fig. 14-1. fig. 14-2a stamens carpel parental generation (p) technique 1 2 3 4

Fig. 14-1

Fig. 14-2a

StamensCarpel

Parentalgeneration(P)

TECHNIQUE

1

2

3

4

Monohybrid Cross for Flower Color

Parentals X

Purple White

100% Purple

F1 Generation

Purple and White Flowered Plants

F2 Generation

X

705 purple and 224 white3:1 Ratio

1. Parents differ for one trait

2. Parents are true breeding

3. Analyze parents for two generations, F1 and F2

4. Examine large number of progeny

Mendel’s Postulates(based on monohybrid crosses)

• Alternative versions of genes (alleles) account for variation in characteristics

• Each individual has two alleles for each gene, one inherited from each parent

• If the individual has two different alleles, one allele is dominant to the other which is recessive

• The two alleles randomly segregate during gamete formation with 50% of the gametes having one allele and 50% having the other allele.

Important Terminology• Genotype – the list of alleles an individual

possesses.

• Phenotype – a measurable characteristic displayed by an individual

• Homozygote – an individual possessing two identical alleles for a gene

• Heterozygote – an individual possessing two different alleles for a gene

Fig. 14-4

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

Example testcross

Independent Assortment vs LinkageP1

F1

Genotype of F1Gametes YR Yr yRyrYR yr

Parental Combination of Alleles

Parental Only 50% Parental 50% Recombinant

Law of Independent Assortment: Each pair of alleles segregates independently of other pairs of alleles during gamete formation

Linked Independent Assort

YR Yr yRyrYR yr

Dihybrid Testcross to

Detect Independent

Assort

YYRR X yyrr

YrRr X yyrr

YR Yr yRyr yr

yyRf

Yyrr

yyrr

YyRr

Eggs

Spermyr

YR

yr

Yr

yR

Phenotypic ratio1:1:1:1

Ratio of parental:Recombinant1:1

Dihybrid

Independent Assortment and Meiosis

What is Mendel’s Law of independent assortment?

A gene is distributed into gametes independently of how other genes are distributed.

Take home message:

Frequency of Dominant Alleles• Dominant alleles are not necessarily more common in

populations than recessive alleles• For example, one baby out of 400 in the United States

is born with extra fingers or toes• The allele for this unusual trait is dominant to the allele

for the more common trait of five digits per appendage• In this example, the recessive allele is far more

prevalent than the population’s dominant allele

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Variations on Mendel

• Allelic Interactions• Sex Linkage• Complete/Partial Linkage• Organellular Genes

Mendel focused on traits based clearly on dominant and recessive alleles. However , the expression pattern of genes for some traits are not so straight forward.

Allelic Interactions

• Complete Dominance• Partial Dominance• Codominance• Multiple Alleles• Pleiotropy• Multigenic traits• Environmental Interactions

Complete Dominance – the heterozygote is indistinguishable from one of the two homozygotes.

• Rh factor – a protein found on the surface of the blood cells in some people.

• Two Alleles– Rh+ - possess the Rh factor– Rh- - lacks the Rh factor

• HeterozygoteRh+,Rh- - possess the Rh factor and has

positive type blood.

Partial DominanceHypercholesterolemia – defect in LDL receptor mediated endocytosis.

Two allelesHc+ - functional LDL receptorHc- defective LDL receptor

HomozygotesHc-,Hc- super high serum cholesterol early onset of heart disease.

Hc+,Hc+ normal serum cholesterol- typical risk of heart disease.

Hc+,Hc- high serum cholesterol – high risks of heart disease.

Multiple Alleles – More than two alleles in the population, although any individual still has only two alleles.

• ABO Blood type – carbohydrate attached to surface protein of blood cells.

• Three Alleles– IA – A type blood– IB – B type blood– IO – O type blood

• Six genotypesHomozygotes HeterozygotesIA,IA IA,IB

IB,IB IA,IO

IO,IO IB,IO

A typeB typeAB typeO type

Codominance – Heterozygote expresses both alleles (with no blending as in partial dominance)

Two non-identical alleles of a gene are both expressed in heterozygotes, so neither is dominant or recessive. Co-dominance may occur in multiple allele systems.

Pleiotropy – one gene may influence multiple traits.

Cystic Fibrosis

Two AllelesCF – healthy allele –functional Cl transporter (dominant)cf – cystic fibrosis allele – non functional Cl transporter (recessive)

Homozygotes for cf exhibit several symptoms

- mucus filled lungs- kidney failure- digestive problems- salty sweat

Genes encoding products used throughout the body are the ones most likely to be pleiotropic.

Mutations in the fibrillin gene cause a disorder called Marfan syndrome.

Fibrillin (long fibers) impart elasticity to the heart skin, blood vessels, tendons and other parts of the body. In Marfan syndrome the fibrillin are defective or not present. The aorta is particularly affected, can rupture during exercise.

Hard to diagnose, affected people are typically, tall, thin , loose jointed. 1 in 5000

Sergei Rachmaninoff

Marfan Syndrome. Become Aware Haris Charalambous, a 6’10’’ Centre, died during a light training session at Toledo University on 9 October 2006. He was 21 years of age. He began his basketball career at Manchester Magic at age 13 and left the club at 18 to take up his scholarship at Toledo. He was later diagnosed with Marfan Syndrome, which is a genetic disease associated with abnormally tall people. At the time, neither the club nor Toledo knew of this situationSyndrome are a tall thin physique, disproportionately long limbs,fingers and toes, flat feet, spinal curvature and abnormally shapednarrow chest.

Multigenic Traits – traits influenced by more than one gene.

Epistasis – Alleles at one gene mask the expression of alleles at a second gene.

Example – black/ brown gene of mice - albino geneB – black is dominant C – allows pigment (dominant)b – brown is recessive c – prevents pigment (recessive)

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

Polygenic Inheritance – many genes influencing a single traitResults in a continuum of variation (height in humans, skin pigmentation in humans, Fat content of milk cows, yield of kernels in corn)

# dark alleles 0 1 2 3 4 5 6 1/64 6/64 15/64 30/64 15/64 6/64 1/64

Multifactoral traits – traits influenced by both the genetic and environment.

ExamplesFlower color in hydrangeaSkin pigmentation in humansMilk yield in cows

• Sex-linked genes follow specific patterns of inheritance

• For a recessive sex-linked trait to be expressed– A female needs two copies of the allele– A male needs only one copy of the allele

• Sex-linked recessive disorders are much more common in males than in females


Fig. 15-4

PGeneration

Generation

Generation

Generation

Generation

Generation

F1

F2

All offspring had red eyes

Sperm

EggsF1

F2

P

Sperm

Eggs

XX

XY

CONCLUSION

EXPERIMENT

RESULTS

w

w

w

w

ww

w w

+

+

++ +

w

ww w

w

w

w

ww

+

++

+ +

+

Reciprocal Crosses

Fruit flies use XY sex chromosomes

X-linked white eye geneR – normal red eyes (dominant)r - white eyes (recessive)

White eye X Red eye Red eye X White eye

♂ ♀ ♂ ♀

Assume flies came from true breeding cultures

White eye X Red eye Red eye X White eye

♂ ♀ ♂ ♀ rY RR RY rr

Rr RY

Rr RY

Sperm

Eggs

r YR

R

50%Red female

50%Red male

Rr rY

Rr rY

Sperm

Eggs

R Yr

r

50%Red female

50%Whitemale

Pedigree Analysis

• A pedigree is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced and described using pedigrees


KeyMale

Female

AffectedmaleAffectedfemale

Mating

Offspring, inbirth order(first-born on left)

Fig. 14-15b

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Widow’s peak No widow’s peak

(a) Is a widow’s peak a dominant or recessive trait?

Ww ww

Ww Wwww ww

ww

wwWw

Ww

wwWW

Wwor

• Pedigrees can also be used to make predictions about future offspring

• We can use the multiplication and addition rules to predict the probability of specific phenotypes


The Behavior of Recessive Alleles

• Recessively inherited disorders show up only in individuals homozygous for the allele

• Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)

• Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair


Fig. 14-16

Parents

Normal Normal

Sperm

Eggs

Normal Normal(carrier)

Normal(carrier) Albino

Aa Aa

A

AAA

Aa

a

Aaaa

a

• If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low

• Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele

• Most societies and cultures have laws or taboos against marriages between close relatives


Cystic Fibrosis

• Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent

• The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes

• Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine


Sickle-Cell Disease

• Sickle-cell disease affects one out of 400 African-Americans

• The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

• Symptoms include physical weakness, pain, organ damage, and even paralysis


Dominantly Inherited Disorders

• Some human disorders are caused by dominant alleles

• Dominant alleles that cause a lethal disease are rare and arise by mutation

• Achondroplasia is a form of dwarfism caused by a rare dominant allele


Fig. 14-17

Eggs

Parents

Dwarf Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd dd

dD

Dd dd

ddDd

d

d

• Huntington’s disease is a degenerative disease of the nervous system

• The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

Huntington’s Disease


Multifactorial Disorders

• Many diseases, such as heart disease and cancer, have both genetic and environmental components

• Little is understood about the genetic contribution to most multifactorial diseases


Genetic Testing and Counseling

• Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease


• For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately

Tests for Identifying Carriers

Fig. 14-18

Amniotic fluidwithdrawn

Fetus

Placenta

Uterus Cervix

Centrifugation

Fluid

Fetalcells

Severalhours

Severalweeks

Severalweeks

(a) Amniocentesis (b) Chorionic villus sampling (CVS)

Severalhours

Severalhours

Fetalcells

Bio-chemical

tests

Karyotyping

Placenta Chorionicvilli

Fetus

Suction tubeinsertedthroughcervix

Newborn Screening

• Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States


fig. 14-1. fig. 14-2a stamens carpel parental generation (p) technique 1 2 3 4

Documents

gene slide

recessive alleles

alleles rh

dihybrid slide

recombinant slide

different alleles

meiosis slide

yryryryryryr slide