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Mendel and Meiosis
Unit 4
Chapter 10
Gregor Mendel
 Austrian monk
 Studied patterns of
heredity (passing on of
characteristics from
parent to offspring)
 Used the common
garden pea in
experiments
Why did Mendel use peas?
 Sexually reproducing:
able to isolate both
male and female
gametes
 Easy to identify traits
(characteristics that are
inherited)
 Short life cycle: able to
be grown quickly
Hybrid
 Any offspring of parents with different traits (ex: tall
plant x short plant)
 Monohybrid cross: cross-pollination (breeding)
between two parents with only one variation
difference (ex: tall plant x short plant)
 Dihybrid cross: cross-pollination (breeding) between
two parents with two variation differences (ex: tall,
green plant x short, yellow plant)
Pea cross-pollination
experiments
PARENT GENERATION (P1)
Tall true breed x short true breed
FILIAL GENERATION (F1)
All tall hybrids
FILIAL GENERATION (F1)
75% tall hybrids, 25% short hybrids
Phenotypes from P1 to F2
Dihybrid Cross
round yellow x wrinkled green
P1
Round yellow
Wrinkled green
All round
yellow
F1
F2
9
Round yellow
3
Round green
3
Wrinkled yellow
1
Wrinkled green
Mendel’s conclusions
 There must be two variations for every trait,
where each variation is called an allele.
 Each offspring inherits only one allele from
each parent.
 The alleles are either dominant or
recessive.
 To show the recessive trait, two recessive
alleles must be inherited.
Dominant and recessive traits
Seed Seed
shape color
Flower
color
Flower
position
Pod
color
Pod
shape
Plant
height
green
inflated
tall
yellow
constricted
short
Dominant
trait
round
yellow
purple
axial
(side)
wrinkled
green
white
terminal
(tips)
Recessive
trait
Homozygous vs. Heterozygous
 Homozygous: inherits two similar alleles
from the parents for a particular gene


Ex: tall allele and tall allele, written as TT
Ex: short allele and short allele written as tt
 Heterozygous: inherits two different alleles
from the parents for a particular gene

Ex: tall allele and short allele, written as Tt
Using a Punnett square
 AA x aa  100% Aa
 Each of the four squares represents 25%
chance of inheritance for one offspring.
a
A
Aa
A
Aa
a
Aa
Aa
Punnett square video
Click on image to start video.
Phenotype vs. Genotype
 Phenotype: physical appearance of the trait

Ex: purple flowers
 Genotype: homozygous or heterozygous
inheritance

Ex: PP, Pp, pp
Dihybrid crosses
Punnett Square of Dihybrid Cross
Gametes from RrYy parent
Ry
RY
RRYY
RRYy
rY
RrYY
ry
RrYy
 A Punnett square
for a dihybrid
cross will need to
be four boxes on
each side for a
total of 16 boxes.
Gametes from RrYy parent
RY
RRYy
RRYy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Ry
rY
ry
Dihybrid crosses
Punnett Square of Dihybrid Cross
Gametes from RrYy parent
Ry
RY
RRYY
RRYy
rY
RrYY
ry
RrYy
RY
F1 cross: RrYy ´ RrYy
Gametes from RrYy parent
RRYy
RRYy
RrYy
Rryy
round
yellow
Ry
RrYY
RrYy
rrYY
rrYy
round
green
rY
RrYy
ry
Rryy
rrYy
rryy
wrinkled
yellow
wrinkled
green
Cells and chromosomes
 A cell with two of each kind of chromosome is
called a diploid cell and has diploid, or 2n,
number of chromosomes.
 Organisms produce gametes that contain one
of each kind of chromosome
Homologous chromosomes
• The two
chromosomes of
each pair in a
diploid cell are
called
homologous
chromosomes.
Homologous chromosomes
• On homologous
chromosomes, the
same types of genes
are arranged in the
same order.
• Because there are
different possible alleles
for the same gene, the
two chromosomes in a
homologous pair are
not always identical to
each other.
Making haploid cells
 Meiosis is the process of producing haploid
gametes with a ½ the amount of DNA as the
parent cell.
 A cell with one of each kind of chromosome is
called a haploid cell and has a haploid, or n,
number of chromosomes.
 Meiosis enables sexual reproduction to occur.
Sexual reproduction
Haploid gametes
(n=23)
Sperm Cell
Meiosis
Meiosis
Egg Cell
Fertilization
Diploid zygote
(2n=46)
Mitosis and
Development
Multicellular
diploid adults
(2n=46)
Meiosis process
Click on image to play video.
Interphase
• During interphase, the
cell replicates its
chromosomes.
• After replication, each
chromosome consists
of two identical sister
chromatids, held
together by a
centromere.
Prophase I
 The chromosomes
coil up and a
spindle forms.
 Homologous
chromosomes line
up with each other
gene by gene along
their length, to form
a four-part structure
called a tetrad.
Prophase I – crossing over
 Chromatids are
wrapped so tightly the
chromosomes can
actually break and
exchange genetic
material in a process
known as crossing
over.
 Crossing over results in
new combinations of
alleles on a
chromosome.
Metaphase I
• The centromere of
each chromosome
attaches to a spindle
fiber.
• The spindle fibers
pull the tetrads into
the middle, or
equator, of the
spindle.
Anaphase I
• Homologous
chromosomes
separate and move
to opposite ends of
the cell.
• This critical step
ensures that each
new cell will receive
only one
chromosome from
each homologous
pair.
Telophase I
• The spindle is broken
down, the
chromosomes uncoil,
and the cytoplasm
divides to yield two
new cells.
• Each cell has half the
DNA as the original cell
because it has only
one chromosome from
each homologous pair.
Prophase II
 A spindle forms in
each of the two new
cells and the spindle
fibers attach to the
chromosomes.
Metaphase II.
 The chromosomes,
still made up of
sister chromatids,
are pulled to the
center of the cell
and line up
randomly at the
equator.
Anaphase II
 The centromere
of each
chromosome
splits.
 The sister
chromatids to
separate and
move to opposite
poles.
Telophase II
• Finally nuclei reform,
the spindles
breakdown, and the
cytoplasm divides.
• Four haploid cells
have been formed
from one diploid
cell
Why meiosis is important
Forms gametes for
sexual reproduction
2. Crossing over during
meiosis which
rearranges allele
combinations so that
the offspring
generations are
genetically different
than the parents.
1.
Nondisjunction leading to trisomy
 This can lead to gamete formations having
too many or too few chromosomes.
 Ex: A gamete with 2 copies of #21
chromosome fertilizes a gamete with 1 copy
of #21. The result is an embryo with trisomy
21. This causes Down Syndrome in humans.
Trisomy leading to monosomy
 A gamete with one copy of the X
chromosome fertilizes a gamete missing a
copy of the X chromosome.
 The result is monosomy X, which in humans
causes Turner Syndrome.
Nondisjunction leading to polyploidy
• When a gamete with an extra set of
chromosomes is fertilized by a normal haploid
gamete, the offspring has three sets of
chromosomes and is triploid.
• The fusion of two gametes, each with an
extra set of chromosomes, produces offspring
with four sets of chromosomes and is a
tetraploid.
• This occurs often in flowering plants, leading
to larger fruit production.