Mendel & Inheritance

Sherry Flint-Garcia

USDA ARS

MU Division of Plant Sciences

Genetic variation

• In the beginning, geneticists studied differences they could see (i.e. morphological differences).

• Individual variants are referred to as phenotypes.– tall vs. short corn plants– red vs. green grapes

Source: USDA

Source: MGDB

Trait

• A broad term encompassing a distribution of phenotypic variation.

• Two types of traits:

Qualitative or DiscreteTrait: Kernel Color

Phenotype: Purple vs. Yellow

Governed by one or few genes

Quantitative or ContinuousTrait: Corn Rootworm Resistance

Phenotype: 1 (no damage) to 6 (severe damage)

Usually governed by many genes

1 2 3 4 5 6Source: USDA

Source: MGDB

Advantages of Plants• Can make controlled crosses.

– Including making inbreds (in many plant species) without the severe effects that are typical in animals.

• One plant can produce tens to hundreds of progeny, and we can store the progeny for long periods of time.

• Less costly and time consuming to maintain than animals. – Generation time is often much shorter.

Source: MGDB

Pre-Mendel

• Blending Theory– Traits present in the parents are mixed (or

blended) in offspring.

• Inheritance of Acquired Characteristics– Traits present in parents are modified,

through use, and passed on to the offspring in their modified form

• They didn’t know about genes or DNA!!!!

Gregor Mendel• Performed several experiments

between 1856 and 1863 that are the basis for what we know about heredity today.

• Published his work in 1866.

• His papers were largely ignored for more than 30 years until other researchers appreciated the significance of his work.

• Described the behavior of particulate (discrete) bodies = genes.

Garden Pea

Pisum sativum

• Many morphological differences.

• Diploid with short generation time.

• Ability to make controlled pollinations.

Each phenotype Mendel studied was controlled by a single gene.

http://staff.tuhsd.k12.az.us/gfoster/standard/pea.gif

Terms• Wild-type is the phenotype that would normally be

expected.• Mutant is the phenotype that deviates from the norm: it 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.

• Phenotype is the observable or measurable characteristic of an individual.

• Genotype is the genetic constitution of an individual.

Terms continued…• An allele is a particular form of a gene.

– The “tall” gene has two alleles: T (tall) & t (short)

• A homozygous individual (homozygote) has two identical alleles, e.g. TT or tt.

• A heterozygous individual (heterozygote) has two different alleles, e.g. Tt.

• The dominant phenotype is expressed in either a homozygous or heterozygous genotype.– TT and Tt condition tall plants. Tall is dominant over short.

• The recessive phenotype is only expressed in a homozygous genotype. – tt conditions short plants. Short is recessive to tall.

Mendel’s Experiment

Parental (Inbred) Lines Round Wrinkled×

ALL Round

F1 progeny

3 Round : 1 Wrinkled

Self-pollinate

Round5474

Wrinkled1850

F2 progeny

Since the F1 progeny are all round, we know that the

round phenotype is dominant and the wrinkled phenotype

is recessive.

Mendel’s Results

Parental Cross F1 Phenotype F2 data

Round x wrinkled seed Round 5474 : 1850

Yellow x green seed Yellow 6022 : 2001

Purple x white flower Purple 705 : 224

Inflated x constricted pod Inflated 882 : 299

Green x yellow pod Green 428 : 152

Axial x terminal flower Axial 651 : 207

Long x short stem Long 787 : 277

All approximately 3 : 1

The 3 : 1 ratio is the key to interpreting Mendel’s data.

Important Observations• F1 progeny are heterozygous, but express only

one phenotype, the dominant one.

• In the F2 generation, plants with both phenotypes are observed: i.e. some plants have recovered the recessive phenotype.

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

Mendel’s Conclusions

• Law of Unit Character– Characteristics of an individual are controlled

by hereditary factors (genes) that maintain their characteristics when passed down. These factors occur in pairs.

• Law of Dominance– Some inherited factors are dominant and can

mask the other, recessive factor.

A a

Mendel’s First Law:Law of Segregation

• Each gamete contains only one factor from each pair of factors (from the parent).

• The pair of factors from a parent separate (or segregate) during reproduction, such that an offspring receives one factor from each parent. Each gamete is equally likely to contain either factor of the pair.

• Fertilization restores the paired condition in the offspring.

First Law Meiosis

• Cellular division resulting in four gametes, each possessing half the number of chromosomes of the original cell.

A Ba b AA BBaa bb

AA bb

aa BB

A b

a B

a B

A b

“Pair of Factors”

“Factor”

“Segregation of Factors”

Round vs. Wrinkled • The wild-type allele (W) is round, and the mutant allele

(w) is wrinkled. W is dominant to w.• The enzyme SBEI (starch-branching enzyme) makes

branched starch (amylopectin).• Round peas (W-) have a functional SBEI, resulting in

amylopectin. Wrinkled peas (ww) have a non-functional SBEI and, therefore, do not make amylopectin.

• Amylopectin causes the seed to shrink uniformly. As a result, round peas shrink uniformly and remain round. Wrinkled peas shrink non-uniformly and become ‘wrinkled.’

Punnett Square

• Grid used to determine the results of simple genetics crosses.

WwWww

WwWww

WWMale

Female

WW ww×

Ww

Round Wrinkled

Round

Female Male

All Round (Ww)

Punnett Square

WwWww

WwWww

WWMale

Female

WwWww

WwWww

WWMale

Female

WW ww×

Ww

Round Wrinkled

Round

Female Male

WW wwWw

Round WrinkledRound

1 2 1

3 round : 1 wrinkled (phenotypic ratio)

Self-pollinate

1 WW : 2 Ww : 1 ww (genotypic ratio)

Chi-Squared (2) Analysis

• Tests if your observations are statistically different from your expectation.

• For example, does the Mendel data (round vs wrinkled) fit the 3:1 hypothesis?

2 = [(observed-expected)2/expected]Observed Expected Expected obs - exp (obs – exp)2

Data Ratio Data

Round 5474 3 5493 19 361Wrinkled 1850 1 1831 19 361

Total 7324 4 2= 722

Look up in 2 tables…The difference between observed and expected is NOT significant. Therefore, the observed data fit the 3:1 hypothesis.

Testcross / Backcross

• Used to determine whether a dominant individual is homozygous or heterozygous.– Cross the dominant individual to a

homozygous recessive individual.

UNKNOWN could be

WW or Wwww

×

ALL round Ww

Wrinkled

Round

Ww

Round

Round Two possible outcomes:

ww

Wrinkled

Unknown was WW

Unknown was Ww

1 : 1

Mendel’s Second Law:Law of Independent Assortment• Each pair of factors (genes) segregates

(assorts) independently of the other pairs in a di-hybrid (tri-hybrid, etc.) cross.

• Different pairs of alleles are passed to offspring independently so that new combinations are possible.

• Example: yellow seed color in pea plants can appear in combination with either the round or wrinkled phenotype.

Independent Assortment During Meiosis-1

A Ba b AA BBaa bb

AA bb

aa BB

A

a

b

B

AA bb

aa BB

A

a

B

b

OR

W = round; w = wrinkled; G = yellow; g = green

Di-hybrid (2-Gene) Cross

RoundYellow

WrinkledGreen

All F1 are Round, Yellow

RoundYellow

315

RoundGreen

108

WrinkledYellow

101

WrinkledGreen

32

×

Self-Pollinate

W is dominant to w; G is dominant to g.

Take It One Gene at a Time

RoundYellow

315

RoundGreen

108

WrinkledYellow

101

WrinkledGreen

32

Round = 423Wrinkled = 133

Yellow = 416Green = 140

Each gene has a 3 : 1 ratio.

Put Two Genes Together

WwGg

WG¼

Wg¼

wG¼

wg¼

F1

Gametes &Frequencies

If a gamete contains W, the probability that it contains G is equal to the probability that it contains g (i.e. W/w and G/g are independent of each other).

The BIG Punnett SquareWG Wg wG wg

WG WWGG WWGg WwGG WwGg

Wg WWGg WWgg WwGg Wwgg

wG WwGG WwGg wwGG wwGg

wg WwGg Wwgg wwGg wwgg

9 3 3 1

Trait-Based Punnett SquareYellow

¾

Green

¼

Round

¾

Round, Yellow

¾ x ¾ = 9/16

Round, Green

¾ x ¼ = 3/16

Wrinkled

¼

Wrinkled, Yellow

¼ x ¾ = 3/16

Wrinkled, Green

¼ x ¼ = 1/16

9 3 3 1

Violations of Mendel’s First Law

• Non-Dominant Gene Action– Partial or incomplete dominance

• Flower color in snapdragon

– Co-dominance • ABO blood groups

• Sex-linked inheritance – Human red-green colorblindness

• Polygenic Inheritance– Human skin color

"Ishihara Color Vision Test," developed by Dr. Shinobu Ishihara

Copyright © 2006 Pearson Prentice Hall Inc

ABC ABc AbC Abc aBC aBc abC abc

ABC 6 5 5 4 5 4 4 3

ABc 5 4 4 3 4 3 3 2

AbC 5 4 4 3 4 3 3 2

Abc 4 3 3 2 3 2 2 1

aBC 5 4 4 3 4 3 3 2

aBc 4 3 3 2 3 2 2 1

abC 4 3 3 2 3 2 2 1

Abc 3 2 2 1 2 1 1 0

Violations of Mendel’s First Law• Organellar Inheritance (Maternal Effect)

– Mitochondria/chloroplast inherited from mother

• Epigenetic– Imprinting, paramutation, gene silencing

• Penetrance, e.g. hemachromatosis

• Expressivity, e.g. timing of onset or severity

• Epistasis– Interaction between different genes and alleles

• one gene masks the effect of another gene • two gene pairs complement each other

Violations of Mendel’s Second Law

• Linkage– Genes are physically linked to one

another on chromosomes– Genes that are close together will

NOT assort independently• Deviation from the 9:3:3:1 ratio

– Recombination (during meiosis) may break these linkages

• Recombination is a function of distance

A

C

A

C

BB

a

c

a

c

bb

L M N O P

l m n o p

Summary of Mendel• Inherited traits are controlled by factors (genes)

present in the gametes.• In a diploid, the pair of factors segregate during

gamete formation, and progeny inherit one factor of each pair of factors (one allele) from the mother and one from the father.

• Each pair of factors assorts independently, such that new combinations are possible in progeny.

• With some exceptions (e.g. due to epigenetics and linkage), Mendel established the foundation of modern genetics.