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Chapter10
Mendel and Meiosis
Section10.1
Mendel’s Laws of Heredity
• Relate Mendel’s two laws to the
results he obtained in his
experiments with garden peas.
• Predict the possible offspring of a
genetic cross by using a Punnett
square.
The Great Mendel
• Gregor Mendel
– Austrian monk; devoted his life to the
studying garden pea plants
– Became the father of Genetics
• Heredity- the passing of characteristics from
parents to offspring
– Traits
• Sexual reproduction- male and female gametes
fuse to form a zygote
– Fertilization
– Genetic material come from both parents
7 Traits of Mendel’s Experiments
• Pollination- transfer of pollen (male
gametes) to the female
reproductive organ in plants
• Cross-pollination- using pollen from
a plant with different traits
Mendel’s Monohybrid Crosses
• Wanted to produce specific hybrids of plant
with different traits
– Tall X Short
– Purple Flowers X White Flowers
– Green Seeds X Yellow Seeds
• Started with P1
– Parents
• Offspring were call F1
– Filial (son/daughter)
– 1st generation
• F1 X F1 = F2
– 2nd generation
3
1
Controlling Heredity
• Each organism has two factors that
control each of its traits.
– these factors are genes and that they
are located on chromosomes.
– Genes exist in alternative forms. We
call these different gene forms alleles
(等位基因).
Dominate vs. Recessive
• For each character, an organism inherits
two alleles, one from each parent.
• If the two alleles differ, then the dominant
allele（显性等位基因）, determines the
organism’s appearance .
– Always a capital letter (A, B, C, etc..)
• The recessive allele（隐性等位基因）,
has no noticeable effect on the
organism’s appearance.
– Always a lowercase letter (a, b, c, etc…)
TT- homozygous
dominant
tt- homozygous
recessive
Alleles are RR, YY, PP, TT, etc…
Alleles are rr, yy, pp, tt, etc…
Mendel’s Monohybrid Crosses
• The law of segregation (分离定律，
P 257）
– Every individual has two alleles of each gene
and when gametes are produced, each
gamete receives one of these alleles.
– During fertilization, these gametes randomly
pair to produce four combinations of alleles.
Tt- heterozygous
Punnett Squares
• Reginald Punnett; English biologist
• Monohybrid cross- following the crossing
of parents while focusing on 1 trait
• A cross with 1 homozygous dominate
and 1 homozygous produces only
heterozygous offspring
• A cross between two heterozygous
produce ¼ homo dominate, ¼ homo
recessive, and ½ heterozygous
P 278
Mendel’s Dihybrid Crosses
• Dihybrid cross-
crossing
parents with two
different traits
and seeing the
percent of
offspring that
get each of the
two traits
• Crossing two
heterozygous
always produces
9/16 dominate for
both traits, 3/16
recessive for one
trait, 3/16
recessive for the
other trait, and
1/16 recessive for
both traits
Mendel’s Dihybrid Crosses
• The law of independent assortment
（自由组合定律，P 260）
– Genes for different traits are inherited
independently of each other
– Shown in meiosis
Nondisjunction （染色体不分离）
• The failure of homologous
chromosomes to separate properly
during meiosis is called
nondisjunction （p. 271）
Fig. 15-13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n–1
n–1
n+1
n–1
n
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
n
Nondisjunction
• Trisomy 21; Down Syndrome
Nondisjunction
• Polyploidy (多倍体)
– Organisms with more than the usual
number of chromosome sets are
called polyploids
– Normally lethal in animals
– Produces interesting results in plants
Hexaploid wheat
六倍体小麦
Triploid watermelon
三倍体西瓜
Section10.2 Meiosis
• Analyze how meiosis maintains a
constant number of chromosomes
within a species.
• Infer how meiosis leads to variation
in a species.
• Relate Mendel’s laws of heredity to
the events of meiosis.
Meiosis: It Made ME
• Normal cells have 46 chromosomes; a
set of 23 from each parent
• Diploid cells
– Two of each chromosome (2n)
• Normal cells cannot be used in
reproduction; DNA will keep doubling
• Haploid cells
– One of each chromosome (n)
– Also known as gametes
• Male gamete is a sperm
• Female gamete is an egg
Genes, Chromosomes, and
Numbers
Homologous chromosomes
– The two chromosomes of each pair in
a diploid cell are called homologous
chromosomes （同源染色体）
The Phases of Meiosis
（减数分裂）
• Why meiosis?
BioFlix: Meiosis
Sexual reproduction:
fusion of two haploid cells to make one
diploid cell; (zygote)
The Phases of Meiosis
• Division in Meiosis I occurs in four
phases:
–
–
–
–
Prophase I
Metaphase I
Anaphase I
Telophase I and cytokinesis
• Meiosis II is very similar to
mitosis
• Prophase II, Metaphase II, Anaphase II,
Telophase II, cytokinesis
Fig. 13-8b
Prophase I
Prophase I
• DNA coils into
chromosomes
• Homologous
chromosomes
match up
• Nuclear
envelope
breaks apart
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore
Prophase I
Prophase I
Crossing over:
-Homologous
chromosomes
swap sections of
genes with nonsister
chromatids
(2 or 3 times on
each homologous
chromosome)
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore
Fig. 13-8b
Prophase I
Metaphase I
• Spindle fibers
attach to the
centromeres
and move
chromosomes
to middle of the
cell
• Homologous
chromosomes
line up side by
side
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore
Fig. 13-8c
Telophase I and
Cytokinesis
Anaphase I
Anaphase I
• Homologous
chromosomes
separate and
move to
opposite sides
of the cell
• Centromeres
are still intact
Sister chromatids
remain attached
Homologous
chromosomes
separate
Cleavage
furrow
Fig. 13-8c
Telophase I and
Cytokinesis
Anaphase I
Telophase I and
cytokinesis
• Reverse of
Prophase I and
cells separate
• Short break
before next
diffusion
Sister chromatids
remain attached
Homologous
chromosomes
separate
Cleavage
furrow
Fig. 13-8d
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
Sister chromatids
separate
Haploid daughter cells
forming
• Meiosis II has the same phases as Mitosis
• Sister chromatids will split and the result if 4 haploid cells,
each with one copy of the 23 chromosomes
Meiosis Provides for
Genetic Variation
Fig. 15-13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n–1
n–1
n+1
n–1
n
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
n
• Genetic recombination (P270)
Nondisjunction （染色体不分离）
• The failure of homologous
chromosomes to separate properly
during meiosis is called
nondisjunction （p. 271）
Nondisjunction
• Trisomy 21; Down Syndrome
Nondisjunction
• Polyploidy (多倍体)
– Organisms with more than the usual
number of chromosome sets are
called polyploids
– Normally lethal in animals
– Produces interesting results in plants
Hexaploid wheat
六倍体小麦
Triploid watermelon
三倍体西瓜