Fig. 14-1. Fig. 14-2a Stamens Carpel Parental generation (P) TECHNIQUE 1 2 3 4.
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Transcript of Fig. 14-1. Fig. 14-2a Stamens Carpel Parental generation (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
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
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Genetic Testing and Counseling
• Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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