Chapter 10
Sexual Reproduction
and Genetics
10.1 Meiosis
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Chromosomes and Chromosome
Number
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Human body cells have 46 chromosomes
DNA on the chromosomes are arranged in
sections that code for a trait; these sections
are genes
Humans have approximately 23,000 genes
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Chromosomes and Chromosome
Number
 Of the 46 human chromosomes each
parent contributes 23 chromosomes
 These chromosomes are paired
Homologous chromosomes—one of two
paired chromosomes, one from each
parent
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Homologous Chromosomes
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Same centromere
position
Same length
Homologous
chromosomes contain
the same genes;
although they may
contain different
versions of the gene
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Chromosomes and Chromosome
Number
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Haploid cells contain only one of the
homologous pair of chromosomes (half the
number of chromosomes)
Diploid cell contain the chromosomes in
pairs (di=two)
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Chromosomes and Chromosome
Number
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Gametes (sex cells) contain the haploid
number of chromosomes (n)
Body cells contain the diploid number of
chromosomes (2n)
Human gametes (sperm and egg) have 23
chromosomes and body cells have 46
chromosomes
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Meiosis
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Gametes are formed
during the process of
meiosis
Meiosis reduces
diploid cells to haploid
cells
Fertilization restores
the diploid number
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Meiosis
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Meiosis involves two consecutive sets of cell
divisions
Meiosis only occurs in the reproductive structures
of organisms who reproduce sexually:
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Most animals
Most plants
Most fungi
Most protists
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Meiosis I and Meiosis II
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Meiosis I is the reduction division; cells
start out diploid and end up haploid
In Meiosis II sister chromatids are
separated (much like mitosis)
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Meiosis I
 Interphase
 Chromosomes replicate.
 Chromatin condenses.
Interphase
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Meiosis I
 Prophase I
 Pairing of homologous
chromosomes occurs.
 Each chromosome consists of two
chromatids.
Prophase I
 The nuclear envelope breaks down.
 Spindles form.
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Meiosis I
 Prophase I
 Crossing over produces exchange of genetic
information.
 Crossing over—chromosomal segments are
exchanged between a pair of homologous
chromosomes.
Tetrads are
groups of
four sister
chromatids
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Meiosis I
 Metaphase I
 Chromosome
centromeres attach
to spindle fibers.
Metaphase I
 Homologous chromosomes line up at the
equator in tetrads
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Meiosis I
 Anaphase I
 Homologous
chromosomes
separate and move
to opposite poles of the cell.
Anaphase I
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Meiosis I
 Telophase I
 The spindles
break down.
Telophase I
 Chromosomes uncoil and form two nuclei.
 The cell divides.
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Meiosis II
 Prophase II
 A second set of
Prophase II
phases begins
as the spindle apparatus forms and the
chromosomes condense.
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Meiosis II
 Metaphase II
 A haploid number
of chromosomes
line up at the equator.
Metaphase II
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Meiosis II
 Anaphase II
 The sister
Anaphase II
chromatids are
pulled apart at the centromere by spindle
fibers and move toward the opposite poles
of the cell.
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Meiosis II
 Telophase II
 The chromosomes
Telophase II
reach the poles, and
the nuclear membrane and nuclei reform.
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Meiosis II
 Cytokinesis results in
four haploid cells,
each with n number
of chromosomes.
Cytokinesis
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Meiosis
 Meiosis consists of two sets of divisions
 Produces four haploid daughter cells that
are not identical
 Results in genetic variation
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Meiosis
 Depending on how the
chromosomes line up at the
equator, four gametes with
four different combinations
of chromosomes can result.
 Genetic variation also is
produced during crossing
over and during
fertilization, when gametes
randomly combine.
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Meiosis and Variation
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Number of possible genetic variations in
the gametes equals:
2n where n is the haploid number
In humans number of possible genetic
combinations in gametes is 223
Add the genetic combinations that exist
when crossing over exists (at 3 per
meiosis) and get (223)3
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Meiosis and Variation, cont
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The possibility that (223)3 variations exists
for each gamete
When fertilization occurs this number must
be doubled 2 x (223)3
You are unique; no one else exists or ever
has existed that is just like you (unless you
have an identical twin).
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Advantages of Asexual
Reproduction
 The organism inherits all of its
chromosomes from a single parent.
 The new individual is genetically identical
to its parent.
 Usually occurs more rapidly than sexual
reproduction
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Advantages of Sexual Reproduction
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Beneficial genes multiply faster over time.
The organisms inherits genes from two
parents and is not genetically identical to
either parent.
Ensures genetic variation
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Chapter 10
Sexual Reproduction
and Genetics
10.2 Mendelian Genetics
27
Gregor Mendel
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Lived in Europe in
what is now Czech
Republic near the
Austrian border.
Father of Genetics
Monk, entered
monastery 1843
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Gregor Mendel
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Failed teacher’s exam
When to U of Vienna
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Studied with physicist Doppler- science through
experiment, applied math to science
Studied with botanist Unger- interest in causes of variation
in plants
Passed teacher’s exam and taught at monastery’s
school; also responsible for school’s garden
Published 1866, mathematics and plant breeding
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Mendel Studied Peas
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Available in many varieties
Self pollinating (can manipulate pollination)
“either-or” inherited traits
Had “true breeder” for parental generation
(P) due to flower structure
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Mendel Studied Peas
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The petals enclose the
stamen (with pollen) so
that cross pollination does
not occur
Cross pollination is easily
accomplished by peeling
back the petals and
moving pollen with a paint
brush
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Inheritance of Traits
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Inheritance of Traits
 The offspring of
this P cross are
called the first filial
(F1) generation.
 The second filial
(F2) generation is
the offspring from
the F1 cross.
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Pea Traits Studied by Mendel
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Inheritance of Traits
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Mendel studied thousands of pea plants for
the seven traits.
He concluded that:
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Genes are in pairs
Different versions of genes (alleles) account for
variation in inherited characteristics
Alleles can be dominant or recessive
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Dominant and Recessive
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Alleles can be dominant or recessive.
An allele is dominant if it appears in the F1
generation when true breeder parents are
crossed.
An allele is recessive if it is masked in the
F1 generation.
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37
Symbols
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To help make genetics easier symbols are used
Capital letters are used for dominant alleles
Lower case letters are used for recessive alleles
The letter to use is based on the dominant trait
Example: purple is dominant to white, P would be
the dominant allele and p the recessive allele
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Homozygous and Heterozygous
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Dominant traits can be homozygous or
heterozygous
Homo= same; the alleles would be the
same, PP
Hetero=different; the alleles would be
different, Pp
For the recessive trait to be expressed both
alleles would be recessive, pp
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Genotype and Phenotype
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Genotype is the organism's gene pairs: PP,
Pp or pp
Phenotype is the outward physical
appearance or expression of the genotype:
purple or white
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Genotype and Phenotype
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If the phenotype
displays the recessive
trait (white) then you
know the genotype; pp
Genotype is pp
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If the phenotype
displays the dominant
trait (purple) then the
genotype could be
homozygous dominant
(PP) or heterozygous
(Pp)
Genotype
is PP or Pp
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Punnett Squares
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Mathematical device for
predicting the results of
genetic cross
Male gametes are written
across the top
Female gametes are
written along the side
Genetic possibilities of the
offspring are in the boxes
Expect 3:1 phenotypic
ratio
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Monohybrid Cross
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Mono= one
One trait is studied at
a time
This one is seed color
Monohybrid crosses
provided evidence for
the Law of
Segregation
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Mendel’s Law of Segregation
Two alleles for each trait separate during
meiosis
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Dihybrid Cross
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Di= two
Two traits are studied
at a time
This one is seed color
(yellow or green) and
seed shape (wrinkled
or round)
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Dihybrid Cross
 Four types of alleles from
the male gametes and four
types of alleles from the
female gametes can be
produced.
 The resulting phenotypic
ratio is 9:3:3:1 which gave
evidence for the Law of
Independent Assortment
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Mendel’s Law of Independent
Assortment
 Random distribution of alleles occurs during
gamete formation
 Genes on separate chromosomes sort
independently during meiosis.
 Each allele combination is equally likely to
occur.
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Mendel’s Law of Independent
Assortment
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Probability
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Genetic crosses predict what to expect in
the phenotypes and genotypes of the
offspring.
Observed results are what you actually see
with the organisms.
The larger the number of offspring the
closer the expected and observed results
usually are.
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Chapter 10
Sexual Reproduction
and Genetics
10.3 Gene Linkage and Polyploidy
50
Gene Linkage
 The linkage of genes on a chromosome results in
an exception to Mendel’s law of independent
assortment because linked genes usually do not
segregate independently.
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Polyploidy
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Polyploidy is the
occurrence of one or
more extra sets of all
chromosomes in an
organism.
Approximately 30% of
flowers are polyploidy
Strawberries are
octoploidy (8n)
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Polyploidy
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Horticultural important
plants are forced to
polyploidy to increase
the size and flavor of
flowers and fruits and
overall vigor of the
plants.
Polyploidy is
uncommon in animals.
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