Mendel and Inheritance

MUPGRET WorkshopJune 13, 2005

Genetic 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.

Trait 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.

Mendel 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.

Mendel 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 Pea Pisum sativum Diploid Differed in seed shape, seed color,

flower color, pod shape, plant height, etc.

Each phenotype Mendel studied was controlled by a single gene.

Terms 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 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 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 SegregationParental Lines

Round Wrinkled

X

All round F1 progeny

Self-pollinate

Round5474

Wrinkled1850

3 Round : 1 Wrinkled

Mendel’s ResultsParent CrossParent Cross FF11

PhenotypePhenotypeFF22 data data

Round x Round x wrinkledwrinkled

RoundRound 5474 : 5474 : 18501850

Yellow x greenYellow x green YellowYellow 6022 : 6022 : 20012001

Purple x whitePurple x white PurplePurple 705 : 224705 : 224

Inflated x Inflated x constricted podconstricted pod

InflatedInflated 882 : 299882 : 299

Green x yellow Green x yellow podpod

GreenGreen 428 : 152428 : 152

Axial x terminal Axial x terminal flowerflower

AxialAxial 651 : 207651 : 207

Long x short Long x short stemstem

LongLong 787 : 277787 : 277

Important 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 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 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 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.

Allele 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 Segregation 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 SegregationParental 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 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. wrinkled The SBEI causes the round vs.

wrinkled phenotype. SBEI = starch-branching enzyme Wrinkled peas result from absence

of the branched form of starch called amylopectin.

When dried round peas shrink uniformly and wrinkled do not.

Round 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. 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

Parents F1 F2 Progeny

WW ww Ww ¼WW ¼Ww ¼wW ¼ww

1: 2 : 1 Genotype = 3: 1 Phenotype

Chi-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/

expected]

Testcross and Backcrossx

x

WW ww

Wwww

Wwww

W w

w Ww ww

w Ww ww

Parents

F1

TestcrossProgeny

Mendel and two genes

xRoundYellow

WrinkledGreen

All F1 Round, Yellow

RoundYellow

315

RoundGreen108

WrinkledYellow

101

WrinkledGreen

32

Mendel and two genes

RoundYellow

315

RoundGreen108

WrinkledYellow

101

WrinkledGreen

32

Round = 423Wrinkled = 133

Yellow = 416Green = 140

Each gene has a 3 : 1 ratio.Each gene has a 3 : 1 ratio.

Punnett Square

Yellow¾

Green¼

Round¾

Round, Yellow¾ x ¾ = 9/16

Round, Green¾ x ¼ = 3/16

Wrinkled¼

Wrinkled, Yellow

¼ x ¾ = 3/16

Wrinkled, Green¼ x ¼ = 1/16

Ratio for a cross with 2 genes Crosses with two genes are called

dihybrid. Dihybrid crosses have genetic

ratios of 9:3:3:1.

Principle of Independent Assortment

Ww Gg

WG¼

Wg¼

wG¼

wg¼

F1

Gametes &Frequencies

If a gamete contains W the probability that it contains G is If a gamete contains W the probability that it contains G is equal to the probability that it contains g.equal to the probability that it contains g.

¼ WG ¼ Wg ¼ wG ¼ wg

¼ WG

WW GG 1/16

WW Gg1/16

WwGG1/16

WwGg1/16

¼ Wg WWGg1/16

WWgg1/16

WwGg1/16

Wwgg1/16

¼ wG WwGG1/16

WwGg1/16

wwGG1/16

wwGg1/16

¼ wg WwGg1/16

Wwgg1/16

wwGg1/16

wwgg1/16

Phenotypes W = Round w = Wrinkled W is dominant to w. G = Yellow g = Green G is dominant to g.

F2 Progeny

Genotype Phenotype

1/16 WWGG + 2/16 WWGg 1/16 WWGG + 2/16 WWGg + 2/16 WwGG + 4/16 + 2/16 WwGG + 4/16 WwGgWwGg

9/16 Round Yellow9/16 Round Yellow

1/16 wwGG + 2/16 wwGg1/16 wwGG + 2/16 wwGg 3/16 Wrinkled 3/16 Wrinkled YellowYellow

1/16 WWgg + 2/16 Wwgg1/16 WWgg + 2/16 Wwgg 3/16 Round Green3/16 Round Green

1/16 wwgg1/16 wwgg 1/16 Wrinkled 1/16 Wrinkled GreenGreen

Principle of Independent Assortment Segregation of the members of any

pair of alleles is independent of the segregation of other pairs in the formation of reproductive cells.

Summary of Mendel Inherited traits are controlled by the

alleles present in the reproductive cells that fuse to form the embryo.

In a diploid, progeny inherit one allele from the mother and one from the father.

Differences in the DNA sequence of two alleles for a gene may result in different phenotypes.

Summary The phenotype is the same if the

gene is inherited from the mother or from the father.

One allele from the diploid is inherited in each reproductive cell.