Gregor Mendel (1822 – 1884)
Father of GeneticsAustrian monk who
worked with pea plants in monastery garden.
Developed first theories on genetic inheritance.
http://history.nih.gov/exhibits/nirenberg/images/photos/01_mendel_pu.jpg
Why Experiment With Pea Plants?
Grow quicklyMany kinds availableSelf-pollinate so they have
both male and female reproductive parts on the same flower and can pollinate themselves. They are true-breeding meaning offspring will have same traits as parent.
Can cross-pollinate so one plant can pollinate another plant to produce offspring.
http://kentsimmons.uwinnipeg.ca/cm1504/Image215.gif
Inherited characteristics (features with different forms in a population) that are passed from parents to offspring.EX: Flower color or seed shape
Studied one characteristic at a time to determine which traits appeared in offspring.
What was Mendel studying?
http://www.cfkeep.org/html/phpThumb.php?src=/uploads/peas_copy.gif&aoe=1&w=
Manually crossed true breeding plants for each characteristic.Example: crossed purple flower plant & white
flower plant.All offspring displayed the same trait of one
parent. In this case, all had purple flowers. White flowers seemed to disappear.
Mendel's First Experiment
http://www.biology.iupui.edu/biocourses/N100/images/10F1.gif
Conclusions:The trait that showed up
most often in the offspring was the DOMINANT trait.
The trait that seemed to disappear or fade away was the RECESSIVE trait.
To determine what happened to the recessive trait, Mendel decided to do another set of experiments.
http://www.jbhs.k12.nf.ca/biology/photos/mvc-006f.jpg
http://www.windows.ucar.edu/earth/Life/images/earlobes.jpg
Allowed the plants produced by his first experiment to self-pollinate.
All purple flowered plants self pollinated:75% of offspring were purple flowered25% of offspring were white flowered
Mendel's Second Experiment
http://www.biology.iupui.edu/biocourses/N100/images/10f2.gif
ConclusionsRecessive trait did not
disappear, it was masked by the dominant trait as it showed up again in the second generation.
Each plant had 2 sets of instructions (one from each parent) for each characteristic.
http://upload.wikimedia.org/wikipedia/commons/thumb/1/17/Punnett_square_mendel_flowers.svg/550px-Punnett_square_mendel_flowers.svg.png
Mendel's PrinciplesInheritance of traits is
predetermined by genes. Genes are passed on from parents.
Some forms of genes are dominant and others are recessive.
Organisms have 2 copies of each gene (one from each parent).
Alleles(different forms for a gene) for different genes segregate independently of one another. (Monohybrid cross) http://www1.umn.edu/ships/updates/
9mendels.gif
AllelesDifferent forms of a gene
EX: freckles or no frecklesDominant allele – expressed
with an UPPER CASE letter.Recessive allele – expressed
with a lower case letter. NOTE: The same letter is
used to express an allele – variations are expressed with the upper or lower case.
http://upload.wikimedia.org/wikipedia/commons/8/83/Punnett_square_(PSF).png
Phenotype vs. GenotypePHENOTYPE:
Physical characteristic – the characteristic that you can see.
EX: Purple flowers
GENOTYPE: The two inherited alleles for a trait. (Cannot be seen)
EX: PP or Pp
http://www.bbc.co.uk/schools/gcsebitesize/science/images/25_environmental_variation.gif
Types of GenotypesHomozygous Dominant:
Two dominant allelesPP or DD or BB
Homozygous Recessive: Two recessive allelespp or dd or bb
Heterozygous: One dominant and one recessive allelePp or Dd or Bb
http://uwadmnweb.uwyo.edu/VetSci/Courses/PATB_4110/4-6/Class_Notes.htm
Organizes all possible genotype combinations for offspring from particular parents.
How to make a Punnett Square crossing a homozygous recessive white flowering pea plant with a heterozygous purple flowering pea plant.
Punnett Square
Incomplete Dominance: One allele is not completely dominant over the other allele. Each allele contributes to the phenotype produced.EX: Snapdragons (white and red produce pink)
One gene may influence more than one trait.EX: in white tigers, one gene codes for fur color and eye
color.
Several genes may work together to produce a trait.EX: human skin, hair and eye color
Exceptions to Mendel's Principles
http://www.estrellamountain.edu/faculty/farabee/biobk/incomdom.gif
MEIOSISCreates the sex cells It is a copying process that produces cells with
½ the number of chromosomes.Helped Walter Sutton determine genes are
located on chromosomes in the nucleus of the cell. Prior to this no one knew where the genetic traits (genes) were located.
http://www2.merriam-webster.com/mw/art/med/meiosis.gif
Meiosis IProphase IMetaphase IAnaphase ITelophase I and
CytokinesisMeiosis II
Prophase IIMetaphase IIAnaphase IITelophase II and
Cytokinesis
Steps of Meiosis
http://www.cps.ci.cambridge.ma.us/CRLS/LC_R/classrooms/AUGUSTINE/Genetics/index_files/frame.html#slide0025.html
Prophase IHomologous
chromosomes find each other and pair up (one chromosome from each parent
Crossing over may occur
Centrioles move toward the poles
Nuclear membrane begins to dissolve
http://www.sciencecases.org/mitosis_meiosis/images/meiosis_a.gif
Metaphase I
Spindle fibers attach to homologous chromosomes.
Homologous chromosomes line up at the equator
http://www.sciencecases.org/mitosis_meiosis/images/meiosis_b.gif
Anaphase I
Homologous chromosomes are separated so each chromosome moves toward opposite poles.
http://www.sciencecases.org/mitosis_meiosis/images/meiosis_c.gif
Telophase IHomologous
chromosomes are completely separated with one chromosome at each pole.
Nuclear membrane re-forms
Cytokinesis takes place and cell divides to form two cells.
http://www.uic.edu/classes/bios/bios100/lecturesf04am/telophase1m.jpg
Prophase IICentrioles move to
polesNuclear membrane
dissolves
NOTE: CHROMOSOMES ARE NOT COPIED AGAIN PRIOR TO PROPHASE II
http://www.sciencecases.org/mitosis_meiosis/images/meiosis_e.gif
Metaphase II
Spindle fibers form and attach to chromosomes
Chromosomes line up at the equator.
http://www.sciencecases.org/mitosis_meiosis/images/meiosis_f.gif
Anaphase II
Chromosomes are pulled apart so each chromatid moves toward opposite poles.
http://www.uic.edu/classes/bios/bios100/lecturesf04am/anaphase2m.jpg
Chromatids reach the poles.Nuclear membrane re-formsCytokinesis occurs
Telophase II
http://www.sciencecases.org/mitosis_meiosis/images/meiosis_h.gif
4 new cellsEach cell has ½ the number of chromosomes
as parent cell (haploid – N)New cells are NOT identical to each other or
to the parents as a result of crossing over.
MEIOSIS Results
MeiosisSex cellsTwo divisions4 genetically
different cells produced
Cells produced have half the number of chromosomes (haploid) 2n n
Somatic cellsOne division2 genetically
identical cells produced
Cells produced have the same number of chromosomes as parents 2n 2n
Meosis vs Mitosis
Mitosis
Sex ChromosomesChromosomes that
carry the genes that determine sex.
In humans:Females: two X
chromosomes (XX)Males: one X
chromosome and one Y chromosome (XY)
Sex of offspring is determined by the male:Egg fertilized by
sperm with X chromosome = FEMALE
Egg fertilized by sperm with Y chromosome = MALE
http://howyoudoin.files.wordpress.com/2008/05/male_female_symbol.jpg
Sex-Linked Disorders
Males have an X and Y chromosome. The Y chromosome does not have all the genes found on the X chromosome, so they only have one copy of those genes on the X. If those genes are damaged, they do not have a backup while females do – they have two X chromosomes. Therefore, males are more likely to inherit these disorders.
Examples:Color blindnessHemophilia
http://www.nlm.nih.gov/MEDLINEPLUS/ency/images/ency/fullsize/9962.jpg
http://member.principalhealthnews.com/Imagebank/Articles_images/Hemophilia_02.gif