Lecture 8: Genetics: Simple inherited patterns variation

1
Lecture 8 Genetics Simple inherited patterns
variation
Text Reading Chapter 14, up to 14.2
You need to be able to explain the following to
yourself and to me (on exams)
Mendels Law of Segregation tracking
heritable traits for three generations parental
generation F1 generation F2 generation,
segregation, allele, dominant, recessive,
phenotype, genotype  Probability multiplicative
rule additive rule Mendels Law of
Independent Assortment independent assortment
of genes, independent assortment of
chromosomes Other dominance relationships partial
or independent dominance codominance Mendel in
the 21st century
Describe the sex life of a pea plant. carpel
stamens self-pollination cross-pollination
true-breeding hybridization List some of the
heritable traits of pea plants studied by
Mendel. flower color seed shape seed color
pod shape pod color stem length flower
position  
Note the slides not covered in Lecture 7 will be
covered in upcoming lectures.
2
Figure 14.1 Gregor Mendel and his garden peas
3
Figure 14.2 Research Method Crossing Pea
Plants(Note for genetic purposes we can often
treat plants like animals, without worrying about
alternation of generations).
The pea plant life cycle self-pollination cross-po
llination true-breeding
Peas normally self-fertilize - pollen (containing
sperm) fertilizes an ovum from the same plant,
or same flower in this case. Mendel manipulated
flowers to ensure cross-fertilization
(hybridization).
Plants that have selfed for several generations
are true-breeding - no variation.
4
Table 14.1 The seven variable characters of pea
plants studied by Mendel. Ignore cross results
for now.
Traits of pea plants flower color flower
position seed color seed shape pod shape pod
color stem length
5
Figure 14.3 When F1 pea plants with purple
flowers are allowed to self-pollinate, what
flower color appears in the F2 generation?
Law of Segregation three generations
segregation dominant, recessive phenotype,
genotype
EXPERIMENT True-breeding purple-flowered pea
plants and white-flowered pea plants were crossed
(symbolized by ?). The resulting F1 hybrids were
allowed to self-pollinate or were
cross- pollinated with other F1 hybrids. Flower
color was then observed in the F2 generation.
RESULTS Both purple-flowered plants and
white- flowered plants appeared in the F2
generation. In Mendels experiment, 705 plants
had purple flowers, and 224 had white flowers, a
ratio of about 3 purple 1 white.
6
Figure 14.4 Alleles, alternative versions of a
gene
Law of Segregation three generations
segregation dominant, recessive phenotype,
genotype
7
Figure 14.5 Mendels law of segregation
Law of Segregation three generations
segregation dominant, recessive phenotype,
genotype
8
Figure 14.5 Mendels law of segregation
Law of Segregation three generations
segregation dominant, recessive phenotype,
genotype
9
Figure 14.6 Phenotype versus genotype
Law of Segregation three generations
segregation dominant, recessive phenotype,
genotype
10
Figure 14.7 The Testcross
Law of Segregation three generations
segregation dominant, recessive phenotype,
genotype
11
Table 14.1 The seven variable characters of pea
plants studied by Mendel. Now note how all 7
show the same 3 to 1 ratio in the F2. Mendel was
generalizing, an important aspect of science.
Traits of pea plants flower color flower
position seed color seed shape pod shape pod
color stem length
12
Probability concepts multiplicative rule additive
rule
Heredity and Probability
Same laws that apply to coin tosses, dice
rolls Each toss or roll is independent Probabili
ties range between 0 and 1 MULTIPLY individual
probabilities to obtain the overall probability
of one or more events occurring together - when
there is an AND , use the Rule of
Multiplication ADD individual probabilities to
obtain the overall probability of an event that
can occur two or more different ways - when there
is an OR, use the Rule of Addition
13
Figure 14.9 Segregation of alleles and
fertilization as chance events. This is the
multiplicative rule, to be used when events are
independent. Also called the and rule.
Probability concepts multiplicative rule additive
rule
14
Probability concepts multiplicative rule additive
rule
P (A and B) P(A) x P(B)
What is the probability that a woman who gives
birth 2 times will have 2 daughters?
P (first birth is a girl and second birth is a
girl) P (first birth is a girl) x P (second
birth is a girl)
Given that the probability of a daughter at any
birth is 1/2
P (A and B) 1/2 x 1/2
P (A and B) 1/4
15
Probability concepts multiplicative rule additive
rule
In a cross between two heterozygous tall pea
plants, what is the probability that an offspring
will be tall (TT OR Tt)?
Heterozygotes are Tt and tall TT homozygotes are
also tall tt homozygotes are short
P (tall) P (Tt or TT) P (Tt) P (TT) P (Tt
or TT) 1/2 1/4 3/4
16
Figure 14.8 Do the alleles for seed color and
seed shape sort into gametes dependently
(together) or independently?
Independent Assortment genes chromosomes
17
Figure 13.10 The independent assortment of
homologous chromosomes in meiosis
Independent Assortment genes chromosomes
18
Figure 14.10 Incomplete dominance in snapdragon
color
dominance relationships partial,
incomplete codominance
19
Codominance in platyfish, Xiphophorus maculatus
dominance relationships partial,
incomplete codominance
Ray, RR
Ray Spot, RS
Spot, SS
20
21st Century Mendel
Moving Mendel into the 21st century The
Mendelian gene that controls stem length in peas
is formally called Le. The two alleles are Le
and le le/le is a dwarf pea plant. This gene has
been recently sequenced. A single base change
changes a single amino acid in an enzyme that is
part of the chain of reactions that makes the
plant growth hormone gibberellin.
21
21st Century Mendel
The chemical pathway for gibberellin synthesis.
Gibberellin is a growth hormone, synthesized in a
long series of steps, each made possible by a
specific enzyme encoded by a specific gene.
22
21st Century Mendel
How does this work, in more detail
Chemical Y
Chemical X
Gibberellin
Le makes a better enzyme 3, that makes a lot of
Gibberellin, and le makes much less. But Le/le
makes enough to grow normally.
23
Lecture 8 Genetics Simple inherited patterns
variation
Text Reading Chapter 14, up to 14.2
You need to be able to explain the following to
yourself and to me (on exams)
Mendels Law of Segregation tracking
heritable traits for three generations parental
generation F1 generation F2 generation,
segregation, allele, dominant, recessive,
phenotype, genotype  Probability multiplicative
rule additive rule Mendels Law of
Independent Assortment independent assortment
of genes, independent assortment of
chromosomes Other dominance relationships partial
or independent dominance codominance Mendel in
the 21st century
Describe the sex life of a pea plant. carpel
stamens self-pollination cross-pollination
true-breeding hybridization List some of the
heritable traits of pea plants studied by
Mendel. flower color seed shape seed color
pod shape pod color stem length flower
position  
Note the slides not covered in Lecture 7 will be
covered in upcoming lectures.