Mendelian Genetics and

Meiosis

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?

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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