Genetics The Scientific Study of Inheritance. Terms Allele Barr body Codominance Dihybrid cross Dominant Epistasis Genotype Heterozygous Homozygous

GeneticsGenetics

The Scientific Study of Inheritance

Terms Terms • Allele • Barr body• Codominance • Dihybrid cross• Dominant• Epistasis• Genotype• Heterozygous • Homozygous

• Inbreeding

• Incomplete dominance

• Linkage

• Locus

• Multi-allelic

• Phenotype

• Pleiotropy

• Polygenic

• Recessive

• Sex-linked

Gregor MendelGregor Mendel

• Monk

• Austria; Czech republic

• 1st to analyze inheritance in a scientific manner

• Scientific method

• Careful record-keeping


• Studied garden peas

– Easy to grow

– Produce lots of offspring

– Easily distinguished characteristics

• Fruit flies - today


• “Parents pass ‘factors’ to their offspring that are responsible for traits”

• ‘Factors’ = genes• Garden peas self-pollinate• True-breedingTrue-breeding = parents

produce offspring identical to themselves


• Control cross-pollination• Cross-pollination produced

hybrids – Called a ‘cross’

• HybridsHybrids = offspring with mixed traits

• TraitsTraits = inherited characteristic

Crossed pure-breeding and got one trait. What happened to the white trait?


• Allowed F1’s to self-pollinate

• Produced F2 generation

• F2’s; 705 purple; 224 white

• 3:1 ratio• The heritable ‘factor’ for

white was ‘masked’ but was not destroyed

Gregor Mendel - 4 Hypotheses:Gregor Mendel - 4 Hypotheses:

1. There are alternate forms for ‘factors’ that control heredity

2. For each characteristic, there are 2 factors inherited; one from each parent

3. A gamete carries only one form for each factor’; during fertilization, the 2 ‘factors’ unite

4. One form of the factor is fully expressed (visible) and the other has no effect

Modern GeneticsModern Genetics• ‘FactorsFactors’ = genesgenes• Alternate ‘forms’Alternate ‘forms’ == allelesalleles • GenesGenes = sections of DNA; code for making

proteins• Expression of proteins determines trait• Dominant Allele= allele that IS expressed;

protein is expressed (made)• Recessive Allele = allele that is NOT expressed

(made); or masked; protein is not made

Structure Of A ChromosomeStructure Of A Chromosome

• Chromosomes are homologous pairs

– Same size, banding, centromere location and genes

• Made of DNA

• Sections of chromosomes are genes

Chromosome 1 Homologue

Gene Allele Allele

Two alleles to a gene; alleles

may be dominant or

recessive

From momFrom dad

• GenotypeGenotype = an organism’s genetic makeup– PP

• PhenotypePhenotype = an organism’s expressed or physical traits– Purple

Modern GeneticsModern Genetics

Mendel’s Principle of Mendel’s Principle of Segregation:Segregation:

Law of SegregationLaw of Segregation

Principle of SegregationPrinciple of Segregation

• All organisms have 2 sets of homologous chromosomes; one from each parent;

– Diploid

• One allele located on each chromosome; one from mom, one from dad

– 2 alleles = 1 gene


• Pairs of alleles separate (segregate) during gamete formation

• 1 form of a ‘factor’ goes into 1 gamete while the other form separates and goes into another gamete (handout)

• LocusLocus = location of a gene on a chromosome; loci (pl.)

• Alleles are at the same locus on each homologous chromosome

• HomozygousHomozygous = both alleles for the trait are the same (homo)

– PP, pp = homozygous

• HeterozygousHeterozygous = the two alleles are different

– Pp = heterozygous


Fertilization Fertilization • During fertilization, the sperm unites with the egg• 1 haploid sperm + 1 haploid egg = 1 diploid diploid

zygotezygote• Which sperm unites with which egg is by random

chance– Flipping a coin

This is too hard to do!!!!

Use The Laws of ProbabilityUse The Laws of Probability

• Probability = chance that something will occur

• How can we predict what will happen easier?

• Punnett SquarePunnett Square

• How does it work, you say?

• I’m so glad you asked ……

Punnett Square Punnett Square

1. Use letters to represent each allele

a. Use the CAPITAL for dominant and small case for recessive

b. Ex. P = purple; p = white

c. T = tall; t = short

d. Y – yellow; y - green

2. Draw a square PURPLE PURPLE x white

3. Determine what letters to use to represent the alleles

Example:

a. Cross a PURPLEPURPLE with a white flower

b. . PURPLEPURPLE is dominant over white in pea plants so use P = PURPLEP = PURPLE and p = white

c. Every gene has 2 alleles so use 2 letters

PPPP pp

Crossing a homozygous purple flower with a homozygous recessive white flower

PPPP pp

X

4. Separate letters (alleles) around the square – this represents segregation

PP PP

p

p

BE CAREFUL HOW YOU

MAKE YOUR LETTERS!!

PPPP

pp

5. Combine the letters (alleles) into each box of the square

PP PP

PURPLE PPPURPLE PP x white pp

p

p

PP PP

PP PP

pp pp

pp pp

6. Determine the results

PP PPPPPP x pp = 4 Pp; and 4 PURPLEPURPLE

p

p

PPpp PPpp

PPpp PPpp

Genotype = 4 Pp

Phenotype = 4 PURPLEPURPLE

1 2

3 4

Purple Purple

Purple Purple

Results:Results:

• Genotype – combination of letters (alleles);

– Pp

• Phenotype – appearance (what do they LOOK like?

– PurplePurple

What If You Crossed heterozygous

purple with heterozygous purple?

PPpp x PPpp

PPppPPpp

Separate letters (alleles) around the

square

PP

PPp

p

p

PP

PPp

Combine the letters (alleles) in the

squares

PP

p

p

PP PPPP PPp

pPP pp

PurplePurple PurplePurple

PurplePurple white

PP

PPp x PPp

p

PPPP PP

PPpp

pp

pppp

pp

PP

Genotypes –

Phenotypes –

1 - PP

2 - Pp

1- pp

3 - PURPLE3 - PURPLE

1 - white

PurplePurple PurplePurple

PurplePurple white

Practice ProblemsPractice Problems

• Tall is dominant to short

• What genotypic and phenotypic results would be expected if you crossed a HOMOZYGOUS tall with a HOMOZYGOUS short?

Practice ProblemsPractice Problems

• What genotypic and phenotypic results would

be expected if you crossed a HOMOZYGOUS tall

with a HOMOZYGOUS short?

T T

t t

T T

Tt Tt

Tt Ttt

t

Genotypes -

4 - Tt

Phenotypes -

4 - tall

100% tall

Tall

Tall

Tall

Tall

• In pea plants, yellow is dominant to green. What results would be expected if you crossed a homozygous yellow with a homozygous green?

• Homozygous = same• Yellow – Y; green – y• Homozygous yellow = YY• Homozygous green = yy

Practice:

Y Y

y

y

Yy Yy

Yy Yy

Yellow Yellow

Yellow Yellow

Genotype – 4 Yy

Phenotype – 4 yellow; 100% yellow

• Black fur is dominant to brown fur in mice. What results should you expect if you crossed a homozygous black with a homozygous brown?– Black is dominant so use B; brown - b– Homozygous black = BB– Homozygous brown = bb

Practice

B B

b

b

BbBb BbBb

BbBb BbBb

Black Black

Black Black

Genotype – 100% Bb

Phenotype – 100% black

Law of Independent Law of Independent AssortmentAssortment

Are Traits Inherited Together (dependently) or Separately (independently)?

Law of Independent Assortment Law of Independent Assortment

• Round (R) is dominant to wrinkled (r)

• Yellow (Y) is dominant to green (y)

• Result from crossing two traits?

– If you inherit a dominant trait does the other trait also have to be dominant?

• Dihybrid cross – result of crossing two traits together

Dihybrid CrossDihybrid Cross

• Homozygous (pure-breeding) round (RR), and yellow (YY) with:

• Homozygous recessive; wrinkled (rr), green (yy)


• Are the two traits inherited together (in a ‘package’) or can they be inherited separately?

• Mendel crossed the P’s (yellow, round x green, wrinkled)

– F1’s were all dominant (yellow, round)

• Allowed the F1’s to self-pollinate


• 9:3:3:1 ratio

• 9/16 = yellow, round

• 3/16 = yellow, wrinkled

• 3/16 = green, round

• 1/16 = green, wrinkled

Independent Assortment:Independent Assortment:

YyRr

YR Yr yR yr

Parent: 1 & 2

YR Yr yR yr

YR

Yr

yR

yr

Law of Independent Assortment

Yy Rr

Yy Rr

YR Yr yR yr

YR YYRR YYRr YyRR YyRr

Yr YYRr YYrr YyRr Yyrr

yR YyRR YyRr yyRR yyRr

yr YyRr Yyrr yyRr yyrr

Law of Independent Assortment:Law of Independent Assortment:

• Each pair of alleles segregates independently of other pairs of alleles

• Can recombine independently of each other

• Genetic Genetic vvaarriiaattiioonn – Biggest cause of genetic variation in sexually Biggest cause of genetic variation in sexually

reproducing organismsreproducing organisms

Independent AssortmentIndependent Assortment

• Budgies inherit two colors INDEPENDENTLY

• Color (Yellow) or no color on the outer surface of the feather

• MelaninMelanin or no melanin in the inner core of the feather

Variation and Patterns of VariationVariation and Patterns of Variation

• Wild type - most common traits in the wild

– Budgies = green feathers

• Knowing patterns and rules of inheritance allows breeders to produce blues, yellows, and whites

Budgie ColorBudgie Color

Two genes inherited separately1. Outside color of feather2. Inside color of feather

Independent assortment; the two characteristics are inherited independently of each other

GreenGreen = Y_B_

Blue = yyB_

Yellow; Y_bb

White; yybb

Test CrossTest Cross

How can We Use Genetics to Determine if Our Organism is Pure-

breeding?

Test crossTest cross

• Mate an individual whose genotype is not known (dominant phenotype) with a homozygous recessive for that trait

• Ex. Is your favorite Labrador a ‘pure’ black or does he carry a recessive allele?

Test crossTest cross

• Cross the unknown with a homozygous recessive

• Eight puppies born, 3 are brown (recessive)

• ?

B B

b

b

Bb Bb

Bb Bb

If the unknown is homozygous (pure) then all the offspring are dominant

B b

b

b

Bb bb

Bb bb

If the unknown is heterozygous (carrier) then some offspring are recessive

Variations of MendelVariations of Mendel

1.1. Complete dominance Complete dominance

2.2. Incomplete dominanceIncomplete dominance

3.3. CodominanceCodominance

4.4. Multiple allelesMultiple alleles

5.5. PleiotropyPleiotropy

6.6. Polygenic inheritancePolygenic inheritance

7.7. LinkageLinkage

Incomplete DominanceIncomplete Dominance

• Dominant allele does not totally mask recessive allele

• Some recessive trait is expressed: blended

RedRed x white = pink pink

Curly hair + straight hair = wavy

Incomplete DominanceIncomplete Dominance

• Heterozygotes express a trait between the dominant and recessive

• Familial hypercholesterolemia

– hh = very high cholesterol

– Hh = mild cholesterol

– HH = low cholesterol; ‘normal’

CodominanceCodominance

• Both traits are EQUALLY dominant;

• Both traits are expressed (not blended)

– Roan color

– Sickle cell

– Blood types

CodomCodominanceinance • Two different traits and both show

equally– Roan color– Blood types

Blood TypesBlood Types

• Antigens = proteins on the surface of red blood cells (RBC’s)

• Antibodies = proteins floating in the plasma of blood that bind with ‘foreign’ proteins (antigens)

• Antibodies stick to ‘foreign’ antigens forming a clot


• ‘B’ into ‘A’ causes a clot

• ‘A’ into ‘B’ causes a clot


• Antibodies will be the opposite of the antigens

– “A” blood will have “B” antibodies

– ‘B’ blood will have ‘A’ antibodies

• Antibodies are like guard dogs; they attack foreign cells with the wrong antigens


• CodominanceCodominance

• Multiple allelesMultiple alleles = 1 gene but three possible allele combinations– A, B, O

Blood Types: PhenotypesBlood Types: Phenotypes

• AntigensAntigens = proteins on the surface of cells (RBC’s)

• Cell-to-cell recognition

• AntibodiesAntibodies = proteins floating in the plasma of blood that bind with ‘foreign’ proteins (antigens)


• Antibodies agglutinate to antigens that are ‘foreign’

• Agglutinate = clot, clump

• “B” into “A” causes agglutination


• Blood type = type of antigens on the surface

• Antibodies will be the opposite of the antigens

• “A” blood will have “B” antibodies

‘A’ antigens

A

‘B’ Antibodies

‘B’ antigens

B

‘A’ Antibodies

‘B’ antigens

B

‘B’ AntibodiesB antibodies attach

to B antigens; causes blood to agglutinate

Person with ‘A’ blood:

‘A’ antigens

A

‘A’ Antibodies

Person with ‘B’ blood:

‘A’ antigens are attacked by ‘A’

antibodies

A, B antigens

AB

No antibodies

Person with AB blood:

No antigens

O

A and B antibodies

Person with O blood:

Blood Types: GenotypesBlood Types: Genotypes

• Dominant allele = I

• Recessive allele = i (inability)

• II, Ii, ii

• Dominant allele can carry A or B

• Ia or IB


• 2 alleles for each gene:

• ‘A’ = IAIA or IA i

• ‘B’ = IBIB or IB i

• ‘AB’ = IAIB

• ‘O’ (zero) = ii

phenotype genotype antigens antibodies Receive

From:

A IAIA

IAi

A Anti-B A or O

B IBIB

IBi

B Anti-A B or O

AB IAIB A,B None A, B, O

O ii None Anti-A

Anti-B

O

How to do Punnett Squares With How to do Punnett Squares With BloodBlood Types: Types:

IA i

IA

IA

IAIA

IAIA

IA i

IA i

Heterozygous

Homozygous

Can 2 people With A and B Can 2 people With A and B Blood Have a Child With O Blood Have a Child With O

Blood?Blood?

IA i

IB

i

IAIB

IA i

IB i

i i

Heterozygous A

Heterozygous B

AB B

A O

Pleiotropy Pleiotropy

• One gene has multiple effects

• Sickle-cell anemia; p. 160

– Abnormal blood cells

– Difficulty breathing

– Brain, heart, kidney damage

PleiotropyPleiotropy:: Heterozygote AdvantageHeterozygote Advantage

• High incidence of sickle-cell in areas where there is a lot of malaria

• Malaria does not effect sickle-cell

• People w/ sickle-cell don’t suffer malaria

Polygenic InheritancePolygenic Inheritance

• Multiple genes produces a continuous effect; very dark-very light

– Skin, hair, eye color

– 6 – 10 alleles

– AABBCC - aabbcc

LinkageLinkage

• Early 1900’s; TH Morgan

• Fruit flies

• Inheritance patterns did not follow Mendelian Laws of Probability (?)

• Genes are linked

LinkageLinkage

• Genes on the same chromosome are inherited together

• Sex – linked genes

• Gene located on the sex chromosome (usually X)

Sex linkage and Punnett Sex linkage and Punnett SquaresSquares

LinkageLinkage

• The sex-linked trait is usually on the X chromosome

• X X = female• X Y = male• XH = ‘normal’• Xh = hemophilia

HemophiliaHemophilia

• Sex-linked, recessive disorder

• ‘Bleeders disease’; lack protein for blood clotting

• Czar Nicholas’ son “Nicki”; p. 168

XH

XH

XH

Y

XH XH XH XH

XH Y XH Y

Normal phenotypes:

XH

XH

Xh

Y

XH XH XH Xh

XH Y Xh Y

Hemophilia phenotype:

Sex-linked Traits:Sex-linked Traits:

• Hemophilia

• Duchenne’s Muscular dystrophy

• Color-blindness

• Mostly males

• Smartness

Sex-linked Traits: Y ChromosomeSex-linked Traits: Y Chromosome

• “Maleness”

Censored

Censored

Censored

Pedigrees Pedigrees

• Tracing traits back over generations

• Dominance does NOT mean that a phenotype is ‘normal’ or more common

• Wild type

PedigreesPedigrees

• Dominance = heterozygote displays the trait

• Recessive expression occurs only if the genotype is homozygous

• bb, tt, ff

PedigreesPedigrees

• Used to predict probability of genetic disorders

• CarriersCarriers = individuals who do not express the trait but have the recessive allele in their genotype

Human DisordersHuman Disorders

• Single gene:

• 2 types;

– Dominant expression

– Recessive expression

Human Disorders: Dominant Human Disorders: Dominant

• Only requires one allele for trait to be expressed

– Polydactyly; multiple fingers

– Achondroplasia; dwarfism, heterozygotes

Human Disorders: DominantHuman Disorders: Dominant

• Most dominant disorders are not lethalMost dominant disorders are not lethal – Huntington’s diseaseHuntington’s disease; midlife

expression, degeneration of the nervous system

– HypercholesterolemiaHypercholesterolemia – high cholesterol; heart disease

Human Disorders: RecessiveHuman Disorders: Recessive

• Homozygous for the disorder to be expressed

– Cystic fibrosis; Cystic fibrosis;

– Sickle cell anemiaSickle cell anemia

– Tay-Sachs diseaseTay-Sachs disease

– PKUPKU

Fetal TestingFetal Testing

• AmniocentesisAmniocentesis = removal of amniotic fluid (surrounds the developing baby); 20 ml– Biochemical tests (spina bifida, infections)– Cells karyotyping (Down’s, Tay-Sachs)


• Chorionic villus samplingChorionic villus sampling (CVS) fetal cells removed from placenta

– Karyotyped quickly


• UltrasoundUltrasound = view of baby

• FetoscopyFetoscopy = direct view of baby

Recessive DisordersRecessive Disorders

• Cystic fibrosis – whites; build up of mucus in lungs, pancreas


• Sickle cell anemiaSickle cell anemia – Black and SE Asia; 1/500 (lethal), 1/10 carrier;

– Codominant – one allele is normal, other forms hemoglobin that crystallizes in low oxygen


• Tay-SachsTay-Sachs – Jewish; lack gene that produces enzyme that breaks down lipids in the brain; causes brain degeneration, lethal by age 3-4

• PKU – phenylketonuria; lack the gene needed to make the enzyme that breaks down phenylalanine. Phenylalanine accumulates causing nervous disorders. Treated with diet

Pedigrees Pedigrees

• Tracing traits back over generations

• Dominance does NOT mean that a phenotype is ‘normal’ or more common

• Wild type

PedigreesPedigrees

• Used to predict probability of genetic disorders

• CarriersCarriers = individuals who do not express the trait but have the recessive allele in their genotype

Pedigree Pedigree

• Family tree• Shows how a trait is passed down from one

generation to the next

= male

= female

Pedigree number 1

Pedigree number 2

Pedigree number 3

Documents

Genetics The Scientific Study of Inheritance. Terms Allele Barr body Codominance Dihybrid cross Dominant Epistasis Genotype Heterozygous Homozygous