p generation: purebred organisms that are€¦ · · 2012-06-04offspring that arise from a f1 x...
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P generation: purebred organisms that are used as the parent generation
F1 generation: offspring of the P generation
F2 generation:
◦ offspring that arise from a F1 x F1 cross
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Gene: part of chromosome that governs expression of a particular trait
eg. hair colour , plant height
Allele: alternate form of a gene
eg. brown hair (B), red hair (b); tall (T), short (t)
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Phenotype: the observable trait or characteristic ( what you can see )
Genotype: the genetic make-up of the trait ( which 2 alleles are present? ) Types of Genotypes
◦ Homozygous: 2 identical alleles are present eg. BB, tt
◦ Heterozygous: 2 different alleles are present eg. Bb, Tt
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Types of Alleles (traits):
Dominant: trait that is fully expressed (visible)
Recessive: trait that is usually masked (hidden)
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Punnett Square:
◦ method of predicting offspring if the genotypes of parents are known
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Cross-fertilized two purebred pea plants that showed short and tall plant heights, respectively Parent (P) generation
First Filial (F1) Generation offspring obtained were all tall
100 % tall (phenotype)
After repeating this experiment while examining other pairs of traits
he observed that one trait was always DOMINANT than the other.
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Principle of Dominance:
When an organism is a hybrid (crossbred) for a pair of contrasting traits, it will show only the dominant trait.
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Hybrid tall plants from F1 generation in 1st experiment self-pollinated
Second Filial (F2) Generation obtained were mostly tall but some were short
75% tall and 25% short ( ratio – 3 tall : 1 short )
After repeating this experiment while examining other pairs of traits
he observed the same ratio of 3 : 1 (called the Mendelian ratio)
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Law (Principle) of Segregation:
Inherited traits are determined by a pair of “factors” that segregate during the formation of gametes.
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Let the tall trait = T (dominant)
Let the short trait = t (recessive)
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P T T x t t male female T T t t F1 100% T t only combination possible all plants were tall tall is dominant; short is masked
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F2 T t x T t
self-pollinated
Possible gametes: T, t T, t
Observed:
ratio 3:1 (tall : short)
3 tall : 1 short
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Alternate forms of a gene exist called ALLELES
For each inherited trait, an organism has two genes (alleles)
one allele is inherited from each parent.
A sperm or egg carries only one allele for each inherited trait because allele pairs segregate (separate) from each other during the production of gametes by meiosis
When two alleles of a pair are different,
◦ one is fully expressed (DOMINANT allele) and
◦ the other is completely masked (RECESSIVE allele)
NOTE: Today, we know this is not true in all circumstances !
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From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic ◦ One characteristic
comes from each parent!!!
P GENERATION (true-breeding parents)
F1 generation
F2
generation
Purple flowers White flowers
All plants have purple flowers
Fertilization among F1 plants (F1 x F1)
3/4 of plants have purple flowers
1/4 of plants have white flowers
Figure 9.3A
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A sperm or egg carries only one allele of each pair
–The 2 alleles for a gene separate during gamete formation, and each gamete gets a different one
–This is the law of segregation
GENETIC MAKEUP (ALLELES)
P PLANTS
F1 PLANTS (hybrids)
F2 PLANTS
PP pp
All P All p
All Pp
1/2 P 1/2 p
Eggs P
p
P
PP p
Sperm
Pp Pp
pp
Gametes
Gametes
Phenotypic ratio 3 purple : 1 white
Genotypic ratio 1 PP : 2 Pp : 1 pp
Figure 9.3B
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(1) Define symbols:
(2) State the cross
(3) Diagram the gametes
(4) Complete the squares
(5) Summarize the results:
Genotype
Phenotype
Monohybrid Cross: Punnett Square Method
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Genotypic ratio:
1EE : 2Ee : 1ee
Phenotypic ratio: 3:1
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In mice, gray coat colour is dominant over white coat colour.
What phenotypic and genotypic ratios would you
expect if: Heterozygous gray mouse crossed with homozygous
white mouse. Heterozygous gray mouse crossed with heterozygous
gray mouse. Work with the people at your table to solve this
question.
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HOMEWORK: complete worksheet