Mendel GeneticsChapter 14

Genetics

The study of heredity

Heredity

Transmission of traitsOne generation to anotherInherited features are the building

blocks of evolution

Historically

Blending of parental contributionsExample: Tall parent + short parent Medium child

Problem

No outside genes All parents traits blendedOver time all members of the species

will look the same.

Variation

Differences in offspring

Vocabulary

Character:Inheritable featureEx: colorTrait: Alternate forms of the characterPurple or white

Vocabulary

True-breeding:Produced same variety as the parentP generation Parental generation

Vocabulary

First filial generation (F1)Offspring from the first crossSecond filial generation (F2)Offspring from the second cross

Vocabulary

Alleles: Alternate versions of the geneDominant: Trait that is expressedRecessive: Trait that is not expressed or hidden

Vocabulary

Homozygous: Pair of the same alleles Heterozygous: Pair of different alleles Genotype: Genetic make-up Phenotype: Appearance of organism

Vocabulary

Hybridization: Crossing of parents that are not alikeHybrids:Offspring with two alleles for traitTestcross:Cross with a homozygous recessive

individual Determines genotype of an

individual.

Vocabulary

Self-fertilization:Fertilization can take place in plant if

undisturbed.Cross-fertilization:Remove the male partsIntroduce pollen from another strain Different traits

Vocabulary

Punnett square: DiagramDisplays allele possibilities of

fertilizations

Vocabulary

Monohybrid: Individuals are heterozygous for one trait Aa Tt Dihybrid: Individuals are heterozygous for two traits AaTt

Gregor Mendel

Austrian monkStudied math & scienceUniversity of ViennaStudied pea plants at the monastery

Why the pea??

1. Has been studiedAble to produce hybrid peas2. Variety with 7 simple & easy to

see traits Purple vs white flower3. Small, easy to grow Short generation time

4. Male & female sex organs located on same plant

Mendel

Chose comparable traits 1. Flower color (white vs purple) 2. Seed color (yellow vs green) 3. Shape of seed (smooth vs wrinkled) 4. Pod color (green vs yellow) 5. Pod shape (inflated vs constricted) 6. Flower location (axial vs terminal) 7. Plant size (tall vs. short)

Mendel’s experiments

Allowed the peas to self-fertilizeUsed true-breeding or pure-breeding

plants

Mendel’s experiment

Crossed plants with alternate forms of characteristics

Example:Tall plants with short plants

Mendel’s experiment

Parental generationPure white flowered plants X pure

purple flowered plants F1 always revealed purple flowered

plantsCrossed the hybrid offspringF2 filial generationSome were purple Some were white

Mendel’s experiment

F1 trait was hidden

F2 trait reappeared

Ratio in the F2 generation 3:1 dominant:recessive 3:1 purple:white

All traits revealed this ratio

Mendel’s experiments

F2 generation self-fertilizedWhite flowers always produce white

flowersPurple flowers 1/3 produced only purple flowers2/3 produced dominant & recessive

flowers in a 3:1 ratio

Mendel’s experiment

Concluded that the F2 generation was really 1:2:1

¼ pure-breeding dominant individuals

½ non-pure breeding¼ pure-breeding recessive

individuals

Mendel’s model

1. Plants did not produce intermediate offspring.

2. Alternate trait was there only not expressed

Mendel’s model

3. Alternate traits segregated in the offspring

4. Mendelian ratio: 3:1 in the F2 generation

¾ dominant ¼ recessive

Mendel’s model

Alleles remain discreteDo not influence the otherDo not blendAre passed on in the gametes

Mendel’s first law of heredity

Law of Segregation:Alternate alleles of a characterSegregate (separate) from each

other & remain distinct.Seen in meiosis when the

homologous chromosomes separateForm gametes

Mendel’s experiment

Crossed dihybridsF1 generation demonstrated

dominant phenotype for both traitsF2 generation showed a 9:3:3:1

phenotype (16 gamete combinations)Each trait showed a 3:1 ratio similar

to a monohybrid cross

Mendel’s second law of heredity

Law of Independent Assortment:Genes located on different

chromosomes Assort independentlyAssuming the genes are on separate

chromosomes

Mendel

Phenotypes may be influenced by many factors

Many different genesEnvironment

Incomplete dominance

Not all chromosomes are dominant or recessive

Heterozygous genotype can cause an intermediate between the parents

Codominance

Effect of both alleles can be seenMN blood groupsMolecules on surface of RBCMM, NN or MNMN see affects of both

Codominance

Tay-Sachs disease (homozygous recessive)

Brain cells unable to break down lipidsLacking enzyme build up lipidsRetardation & early deathHeterozygous 50% the normal enzyme levelsSurvive

Tay Sachs

1 in 300,000 births in the US1 in 3500 births in Ashkenazi Jews1 in 28 are carriers in this population

Multiple alleles

ABO blood typeGene codes an enzyme Adds a sugar to lipidsLocated on the surface of the RBCSugars act as recognition markers for

the immune system

ABO

3 gene alleles4 different blood typesI is the enzymeIA (allele) adds galactoseIB (allele) adds galactosaminei (allele) has no sugar

ABO

Type A IAIA HomozygousType A IAi HeterozygousType B IBIB HomozygousType B IBi HeterozygousType AB IAIB HeterozygousType O ii Homozygous

Rh blood group

Cell surface marker on the RBC85% have the markerRh +Rh - does not have the markerIf a Rh- person gets blood that is Rh

+ Develops antibodies against Rh+

blood.

ABO

Problem Rh- mother gives birth to a child that is Rh

+ (Rh+ dad) She has built up antibodies They could cross into the babies blood. Erythroblastosis fetalis: Babies blood clumps due to antibodies

against it’s Rh factor RhoGam

Pleiotropic

Allele has more than one effect on the phenotype

One gene has many effects Peas: gene for flower color Codes for seed cover color Yellow mice Gene for yellow fur Same for lethal developmental defect So homozygous dominant would die

Pleiotropic

Inherited diseases that one gene produces many symptoms

Sickle cell anemia Anemia Joint pain/swelling Heart failure Splenomegaly Renal failure

Sickle cell

Single aa change in beta-globin of hemoglobin

Causes hemoglobin to be stickySickle cell shapeHigher incidence to people of African

decent 1/500Heterozygous for the disease Have greater resistance to malaria

Pleiotropic

Cystic fibrosisMutation in the gene that encodes

the chloride ion trans membrane channel Increased mucous Salty sweat Liver/pancreatic failure SOB

Epistasis

One gene can interfere with the expression of another gene

Interaction between two non-allelic genes

Controls phenotypic expression of a single trait

Epistasis

Corn (Zea Mays)Purple pigment called anthocyanin

pigmentRequires two working enzyme genes

to produce the colorDominant alleles have functional

genesRecessive alleles have non-functional

genes

Epistasis

Both dominant genes present Corn will be purple (AABB, AaBb)One dominant & one recessiveCorn will be white. (aaBb, aaBB,

Aabb, AAbb)9:7(purple:white)9/16 vs 7/16

Epistasis

Labrador retrievers has two genes that affect fur, nose

Epistasis

E gene is the gene for colorEE or Ee genotype Dark pigment will be depositedee no pigment

Epistasis

B gene determines darkness of pigment

Distributes melanosomes (hair)EEBB, EeBb will be a black labEEbb, Eebb will be a chocolate labeeBB, eeBb will have yellow fur/black

noseeebb will have yellow fur/brown nose

Fig. 14-12

BbCc BbCc

Sperm

EggsBC bC Bc bc

BC

bC

Bc

bc

BBCC

1/41/4

1/41/4

1/4

1/4

1/4

1/4

BbCC BBCc BbCc

BbCC bbCC BbCc bbCc

BBCc BbCc

BbCc bbCc

BBcc Bbcc

Bbcc bbcc

9 : 3 : 4

Polygenes

Additive effect of two or more genes determines a single phenotypic character.

Continuous variation

When multiple genes jointly influence a character

A range in the degree of expressionSuch as height or weightQuantitative traits: Traits that cause a range in

phenotype

Continuous variation

• Three genes with the dark-skin allele (A, B, C)

• Contribute to the phenotype • A cross between two AaBbCc individuals • Produce offspring covering a wide range of

shades.• Range of phenotypes forms a normal

distribution.

Continuous variation

Environmental effects

Some alleles are heat sensitive.Artic fox makes fur pigment only

when it is warm During the winter it is white/summer

brown

Environmental effects

Siamese catsHeat sensitive enzyme that codes for

MelaninAbove 330C it is inactiveEar tips, nose are colder so they are

darker

Fig. 14-14

Mendelian Inheritance in humans is difficult to study because:

1. Generation time is 20 years.2. Humans produce relatively few

offspring.3. Breeding experiments are

impossible.

Pedigree

Graphical representation of mating over multiple generations for a particular trait

Male

Female AffectedFemale

AffectedMale

Mating

Offspring, inbirth order(first-born on left)

Pedigree

Hemophilia:Bleeding disorderAffects one protein in series of

proteins to clot bloodSex linked genetic abnormalityX-linked recessive alleleHeterozygous females are carriers

but do not have the disease

Human genetics does follows Mendelian principlesMost genetic disorders are recessiveMajority of recessive disorders are

born to heterozygous parents that are symptom free

Deafness in Martha’s Vineyard Single gene Parents are heterozygous for

deafness 25% chance of having a deaf child

Recessive disorders

Cystic Fibrosis 1/1800 European Americans

Albinism 1/22000PKU 1/10,000

Fig. 14-16

Parents

Normal Normal

Sperm

Eggs

NormalNormal(carrier)

Normal(carrier) Albino

Aa Aa

A

AAA

Aa

a

Aaaa

a

Dominant disorders

Not too commonHuntington diseaseAltered protein in nerve cells of the

brainLeads to neural degenerationMental deterioration and

uncontrollable movementsAge of onset around 40-50

Dominant disorders

AchondroplasiaForm of dwarfismHead and torso develop normallyArms and legs are short1/25,000

Genetic counseling

Identifies parents at a riskProduce a child with a genetic

disorderHelps parents plan

Amniocentesis

Needle removes fluid from the pregnant female

Analyzes fluid for genetic anomaliesNeedle is guided by ultrasound.

Amniocentesis

Fig. 14-18a

Fetus

Amniotic fluidwithdrawn

Placenta

Uterus Cervix

Centrifugation

Fluid

Fetalcells

Severalhours

Severalweeks

Severalweeks

Bio-chemical

tests

Karyotyping

(a) Amniocentesis

Chorionic villi sampling

Can be done earlierRemoves cells from the membrane of

placentaLess invasive

Genetic counseling

Identifies aneuploidyHelps identify enzyme problems such

as PKU (phenylketouria) Missing enzyme to break down

phenylalanineTay-Sachs disorder missing the

enzyme to break down gagliosides