Mendelian Genetics and Meiosis

Gregor Mendel (1822 – 1884) Father of Genetics Austrian 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 quickly Many kinds available Self-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

What was Mendel studying? 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. http://www.cfkeep.org/html/phpThumb.php?src=/uploads/peas_copy.gif&aoe=1&w=

Mendel's First Experiment 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. 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

Mendel's Second Experiment Allowed the plants produced by his first experiment to self-pollinate. All purple flowered plants self pollinated: 75% of offspring were purple flowered 25% of offspring were white flowered http://www.biology.iupui.edu/biocourses/N100/images/10f2.gif

Conclusions Recessive 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 Principles Inheritance 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

Alleles Different forms of a gene EX: freckles or no freckles Dominant 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. Genotype PHENOTYPE: 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 Genotypes Homozygous Dominant: Two dominant alleles PP or DD or BB Homozygous Recessive: Two recessive alleles pp or dd or bb Heterozygous: One dominant and one recessive allele Pp or Dd or Bb http://uwadmnweb.uwyo.edu/VetSci/Courses/PATB_4110/4-6/Class_Notes.htm

Punnett Square 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.

Exceptions to Mendel's Principles 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 http://www.estrellamountain.edu/faculty/farabee/biobk/incomdom.gif

MEIOSIS Creates 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

Steps of Meiosis Meiosis I Meiosis II Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Meiosis II Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis

http://www. cps. ci. cambridge. ma http://www.cps.ci.cambridge.ma.us/CRLS/LC_R/classrooms/AUGUSTINE/Genetics/index_files/frame.html#slide0025.html

Prophase I Homologous 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 I Homologous 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 II Centrioles move to poles Nuclear 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

Telophase II Chromatids reach the poles. Nuclear membrane re-forms Cytokinesis occurs http://www.sciencecases.org/mitosis_meiosis/images/meiosis_h.gif

MEIOSIS Results 4 new cells Each 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.

Meosis vs Mitosis Meiosis Mitosis Sex cells Two divisions 4 genetically different cells produced Cells produced have half the number of chromosomes (haploid) 2n  n Somatic cells One division 2 genetically identical cells produced Cells produced have the same number of chromosomes as parents 2n  2n

Sex Chromosomes Chromosomes 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 Examples: Color blindness Hemophilia 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. http://www.nlm.nih.gov/MEDLINEPLUS/ency/images/ency/fullsize/9962.jpg http://member.principalhealthnews.com/Imagebank/Articles_images/Hemophilia_02.gif