Download File

Mendel and
Heredity
Chapter 12
12.1 Objectives:
• 1.) summarize the importance of
Mendel’s experiments
• 2.)Differentiate between genes and
alleles.
• 3.) Explain that alleles determine what
physical traits are expressed in an
individual.
A Brief History
• In the past, people did not understand how
traits were inherited. Two theories :
• Blending Theory-offspring are a straight mix
• Particulate Theory- traits are inherited as
“particles” and offspring receive a “piece”
• Then Gregor Mendel studied pea plants and
learned some things about genetics
Basic Terms to Know…
• Genetics – the study of heredity
• Heredity – the passing of characteristics
from parents to offspring
• Traits – the characteristics that are
inherited
• Genes – segments of DNA (chemical factors)
that determine traits
Who was Gregor Mendel?
• He discovered how traits
are inherited
• Gregor Mendel –
– Born 1822 – Austria
– Lived in a Monastery – in
charge of garden.
– Worked with the pea plants
and studied their traits and
how they are passed from
parent to offspring.
“Father of
Genetics”
Mendel chose pea plant traits that
had two forms of each trait…
Trait
Dominant Characteristic
Recessive Characteristic
Seed Shape
Round
Wrinkled
Seed Color
Yellow
Green
Seed Coat Color
Gray
White
Pod Shape
Smooth
Constricted
Pod Color
Green
Yellow
Flower Position
Axial
Terminal
Plant Height
Short
Tall
Therefore, there are 2 alleles for every pea plant trait.
Basic Terms to Know…
• Alleles – alternative or different forms
of a gene that produce different
characteristics for each trait
• Each parent donates one set of
instructions (genes) to the offspring.
The fertilized egg would then contain 2
genes for every characteristic – one
from each parent. The two genes that
code for the same characteristic are
the alleles.
Plant Reproduction
• Pea plants can be self-fertilized or
cross-fertilized
Plant Reproduction
• Pea plants reproduce through self-pollination or
cross-pollination
• Pea plants Self-Pollinate (1 Parent)
– True-Breeding – identical offspring every
generation, plants that look like themsleves
• Pea plants can also Cross-Pollinate – pollen from
one plant is used to fertilize the egg cells in
another plant (2 Parents)
• Hybrids-offspring of two true-breeding
plants that are cross pollnated
Mendel’s Experiments
• Mendel cross-pollinated two truebreeding plants (1 white, 1 purple)
that had the contrasting trait of
flower color
• He found that all the offspring
were purple.
• He allowed the F1 offspring to
self-pollinate- the offspring were
mostly purple (75%) and some
white (25%)
P
Generation
F1
Generation
F2
Generation
Mendel’s Experiment
Mendel’s Experiments
• When Mendel crossed plants with contrasting
characteristics for the same trait (like
flower color), the resulting offspring had only
one of the characteristics.
– For example: Purple x White= All Purple
From these experiments, Mendel drew
2 conclusions…
Mendel’s Conclusions
1. Inheritance is determined by factors
that are passed from one generation to
the next. (today we call them genes)
Mendel’s Conclusions
1. Each gene has two alleles and the
alleles are either Dominant or
Recessive. (Principle of Dominance)
•
The trait that is dominant will appear over
the recessive trait. (The dominant allele
always overpowers the recessive allele)
•
An organism will have the recessive trait
only when the dominant allele is NOT
present
Meiosis Connection!
• Mendel did not understand how
chromosomes separate during meiosis but
he learned something about the process
• Chance decides which alleles will end up in
each gamete through meiosis
• Law of segregation-when an organism
produces gametes, each pair of alleles is
separated and each gamete has an equal
chance of receiving either one of the
alleles.
12.2 Objectives
• 1. Differentiate between genotype and
phenotype
• 2. Differentiate between dominant and
recessive alleles
• Apply probability in predicting the
outcomes of offspring inheriting certain
traits
• Create a punnett square and use it to
analyze the probability of offspring
inheriting traits
12.2 Probability and Punnett
Squares
Genetics and Probability
• Mendel realized that the principles of
probability can be used to explain
genetics…
Probability - is the mathematical chance or
likelihood that a particular event will occur
(p)
*Can be written as a fraction or ratio
p = 2/3
p = 2:3
p = 2 out of 3
Practice Problems
1. You flip a coin once. What is the probability of the
coin showing tails?
½ or 1 out of 2
2. What is the probability of flipping two coins and
having both show heads?
½ x ½ = ¼ or 1 out of 4
3. What is the probability of flipping two coins and
having one show heads and the other show tails?
½ x ½ = ¼ or 1 out of 4
4. What is the probability of flipping heads three times
in a row?
½ x ½ x ½ = 1/8 or 1 out of 8
How is all of this related to genetics?
• The same method is used to calculate
the probability that an offspring will
inherit a certain allele.
• The way in which alleles segregate is
completely random.
• The principles of probability can be
used to predict the outcomes of genetic
crosses….which leads us to….
PUNNETT SQUARES!!!
Genotype/Phenotype
• Genotype – the genetic makeup of an
organism.
– The inherited information an organism contains
in the form of genes and alleles.
• Phenotype – the physical characteristics
of an organism. The way an organism looks.
Codes For
DNA/Chromosomes
(Genotype)
Red
(Phenotype)
Representing the Different
Alleles…
1. Use the same letter for the different alleles of
a gene.
Plant Height = Tall or Short
We would use the letter “T” for plant height
2. Use a capital letter for the dominant allele
T = Tall
3. Use a lowercase letter for the recessive allele
t = Short
You cannot always know an organism’s
Genotype by simply looking at its Phenotype…
• Homozygous – having 2 identical alleles for
the same trait
• Heterozygous – having 2 different alleles for
the same trait
Genotype
Homozygous Dominant
Homozygous Recessive
Heterozygous
TT
tt
Tt
Phenotype
Tall
Short
Tall
How to Make a Punnett Square
1. Draw a square that is divided into 4
sections
2. Write the genotype of the alleles of
the father along the top of the box
3. Write the genotype of the alleles from
the mother along the left hand side of
the box
4. Fill in the inside of the box with one
allele from the top of the box and one
allele from the left hand side of the
box.
Let’s try one….BB x Bb
B
B
B
BB
BB
b
Bb
Bb
Mendel’s first experiments were crosses between 2 parent plants
that differed in 1 trait. Mendel chose this trait to be plant
height (H).
 P-Generation:
 Phenotypes: Tall x Short
 Genotypes: TT x tt
 This produced the F1 Generation:
 Phenotypes: All Tall!
 Possible Genotypes: TT or Tt
Hybrid: offspring of crosses between parents of different
traits have a heterozygous genotype Tt
 How do we know which one???
 Mendel allowed the F1 generation to self-pollinate and he
produced the F2 generation.
 Phenotypes: 3 Tall and 1 Short
 (How did the short reappear???)
We can use Punnett Squares to
Figure it Out!
P Generation = TT x tt
T
T
t
Tt
Tt
t
Tt
Tt
The only possible Genotype for the F1 Generation is Tt
Could we figure out the F2 Generation?
Yes! We would allow the F1 generation to selfpollinate and produce the F2 generation (Tt x Tt)
T
t
T
TT
Tt
t
Tt
tt
Genotypes of F2: TT, Tt, and tt
Phenotypes of F2: 3 Tall and 1 short!
The numbers in the genotypes and
phenotypes can also be represented
as ratios…
• Genotypic Ratio:
– 1 TT : 2 Tt : 1 tt
– We say that the ratio is 1 : 2 : 1
(Homozygous Dominant : Heterozygous :
Homozygous Recessive)
• Phenotypic Ratio:
-3 Tall : 1 Short
-We say that the ratio is 3 : 1
(Dominant Trait : Recessive Trait)
Stop & Think…
1) What are dominant and recessive alleles?
2) Use the terms “heredity, genetics, and
traits” in ONE sentence.
3) What is the difference between the
genotype and phenotype of an organism?
4) What is the relationship between truebreeding organisms and a homozygous
genotype?
Stop & Think…
1) How do geneticists use the principles of
probability?
2) What is the probability that a tossed
coin will come up tails twice in a row?
3) Why do geneticists use Punnett Squares?
4) What do the boxes in a Punnett Square
represent?
5) What is the probability of two
heterozygous parents producing a
heterozygous offspring?
12.3 Exceptions to Mendel’s
Genetics
• Objectives:
• 1. Explain codominance and use probability to
predict this type of inherited trait
• 2. Explain incomplete dominance and use
probability to predict this type of inherited trait
• 3.Explain multiple alleles and how it relates to
heredity
• 4.Explain what polygenic traits are and how they
relate to heredity
A Summary of Mendel’s Principles
1. The inheritance of traits is determined by
genes, which are passed from parents to
offspring.
2. Law of Dominance – When two or more alleles
for a gene exist, some alleles may be dominant
and others may be recessive.
3. Law of Segregation – Organisms inherit two
copies of each gene (one from each parent.)
These genes are segregated (separated) from
each other when gametes are formed during
meiosis.
4. Law of Independent Assortment – The alleles
for different genes separate independently of
one another. (This is why we have to FOIL
when completing a dihybrid Punnett Square!)
Exceptions to Mendel’s Rules
• Not all genes show simple patterns of
dominant and recessive alleles.
• The majority of genes have more than
two alleles.
• Traits are also sometimes controlled by
more than one gene.
Exceptions to Mendel’s Rules
1) Incomplete Dominance
 One allele is not completely dominant over another.
 There is an appearance of a third and different
phenotype.
 The heterozygous phenotype is somewhere in
between the two homozygous phenotypes.
 Both alleles of the gene combine to form a new
trait.
 Example: Red, White, and Pink Snapdragons
Exceptions to Mendel’s Rules
2) Codominance
Both alleles contribute to the phenotype of the
heterozygous organism.
 The offspring have a phenotype that expresses both
alleles equally.
 Examples:
 Roan cattle
red x white -> roan


Black and white chickens
Exceptions to Mendel’s Rules
3) Multiple Alleles
 Genes that have more than two alleles for a
specific trait.
 Individuals still only inherit two alleles (one
from each parent); there are just more
possible alleles to inherit!
Exceptions to Mendel’s Rules
3) Multiple Alleles
 Examples: Rabbit fur color has 4 different
alleles

Alleles: C, cch, ch, c
Multiple Alleles (cont’d)
• Human Blood Type
ALLELE
IA
IB
i
CODES FOR
Type "A" Blood
Type "B" Blood
Type "O" Blood
GENOTYPES
IAIA or IAi
RESULTING PHENOTYPES
Type A
IBIB or IBi
Type B
IAIB
Type AB
ii
Type O
The alleles IA and IB are always expressed – they
are codominant. Both IA and IB are dominant to i.
Exceptions to Mendel’s Rules
4) Polygenic Traits
 Traits controlled by two
or more genes.
 The genes can be on the
same chromosome or
different chromosomes.
 All heterozygotes are
intermediate in
phenotype.
 Example: Skin color in
humans
Stop & Think…
1) What is the difference between the
heterozygous phenotype in incomplete
dominance versus codominance?
2) Which exceptions to Mendel’s principles
is demonstrated with blood type in
humans?
3) Why does skin color in humans not follow
Mendel’s basic principles of genetics?
Fruit Fly Lab
• http://vital.cs.ohiou.edu/steamwebsite/downl
oads/FruitFlyGeneticsFlash/fruitflygenetics.ht
ml
• Jeopardy Review
• http://www.superteachertools.com/jeopardyx
/jeopardy-reviewgame.php?gamefile=1297971268