Download sect10.1mendel - MissDavisNHSScience

Chapter 10: Mendel & Meiosis
Section 10.1:
Mendel’s Laws of Heredity
*The study of heredity actually began in an Austrian
Monastery in the mid 19th century.
I.
Mendel’s Interests and Experiments w/ Pea
Plants
*Gregor Mendel carried out the
first important studies of heredity
A. General Terms
1. Heredity - the passing on of
characteristics from
parents to offspring.
2. Genetics - the branch of biology which studies heredity.
3. Traits - characteristics that are inherited.
B. Choosing the right subject:
*Mendel studied many types of plants before deciding on
using pea plants.
1. Why Peas?
a) peas reproduce sexually; they
have two distinct sex cells, male
& female
b) in pea plants, both male and female gametes are on
the same flower; therefore pollination is very easy to
manipulate.
c) peas reproduce quickly, thus the results can be seen
quickly.
2. Gametes - the sex cells; egg (ovule) and sperm/pollen.
*in plants:
a) female gamete = ovule
b) male gamete = pollen grain
Parts of a flower:
3. Pollination - the transfer of the male pollen grain to the
pistil of a flower.
a) self - pollination - a reproductive process in which a
plant/ flower fertilizes itself.
b) cross - pollination - pollination between different
plants/ flowers of the same species.
4. Fertilization - the uniting of male and female gametes; in
plants it occurs when the male gamete of the pollen grain
meets and fuses with the female gamete in the ovule.
*upon fertilization the ovule becomes a seed.
C. What Mendel Did: The Experiment
1. Mendel chose several characteristics of pea plants and
studied each characteristic’s contrasting traits separately.
Characteristic
Dominant
Recessive
1) Seed texture
Round
Wrinkled
2) Seed color
Yellow
Green
3) Seed coat color
Colored
White
4) Pod appearance
Inflated
Constricted
5) Pod color
Green
Yellow
6) Position of flowers
Axial
Terminal
7) Stem length
Tall
Short
Mendel’s Characteristics:
2. Mendel grew plants that were pure or true-breeders.
True breeders or pure plants - plants that produce
offspring that are identical to themselves.
ex) pea plants that are pure for tallness will always
produce offspring that are tall
*when all plants are pure for a specific trait , that trait is
called a strain.
3. Mendel produced strains via self-pollination.
4. Mendel was able to get 14 strains, one for each
contrasting trait. Each of the 14 strains were called the
Parental or P1 generation. This is because these strains
serve as the parents or parent plants for his experiments.
5. The next step Mendel took was to cross-pollinate these
pure strains w/ the pure strains of their contrasting traits.
ex) P1 Tall
X
P1 Short
*The offspring of this mating is called the 1st Filial
Generation or F1 Generation.
Filial = sons or daughters
6. Mendel then self-pollinated the F1 generation and termed
the offspring from this self-pollination the Second Filial
Generation or F2 Generation.
Summary:
1) Mendel produced plants pure for each trait
2) He self-pollinated them and called them the
P1 generation
3) He cross-pollinated the contrasting P1’s to get an
F1 generation: P1 --> F1
4) Mendel then self-pollinated the F1’s and got the
F2 generation: F1 --> F2
Mendel’s Experiment:
Mendel’s Experiment:
II. Mendel’s Results and Conclusions
*hybrid - the offspring of parents that have different forms
of a trait, such as tall and short height.
A. A dry run of Mendel’s Experiment
1. The First Generation
In one experiment, Mendel crossed a pea plant that
was pure for tallness with a pea plant that was pure
for shortness:
P1 generation --> Tall Plant x Short Plant
* the result of this cross was ALL TALL plants; as if the short
plant never existed.
Therefore:
P1 --> Tall x Short
F1 = ALL TALL
*Mendel did not stop here, however, if he did he would have
proposed that Tall X Short will always = Tall; but this is not
actually the case.
2. The Second Generation
Mendel decided to self-pollinate the F1 Generation and
was surprised by his findings.
The F2 Generation was:
3/4 = Tall
1/4 = Short
(3:1 ratio)
*the short trait that had been “lost” now reappeared!!!!!!
Summary:
P1
F1
F2
Tall X Short (cross-pollination)
ALL TALL (self-pollinated)
3/4 Tall: 1/4 Short
Tall vs. Short Experiment:
Purple vs. White Flowers:
a) this pattern occurred in thousands of pollinations.
Mendel termed the plants in the F1 generation Dominant.
Dominant - the trait that occurs the most frequently.
b) the trait that was lost, but reappeared in the F2
generation was termed recessive.
B. Mendel’s Three Principles/ Laws:
*from his results, Mendel concluded that 3 principles or
laws governed inheritance:
1) The Principle or Law of Dominance & Recessiveness
2) The Principle or Law of Segregation
3) The Principle or Law of Independent Assortment
1. The Principle or Law of Dominance and Recessiveness
a) Mendel declared that some “factor” was responsible
for controlling which characteristics would be expressed
b) Each characteristic has 2 traits;
ex) height - tall or short
c) Mendel concluded that each characteristic is the
result of the interaction of the pair of the two traits.
d) Mendel proposed that one factor in a pair may mask
other factor, thus preventing it from having an effect or
being able to be seen or observed.
e) Recessive - the factor which was masked or could not
be observed or physically seen.
f) Dominant - the factor which masked the other factor and
thus was readily observed, or physically seen.
2. The Principle or Law of Segregation
a) Mendel wanted to know why some traits could disappear
in the F1 generation, but then reappear in the F2
generation.
b) He figured that if each parent had 2 “factors,” then each
offspring must have 2 as well. Each parent could not pass
BOTH of their factors to the offspring or else the offspring
would result in 4 factors
Parent X Parent
(2 factors) (2 factors)
Offspring
(2 factors)
NOT
Parent X Parent
(2 factors) (2 factors)
Offspring
(4 factors)
c) Mendel proposed that each reproductive cell received
only one factor for each characteristic, so if the parent cell
had the following:
Characteristic
Dominant Factor Recessive Factor
Pod color
Green
Yellow
Stem length
Tall
Short
Assuming that the parents are heterozygous ( each one has
one of each factor), then the reproductive cell would receive
only ONE factor for each characteristic.
Therefore:
for pod color, the reproductive cell will have
either the green factor OR the yellow factor
(not both), and for stem length the cell will get
either the tall OR the short factor, not both.
d) Mendel defined this as the Principle of Segregation - so
that the two factors for a characteristic segregate or
separate during the formation of eggs and sperm.
3. Principle or Law of Independent Assortment
a) As part of his experiment, Mendel crossed plants w/ 2
different characteristics to see if one would have an effect
on the other.
ex) he used height and seed color
Characteristic
Height
Seed Color
(D) Traits ( R)
Tall or Short
Yellow or Green
b) What these crosses illustrated was that all dominant
factors did not necessarily appear together, therefore, you
could have a tall plant w/ green seeds or a short yellow
seeded one.
These traits acted Independently of each other.
c) This served as the basis for Mendel’s Principle or Law of
Independent Assortment - where factors for different
characteristics are distributed to reproductive cells
independently of one another.
III. Chromosomes & Genes
1. For the most part, Mendel’s findings agree w/ what is
known today about genes and chromosomes.
Unfortunately for Mendel, he was considered a FREAK! He
had no solid basis to explain his findings and he had no
knowledge of what genes or chromosomes were.
2. Mendel would not receive credit for his findings until
Walter Sutton proposed the Chromosome Theory.
3. Today, Mendel’s “characteristics” or “traits” have come to
be known as GENES.
genes - a segment of DNA on a chromosome which
controls a particular hereditary trait.
4. Because chromosomes are paired, genes are often paired
as well.
5. Mendel’s “factors” have come to be known as ALLELES.
Alleles - an alternative form of a gene represented by
letters:
Dominant traits = CAPITALS
Recessive traits = lowercase
IV. Genotype & Phenotype
1. Phenotype - the external appearance or physical
appearance of an organism.
2. Genotype - the genetic makeup of an organism;
“combination of the genes.”
The genotype consists of two alleles:
ex) the dominant tall pea plant would be:
TT or Tt
the recessive short plant would be tt
3. Homozygous - if both alleles of pair are the same, the
organism is described as homozygous.
ex) TT = homozygous dominant
tt = homozygous recessive
4. Heterozygous - if the alleles of the pair are NOT the
same, the organism is said to be heterozygous.
ex) Tt = heterozygous
V. Probability, Monohybrid Crosses, and Dihybrid Crosses
A. Probability
1. Probability - the likelihood that a certain event will occur.
2. Equation Probability is determined by the following equation:
Probability = # of one kind of event or item
# of total events or items
ex) given: yellow = 6,022
green = 2,001
total
8,023
The probability for yellow would be:
P = # of one kind = yellow (6,022)
Total #
all (8,023)
= 0.75 or 75%
What would be the Probability of getting green?
How would you set up the equation?
P = #of one kind
Total #
=
green (2,001) = 0.25 or 25%
all (8,023)
B. Monohybrid Crosses
1. Monohybrid Crosses - a cross between individuals that
involves ONE (1) pair of contrasting traits.
ex) yellow vs. green seed color
2. Punnett Square - grid type diagram used to help
biologists in determining probabilities. Devised by Reginald
Punnett in 1905.
Punnett Squares:
Example #1)
Homozygous X Homozygous
(dominant)
(recessive)
TT (Tall)
tt (short)
Steps:
1) draw a square and divide it into 4 sections:
2) Represent the alleles by placing one of the parents
on top of the boxes, and the other parent to the left of
the boxes. Be sure that each letter gets it’s own
“box.”
T
t
t
T
3) Fill in each box with the letter that is above it and the
letter that is aside of it. Each box should have two letters.
What are the Phenotypes?
What are the Genotypes?
Example #2) Guinea Pig fur texture
R = dominant rough fur
r = recessive smooth fur
Homozygous X Heterozygous
(dominant) RR
Rr
RR
What are the phenotypes?
What are the Genotypes?
What is the Ratio?
X
Rr
R
r
R
R
RR
rough
RR
rough
Rr
rough
Rr
rough
Phenotypes: all will have rough fur (4/4)
Genotypes: RR - homozygous dominant
Rr - heterozygous
Ratios: Phenotype: 4 Rough or 4/4 Rough
Genotype: 2RR : 2Rr
Example #3)
Fur color in Rabbits
B = Black (dominant)
b = brown (recessive)
Heterozygous X Heterozygous
What is the Phenotype ratio?
What is the Genotype Ratio?
B
b
B
BB
Black
Bb
Black
b
Bb
Black
bb
brown
Phenotype Ratio: 3 Black : 1 brown
Genotype Ratio:
1 BB : 2 Bb : 1 bb
What would you get if:
BB x bb
Phenotypes: ?
Genotypes: ?
What if:
BB x Bb
?
The Test Cross: