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The Chromosomal basis of inheritance provides an
understanding of the pattern of passage (transmission) of
genes form parent to offspringHeredity Part 1
Ideas about inheritance
Blending Inheritance Particulate Inheritance
The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)
• The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)
• Mendel documented a particulate mechanism through his experiments with garden peas
Gregor Mendel
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
https://www.youtube.com/watch?v=2xpTz7SUbnc
Why study pea plants?
◦There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits
◦Mating of plants can be controlled
◦Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
◦Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another
TECHNIQUE
RESULTS
Parentalgeneration(P) Stamens
Carpel
1
2
3
4
Firstfilialgener-ationoffspring(F1)
5
•Mendel chose to track only those characters that varied in an either-or manner
•He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)
Table 14-1
Fig. 14-3-3
EXPERIMENT
P Generation
(true-breeding parents) Purple
flowers Whiteflowers
F1 Generation
(hybrids) All plants hadpurple flowers
F2 Generation
705 purple-floweredplants
224 white-floweredplants
Fig. 14-4
Allele for purple flowers
Homologouspair ofchromosomes
Locus for flower-color gene
Allele for white flowers
Fig. 14-8
EXPERIMENT
RESULTS
P Generation
F1 Generation
Predictions
Gametes
Hypothesis ofdependentassortment
YYRR yyrr
YR yr
YyRr
Hypothesis ofindependentassortment
orPredictedoffspring ofF2 generation
Sperm
Sperm
YR
YR
yr
yr
Yr
YR
yR
Yr
yR
yr
YRYYRR
YYRR YyRr
YyRr
YyRr
YyRr
YyRr
YyRr
YYRr
YYRr
YyRR
YyRR
YYrr Yyrr
Yyrr
yyRR yyRr
yyRr yyrr
yyrr
Phenotypic ratio 3:1
EggsEggs
Phenotypic ratio 9:3:3:1
1/21/2
1/2
1/2
1/4
yr
1/41/4
1/41/4
1/4
1/4
1/4
1/43/4
9/163/16
3/161/16
Phenotypic ratio approximately 9:3:3:1315 108 101 32
P Generation Yellow-roundseeds (YYRR)
Y
F1 Generation
Y
R R
R Y
r
r
r
y
y
y
Meiosis
Fertilization
Gametes
Green-wrinkledseeds ( yyrr)
All F1 plants produceyellow-round seeds (YyRr)
R R
YY
r ry y
Meiosis
R R
Y Y
r r
y y
Metaphase I
Y Y
R Rrr
y y
Anaphase I
r r
y Y
Metaphase IIR
Y
R
y
yyy
RR
YY
rrrr
yYY
R R
yRYryrYR1/41/4
1/41/4
F2 Generation
Gametes
An F1 F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologouschromosomes assortindependently during gameteformation.
LAW OF SEGREGATIONThe two alleles for each geneseparate during gameteformation.
1
2
33
2
1
Segregation and Independent Assortment of chromosomes result in genetic variation
Vocab
DominantRecessiveHomozygousHeterozygousGenotypePhenotypeMonohybrid CrossTest CrossDihybrid Cross
Fig. 14-7
TECHNIQUE
RESULTS
Dominant phenotype, unknown genotype:
PP or Pp?
Predictions
Recessive phenotype, known genotype: pp
If PP If Ppor
Sperm Spermp p p p
P
P
P
p
Eggs Eggs
Pp
Pp Pp
Pp
Pp Pp
pp pp
or
All offspring purple 1/2 offspring purple and1/2 offspring white
Test Cross: used to determine genotype of individuals with dominant phenotype
Fig. 14-9
Rr RrSegregation of
alleles into eggs
Sperm
R
R
R RR
R rrr
r
r
r1/2
1/2
1/2
1/2
Segregation ofalleles into sperm
Eggs1/4
1/4
1/41/4
Rules of Probability can be applied
Rule of multiplication◦The probability of a compound event (like
flipping 2 coins and both coins landing heads up) is the product of the separate probabilities of the independent events ½ x ½ = ¼
Rule of addition◦The probability that an event can occur in two
or more alternative ways (like the chance of rolling a 1 or a 2 when rolling a die) is the sum of the events 1/6 + 1/6 = 2/6 = 1/3
We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters
A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Example
Dihybrid Cross: TtGg X ttGG
What is the probability of producing offspring that is heterozygous for both traits.
You could do a dihybrid Punnett square.You could do 2 monohybrid Punnett squares
then use the rule of multiplication.
Example
2 monohybrids…◦Tt X tt probability of Tt offspring = ½
◦Gg X GG probability of Gg offspring = ½
◦Probability of both TtGg = ½ X ½ = ¼
Or do dihybrid – 16 box Punnett Square
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