Download Mendel and Meiosis

Slide 1
Mendel and Meiosis
Slide 2
Mendel: An Austrian Monk
Why offspring resemble their parents?
 Pea plants to study inheritance of
characteristics.
 Traits transferred.

Slide 3
Genetic Terms

Heredity: The passing of characteristics from
parents to offspring.

Genetics: Branch of biology that studies heredity.

Traits: Characteristics that are inherited.

Gametes: Sex Cells (two distinct cells –male and
female)
Slide 4
Plant Fertilization

Male gamete is pollen (produced by anther)

Female gamete is ovule (produced in the pistil)

Pollination: The transfer of male pollen grains to the pistil of
a flower.

Fertilization: The uniting of male and female gametes.
Occurs when male gamete in the pollen grain meets and
fuses with the female gamete in the ovule.

After the ovule gamete is fertilized, it matures into a seed.
Slide 5
Genetics Terms

Hybrid- Offspring of crosses between
parents with different traits

Purebred- Organisms that carry only one
variation of a characteristic.
Slide 6
Mendel’s Experiment:
Why Pea Plants?

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Reproduce sexually (sex cells)
Male and female gametes are in same flower
Reproductive parts are enclosed tightly together.
Reproduce by self-pollination (Male and Female
coming from same plant)
 Mendel could manipulate

Cross-Pollination: Breed-or-cross, one plant with
another
– Mendel opened petals and removed anthers
– Dusted the pistil with pollen from plant he wanted to
cross with
– Covered with bag
Slide 7
Mendel a careful researcher

Controlled experiments and peas

One trait at a time

Analyzed data mathematically

Used plants that were tall for many
generations (true breeding for tallness). Used
plants that were short for many generations
(true breeding for shortness.)
Slide 8
Mendel’s Monohybrid Cross:
Mendel crossed tall and short
plants to produce new plants.
Hybrids: Offspring of parents that have
different forms of a trait. (short and tall).
 Mono (one):
 1st experiments monohybrids
 Each parent plants differed by one single trait
(height).

Slide 9
Mendel’s Monohybrid Cross

(P1=“parents”)
– Short pea plant x Tall pea plant
first generation (F1=“filial”)
Result: All tall pea plants
 F1(first generation) Self-pollinate
– Tall pea plant x Tall pea plant
– 3/4th’s Tall
– 1/4th Short

F2(second generation)
Slide 10
Crosses
P1 generation—original parents
“P” parent
 F1 generation—Offspring of parents
“F” filial—son or daughter
 F2 generation—Cross 2 from F1 generation.

Slide 11
Trait patterns Mendel Observed
In every case, he found that one trait of a pair seemed to disappear
in the F1 generation, only to reappear unchanged in ¼ of the F2
plants.
Slide 12
Role of Unit factors
Two factors control each traits
 Genes located on chromosomes.
 Different forms of genes are called
ALLELES.
Example: Alleles for height

Two alleles for tallness
Two alleles for shortness
One allele for tallness and one for shortness

Alleles are located on different copies of
the chromosomes
Slide 13
The Rule of Dominance
trait: Observed trait –
Masks recessive
 Dominant
(Example: Mendel’s F1—All tall plants/tall allele is
dominant trait)
 Recessive trait: Trait hidden by
dominant trait
(Example: Mendel’s F1—All tall plants/short allele that reappears in F2 is
recessive trait)
Slide 14
Allele Shorthand
– Same letter for different alleles
– Upper case for dominant allele
– Lower case for recessive allele
– Dominant written first
Example: Allele for tallness = T
Allele for shortness = t
Tt
Slide 15
Law of Segregation (Mendelian)
Each organism has two different alleles, it can
produce two different types of gametes.
During fertilization, male and female gametes
randomly pair to produce four combinations of
alleles.
Slide 16
Phenotype
Greek words phainein, meaning “to show,”
and typos, meaning “model.”
 The visible characteristics (appearance and
behavior) of an organism makes up it
phenotype.
 Example:

Round, Wrinkled
Yellow, Green
Brunette, Blonde
Blue Eyes, Brown Eyes
Slide 17
Genotype
From the Greek words gen or geno, meaning
“race,” and typos, meaning “model.”
 The genetic characteristics of an organism
make up its genotype.

Example:
Genotype of a tall plant that has two alleles for tallness is
TT.
Genotype of a tall plant that has one allele for tallness and
one allele for shortness is Tt.
Slide 18
Homozygous
Trait for two alleles is the
same
Example:
Two alleles for tallness (TT)
homozygous dominant
Two alleles for shortness (tt)—
homozygous recessive.
Slide 19
Heterozygous
Two different alleles for one trait.
Tt
Slide 20
Punnett Squares.
(1905) Reginald Punnett, an English
biologist, created way to expected
proportions of possible genotypes in the
offspring of cross-Punnett Square.
 Know the genotypes of the parents, you can
use a Punnett square to predict the possible
genotypes of their offspring.

Slide 21
Making a Punnett Square
Slide 22
Monohybrid Crosses
Mendel’s cross Tt x Tt
 Half the gametes of each parent would
contain the T allele, and the other half
would contain the t allele.

Gametes that each
parent forms
One parent
It doesn’t matter which
Tt x Tt
set of gametes are on top
Other Parent and which are on the side.
Slide 23
Determining Phenotypes
(characteristics)

If organism has at least one
dominant allele, dominant trait
will be expressed.
(TT, BB, Tt, Bb )

For the recessive trait to be
expressed, organism must
lack dominant allele and have
two recessive alleles. (bb or
tt)

Of the offspring ¼ will be
homozygous dominant
(TT/BB) 2/4 or ½ will be
heterozygous (Tt, tT, Bb, bB)
and ¼ will be homozygous
recessive.
Slide 24
Mendel’s Dihybrid Crosses
Di means “two.”
 Dihybrid cross: A cross involving two different traits.
 Mendel did true-breed dihybrid cross between
round yellow seeds (RRYY) and wrinkled green
seeds (rryy)
 Mendel’s results of dihybrid cross:

– P1 round yellow x wrinkled green
– F1 All round yellow (round yellow dominant)
– F2 9 round yellow, 3 round green, 3 wrinkled
yellow, 1 wrinkled green.
Mendel found 2 dominate traits for round and yellow seeds.
Slide 25
Mendel’s dihybrid cross
Dihybrid cross led to Mendel’s Law of Independent Assortment.
Slide 26
Dihybrid Crosses
Think of the RRYY x rryy cross (Round
Yellow x Wrinkle Green seeds)
 Mendel found that seed shape and seed
color would be inherited independently of
each other.
 Punnett square you will need four boxes on
each side.

Slide 27
Dihybrid Cross
Slide 28
Law of Independent
Assortment
Mendelian principle stating that genes for
different traits are inherited independently
of each other.
Example: Genotype RrYy produces gametes:
Rr will separate
Yy will separate
Recombine in 4 different ways.
Slide 29
Dihybrid Cross
Punnett squares are good for showing all the possible combinations
of gametes and the likelihood that each will occur. However, you
don’t get the exact ratio of results shown in the square.
Slide 30
Probability

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Genetics follows the rules of chance.
Probability or chance that an event will occur
can be determined by dividing the number of
desired outcomes by the total number of
possible outcomes.
desired # of outcomes / total # of possible outcomes
(Example: Toss a coin the probability of getting heads
would be one in two chances, written 1:2 or ½.

A Punnetts square can be used to determine
the probability of the event.
Slide 31
Probability
Slide 32
Meiosis
Slide 33
Genes

Tens of thousands of genes
 Lined up on chromosomes
Slide 34
Chromosomes

Occur in pairs (Male,
Female)

DIPLOID—A cell with
two of each kind of
chromosome is said to
be diploid, or 2n,
number of
chromosomes
Slide 35
Slide 36
Gametes
Male (sperm) and Female (egg)
 Contain one of each kind of chromosomes.
 A cell with one of each kind of chromosome
is called a HAPLOID and is said to contain a
haploid, or n, number of chromosomes.

Slide 37
Slide 38
Chromosome Numbers of Some
Common Organisms
Organism
Fruit Fly
Garden Pea
Corn
Tomato
Leopard Frog
Apple
Human
Chimpanzee
Dog
Adder’s tongue
fern
Body Cell (2n)
8
14
20
24
26
34
46
48
78
1260
Gamete (n)
4
7
10
12
13
17
23
24
39
630
Slide 39
Homologous Chromosomes

Pair chromosomes are
called homologous
chromosomes—
determine phenotype.
 Gene for same trait
– same order,
– chromosomes in a
homologous pair are not
always identical.

(Chromosome 4
contains 3 traits
Mendel Studied)
Slide 40
Slide 41
Meiosis
From the Greek word meioun,
meaning “to diminish”.
Cell division that results in a
gamete containing half the
number of chromosomes of its
parents.
Slide 42
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Meiosis
Divisions: Meiosis I and Meiosis II
Begins with
one diploid (2n) cell
four haploid (n) cells.
Sex cells (gametes) haploid.
Sperm fertilizes an egg-results in zygote
(diploid)
Zygote develops by MITOSIS into a multicellular organism.
Reproduction —Production and subsequent
fusion of haploid sex cells.
Slide 43
Interphase

Chromosomes replicate
 Chromosome
– two identical sister chromatids held together by a
centromere
Slide 44
Prophase I

Chromosomes coil up and a
spindle forms.
 Homologous chromosomes
comes together, matched
gene by gene, to form a
four-part structure called a
tetrad.
 Chromatid pair so tight that
sometimes non-sister
chromatids from
homologous chromosomes
sometimes exchange
genetic material in a
process known as
crossing over.
Slide 45
Crossing Over

Exchange of genetic material
 Any location
 Several locations at once
 Humans-Two to three crossovers for each pair of
homologous chromosomes.
Slide 46
Metaphase I

Centromere attaches to a spindle fiber
 Spindle fibers pull the tetrads into the middle, or
equator, of the spindle.
 Chromosomes are lined up side by side as
tetrads.
Slide 47
Anaphase I

Chromosomes
separate and move to
opposite ends of the
cell.
 Centromeres holding
the sister chromatids
together do not split
like they do in
anaphase of mitosis.
 Ensures that each new
cell have only one
chromosome from
each homologous pair.
Slide 48
Telophase I

Spindle is broken down
 Chromosomes uncoil
 Cytoplasm divides  2
new cells.
 Half of genetic
information of original
cell (one chromosome
from each homologous
pair)
 Another cell division
needed
Slide 49
Meiosis II
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Newly formed cells go through short interphase
(chromosomes don’t replicate)
Prophase II—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—Centromere of each chromosome splits,
allowing sister chromatids to separate and move to opposite
poles.
Telophase II—Nuclei reform, spindles break down, and
cytoplasm divides.
Slide 50
Slide 51
Meiosis Results
Four haploid sex cells have been formed
from one original diploid cell.
 Each haploid cell contains one chromosome
from each homologous pair.
 Haploid cells become gametes, transmitting
the genes they contain to offspring.

Slide 52
Slide 53
Slide 54
Genetic Recombination

Gene combinations vary based on how chromosomes
lines up during metaphase I (random).
 As number of chromosome increase the number of gene
combinations increase.
Example: Pea Plants (7 n) 27= 128 x 128 =16,384 different offspring
Sperm Egg
Human (23n) 223= 8 million x 8 million =70 trillion
possible zygotes.

Reassortment of chromosomes and the genetic
information they carry, either by crossing over or by
independent segregation of homologous chromosomes is
called GENETIC RECOMBINATION.
 Provides genetic variation
Slide 55
Slide 56
Slide 57
Mistakes in Meiosis
Nondisjunction (failure of homologous
chromosomes to separate) results in
gametes with either an extra chromosome
or a missing chromosome.
 Extra chromosomes often survive; those
lacking one or more usually do not.
 TRISOMY—Extra Chromosome
e.g.—Extra chromosome 21—Down’s
Syndrome

Slide 58
Mistakes in Meiosis Continued…
MONOSOMY—When a gamete with a
missing chromosomes fuses with a normal
gamete during fertilization, resulting zygote
lacking chromosome.
 Most zygotes don’t survive; if do organisms
generally does not.
 Turner syndrome—human females with only
one X chromosome.

Slide 59
Slide 60
Mistakes in Meiosis

Lack of separation chromosomes- Gamete inherits
a complete diploid set of chromosomes.

Triploid—gamete with extra set of chromosome is
fertilized by a normal haploid gamete, resulting
offspring has a set of three chromosomes.

Tetraploid—Fusion of two gametes, each with an
extra set of chromosomes, produces offspring with
four sets of chromosomes.
Slide 61
Polyploidy
Organisms with more than the usual number
of chromosome sets are called
POLYPLOIDY.
 Animals almost always cause death in
zygotes
 Plants happens often; Larger and healthier.
Great commercial value.
 Plant breeders artificially produce polyploidy
plants using chemicals that cause nondisjunction.
