Title: Meiosis
1Meiosis
- Meiosis involves two consecutive sets of cell
divisions - Meiosis only occurs in the reproductive
structures of organisms who reproduce sexually - Most animals
- Most plants
- Most fungi
- Most protists
2Meiosis I and Meiosis II
- Meiosis I is the reduction division cells start
out diploid and end up haploid - In Meiosis II sister chromatids are separated
(much like mitosis)
3Meiosis I
Interphase
4Meiosis I
- Pairing of homologous chromosomes occurs.
- Each chromosome consists of two chromatids.
Prophase I
- The nuclear envelope breaks down.
5Meiosis I
- Crossing over produces exchange of genetic
information.
- Crossing overchromosomal segments are exchanged
between a pair of homologous chromosomes.
Tetrads are groups of four sister chromatids
6Meiosis I
- Chromosome centromeres attach to spindle fibers.
Metaphase I
- Homologous chromosomes line up at the equator in
tetrads
7Meiosis I
Anaphase I
8Meiosis I
Telophase I
- Chromosomes uncoil and form two nuclei.
9Meiosis II
Prophase II
10Meiosis II
Metaphase II
11Meiosis II
Anaphase II
12Meiosis II
Telophase II
13Meiosis II
- Cytokinesis results in four haploid cells, each
with n number of chromosomes.
Cytokinesis
14Meiosis
- Meiosis consists of two sets of divisions
- Produces four haploid daughter cells that are not
identical
- Results in genetic variation
15Meiosis
- 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.
16Meiosis and Variation
- 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
17Meiosis and Variation, cont
- 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).
18Advantages 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
19Advantages of Sexual Reproduction
- Beneficial genes multiply faster over time.
- The organisms inherits genes from two parents and
is not genetically identical to either parent. - Ensures genetic variation
20Chapter 10Sexual Reproduction and Genetics
21Gregor Mendel
- Lived in Europe in what is now Czech Republic
near the Austrian border. - Father of Genetics
- Monk, entered monastery 1843
22Gregor Mendel
- Failed teachers exam
- When to U of Vienna
- Studied with physicist Doppler- science through
experiment, applied math to science - Studied with botanist Unger- interest in causes
of variation in plants - Passed teachers exam and taught at monasterys
school also responsible for schools garden - Published 1866, mathematics and plant breeding
23Mendel Studied Peas
- Available in many varieties
- Self pollinating (can manipulate pollination)
- either-or inherited traits
- Had true breeder for parental generation (P)
due to flower structure
24Mendel Studied Peas
- 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
25Inheritance of Traits
26Inheritance 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.
27Pea Traits Studied by Mendel
28Inheritance of Traits
- Mendel studied thousands of pea plants for the
seven traits. - He concluded that
- Genes are in pairs
- Different versions of genes (alleles) account for
variation in inherited characteristics - Alleles can be dominant or recessive
29Dominant and Recessive
- 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.
30(No Transcript)
31Symbols
- 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
32Homozygous and Heterozygous
- Dominant traits can be homozygous or heterozygous
- Homo same the alleles would be the same, PP
- Heterodifferent the alleles would be different,
Pp - For the recessive trait to be expressed both
alleles would be recessive, pp
33Genotype and Phenotype
- 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
34Genotype and Phenotype
- If the phenotype displays the recessive trait
(white) then you know the genotype pp
- If the phenotype displays the dominant trait
(purple) then the genotype could be homozygous
dominant (PP) or heterozygous (Pp)
Genotype is PP or Pp
Genotype is pp
35Punnett Squares
- 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 31 phenotypic ratio
36Monohybrid Cross
- Mono one
- One trait is studied at a time
- This one is seed color
- Monohybrid crosses provided evidence for the Law
of Segregation
37Mendels Law of Segregation
Two alleles for each trait separate during meiosis
38Dihybrid Cross
- Di two
- Two traits are studied at a time
- This one is seed color (yellow or green) and seed
shape (wrinkled or round)
39Dihybrid 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 9331 which
gave evidence for the Law of Independent
Assortment
40Mendels 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.
41Mendels Law of Independent Assortment
42Probability
- 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.
43Chapter 10Sexual Reproduction and Genetics
- 10.3 Gene Linkage and Polyploidy
44Gene Linkage
- The linkage of genes on a chromosome results in
an exception to Mendels law of independent
assortment because linked genes usually do not
segregate independently.
45Polyploidy
- 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)
46Polyploidy
- 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.