Transcript Slide 1
9.1 The science of genetics has ancient roots
Pangenesis was an early explanation for inheritance
• It was proposed by Hippocrates
• Particles called pangenes came from all parts of the organism
to be incorporated into eggs or sperm
• Characteristics acquired during the parents’ lifetime could be
transferred to the offspring
• Aristotle rejected pangenesis and argued that instead of
particles, the potential to produce the traits was inherited
Blending was another idea, based on plant breeding
• Hereditary material from parents mixes together to form an
intermediate trait, like mixing paint
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1860’s Mendel’s Methods:
1. Experimentally rigorous
2. Mathematically rigorous
Proposed Particulate Hypothesis:
Parents pass on separate and
distinct factors (genes)
responsible for traits to their
offspring. Factors retain their
trait generation after
generation.
P generation
(true-breeding
parents)
Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1 ´ F1)
F2 generation
3
–
4
of plants
have purple flowers
1
–
4
of plants
have white flowers
Mendel observed that
• Parental generation: purple flowers white flowers
• F1 generation: all plants with purple flowers
• F2 generation: 3/4 of plants with purple flowers
1/4 of plants with white flowers
Mendel needed to explain
• Why one trait seemed to disappear in the F1 generation
• Why that trait reappeared in one quarter of the F2
offspring
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Mendel’s Law of Inheritance
Four Hypotheses
1. Genes are found in alternative versions called alleles; a
genotype is the listing of alleles an individual carries for a
specific gene
2. For each characteristic, an organism inherits two alleles,
one from each parent; the alleles can be the same or
different
– A homozygous genotype has identical alleles
– A heterozygous genotype has two different alleles
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Mendel’s Laws of Inheritance
Four Hypotheses
3. If the alleles differ, the dominant allele determines the
organism’s appearance, and the recessive allele has no
noticeable effect
– The phenotype is the appearance or expression of a trait
– The same phenotype may be determined by more than one
genotype
4. Law of segregation: Allele pairs separate (segregate) from
each other during the production of gametes so that a
sperm or egg carries only one allele for each gene
Copyright © 2009 Pearson Education, Inc.
Genetic makeup (alleles)
pp
PP
P plants
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
1–
2
Gametes
1–
2
P
Sperm
F2 plants
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
P
p
P
PP
Pp
p
Pp
pp
Eggs
p
9.4 Homologous chromosomes bear the alleles for each
character
For a pair of homologous chromosomes, alleles of a
gene reside at the same locus
• Homozygous individuals have the same allele on both
homologues
• Heterozygous individuals have a different allele on each
homologue
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Gene loci
Genotype:
Dominant
allele
P
a
B
P
a
b
PP
Homozygous
for the
dominant allele
aa
Homozygous
for the
recessive allele
Recessive
allele
Bb
Heterozygous
Student Practice
Should this dog be called Spot?
Background: In Labrador retrievers a
black coat(B) is dominant to a brown
coat(b).
• What is the genotype(s) of a dog with a brown
coat?
• What is the genotype(s) of a dog with a black
coat?
• If you wanted to determine the exact
genotype of a dog with a black coat, what
genetic cross (mating) would you perform?
A testcross determines unknown genotypes
Testcross
• Mating between an individual of unknown genotype and
a homozygous recessive individual
• Will show whether the unknown genotype includes a
recessive allele
Copyright © 2009 Pearson Education, Inc.
Testcross:
B_
Genotypes
bb
Two possibilities for the black dog:
BB
B
Gametes
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
Uncovering the law of independent assortment by
tracking two characters at once
Consider the following:
• Parental generation: round yellow seeds wrinkled green
seeds
• F1 generation: all plants with round yellow seeds
• F2 generation: 9/16 of plants with round yellow seeds
3/16 of plants with round green seeds
3/16 of plants with wrinkled yellow seeds
1/16 of plants with wrinkled green seeds
Mendel needed to explain
• Why nonparental combinations were observed
• Why a 9:3:3:1 ratio was observed among the F2 offspring
Copyright © 2009 Pearson Education, Inc.
Law of independent assortment
• Each pair of alleles segregates independently of the other
pairs of alleles during gamete formation
• For genotype RrYy, four gamete types are possible: RY, Ry,
rY, and ry
Copyright © 2009 Pearson Education, Inc.
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P
generation
rryy
RRYY
ry
Gametes RY
F1
generation
rryy
RRYY
ry
Gametes RY
RrYy
RrYy
Sperm
Sperm
1
–
2
F2
generation
1
–
2
RY
1
–
2
1
–
4
ry
RY
Eggs
1
–
2
ry
1
–
4
RY
1
–
4
rY
1
–
4
Ry
Eggs
Hypothesized
(not actually seen)
1
–
4
RY
1
–
4
rY
1
–
4
Ry
1
–
4
ry
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
9
––
16
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
(support hypothesis)
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled