Mendel’s Legacy

Genetics is everywhere these days – and it will continue to become even more important in decades to come.

So wouldn’t it be nice if people understood it better?

Terms to Know and Use

• Genetics – the study of heredity

• Heredity – the passing of traits from the

parents to their offspring

• Trait - variations of a gene: (i.e. black or brown hair) determined by alleles

• Allele – different forms a gene, often expressed as Y or y, X or x, Z or z

• Gene - a heritable feature on a chromosome:

(i.e. hair color)

• Chromosome - strand of DNA that codes for genes

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

•Monohybrid cross – cross involving parents

differing in only 1 trait

•True-breeding - organisms that always pass

the same genes to their offspring

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

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

•Homozygous – when both alleles of a gene are

the same (i.e. pure, TT)

•Heterozygous – when the two alleles are not

the same (i.e. hybrid, Tt)

• Genotype - the type of alleles on a chromosome: (gene makeup)

• Phenotype - The way a genotype is expressed: (physical appearance)

Gregor Johann Mendel“Father of Genetics”

• 1822- 1884• Austrian monk• Experimented with

pea plants• Trained in math and

science

Garden Pea Experiments 1856-64

Why pea plants? (Pisium sativum)

Because they…

1) Were easy to grow

2) Produced a large number of offspring

3) Matured quickly

4) Had both male and female parts

The Problem

T.A. Knight, a scientist, saw that if he crossed

true bred true bred

purple pea plant (P) x white pea plant (P)

ALL the offspring would be purple (F1).

The Problem (cont’d)

If he then crossed the purple (F1) offspring:

hybrid hybrid

purple pea plant (F1) x white pea plant (F1)

Most offspring are purple (F2) & few white (F2).

Mendel’s Answer

Mendel used math with science to explain heredity.

He counted: 705 purple

224 white

total 929 pea plants

Thus he saw a 3:1 ratio.

Mendel’s Answer (cont’d)

To explain this ratio he came up with Rules of Heredity

1) Parents transmit “genes” to offspring

2) Each individual has 2 genes (1 from each parent)

3) Some “genes” are dominant and others recessive

P P

p

p

Pp

Pp

Pp

Pp

Fat

her

cont

r ibu

tes:

Mother contributes:

oror

P Generation

P p

P

p

PP

Pp

Pp

pp

Fat

her

cont

r ibu

tes:

Mother contributes:

oror

F1 Generation

Lucky or Right On?

Mendel repeated his experiments while observing other traits such as:

• height of plant (tall vs. short)• pod appearance (inflated vs. constricted)• pod color (green vs. yellow)• seed texture (round vs. wrinkled)• seed color (yellow vs. green)

Pea Characteristics

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

Mendel's Laws of Inheritance

• Law of Segregation - states that the two alleles separate when gametes (sperm/egg) form. Thus, a gamete receives only one allele from each parent. (test cross)(test cross)

• Law of Independent Assortment - states that different alleles (traits) separate independently. Thus, color, height, pod shape, etc. are not connected together. (dihybrid cross)(dihybrid cross)

Determining Unknown Genotypes

• How do you know if a dominant phenotype is homozygous or heterozygous?

• Scientists can perform a test cross, where they cross the unknown with a recessive (known) phenotype.

a a

A

A

Aa

Aa

Aa

Aa

Fat

her

cont

r ibu

tes:

Mother contributes:

oror

OPTION 1:Test Cross

a a

A

a

Aa

aa

Aa

aa

Fat

her

cont

r ibu

tes:

Mother contributes:

oror

OPTION 2:Test Cross

Mendel's Laws of Inheritance

•Law of Segregation - states that the two alleles separate when gametes (sperm/egg) form. Thus, a gamete receives only one allele from each parent. (test cross)(test cross)

•Law of Independent Assortment - states that different alleles (traits) separate independently. Thus, color, height, pod shape, etc. are not connected together. (dihybrid cross)(dihybrid cross)

SB

Sb

sB

SB

sb

sb

Fat

her

cont

r ibu

tes:

Mother contributes:Dihybrid Cross

Sb sBSSBB SSBb SsBB SsBb

SSbB SSbb SsbB Ssbb

sSBB sSBb ssBB ssBb

sSbB sSbb ssbB ssbb

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

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

Chromosomes = DNA

Meiosis I

One diploid sex cell divides……

Meiosis II

Result: One diploid cell = four haploid cells

Crossing Over in Meiosis I

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

Any Questions?

• “The problem with the gene pool is that there is no lifeguard.” —Unknown

• “Life can only be understood backwards; but it must be lived forwards.”

—Kierkegaard