ch 14 gregor mendel and inheritance study
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Ch 14 Gregor Mendel and Inheritance Study. Began research late 1850’s St. Thomas (Augustinian) monastery, Brno, Yugoslavia. 1. “blending” hypothesis genetic material from the two parents blends together (like blue and yellow paint blend to make green). - PowerPoint PPT PresentationTRANSCRIPT
Ch 14 Gregor Mendel and Inheritance Study
Began research late 1850’s
St. Thomas (Augustinian) monastery, Brno, Yugoslavia
• What genetic principles account for the passing of traits from parents to offspring?
•Mendel’s pea breeding data• supported the particulate mechanism
1. “blending” hypothesis–genetic material from the two parents blends together (like blue and yellow paint blend to make green)
2. “particulate” hypothesis–parents pass on discrete heritable units (genes)
Scientific approach
Asked question: how are traits inherited?
Used convenient model system: garden peas
-controlled breeding
-a variety of either/or visible traits
-quantitative data
Derived conclusions based on the data
For example
To examine inheritance of flower color
Cross-fertilized (hybridized):
true breeding purple x true breeding white
Measurable character: phenotype
LE 14-2
Removed stamensfrom purple flower
Transferred sperm-bearing pollen fromstamens of whiteflower to egg-bearing carpel ofpurple flower
Carpel Stamens
Parentalgeneration(P)
Pollinated carpelmatured into pod
Planted seedsfrom pod
Examinedoffspring:all purpleflowers
Firstgenerationoffspring(F1)
Materials & Methods
100% purplephenotype
“True-breeding”
plants that produce offspring of the same variety when they self-pollinate
• True-breeding parents– P (generation)
• Hybrid offspring (cross between 2 different true-breeding parents)
– F1 (generation)
• Progeny of F1 self-pollination– F2 (generation)
Nomenclature
LE 14-3
P Generation
(true-breedingparents)
F1 Generation
(hybrids)
F2 Generation
Purpleflowers
Whiteflowers
All plants hadpurple flowers
Results fromtwo crosses
F1 x F1
F2
P1 x P2
• Only purple flowers in the F1 hybrids– Purple: dominant trait– White: recessive trait
• Same pattern of dominant inheritance- six other pea plant phenotypes, each represented by two
traits
Mendel’s “heritable factors” (= genes)
(though he did not know the existence of DNA)
Conclusions
Mendel’s Model
• Inheritance patterns obeyed rules of probability
1. Assumes each trait has two variants
Current knowledge: phenotype is controlled by genes (genotype)
paternal and maternal alleles (genes)=two variantspositioned at chromosomal locus
LE 14-4
Allele for purple flowers
Homologouspair ofchromosomes
Allele for white flowers
Locus for flower-color gene
2. Offspring inherit two alleles, one from each parent
P= purple (dominant allele) p= white (recessive allele)
Homozygous if PP or pp (noun; homozygote)
Heterozygous if Pp (noun: heterozygote)
PP=_____________________________
pp=___________________________
3. One of the alleles determines phenotype
4. Law of segregationtwo alleles for any given phenotype separate duringgamete formation.
Thus, offspring inherit one allele from each parent.
During anaphase I
When do alleles first separate from each other?
Convenient quantitative way to predict outcome of crosses
Punnett Square
1. Name alleles with single letter2. Write down genotype of each parent3. Fill in boxes with all possible combinations of alleles
according to parental genotypes
On Board
What is the frequency of genotypes of the offspring of the parental cross?
What is the frequency of phenotypes of the offspring of a parental cross?
What is the frequency of genotypes of the F1 cross progeny?
What is the frequency of phenotypes of the F1 cross progeny?
50% heterozygous; 25% dominant and recessive homozygous
75% purple, 25% white (3:1)
100% heterozygous
100% purple
LE 14-5_2
Appearance:
P Generation
Genetic makeup:
Gametes
F1 Generation
Appearance:Genetic makeup:
Gametes:
F2 Generation
Purple flowersPp
P p1 21 2
P p
F1 sperm
F1 eggsPP Pp
Pp pp
P
p
3 : 1
PurpleflowersPP
Whiteflowerspp
P p
LE 14-6Phenotype
Purple
Purple3
Purple
Genotype
PP(homozygous
Pp(heterozygous
Pp(heterozygous
pp(homozygous
1
2
1
Ratio 1:2:1
White
Ratio 3:1
1
• How can we determine the genotype of an individual with a dominant phenotype?
Genotype possibilities (purple flower color)?
PP or Pp
Test cross
Hybridize to individual with homozygous recessive individual (pp)
Do Punnett for each. Outcome?
LE 14-7
Dominant phenotype,unknown genotype:
PP or Pp?
If PP,then all offspring
purple:
p p
P
P
Pp Pp
Pp Pp
If Pp,then 1
2 offspring purpleand 1
2 offspring white:
p p
P
Ppp pp
Pp Pp
Recessive phenotype,known genotype:
pp
Monohybrid vs dihybrid
Heterozygous for 1 character Heterozygous for 2 characters
Set up a cross betweenheterozygous purple flowered pea plants
Pp x Pp
Do a Punnett Square: determine genotypic and phenotypic ratios.
What are patterns of inheritance 2 characters?
Given: true breeding Pfor pea color & shape
Dominant charactersYellowRound
Recessive charactersgreenwrinkled
Set up Parental Cross
Set up F1 cross
What is phenotypic frequency of the offspring?
LE 14-8
P Generation
F1 Generation
YYRR
Gametes YR yr
yyrr
YyRr
Hypothesis ofdependentassortment
Hypothesis of independent assortment
SpermEggs
YR
Yr
yrYR
YR
yr
Eggs
YYRR YyRr
YyRr yyrr yR
yrPhenotypic ratio 3:1
F2 Generation(predictedoffspring)
YYRR YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
Phenotypic ratio 9:3:3:1
YR Yr yR yr
Sperm
12
14
14
14
14
1 43
4
12
12
12
14
916
316
316
316
14
14
14
• Observation of a dihybrid cross - Each pair of alleles segregates independently of
other pairs of alleles during gamete formation
- Mendel proposed the law of independent assortment
•Note: Applies only to genes on different, nonhomologous chromosomes
Genes located near each other on same chromosome tend to be inherited together: linkage
(Fast forward) Established at metaphase I & II
Autosomes (22 pr)
Sex chromosomes
Inheritance of Sex-Linked Genes
Sex chromosomesCarry genes that determine sexual phenotype & other traits
Sex-linked genes:Any gene on sex chromosome (usually refers to X-chromosome because of bigger size, more
genes)
LE 15-10
Sperm
Ova
Sperm
Ova
Sperm
Ova
How to show inheritance of sex-linked traits:follow X and Y chromosomes in a cross
• Sex-linked recessive disorders
–Color blindness–Duchenne muscular dystrophy–Hemophilia
Present hemophilia problem
Mother carrier/ father normal
We’re celebripeas!Questions?