Mendel and His PeasMendel and His Peas

MUPGRET Workshop

Feb. 7, 2004

Genetic variationGenetic variation

In the beginning geneticists studied differences they could see in plants.

These differences are called morphological differences.

Individual variants are referred to as phenotypes, ex. tall vs. short plants or red vs. white flowers.

TraitTrait

A broad term encompassing a distribution of phenotypic variation.

Example:– Trait: Disease resistance– Phenotype: resistant vs. susceptible– Morphological differences associated with the

trait might include fungal infection, fungal growth, sporulation, etc.

MendelMendel

Monk at the St. Thomas monastery in the Czech Republic.

Performed several experiments between 1856 and 1863 that were the basis for what we know about heredity today.

Used garden peas for his research.Published his work in 1866.

MendelMendel

Results are remarkably accurate and some have said they were too good to be unbiased.

His papers were largely ignored for more than 30 years until other researchers appreciated its significance.

Garden PeaGarden Pea

Pisum sativumDiploidDiffered in seed shape, seed color, flower

color, pod shape, plant height, etc.Each phenotype Mendel studied was

controlled by a single gene.

TermsTerms

Wild-type is the phenotype that would normally be expected.

Mutant is the phenotype that deviates from the norm, is unexpected but heritable.

Notice that this definition does not imply that all mutants are bad in fact many beneficial mutations have been selected by plant breeders.

Advantages of plantsAdvantages of plants

Can make controlled hybrids.Less costly and time consuming to maintain

than animals.Can store their seed for long periods of

time.One plant can produce tens to hundreds of

progeny.

Advantages of plantsAdvantages of plants

Can make inbreds in many plant species without severe effects that are typically seen in animals.

Generation time is often much less than for animals.– Fast plants (Brassica sp.)– Arabidopsis

Principle of SegregationPrinciple of Segregation

Parental Lines

Round Wrinkled

X

All round F1 progeny

Self-pollinate

Round5474

Wrinkled1850

3 Round : 1 Wrinkled

Mendel’s ResultsMendel’s Results

Parent CrossParent Cross FF11 Phenotype Phenotype FF22 data data

Round x wrinkledRound x wrinkled RoundRound 5474 : 18505474 : 1850

Yellow x greenYellow x green YellowYellow 6022 : 20016022 : 2001

Purple x whitePurple x white PurplePurple 705 : 224705 : 224

Inflated x Inflated x constricted podconstricted pod

InflatedInflated 882 : 299882 : 299

Green x yellow podGreen x yellow pod GreenGreen 428 : 152428 : 152

Axial x terminal Axial x terminal flowerflower

AxialAxial 651 : 207651 : 207

Long x short stemLong x short stem LongLong 787 : 277787 : 277

Important ObservationsImportant Observations

F1 progeny are heterozygous but express only one phenotype, the dominant one.

In the F2 generation plants with both phenotypes are observedsome plants have recovered the recessive phenotype.

In the F2 generation there are approximately three times as many of one phenotype as the other.

3 : 1 Ratio3 : 1 Ratio

The 3 : 1 ratio is the key to interpreting Mendel’s data and the foundation for the the principle of segregation.

The Principle of SegregationThe Principle of Segregation

Genes come in pairs and each cell has two copies.

Each pair of genes can be identical (homozygous) or different (heterozygous).

Each reproductive cell (gamete) contains only one copy of the gene.

Principle of SegregationPrinciple of Segregation

Either copy of the gene is equally likely to be included in a gamete.

One male and one female gamete combine to generate a new individual with two copies of the gene.

AlleleAllele

One of two to many alternative forms of the same gene (eg., round allele vs. wrinkled allele).

Alleles have different DNA sequences that cause the different appearances we see.

Mendel’s Principle of Mendel’s Principle of SegregationSegregation

In the formation of gametes, the paired hereditary determinants separate (segregate) in such a way that each gamete is equally likely to contain either member of the pair.

Principle of SegregationPrinciple of Segregation

Parental Lines

Round (WW) Wrinkled (ww)

X

All round F1 progeny (Ww)

Self-pollinate

Round (WW + Ww)5474

Wrinkled (ww)1850

3 Round : 1 Wrinkled

Punnett SquarePunnett SquareA (½) a (½)

A (½) AA

(½ x ½ = ¼)

Aa

(½ x ½ = ¼)

a(½) Aa

(½ x ½ = ¼)

aa

(½ x ½ = ¼)

Male

Female

¼ AA :½ Aa : ¼ aa¼ AA :½ Aa : ¼ aa

Round vs. wrinkledRound vs. wrinkled

The SBEI causes the round vs. wrinkled phenotype.

SBEI = starch-branching enzymeWrinkled peas result from absence of the

branched form of starch called amylopectin.When dried round peas shrink uniformly

and wrinkled do not.

Round vs. wrinkledRound vs. wrinkled

The non-mutant or wild-type round allele is designated W.

The mutant, wrinkled allele is designated w.Seeds that are Ww have half the SBEI of

wild-type WW seeds but this is enough to make the seeds shrink uniformly.

W is dominant over w.

Round vs. wrinkledRound vs. wrinkled

An extra DNA sequence is present in the wrinkled allele that produces a non-functional SBEI and blocks the starch synthesis pathway at this step resulting in a lack of amylopectin.

A Molecular View A Molecular View

Parents F1 F2 Progeny

WW ww Ww ¼WW ¼Ww ¼wW ¼ww

1: 2 : 1 Genotype = 3: 1 Phenotype

Chi-Squared AnalysisChi-Squared Analysis

Tests if your observations are statistically different from your expectation.

For example does the Mendel data fit the 3:1 hypothesis?

Chi-squared =

[(observed-expected)2/expected2]