
the pea plants by removing
the male reproductive parts
and dusting pollen from
another plant onto the
flower
 This produced seeds with
two different parents
 Mendel was able to study
the results of different
characteristics these
plants had
Genes and Dominance
 Mendel studied seven different pea
plant traits
 Trait: specific characteristic (seed
color, height, etc.) that varies from
one individual to another
 Mendel crossed seven contrasting
characteristics and studied the
results
 Hybrids: offspring of crosses
between parents with different
traits
1. Biological inheritance is determined by factors
that are passed from one generation to the next
2. Principle of dominance – some alleles are
dominant and others are recessive
Alleles: different forms of a gene
 A recessive allele will only be evident when a
dominant allele is not present

Chromosome Number Basics..
= each chromosome from the male
parent has a corresponding chromosome from the
female parent (a match!)
 A cell that contains both sets of homologous
chromosomes is said to be diploid.
 The number of chromosomes in a diploid cell is
sometimes represented by the symbol 2N.
 For Drosophila, the diploid number is 8, which can be
written as 2N=8.
Chromosome Number Basics..
 The gametes of sexually reproducing organisms
contain only a single set of chromosomes, and
therefore only a single set of genes.
 These cells are haploid. Haploid cells are represented
by the symbol N.
 For Drosophila, the haploid number is 4, which can be
written as N=4.
 Meiosis is a process of reduction
division in which the # of chromosomes
per cell is cut in half through the
separation of homologous chromosomes
in a diploid cell


Meiosis involves two divisions, meiosis I and
meiosis II.
By the end of meiosis II, the diploid cell that
entered meiosis has become 4 haploid cells
(gametes).
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Phases of Meiosis I
 Meiosis I
Interphase I
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I and
Cytokinesis
Prophase I (Meiosis I)
 Chromosome
replication occurs
during interphase
 Tetrad – formed when a
chromosome pairs with its
homologous chromosome
 There are 4 chromatids in
a tetrad.


When homologous chromosomes form tetrads in
meiosis I, they swap portions of their chromatids
in a process called crossing-over.
Crossing-over produces new combinations of
alleles.
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 Spindle fibers
attach to the
chromosomes,
which are lined
up in the
middle of the
cell
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 The fibers pull
the homologous
chromosomes
toward opposite
ends of the cell.
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 Nuclear membranes form
 The cell separates into
two cells
 End result of Meiosis I:
two cells that have
chromosomes and alleles
different from each other


The two cells produced by meiosis I now enter
a second meiotic division.
Unlike meiosis I, neither cell goes through
chromosome replication.
 Meiosis II
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
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Anaphase II
Telophase II
and
Cytokinesis
 Nuclear Membrane
dissolves
 Spindle forms
 Just like old times!
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The
chromosomes
line up in the
center of cell.
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The sister
chromatids
separate and
move toward
opposite ends of
the cell.
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 Meiosis
II
results in four
haploid (N)
daughter cells.
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Gamete Formation
 Meiosis produces 4 genetically different haploid cells
 In male animals, meiosis results in four equal-sized
gametes called sperm.
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 In many female animals, only one egg
results from meiosis. The other three cells,
called polar bodies, are usually not
involved in reproduction.
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Questions! (Page 278)
 1. 4 haploid cells genetically different from one another and
the original cell
 2. Mitosis produces two genetically identical diploid cells;
meiosis produce for genetically different haploid cells
 3. Diploid: two sets of chromosomes
 Haploid: one set of chromosomes
 4. Homologous chromosomes pair up and form tetrads,
which may exchange portions of their chromatids results in
the exchange of alleles between the homologous
chromosomes
 5. Both sperm and egg cells have 23 chromosomes because
they are gametes, which are haploid cells. A white blood cells
has 46 chromosomes because it is a diploid body cell
Questions! (page 266)
 1. Dominant: form of an allele who trait always shows up if it is
present. Recessive: form of an allele whose trait shows up only
when the dominant allele is not present
 2. Separation of paired alleles
 Alleles are separated during gamete formation with the result that
each gamete carries only a single allele from the original pair
 3. Factors that are passed from one generation to the next
 4. Mendel cut away the male parts of one flower; then dusted it
with pollen from another flower
 5. Only ¼ of the possible gamete formations did not have a
dominant allele
 6. True-breeding pea plants have two identical alleles for one
gene, so in a genetic cross each parent contributed only one
form of a gene, making inheritance patterns more detectable
Mitosis results in the production
of two genetically identical
diploid cells. Meiosis produces
four genetically different
haploid cells.
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Comparing Mitosis and Meiosis
 Mitosis



Cells produced by mitosis have the same
number of chromosomes and alleles as the
original cell.
Mitosis allows an organism to grow and
replace cells.
Some organisms reproduce asexually by
mitosis.
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Comparing Mitosis and Meiosis
 Meiosis
Cells produced by meiosis have half the
number of chromosomes as the parent
cell.
 These cells are genetically different
 Meiosis is how sexually reproducing
organisms produce gametes.

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Parent
F1
F2
•Parent (P) generation = the first 2 individuals that mate in a
genetic cross
•F1 generation = first offspring from a cross between the (P)
generation
•F2 generation = offspring from crosses among individuals of the
F1 generation
 Genotype: A set of alleles that determines the
expression of a particular characteristic or trait (MMRr)
 Phenotype: physical characteristics (tall, green, etc.)
 Homozygous dominant: MM (two capital); dominant
trait expressed
 Heterozygous dominant: Mm (one capital, one
lowercase); dominant trait expressed
 Homozygous recessive: mm (two lowercase); recessive
trait expressed
 Eye color:
 Brown = dominant BB, Bb
 Blue, green, hazel = recessive bb
 Facial Features
 Freckles = dominant FF, Ff
 No freckles = recessive ff
 Dimples = dominant DD, Dd
 No dimples = dd
 Hair Color
 Dark hair = dominant HH, Hh
 Blonde hair = recessive hh
What does the symbol 2N represent?
What is our haploid number?
How many cells does meiosis result in?
Are these cells haploid or diploid cells?
What are the female haploid cells created during
meiosis?
6. What are the male haploid cells created during
meiosis?
7. During what specific phase of meiosis do tetrads
form?
8. What process during meiosis creates genetic variance
across generations?
1.
2.
3.
4.
5.
Genotype
Punnett Squares
 Diagram that shows the result
of a genetic cross
 Simple Punnett Square – one
trait examined
 Two generations shown
 Letters represent alleles
(genotype)
Parent
Generation
F1
Generation
 Breakdown of cross results
 Let’s cross two of the offspring from the F1 generation
genes that segregate
independently do not influence each
other’s inheritance
 Dihybrid Cross
• Cross between 2 heterozygous
dominant individuals crossed
• 2 traits crossed
• 16 box square
 The phenotype ratio predicted
for dihybrid cross is 9:3:3:1.
 9 offspring that exhibit both
dominant traits
 3 offspring that have one dominant,
one recessive trait
 3 offspring that have one dominant,
one recessive (opposite of last)
 1 offspring that is recessive for both
traits
Rr(Yy)
take the first R and multiply it out with the two Y’s
Ry
RY
Rr(Yy)
rY
ry
now focus on the next r and multiply it out by the two Y’s
 Set up the Punnett
Square with the
allele combinations
you got from the
FOIL method.
 Begin to cross the
parents to create the
new generations
 If one parent is homozygous recessive, and other
parent is homozygous dominant = F1 will be
heterozygous dominant
 If crossing two heterozygous dominant,
ratio will always be
 Break down genotype into
Trait Lab
 Widows Peak
 Bent Little Finger
 Hitchhiker’s thumb
 Tongue Rolling
 Tongue Folding
 Dimple Chin
 Long Eyelashes
 Free Ear Lobe
 Three traits examined
 64 box Punnett Square
Ratio of a
cross
27 : 9 : 9 : 9 : 3 : 3 : 3 : 1
 27
= all dominant
 9 = 2 dominant, 1 recessive
 3 = 1 dominant, 2 recessive
 1 = recessive
heterozygous phenotype is
somewhere in between the
two homozygous
phenotypes
•Example: red, white, and
pink flowers
 Codominance: situation in which both alleles
contribute to the phenotype
 Example: black and white chicken feathers 
“erminette”
 Appear separately, not as a “blend”
 Multiple alleles: genes that have more than
two alleles
 Example: blood type
 DOES NOT MEAN the individual can have more
than two alleles (hh, AA, Aa)
 Type A – has the A antigen on red cells
 Type B – has the B antigen on red cells
 Type AB – has both A and B antigens on red cells
 Type O – has neither A nor B antigens on red cells
 Rh (rhesus) is a protein
found on the surface of
red blood cells. is a single
gene with two alleles, “+”
and “-”
 Rh+ is dominant
 Rh- is recessive
 Named after the
monkeys used to study
the blood types
Caucasians
African
American
Hispanic
Asian
O+
37%
47%
53%
39%
O-
8%
4%
4%
1%
A+
33%
24%
29%
27%
A-
7%
2%
2%
0.5%
B+
9%
18%
9%
25%
B-
2%
1%
1%
0.4%
AB +
3%
4%
2%
7%
AB -
1%
0.3%
0.2%
0.1%
 To prevent this condition, she can take a drug that keeps
her from developing the antibodies to the Rh+ blood
group
 Range of eye color




caused by
particular
combinations of
alleles
Locus (plural loci)
is the specific
location of a gene
Eye color is found
on different loci
Alleles  B/b and
G/g
Come together and
give eye color
 Two eye colors
 Heterochromia
 Considered abnormal
and may be pathological
 Born with two different
colored eyes 
congenital heterochromia
 Height
 # of Fingers (d)
 Poison ivy susceptibility
(R)
Karyotype
 A set of photographs
of chromosomes
grouped in
homologous pairs
 Used to analyze
chromosomes
 46 (diploid)
chromosomes in
humans
 Sperm (haploid) = 23
chromosomes
 Egg (haploid) = 23
chromosomes
Pairs 1 through 22 = Autosomes
23rd pair = Sex
Chromosomes
 Sex linkage is the phenotypic expression of an allele
related to the chromosomal sex of the individual.
 Different from autosomal trait inheritance, where
both sexes have the same probability of
inheritance.
 In mammals:
 female is the homogametic sex, with two X
chromosomes
 male is heterogametic, with one X and one Y
chromosome
 A male or female child of a heterozygous mother
affected with an X-Linked dominant trait has a
chance of inheriting the mutation
children of an affected father will be
inherit the affected X chromosome (daughters possess
their fathers' X-chromosome).
children of an affected father will be
affected (sons do not inherit their fathers' Xchromosome).
 Because the Y-chromosome is small and does not
contain many genes, few traits are Y-linked, and Ylinked diseases are rare
 Since the only humans who have a Y chromosome are
males, Y-linked traits are passed only from father
to son
 X-Linked Genes
 Linked to the
(no kidding?)
 No male will get an X-linked trait from his father, only from his
mom
 Mom’s have 2 X’s to donate  50% probability of inheritance
for males and females
 Y-Linked Genes
 Linked to the
Small chromosome, not many traits
 Passed from father to son
Examples of X and Y Linked
Traits
 X-linked
 Hemophilia
 Rickets (dominant)  bone deformity
 Rett Syndrome  growth failure, small hands, feet, head
 Male pattern baldness (recessive)
 Colorblindness
 Y-linked
 Retinitis pigmentosa  damaged retina, vision
impairment
 Azoospermia  immobility of the sperm or inability to
produce sperm
•RECESSIVE TRAIT
Mendel’s 3 Laws
 Law of Dominance – he figured out that some traits
are dominant, some are recessive
 Law of Independent Assortment – alleles separate
independently during meiosis
 Homologous chromosomes lining up in the middle of
the cell varies from cell to cell because there’s NO SET
WAY they have to arrange.
 Law of Segregation – the two copies of genes present
in an individual’s cells segregate during the formation
of gametes (mom gives one set, dad gives another).
Polygenic Traits
 Polygenic traits are
controlled by two or more
than two genes at different
loci on
different chromosomes.
 These genes are described as
polygenes  “poly” means
many!
 Examples are human height,
weight, skin color
 Eye color is a polygenic trait
with multiple alleles.
Probability and Mendelian Genetics
 Probability is the likelihood that an event will occur
 When using Punnett Squares, you are showing
the probability (likelihood) of certain traits
appearing in offspring with 2 parents are
crossed
 Punnett Squares don’t show actual results –
they show possible results.

If enough is known about the family genetics, then
Punnett Squares are much more accurate.
methemoglobinemia
 Chart that shows the relationships within a family
 Family tree!
 Used to determine genotypes of family members for certain
traits
Interpreting a Pedigree Chart
1. Determine if the pedigree chart shows an
autosomal or X-linked disease:
 If mostly males in the pedigree are
affected the disorder is X-linked because males
have only one X and therefore cannot be a
carrier  no dominant allele to cover up the
recessive
 If it is a 50/50 ratio between men and
women the disorder is autosomal.
Is it Autosomal or X-linked?
Autosomal – roughly
50/50 male and female
X-linked Trait – males seen with
it
2. Determine whether the disorder is
dominant or recessive:
 If the disorder is dominant, one of the
parents must have the disorder AND the
disorder will be seen across all the generations
 If the disorder is recessive, neither
parent has to have the disorder because
they can be heterozygous AND the trait will
NOT be seen across all generations
 Meiosis: 1 diploid cell  4 haploid cells
1 cell with 46 chromosomes
2 cells with 46 chromosomes
4 cells with 23 chromosomes
 2 DIVISIONS! (meiosis I and meiosis II)
 Mitosis: 1 diploid cell  2 diploid cells
 1 cell with 46 chromosomes  2 cells with 46
chromosomes
Male and Female Gametes
23