Mendelian Genetics

http://www.youtube.com/watch?v=l1nYsVeLnBEStart at 2:00

Why Peas?• Small• Easy to grow• Short Life Cycle• Can Self or Cross pollinate• Many easily observed “either/or” traits

• “True breeding” = Pure breeding, parents will pass on the same traits to the offspring every time.

Monohybrid Cross• Cross between parents that differ only in one

characteristic

• First Cross • Between Parental (P) Generations• Cross one Green Pod plant with one Yellow Pod Plant• Results? • Offspring (F1 generation) with all yellow pea pods

• Second Cross• F1 Generation plants Self Fertilize• Results?• Offspring (F2 generation) with 3:1 ratio yellow

to green pod

• Dominant Trait = trait visible in the F1 generation (yellow)

• Recessive Trait =trait that reappeared in the F2 Generation (green)

Principle of Segregation• During Meiosis homologous chromosomes will

separate into different gametes so that alleles for a given trait will appear in different gametes

• This allows each parent to contribute one allele for each trait

• Allele = alternate form of a gene (yellow and green are 2 different alleles for the pea color gene)

• Dominant = capital letter (Y = yellow)• Recessive = lower case letter (y = green)• An individual inherits 2 alleles for each trait (1 from

each parent)

YyY Y yyx

• Genotype = genetic makeup of an individual (YY, Yy or yy)

– Homozygous = both alleles are the same• YY or yy

– Heterozygous = both alleles are different• Yy

• Phenotype = Appearance of observable characteristics– If both alleles are the same, the trait they code for will be

expressed • YY = yellow and yy = green

– If both alleles are different, the dominant trait will be expressed• Yy = Yellow

Punnett Squares• Show the expected proportions of possible

genotypes in the offspring of 2 individuals.• Yellow Pea (YY) x Green Pea (yy)• Result 100% offspring genotype will be Yy

y

y

YY

Y Y

Y Y

y y

y y

Time to Practice!

Principle of Independent Assortment• During Gamete formation alleles for one

characteristic assort independently of alleles for another characteristic

• Allows different traits to be inherited separately

Dihybrid Cross

• Involves two independently assorting traits• Cross 2 Heterozygous Round Yellow Pea Plants R = round r = wrinkledY = Yellow y = green

RrYy x RrYy

• Need to identify all the possible allele combinations that can be in the gametes

• RrYy• RY Ry rY ry

SetupRY

RY

Ry

Ry

ry

ry

rY

rY

R YR Y R YR y RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

Results

• Round Yellow

• Round Green

• Wrinkled Yellow

• Wrinkled Green

9

3

3

1

A dihybrid cross between 2 heterozygous organisms always results in a 9:3:3:1 ratio

More Practice!

Set up a Dihybrid cross between a heterozygous Yellow Wrinkled pea and a Green heterozygous Round pea plant.

Yyrr x yyRr

Yr Yr

yr

yryr

yR

yRyr

More Practice!

Set up a Dihybrid cross between a heterozygous Yellow Wrinkled pea and a Green heterozygous Round pea plant.

Yyrr x yyRr

Yr Yr

yr

yryr

yR

yRyr

YyRr

YyRr

YyRr yyRr yyRr

yyRr yyRrYyRr

Yyrr Yyrr

Yyrr Yyrr

yyrr yyrr

yyrr yyrr

More Fun with Punnett Squares• Incomplete dominance– Occurs when both alleles contribute to the phenotype

of a heterozygote to create an intermediate trait– Example: Red flowers x white flowers

= pink flower offspringCRCR = Red CWCW = White CRCW = Pink

• Co dominance– Both alleles are dominant and are both expressed in

the heterozygote– Example: red flower x white flower

= red and white flower offspringRR = Red R’R’ = White RR’ = Red and

white

What type of dominance is this?• A roan cow is the result of a cross between a

red cow and a white cow.• A roan horse is the result of a cross between a

red horse and a white horse.Co dominance!

What type of dominance is this?

• In human blood types, a homozygote type A person is crossed with a homozygote type B person

• Result = heterozygote with type AB bloodCo-dominance!

Multiple Alleles

• Some genes have more than 2 allelic forms

• Example – human blood types

• A, B, O alleles• O is the recessive allele

Type A

AA AO

Type B

BB BO

Type AB

AB

Type O

OO

Predict the potential blood types of the offspring of a Type A and a Type B person

• AA x BB• AO x BB• AA x BO• AO x BO

• 100% AB• 50% AB, 50% BO• 50% AB, 50% AO• 25% AB, 25% AO,

25% BO, 25% OO

X- linked traits• Traits whose only gene is carried on the X

chromosome.• More frequently expressed among males (only 1 X)• Examples: colorblindness and hemophilia

X-linked Punnett Square

• Colorblindness: B = normal vision b= colorblind

• Mom (not color blind but carries the allele) Dad (not color blind)XBXb XBY

XB Xb

XB

Y

XB XB XB Xb

XbXB Y Y

X- inactivation• One X chromosome in females randomly becomes

inactivated in each cell (genes won’t be expressed)• Barr Bodies = inactivated, condensed X• Coloring in Calico cats = black and orange color alleles

are on the X chromosome (female cats can inherit one X with orange allele and 1 X with black allele – splotches result from X inactivation)

• Why don’t we usually see Calico males?

Mrs. Colvin’s Cats

Just to make things a little more interesting…• Linkage• Genes on the same chromosome are linked and therefore

inherited together• Don’t follow Mendelian ratios• Is there a way to “unlink” genes?– Crossing over!

Epistasis• Where one trait prevents the expression of

another trait.• Example:– If you inherited two genes, one for baldness and

another for curly hair, will you see the curly hair gene expressed?

– the baldness gene will prevent the expression of the curly hair gene.

Multifactorial Inheritance• Polygenic– Traits controlled by more than one gene • eye color, height, weight, intelligence, hair color, skin

color, metabolism

– Don’t follow mendelian ratios of expression• Distribution in a population = bell shaped curve