Unit 4a - Genetics
Chapter 10 – Sexual
Reproduction and
Genetics
10.1 - Meiosis
Essential Questions:
 How does the reduction in
chromosome number happen during
meiosis?
 What are the stages of meiosis?
 What is the importance of meiosis in
providing genetic variation?
DNA, Chromosomes, & Genes
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Each human cell has a set of 46
homologous chromosomes.
Each chromosome is a single strand
of DNA that is tightly wound.
One chromosome comes from each
parent (23 sets, 46 individual).
Genes for specific traits are located
on the chromosomes.
The traits are represented by alleles.
Homologous Chromosomes
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A pair of
chromosomes is
made up of two
homologues.
The pair is called
“homologous
chromosomes.”
Haploid and Diploid Numbers
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Diploid Number (2n) – the
number of chromosomes
found in a somatic cell (body
cell).
Haploid Number (n) – the
number of chromosomes
found in a gamete (sex cell).
Haploid and Diploid Cells
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Gametes are haploid
(n) sex cells (not
somatic). There is a
male cell – sperm, and
a female cell – egg.
When the two cells
meet, they fertilize
and create a diploid
(2n) zygote.
How Many Chromosomes Do
We Have?
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Each species has its own number of chromosomes
The complexity of the organism does NOT correspond
to the number of chromosomes.
Organism
Mosquito
Onion
Corn
Frog
Sunflower
Cat
Human
Plum
Dog
Sugar Cane
Goldfish
Gamete (Sex Cell)
Body Cell
Haploid Number (n)
3
8
10
13
17
19
23
24
39
40
47
Diploid Number (2n)
6
16
20
26
34
38
46
48
78
80
94
Meiosis
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MEIOSIS is a special type of division which separates one
copy of each homologous chromosome into each new
"gamete."
During meiosis, the number of chromosomes is reduced by
half. It becomes haploid (n).
The number is returned to the full amount - diploid (2n).
when the two gametes fuse during fertilization.
Cells that become gametes are also called germ cells.
Cells made by meiosis are found in the ovaries or testes.
Sexual reproduction occurs only in eukaryotes.
Comparing Meiosis and Mitosis
Meiosis
Property
Mitosis
Meiosis
DNA Replication
Interphase
Interphase
Number of Divisions
1
2
Synapsis of Homologous
Chromosomes
No
Happens during crossing
over between non-sister
chromatids in Prophase I
Number of daughter cells 2, identical and
and genetic make up
diploid (2n)
4, genetically different
and haploid (n)
Role in the body
Produces gametes and
assures genetic diversity
in sexual reproduction.
Growth and repair of
the body.
Phases of Meiosis
A)Parent Cell
B)Meiosis I
C)Meiosis II
Meiosis Creates Variety!
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During Prophase I, the homologous
chromosomes come together to form tetrads.
Bits of chromosomes can “cross over” and
switch places.
10.2 – Mendelian Genetics
Essential Questions
What is the significance of Gregor Mendel’s
experiments to the study of genetics?
What is the law of segregation and the law
of independent assortment?
What are the possible offspring from a cross
using a Punnett square?
Gregor Mendel (1822-1884)
He was a monk and lived in a
monastery.
Mendel performed the first
“mathematical/ scientific” study of
how traits are inherited.
He used pea plants with 7 select
genotypes (the genetic make up
of an individual) & phenotypes
(the physical appearance) to study
cross-breeding of the plants.
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More Key Terms to Know!
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Allele – the different varieties of a gene. Mendel called these
“factors.” Expressed as a letter (T for tall, R for round, etc.).
Dominant Allele – if present in a pair, will instruct for a certain trait.
Shown with a capital letter (R).
Recessive Allele – will only show up as a phenotype if paired with
another recessive allele. If paired with a dominant allele, it will “hide”
and possibly surface in later generations. Shown with a lowercase
letter (r).
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Homozygous – same or purebred. Ex: RR or rr
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Heterozygous – different or hybrid. Ex: Rr
4 Rules of Mendelian Genetics
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The Rule of Unit Factors – Two factors (dominant and
recessive gene pairs called “alleles”) control traits. Half
of each pair is “donated” by the parent.
The Rule of Dominance – For every allele, one part is
dominant and one part is recessive. Dominant traits are
shown as capital letters (TT). Recessive traits are shown
as lower case (tt). The dominant trait is written first.
The Law of Segregation - states that allele pairs
separate (or segregate) during gamete formation, and
randomly unite at fertilization.
The Law of Independent Assortment - states that
traits are transmitted to offspring independently of one
another (wrinkled seed vs. smooth and tall plant vs.
short).
Traits that Mendel Studied
Trait
Dominant
Recessive
Seed Shape (R)
Round
Wrinkled
Seed Color (Y)
Yellow
Green
Flower Color (P)
Purple
White
Flower Position (A)
Axial (side)
Terminal (tips)
Pod Color (G)
Green
Yellow
Pod Shape (I)
Inflated
Constricted
Plant Height (T)
Tall
Short
Drawing a Punnett Square
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1. Determine the genotypes of the parent organisms
and write down your "cross" (mating).
2. Draw a p-square.
3. Divide the letters of the genotype for each parent
& put them outside the p-square.
4. Determine the possible genotypes of the offspring
by filling in the p-square.
5. Summarize results (genotypes & phenotypes of
offspring).
Drawing a Punnett Square
1) Pure Breeding Tall x Pure Breeding Short = TT x tt
2) Draw your square. 3) Add parent genotype & 4) Determine offspring…
T
T
t
Tt
Tt
t
Tt
Tt
5) Analyze the outcome. Offspring = 100% Tt (heterozygous or “hybrid”).
They will all be tall, but carry the short gene.
Monohybrid Cross
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1st Generation or P
Pure (homozygous) parents of different
traits reproduce.
100% of offspring (F1) are hybrids with both
traits, but display the dominant tall trait:
T (tall)
T (tall)
t (short)
Tt
Tt
t (short)
Tt
Tt
Monohybrid Cross
Monohybrid Cross
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2nd Generation or F1
The heterozygous offspring of the first cross
reproduce.
Results (F2) are 1:2:1 genotype; 3:1 phenotype.
T (tall)
t (short)
T (tall)
TT (pure tall)
Tt (hybrid tall)
t (short)
Tt (hybrid tall)
tt (pure short)
Monohybrid Cross
Dihybrid Cross with 2 Traits
9:3:3:1
Phenotype
Punnett Squares - FYI
In the early 1900’s Dr.
Reginald Punnett
developed the now
famous square to
predict the statistical
outcome of a cross.
10.3 – Gene Linkage & Polyploidy
Essential Questions:
 How does the process of meiosis
produce genetic recombination?
 How can gene linkage be used to create
chromosome maps?
 Why is polyploidy important to the field
of agriculture?
Genetic Recombination
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The new combination of genes produced by
crossing over and independent assortment is
called genetic recombination.
The possible combinations of gene pairs is 2n
where n is the number of chromosomes.
Peas have 7 pairs of chromosomes = 27 = 128
combinations. When you consider male plant and
female, then you have 128 x 128 = 16,384 after
fertilization.
Humans have 223 x 223 = +70 trillion!
Gene Linkage
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Genes that are close together on the same
chromosome are said to be linked.
They usually (but don’t always) travel
together during gamete formation.
Linked genes do no segregate
independently and break Mendel’s rule.
Crossing over occurs more between genes
that are far apart.
Polyploidy
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Polyploidy is the
occurrence of one or more
extra sets of chromosomes.
In humans, polyploidy is
lethal.
1 in 3 flowering plant
species are polyploid.
Many polyploid plants have
desirable traits like
increased vigor and size.
Non Disjunction
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Nondisjunction - The normal separation of
chromosomes in is termed disjunction. When the
separation is not normal, it is called nondisjunction. This
results in production of gametes with more or less of the
usual amount of chromosomes.
 trisomy (an extra chromosome)
 monosomy (missing a chromosome)
 triploidy (an extra SET of chromosomes = 3n)
 tetraploidy = 4n.
*#3 & #4 are also called “polyploidy”
Non Disjunction & Polyploidy
Genetic Diseases
Non-disjunction and polyploidy can cause several medical
conditions in humans:
Down Syndrome - trisomy of chromosome 21
Patau Syndrome - trisomy of chromosome 13
Edward Syndrome - trisomy of chromosome 18
Klinefelter Syndrome - an extra X chromosome in males
Turner Syndrome - only one X chromosome present in
females
XYY Syndrome - an extra Y chromosome in males
Triple X Syndrome - an extra X chromosome in females
Chromosomal Disorders
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This link
describes
polyploid
disorders
and deletion
syndromes...