Mendelian genetics 2007.ppt

Gregor Johann Mendel

• 1822- 1884• Austrian monk• Experimented with pea

plants

• He thought that ‘heritable factors’ (genes) retained their individuality generation after generation

Time Line

• 1831 Charles Darwin joins crew of Beagle• 1839 Schleiden and Schwann propose the Cell Theory• 1847 Semmelweiss proposes that infection is spread by contaminated hands of

physicians.• 1856 Mendel begins hybridization studies with garden peas• 1857 Louis Pasteur (France) introduces the Germ Theory of Disease. • 1859 Darwin publishes Origin of Species• 1865 Mendel presents presents his results in transmission of phenotypic traits

between the generations to the Brünn Society of Natural Sciences.

• 1900 Hugo de Vries in Holland, William Bateson in Great Britain, Franz Correns in Germany, and Erich Tschermak in Austria acknowledged Mendel's legacy, and hailed him as the true father of classical genetics.

Terms to Know and Use

• Gene – A DNA blueprint controlling synthesis of a protein

• Trait - variant for a gene: i.e. a purple flower, determined by alleles

• Dominant trait - expressed over recessive trait when both are present

• Recessive trait - not expressed when the dominant trait is present

• Co-Dominant – expressed as blended traits

• Allele - a variation of a gene responsible for different traits, often represented as A or a

• Locus - location of a gene,or allele, on a chromosome

• Chromosome - strand of DNA containing the genes

• Haploid - one copy of a chromosome

• Diploid - two copies of a chromosome

• Gamete - a spermatozoa or oocyte (egg) cell, they are haploid

• Zygote - cell resulting from the fusion of two gametes, they are diploid

• Genotype - the type of alleles on a chromosome: genetic makeup

• Phenotype - The way a genotype is expressed: i.e. the color of a flower

• True breeding line - organisms that always pass the same genotype to their offspring

• Hybrid - offspring resulting from crossbreeding two true breeding lines: F1

Garden Pea Experiments 1856-64

• Mendel disagreed with the “Blending Theory” of inheritance.

• Started with 34 kinds peas Pisium sativum

• After 2 years he had 22 purebreds

http://www.dnaftb.org/dnaftb/1/concept/index.html

Pea Characteristics

Trait on the left is dominant. Trait on the right is recessive.

Flower Parts

Mendel’s Hypotheses• There are alternate forms

of ‘genes’=alleles

• For each trait, organisms have 2 genes, one from mom & one from dad

• Pollen and egg each carry 1 allele/trait because alleles segregate

• When only one allele is expressed & other has no noticeable effect, it is dominant

• http://www.sumanasinc.com/webcontent/anisamples/nonmajorsbiology/independentassortment.html

Mendel’s Experiments

1. Plants must possess constant differentiating characteristics.

2. The hybrids of such plants must, during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection.

3. The hybrids and their offspring should suffer no marked disturbance in their fertility in the successive generations.

Mendel's Laws of Inheritance• Law of Segregation during gamete

formation allele pairs separate or segregate, into different gametes (Demonstrated with a “test cross”).

• Law of Independent Assortment suggested that each allele pair segregates independently of other gene pairs during gamete formation (Demonstrated with a dihybrid cross).

• http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/independentassortment.html

A A

A

A

AA

AA

AA

AA

Fat

h er

c ont

r ibu

t es:

Mother contributes:

oror

True Breeding

a a

A

A

Aa

Aa

Aa

Aa

Fat

h er

c ont

r ibu

t es:

Mother contributes:

oror

Cross Breeding

Monohybrid Cross

Test Cross

Mendel's Laws of Inheritance

• Law of Segregation during gamete formation allele pairs separate or segregate, into different gametes (Demonstrated with a “test cross”).

• http://www.sumanasinc.com/webcontent/anisamples/nonmajorsbiology/independentassortment.html

• Law of Independent Assortment suggested that each allele pair segregates independently of other gene pairs during gamete formation (Demonstrated with a dihybrid cross).

• http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/independentassortment.html

Dihybrid Cross

Two chromosomes of one parent are represented on the left.Possible alleles passed on to the offspring are on the right.(Consider smooth or wrinkled peas AND tall or short plants)

Smooth Tall Smooth Smooth wrinkled wrinkledTall short Tall short

SB

Sb

sB

SB

sb

sb

Fat

h er

c ont

r ibu

t es:

Mother contributes:Dihybrid Cross

Sb sBSSBB SSBb SsBB SsBb

SSbB SSbb SsbB Ssbb

sSBB sSBb ssBB ssBb

sSbB sSbb ssbB ssbb

Chromosomes = DNA

Meiosis I

One diploid sex cell divides……

Meiosis II

Result: One diploid cell = four haploid cells

Crossing Over in Meiosis I

DO ALL GENES ASSORT INDEPENDENTLY?

• Genes on the same chromosome tend to be inherited together = linked

• Sex-linked genes: color blindness, MD, hemophilia. X Y

• X

X

Credits

• Mendel Image: pw1.netcom.com/~aguldo/ agga/bt/txt/bt1899.htm • Pea trellis: http://www.floridata.com/ref/p/images/pisu_sa1.jpg• Experiments in Plant Hybridization (1865)

by Gregor Mendel Read at the meetings of February 8th, and March 8th, 1865 to the the Natural History Society of Brünn (paper can be read at: http://www.mendelweb.org/home.html

• MendelWeb, edited by Roger B. Blumberg. (http://www.netspace.org/MendelWeb/, Edition 97.1 1997)

• The Biology Project: http://www.biology.arizona.edu/default.html• Meiosis Images: www.micro.utexas.edu/.../bio304/ genetics/meiosis.4.gif • Crossing Over image:

http://medlib.med.utah.edu/block2/biochem/Formosa/Figures/Lecture5/5-15%20Meiosis.JPG• Monohybrid, dihybrid, test cross & pea flower: ntri.tamuk.edu/homepage-

ntri/lectures/ biology/test-cross.gi