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Mendel and the Gene Idea
1

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What genetic principles account for the transmission of traits from parents to offspring?
One possible explanation of heredity is a “blending” hypothesis - The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green
An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea - Parents pass on discrete heritable units, genes
2

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Documented a particulate mechanism of inheritance through his experiments with garden peas
Gregor Mendel’s monastery garden.
Fig. 2.2
3

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Gregor Johann Mendel (1822-1884)
–
Augustinian monk, Czech Republic
–
Foundation of modern genetics
–
Studied segregation of traits in the garden pea ( Pisum sativum ) beginning in 1854
–
Published his theory of inheritance in 1865.
“Experiments in Plant Hybridization”
–
Mendel was “rediscovered” in 1902
–
Ideas of inheritance in Mendel’s time were vague. One general idea was that traits from parents came together and blended in offspring. Thus, inherited information was predicted to change in the offspring, an idea that
Mendel showed was wrong. “Characters,” or what we now call alleles, were inherited unchanged. This observation and the pattern of inheritance of these characters gave us the first definition of a gene
4

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•
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Variation is widespread in nature
Observable variation is essential for following genes
Variation is inherited according to genetic laws and not solely by chance
Mendel’s laws apply to all sexually reproducing organisms
5

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Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
Mendel chose to work with the garden pea (Pisum sativum)
–
Because they are available in many varieties, easy to grow, easy to get large numbers
–
Because he could strictly control which plants mated with which
6

Parental generation
(P)
3 Pollinated carpel matured into pod
1 Removed stamens from purple flower
2 Transferred spermbearing pollen from stamens of white flower to eggbearing carpel of purple flower
Carpel
(female)
Stamens
(male)
First generation offspring
(F
1
)
4 Planted seeds from pod
5 Examined offspring: all purple flowers
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Statistical analyses:
–
–
Worked with large numbers of plants counted all offspring
– made predictions and tested them
Excellent experimentalist
–
– controlled growth conditions focused on traits that were easy to score
– chose to track only those characters that varied in an “either-or” manner
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9

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•
•
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Character: a heritable feature, such as flower color
Trait: a variant of a character, such as purple or white flowers
Each trait carries two copies of a unit of inheritance, one inherited from the mother and the other from the father
Alternative forms of traits are called alleles
10

Dominant
Recessive
11

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Mendel also made sure that he started his experiments with varieties that were “true-breeding”
X X
X X
X X
12

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Phenotype – observable characteristic of an organism
Genotype – pair of alleles present in and individual
Homozygous – two alleles of trait are the same
(YY or yy)
Heterozygous – two alleles of trait are different
(Yy)
Capitalized traits = dominant phenotypes
Lowercase traits= recessive phenotypes
13

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Generations:
–
P = parental generation
–
–
F1 = 1st filial generation, progeny of the P generation
F2 = 2nd filial generation, progeny of the F1 generation
(F3 and so on)
Crosses:
–
–
Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait.
Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits.
14

Figure 14.6
1
3
Phenotype
Purple
Genotype
PP
(homozygous)
1
Purple
Pp
(heterozygous)
Pp
(heterozygous)
2
Purple
White
Ratio 3:1 pp
(homozygous)
Ratio 1:2:1
1
15

16

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In a typical breeding experiment Mendel mated two contrasting, true-breeding varieties, a process called hybridization
The true-breeding parents are called the P generation
The hybrid offspring of the P generation are called the F1 generation
When F1 individuals self-pollinate the F2 generation is produced
17

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When Mendel crossed contrasting, true-breeding white and purple flowered pea plants all of the offspring were purple
When Mendel crossed the F1 plants, many of the plants had purple flowers, but some had white flowers
A ratio of about three to one, purple to white flowers, in the F2 generation
EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by

). The resulting F
1 in the F
2 hybrids were allowed to self-pollinate or were crosspollinated with other F
1 generation.
hybrids. Flower color was then observed
P Generation
(true-breeding parents) Purple flowers

White flowers
F
1
Generation
(hybrids)
All plants had purple flowers
RESULTS Both purple-flowered plants and whiteflowered plants appeared in the F
2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.
F
2
Generation
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In the F1 plants, only the purple trait was affecting flower color in these hybrids
Purple flower color was dominant, and white flower color was recessive
Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed among the F2 offspring
There are four related concepts that are integral to this hypothesis
19

1.
Alternative versions of genes account for variations in inherited characters, which are now called alleles
Allele for purple flowers
Locus for flower-color gene
Homologous pair of chromosomes
Allele for white flowers
2.
For each character an organism inherits two alleles, one from each parent, A genetic locus is actually represented twice
3.
If the two alleles at a locus differ, the dominant allele determines the organism’s appearance
4.
The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
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Mechanism of gene transmission
Gametogenesis: alleles segregate
Fertilization: alleles unite
21

• Classical Punett's Square is a way to determine ways traits can segregate
Parental P
0 cross
P P p p
F
1 cross
P p
P p
Determine the genotype and phenotype
23

• Classical Punett's Square is a way to determine ways traits can segregate
Parental P
0 cross p p
P
Pp
Pp
F
1 cross
P
Pp
Pp
P p
P p
Determine the genotype and phenotype
24

• Classical Punett's Square is a way to determine ways traits can segregate p p
Parental P
0 cross
P
Pp
Pp
P
Pp
Pp
P p
F
1 cross
P
PP
Pp p
Pp pp
Determine the genotype and phenotype
25

Mendel’s Law Of Segregation, Probability And The
Punnett Square
Each true-breeding plant of the parental generation has identical alleles, PP or pp .
Gametes (circles) each contain only one allele for the flower-color gene.
In this case, every gamete produced by one parent has the same allele.
P Generation 
Appearance:
Genetic makeup:
Purple flowers
PP
White flowers pp
Gametes:
P p
Union of the parental gametes produces F
1 hybrids having a Pp combination. Because the purpleflower allele is dominant, all these hybrids have purple flowers.
When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.
This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an
F
1

F
1
( Pp

Pp ) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm.
Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F
2 generation.
F
1
Generation
Appearance:
Genetic makeup:
Gametes: 1 /
2
P
Purple flowers
Pp
1 /
2 p
P
F
2
Generation
P
F
1 eggs p
PP
Pp
3 : 1
F
1 sperm p
Pp pp
26

White
(pp)
Purple
(PP) p
Gametes p
Gametes
P
P
Pp Pp
Pp Pp
F
1 generation
All purple
Purple
(Pp)
Purple
(Pp)
P
Gametes p
P
Gametes p
PP Pp
Pp pp
F
2 generation
¾ purple, ¼ white
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Smooth and wrinkled parental seed strains crossed.
•
Punnett square
–
F1 genotypes: 4/4 Ss
–
F1 phenotypes: 4/4 smooth
28

Mendel Observed The Same Pattern In Characters
29

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In pea plants with purple flowers the genotype is not immediately obvious
A testcross
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Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
–
Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait
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To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype:
Alternative 1 – Plant with dominant phenotype is homozygous
Recessive phenotype pp
Gametes p p
Gametes
P P
Dominant
Phenotype
PP
?
Dominant
Phenotype
Pp
?
Alternative 2 – Plant with dominant phenotype is heterozygous
Recessive phenotype pp
Gametes p p
Gametes
P p
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To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype:
Alternative 1 – Plant with dominant phenotype is homozygous
Recessive phenotype pp p
Gametes p
Gametes
P P
Pp Pp
Pp Pp
Dominant
Phenotype
PP
If all offspring are purple; unknown plant is homozygous.
?
?
Alternative 2 – Plant with dominant phenotype is heterozygous
Dominant
Phenotype
Pp
Recessive phenotype pp
Gametes p p
Gametes
P p
Pp pp
Pp pp
If half of offspring are white; unknown plant is heterozygous.
32

APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross.
Dominant phenotype, unknown genotype:
PP or Pp ?

Recessive phenotype, known genotype: pp TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example).
By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.
RESULTS
If PP , then all offspring purple: p p
P
P
Pp
Pp
Pp
Pp
P then 1 ⁄
2 and 1 ⁄
2
If Pp , offspring purple offspring white: p p
Pp Pp p pp pp
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Mendel derived the law of segregation by following a single trait
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2 alleles at a single gene locus segregate when the gametes are formed
–
The F1 offspring produced in this cross were monohybrids, heterozygous for one character
•
Mendel identified his second law of inheritance by following two characters at the same time
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Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently
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Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
34

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With his monohybrid crosses, Mendel determined that the 2 alleles at a single gene locus segregate when the gametes are formed.
With his dihybrid crosses, Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently.
35

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For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow
(Y) is dominant to green (y).
Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds.
36

•
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For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow
(Y) is dominant to green (y).
Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds.
37

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If dependent segregation (assortment) occurs:
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Alleles at the 2 gene loci segregate together, and are transmitted as a unit.
–
Therefore, each plant would only produce gametes with the same combinations of alleles present in the gametes inherited from its parents:
Parents
Parental Gametes
F
1
Offspring
F
1
Offspring’s Gametes
R R
Y Y
R
Y
R r
Y y r r y y r y
R
Y r y
What is the expected phenotypic ratio for the F
2
?
38

2
R
Y r y
R
Y
R R
Y Y
R r
Y y r y
R r
Y y r r y y
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Alleles at the 2 gene loci segregate (separate) independently, and are
NOT transmitted as a unit. Therefore, each plant would produce gametes with allele combinations that were not present in the gametes inherited from its parents:
Parents
Parental Gametes
F
1
Offspring
F
1
Offspring’s
Gametes
R R
Y Y
R
Y r r y y r y
R
Y
R y r
Y
R r
Y y r y
What is the expected phenotypic ratio for the F
2
?
40

Dihybrid cross - parental generation differs in two traits example-- cross round/yellow peas with wrinkled/green ones
Round/yellow is dominant
RY R y
RY
R y r Y ry r Y ry
What are the expected phenotype ratios in the F
2 round, yellow = generation?
round, green = wrinkled, yellow = wrinkled, green =
41

2
RY
RY
RR
YY
R y
RR
Y y r Y
R r
YY ry
R r
Y y
R y
RR
Y y
RR yy
R r
Y y
R r yy r Y
R r
YY
R r
Y y rr
YY rr
Y y ry
R r
Y y
R r yy rr
Y y rr yy
Phenotypic ratio is 9 : 3 : 3 : 1
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How are two characters transmitted from parents to offspring?
–
As a package?
–
Independently?
A dihybrid cross
–
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Illustrates the inheritance of two characters
Produces four phenotypes in the F2 generation
P Generation YYRR
Gametes YR
 yr yyrr
EXPERIMENT Two true-breeding pea plants — one with yellow-round seeds and the other with green-wrinkled seeds —were crossed, producing dihybrid F
1 plants. Self-pollination of the F
1 dihybrids, which are heterozygous for both characters, produced the F
2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color ( Y ) and round shape ( R ) are dominant.
CONCLUSION The results support the hypothesis ofindependent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.
F
1
Generation
Hypothesis of dependent assortment
1 ⁄
2
YR 1 ⁄
2
Sperm yr
F
2
(predicted
Eggs
Generation
1 ⁄
2
YR offspring)
1 ⁄
2 yr
YYRR YyRr
YyRr yyrr
YyRr
Hypothesis of independent assortment
Eggs
1 ⁄
4
YR 1 ⁄
4
Yr
Sperm
1 ⁄
4 yR 1 ⁄
4 yr
1 ⁄
4
YR
YYRR YYRr YyRR YyRr
1 ⁄
4
Yr
YYrr YYrr YyRr Yyrr
1 ⁄
4 yR
YyRR YyRr yyRR yyRr
3 ⁄
4
1 ⁄
4
Phenotypic ratio 3:1
1 ⁄
4 yr
9 ⁄
16
YyRr
3 ⁄
16
Yyrr
3 ⁄ yyRr
16 yyrr
1 ⁄
16
Phenotypic ratio 9:3:3:1
315 108 101 32 Phenotypic ratio approximately 9:3:3:1 43

Dihybrid cross
•
F
2 generation ratio:
9:3:3:1
44

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Mendel’s dihybrid crosses showed a 9:3:3:1 phenotypic ratio for the F2 generation.
Based on these data, he proposed the Law of
Independent Assortment, which states that when gametes form, each pair of hereditary factors
(alleles) segregates independently of the other pairs.
45

Laws Of Probability Govern Mendelian Inheritance
•
Mendel’s laws of segregation and independent assortment reflect the rules of probability
–
The multiplication rule

States that the probability that two or more independent events will occur together is the product of their individual probabilities
–
The rule of addition

States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
46

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The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself
•
Following the self-hybridization of a heterozygous purple pea plants ( P p), what is the probability that a given offspring will be homozygous for the production of white flowers
(pp)?
•
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Probability that a pollen seed will carry p: ½
Probability that an egg will carry p: ½
•
•
Probability that the offspring will be pp:
1/2 X 1/2 = 1/4
47

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The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities
•
Following the self-hybridization of a heterozygous purple pea plant ( P p), what is the probability that a given offspring will be purple ?
•
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Probability of maternal P uniting with paternal P : 1/4
Probability of maternal p uniting with paternal P : 1/4
Probability of maternal P uniting with paternal p: 1/4
•
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Probability that the offspring will be purple :
1/4 + 1/4 + 1/4 = 3/4
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Can be determined using these rules
Rr
Segregation of alleles into eggs
 
1 ⁄
2
Eggs
R
1 ⁄
2 r
1 ⁄
2
R
Sperm
1 ⁄
2 r
R r
1 ⁄
R
4
R
1 ⁄
4
R r
1 ⁄
4 r r
1 ⁄
4
Rr
Segregation of alleles into sperm
49

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•
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Genes are distinct entities that remain unchanged during crosses
Each plant has two alleles of a gene
Alleles segregated into gametes in equal proportions, each gamete got only one allele
During gamete fusion, the number of alleles was restored to two
50

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Mendel’s Principle of Uniformity in F1:
–
–
F1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents.
Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds
(alleles).
•
Mendel’s Law of Segregation:
–
–
Recessive characters masked in the F1 progeny of two true-breeding strains, reappear in a specific proportion of the F2 progeny.
Two members of a gene pair segregate (separate) from each other during the formation of gametes.
•
Mendel’s Law of Independent Assortment:
–
–
Alleles for different traits assort independently of one another.
Genes on different chromosomes behave independently in gamete production.
51

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•
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Incomplete dominance
Codominance
Multiple alleles
Polygenic traits
Epistasis •
•
•
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Pleiotropy
Environmental effects on gene expression
Linkage
Sex linkage
52

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Neither allele is dominant and heterozygous individuals have an intermediate phenotype
For example, in Japanese “Four o’clock”, plants with one red allele and one white allele have pink flowers:
P Generation
Red
C R C R

White
C W C W
Gametes C R C W
F
1
Generation
Gametes
1 ⁄
2
C R
1 ⁄
2
C R
Pink
C R C W
F
2
Generation
Eggs
1 ⁄
2
C R 1 ⁄
2
C R Sperm
1 ⁄
2
C R
C R C R C R C W
1 ⁄
2
C w
C R C W C W C W
53

C R
Gametes
C W
C R C R
C W C W
F
1 generation
All C R C W
C R
Gametes
C R C R
C W
C R C W
C R C W C W C W
F
2 generation
1 : 2 : 1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
54

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Neither allele is dominant and both alleles are expressed in heterozygous individuals
Example ABO blood types
55

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Most traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. These are called polygenic traits.
Polygenic traits often show continuous variation, rather then a few discrete forms:
56

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•
•
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Type of polygenic inheritance where the alleles at one gene locus can hide or prevent the expression of alleles at a second gene locus.
Labrador retrievers one gene locus affects coat color by controlling how densely the pigment eumelanin is deposited in the fur.
A dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat
However, a second gene locus controls whether any eumelanin at all is deposited in the fur. Dogs that are homozygous recessive at this locus (ee) will have yellow fur no matter which alleles are at the first locus:
57

ee
No dark pigment in fur eebb
Yellow fur
E_
Dark pigment in fur eeB_
Yellow fur
E_bb
Brown fur
E_B_
Black fur
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
58

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This is when a single gene locus affects more than one trait.
For example, in Labrador retrievers the gene locus that controls how dark the pigment in the hair will be also affects the color of the nose, lips, and eye rims.
59

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The phenotype of an organism depends not only on which genes it has (genotype), but also on the environment under which it develops.
•
Although scientists agree that phenotype depends on a complex interaction between genotype and environment, there is a lot of debate and controversy about the relative importance of these 2 factors, particularly for complex human traits.
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