Welcome back to IB 150...

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Welcome back to IB 150...
Johns Hopkins Applied Physics Lab
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Lecture 6 Simple patterns of inheritance
Assigned Readings Ch. 14, up to 14.2
Understand the Essence of genetics. allele,
mutation, genotype, heterozygote, homozygote,
phenotype 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  
Understand Mendels experimental design.
tracking heritable traits for three
generations Parental Generation F1 Generation
F2 Generation  Understand how Mendel built a
model of inheritance from his experimental
observations. dominant recessive appearance
vs. genetic makeup (phenotype vs. genotype)
allele Relate Mendels Law of Segregation and Law
of Independent Assortment to events that occur
during meiosis. Understand how Mendelian
inheritance reflects laws of probability. Rule
of Multiplication Rule of Addition
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Essence of genetics
Where does fundamental variation come from? Any
gene can mutate (e.g. during DNA replication
before meiosis) to produce a gene with a
different DNA sequence ORIGINAL (DNA in black,
gene in red) ...GGGAATGTACTAAAAAATGATCCTACACATA M
UTANT ...GGGAATGTACTAAAAAATGATACTACACATA...
We now have two alleles of the same gene.
Anytime there is DNA sequence variation in a
gene, there are alleles.
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An example haploid organism White button
mushroom - Agaricus bisporus - white button and
brown crimini (also large crimini are
portabellos).
Www.botit.botany.wisc.edu/.../images/agfig6.jpg
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Essence of genetics (continued)
Different alleles can produce different effects
on the organism. Lets assume that our little
gene controls color in a mushroom that is
haploid ORIGINAL (DNA in black, gene in
red) ...GGGAATGTACTAAAAAATGATCCTACACATA... MUTANT
...GGGAATGTACTAAAAAATGATACTACACATA...
crimini
white button
The original allele produces a purple color, and
the mutant produces a red color. The gene
description original or mutant is the genotype
the appearance is the phenotype.
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Essence of genetics (continued)
But a lot of the time we do not know which is the
original allele, and which the mutant - we
werent there when the mutation occurred! So, we
just call the alleles by some descriptive name.
Like... BROWN ...GGGAATGTACTAAAAAATGATCCTACACATA.
.. WHITE ...GGGAATGTACTAAAAAATGATACTACACATA...
Keep in mind that these are alleles of the Color
gene.
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Essence of genetics (continued)
Geneticists also use abbreviations a lot. Lets
use C here to stand for the color gene, and use
superscripts to note the alleles. (Not the only
way).
Genotype Phenotype CB
brown CW white
One might think we could call these the C or
A alleles, because those are the base
differences that cause the phenotype differences,
but often alleles differ by more than one base,
and we also often dont know what the relevant
base is. But this is done in some curcumstances.
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Essence of genetics (continued)
But many organisms are diploid - this adds a
complication - there are 3 combinations of 2
alleles! Assume a hypothetical gene A with two
alleles A1 and A2. Three genotypes
A1A1
A1A2
A2A2
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Essence of genetics (continued)
But many organisms are diploid - this adds a
complication - there are 3 combinations of 2
alleles! Assume a hypothetical gene A with two
alleles A1 and A2. Three genotypes
A1A1 - homozygote
A1A2 - heterozygote
A2A2 - homozygote
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Essence of genetics (continued)
How are genotype and phenotype related in
diploid-dominant organisms with heterozygotes and
homozygotes? Lets look at some different cases.
Snapdragons
Phenotype
red
pink
white
Genotype
CRCR
CWCW
CRCW
Here there is a one-to-one relationship between
genotype and phenotype.
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Essence of genetics (continued)
But the kind of one-to-one relationship we see in
snapdragons is not the only possible
relationship. Lets look at another different
case. Garden peas
Phenotype
purple
purple
white
Genotype
pp
Pp
PP
The P allele of the P gene is dominant to the p
allele. The p allele is recessive. Note also
the use of upper and lower case letters instead
of superscripts to name alleles.
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Essence of genetics cont.)
The importance of the number 1/2 The
fundamental ratio of paternal and maternal
chromosomes in gametes produced by a given
individual is 11. This is because of
independent assortment. Thus if the individual
is an Aa heterozygote for a gene on this
chromosome, half the resulting gametes will be A
and half a.
Fig. 13.7
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Essence of genetics (final)
The rules of inheritance, how the features of
parents are passed on to offspring (but
inexactly), were discovered by careful
experiments called controlled crosses or just
crosses. In a cross, a pair of parents are
chosen and their phenotypes recorded, and then
allowed to produce offspring. The different
phenotypes and numbers of each phenotype are
carefully studied and counted. The outcome is
an understanding of the rules.
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Gregor Mendel and his peas - Mendel did the first
controlled crosses, in garden peas.
www.hort.purdue.edu/.../large/peaflower.jpg
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Pea life cycle alteration of generations, but
can be taught like that of an animal, except that
the same individual plant produces both pollen
and ova, so self-fertilization - selfing - is
possible.
pollen
ova
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Mendels basic method
Fig. 14.2
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Seven characters in pea plants studied by Mendel
Table 14.1
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Sample results from one of Mendels experiments
F1 first filial generation
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Fig. 14.4
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Fig. 14.5
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Seven characters in pea plants studied by Mendel
Table 14.1
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Fig. 14.6
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Is it PP or Pp?
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Fig. 14.7 The Testcross
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Fig. 14.8 Independent assortment
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one coin toss genotype of one gamete
probability that two PP alleles will come
together at fertilization probability that two
coin tosses land heads up 1/2 x 1/2
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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
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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
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P (A or B) P (A) P (B)
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
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Mendels discoveries (1865)

• Alleles sexually reproducing diploid organisms
  have two copies of each gene, one from the father
  and one from the mother they may be the same or
  different
• Dominance if an organism has two different
  alleles, one may be fully expressed while the
  other has no effect (implies same phenotype may
  not be same genotype)
• The two copies of a gene an organism become
  separated during gamete formation (Law of
  Segregation, but there are exceptions)
• Different traits are inherited independently
  (Law of independent assortment, but there are
  many exceptions to this rule)
• Inheritance reflects the rules of probability

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Why was Mendel successful?

• Chose pea plants as his model organism
• Chose traits with distinctly different forms
• Followed traits for several generations
• Quantitative analysis of results
• Developed a model
• Published quickly

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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.
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The chemical pathway for gibberellin synthesis.
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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.
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Lecture 6 Simple patterns of inheritance
Assigned Readings Ch. 14, up to 14.2
Understand the Essence of genetics. allele,
mutation, genotype, heterozygote, homozygote,
phenotype 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  
Understand Mendels experimental design.
tracking heritable traits for three
generations Parental Generation F1 Generation
F2 Generation  Understand how Mendel built a
model of inheritance from his experimental
observations. dominant recessive appearance
vs. genetic makeup (phenotype vs. genotype)
allele Relate Mendels Law of Segregation and Law
of Independent Assortment to events that occur
during meiosis. Understand how Mendelian
inheritance reflects laws of probability. Rule
of Multiplication Rule of Addition