Download Meiosis

General Overview
 Goal is to produce 4 unique gametes
 Occurs in germ cells in testes and ovaries.
 Genetic information is exchanged between maternally
and paternally inherited traits.
 Contributes to variability.
Homologous Chromosomes
 Same size/length
 Same centromere position
 Carry the same information
Meiosis I: The Reduction Division
 Results in reduction of chromosome number from
diploid (cell has two copies of each chromosome, one
maternal and one paternal chromosome)
 …to haploid (cell has only one copy of each
chromosome, either maternally-derived or paternallyderived chromosome)
 Chromosomes are two sister chromatids at this point
Interphase
 DNA is replicated.
 Each chromosome is copied
 There is now the equivalent of four copies of each
chromosome in the cell
Prophase I
 Chromatin condenses
 Homologous chromosomes pair up
Crossing Over
 Occurs in Prophase I
 Paternal and Maternal homologous chromosomes are
paired up and intertwine, exchanging information to
create new and unique chromosomes.
Metaphase I
 Chromosomes attach to spindle fibers at the
centromere.
 Homologous chromosomes line up at the equator.
Anaphase I
 Homologous chromosomes move to opposite poles of
the cell
Telophase
 Spindle fibers break down.
 Chromosomes uncoil into nuclei
 Cell divides into two intermediate cells each with 46
chromosomes.
Prophase II
 Spindle fibers form.
 Chromosomes condense
Metaphase II
 Haploid chromosomes line up on the equator
Anaphase II
Sister chromatids are pulled apart at the centromere and
moved to opposite poles of the cell.
Telophase II
 Chromosomes reach the poles
 Spindle fibers disappear and nuclear membranes
reform.
Finally…
 Meiosis results in four genetically unique haploid cells,
each with n chromosomes.
Bell work 12/7
Suppose that for an organism, 2N = 24. How
many chromosomes do the organism’s
gametes contain? Explain.
Importance of Meiosis
 Genetic diversity.
 Genetic issues possessed by one parent may not be
passed on to offspring.
 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
Mendelian Genetics
 Gregor Mendel: Austrian, Monk, Awesome.
 “Father of genetics” (he kind of only discovered simple
genetics that aren’t really the common type in humans
but whatev)
 Published his findings on pea plants.
I am really into the
genetics of peas.
Inheritance of Traits
 Mendel selectively bred yellow and green pea plants,
removing male organs of the yellow plants to prevent
self-fertilization.
 The yellow and green plants were known as the parent
or P generation- first generation.
 Pea plants are true-breeding: they produce offspring
with only one form of a trait.
First Filial Generation (F1)
 The offspring of the P generation is the first filial
generation.
 When yellow and green pea plants are crossed, the
resulting seeds were always yellow.
 Mendel crossed the F1 generation with itself to get
the F2 generation….
F2 Generation
 Mendel noticed that in the F1 generation, the green
trait seemed to disappear. To test this, he crossed F1
plants together.
 These offspring were called the F2 generation.
 These seeds showed a near perfect 3:1 yellow:green
ratio. The trait was not gone, simply masked in the
generation before.
Dominance
 The yellow trait in this case was dominant and masks
the green color trait.
 Only individuals with homozygous green genotype
(yy) would display a green phenotype.
 Homozygous= 2 of the same alleles
 Any heterozygous (Yy) or homozygous yellow (YY)
individuals would appear yellow.
 Heterozygous= 2 different alleles
Genotype and Phenotype
 Genotype: the pairs of alleles (alternative forms of a
gene) that an individual possesses.
 Phenotype: the outward expression of the allele pairs
an individual possesses.
Mendel’s Law of Segregation
 every individual possesses a pair of alleles for any
particular trait; each parent passes a randomly
selected copy (allele) of only one of these to its
offspring.
 Alleles separate during meiosis.
Mendel’s Law of Independent
Assortment
 Allele pairs separate independently during the
formation of gametes. This means that traits are
transmitted to offspring independently of one another.
Gene Linkage
 Genes that are close together will not assort
independently. They are often inherited together.
 The farther apart the two genes are, the less likely they
are to be inherited together. This is due to crossing
over.
Monohybrid Cross
 Crossing two hybrid (heterozygous) individuals
 Yy x Yy: Phenotypic ratio is 3:1
Dihybrid Cross
 Crossing two individuals hybrid for two traits.
 AaBb x AaBb
 Phenotypic ratio is 9:3:3:1