Chapter 13 –

An Introduction to Heredity: Meiosis

Heredity: The transmission of traits from one generation to the next

GenesGenes are DNA are DNA segmentssegments

A A locuslocus is a gene’s is a gene’s specific location specific location along the length of a along the length of a chromosomechromosome

Offspring acquire Offspring acquire genes from parents genes from parents by inheriting by inheriting chromosomeschromosomes

Characteristics of Asexual ReproductionAsexual reproduction = form of cell division; A system of cloning and creating exact genetic replicas so there exists no genetic variation

Single-celled organisms can reproduce to create new organisms

Budding is a form of asexual reproduction in multi-cellular organisms

Characteristics of Sexual Reproduction

2 parents must give rise to offspring; each parent giving a haploid set of chromosomes

Offspring has greater genetic variation

Two haploid gametes (reproductive cells) must combine to create a new diploid organism.Each cell of a diploid

organism must have 2 sets of chromosomes. One set donated from the mom, the other set donated from dad.

Homologous Chromosomes: a pair of chromosomes containing the same linear gene sequences, each derived from one parent.

Human cells have a diploid number of 46

2N = 46

This means each cell has 23 homologous pairs

There is one exception…

The sex chromosomes (pair # 23), is homologous in females (XX)

but is only hemizygous in males (XY)

Karyotypes (such as these) are pictures of an individual’s chromosomes.

Autosomes are non-sex chromosomes (pairs 1- 22 in humans)

Meiosis = A form of nuclear division that results in the reduction of chromosome number by half (from diploid cell to a haploid cell)

Let’s review mitosis = a form of nuclear division, whereby the chromosome number and genetic content in the organism remains the same in both parent and daughter cells.

How does mitosis and meiosis compare?

How does meiosis increase genetic diversity?

Crossing over during prophase I

Independent Assortment or random alignment of homologous chromosomes along metaphase plate during metaphase I

Segregation (separation) of homologous chromosomes in Anaphase I and segregation of sister chromatids in Anaphase II

Random Fertilization

Genetic diversity in a species increases the species’ chances of survival and prevents extinction.

In a diverse gene pool, there is a higher probability of a gene that increases fitness during times of instability or stress.

Varieties in Sexual Life Cycles

Type 1: Dominant Stage (stage it spends most of its life in) = Diploid Multicellular (Ex. Humans)

2N

2N

Diploid Multicellular

Meiosis/cytokinesis

N

N

Haploid unicellularFertilization

Mitosis/Cytokinesis

Varieties in Sexual Life Cycles

Type 2: Dominant Stage = Haploid Multicellular (Ex. Fungi and Algae)

N

N

Haploid multicellular

One cell from multicellular haploid cluster is designated a haploid gamete

fertilization

2N

Diploid Single Cell

Meiosis/Cytokinesis

N N

N N

Mitosis/cytokinesis

Sordaria (fungi) – lab #3 Example of Type 2 Life Cycle

Gene that codes for ascospore color

Black (+) = wildtype black spore Tan (tn) = mutant brown spore

Sordaria is a haploid organism, so fungi’s phenotype for spore color is dependent on the single gene that it inherits

Example of Type 2 Life Cycle

Black sporesTan sporesAreas where both haploid strands can fuse, become diploid, and undergo immediate meiosis/cross over.

Both haploid strands grown on an agar plate

Example of Type 2 Life Cycle

Two haploid genes fuses to become

diploid

Cross over might occur

Completion of meiosis

mitosis

Ascus containing 8 ascospores

Ascopore pattern w/o crossover

tntn

++

DNA replication

tntn

++

Meiosis I

Meiosis II

Meiosis II

tn

tn

+

+

Mitosis

Mitosis

Ascospore with 4X4 pattern

tn

tn

+

+

tn

tn

+

+

Gene expression

tn

tn

+

+

tn

tn

+

+

Ascopore pattern #1 w/ crossover

tntn

++

DNA replication

tn

++

Meiosis I

Meiosis II

Meiosis II

tn

+

+

Mitosis

Mitosis

Ascospore with 2X2x2x2 pattern

tn

+

tn

+

tn

+

+ tn

+

Cross-over

+

tn

tn

+

+

tn

tn

+

+

tn

Ascopore pattern #2 w/ crossover

tntn

++

DNA replication

tn

++

Meiosis I

Meiosis II

Meiosis II

tn

+

+

Mitosis

Mitosis

Ascospore with 2x4x2 pattern

tn

+

tn

+

tn

+

+ tn

+

Cross-over

+

tn

tn

+

+

tn

tn

+

+

tn

Varieties in Sexual Life CyclesType 3: Alternation of Generation = Dominant haploid AND diploid multicellular stages

Diploid multicellular stage = Sporophyte

2N

2N Meiosis/cyt

Meiosis/cyt

N

N

4 Haploid spores

Mitosis/cyt

Mitosis/cyt

Haploid multicellular stage = Gametophyte

2N

2N

One cell from multicellular haploid cluster is designated a haploid gamete

fertilization

Mitosis/Cytokinesis

N

N

What can go wrong in meiosis?

No

Non-disjunction Disorders

Meiosis I - Failure to separate

Meiosis II Failure to separate

Abnormal Gametes

Definition: When members of homologous chromosomes fail to separate during Meiosis I – or – when sister chromatids fail to separate during Meiosis II.

Examples: Down Syndrome, Turner’s syndrome, Klinefelter’s syndrome

Normal Gametes

Polyploidy

Polyploidy is a term that describes the inheritance of more that two sets of chromosomes due to non-disjunction during meiosis

Interpret these karyotypes

Klinefelter’s syndrome

Interpret these karyotypes

Down Syndrome

Try these on-line activities

• http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping2.html

X-inactivation (in mammals)

In female mammals, one of the X-chromosomes turns “off” and condenses into a compact barr body.

The barr body is reactivated in the ovary cells only during meiosis for reproduction.

Two X-chromosomes (autosomes not shown)

Barr body due to X-inactivation

Other non-disjuntion disorders…- XO individual (missing 1-X chromosome) –

physically female (same phenotype as normal female who has barr body), but sterile since there is no barr body to reactivate during puberty

- XXX female – sterile - XYY male – abnormally tall- XXY male – one X becomes a barr body,

so phenotypically male, but is sterile when barr body is reativated. He has abnormally small testes

What is a mutation?

Mutations are changes in the DNA. During meiosis, there are 4 different types of chromosomal mutations that can occur.

A

A

A

A

B

B

B

B

A B

A B

A B

C

C

C

C

C

C

D

D

D D

E E

EDB CE

E E

F

F

F

F

F

FG H

G H

G H

G H

G H

G H

1. Deletion: A segment of the chromosome is removed (not justone nuclotide)

C D E F G H

A B

C D E F G HM N O

M N O P Q R P Q R

3. Inversion: A segment within a chromosome is reversed

2. Duplication: A segment of the chromosome is repeated

4. Translocation: A segment from one chromosome moves to another, non-homologous one