Chapter 13 – An Introduction to Heredity: Meiosis

Heredity: The transmission of traits from one generation to the next Genes are DNA segments A locus is a gene’s specific location along the length of a chromosome Offspring acquire genes from parents by inheriting chromosomes

Characteristics of Asexual Reproduction Asexual 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.pair chromosomes samelinear genesequences 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 in humans)

MeiosisMeiosis = 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.mitosis How does mitosis and meiosis compare?mitosis and meiosis

How does meiosis increase genetic diversity? Crossing over during prophase I Independent Assortment 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 Diploid Multicellular Meiosis/cytokinesis N N Haploid unicellular Fertilization 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 NN NN 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 spores Areas 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 tn + + DNA replication tn + + Meiosis I Meiosis II tn + + Mitosis Ascospore with 4X4 pattern tn Gene expression tn

Ascopore pattern #1 w/ crossover tn + + DNA replication tn + + Meiosis I Meiosis II tn + + Mitosis Ascospore with 2X2x2x2 pattern tn Cross-over + tn

Ascopore pattern #2 w/ crossover tn + + DNA replication tn + + Meiosis I Meiosis II tn + + Mitosis Ascospore with 2x4x2 pattern tn Cross-over + tn

Varieties in Sexual Life Cycles Type 3: Alternation of Generation = Dominant haploid AND diploid multicellular stages Diploid multicellular stage = Sporophyte 2N Meiosis/cyt N N 4 Haploid spores Mitosis/cyt Haploid multicellular stage = Gametophyte 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 /activities/karyotyping/karyotyping2.htmlhttp:// /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 AB AB AB C C C C C C D D DD EE ED BC E EE F F F F F FGH GH GH GH GH GH 1.Deletion: A segment of the chromosome is removed (not just one nuclotide) CDEFGH AB CDEFGH MNO MNOPQRPQR 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