Transcript No Slide Title

Why are peas useful for genetic studies?
(What contributed to Mendel’s success?
1. Normally self-fertilize
2. One can manipulate the crosses (matings) by cutting off
stamens (male part) and fertilizing the carpel (female part) with
pollen from another plant.
3. Mendel used “true-breeding” varieties.
Phenotypes
What do we call
each of these
generations?
Genotypes
Purple flower X white flower
P (parental)
PP X pp
Purple flowers (cross w. self)
F-1 generation
Pp X Pp
Purple and white flowers
F-2 generation
PP, Pp, pP, and pp
Which trait is dominate?
purple
What ratio of purple/
white flowered plants
would you expect in the F2
generation?
3:1
Why?
See genotypes
What is the difference between a plant that homozygous
versus heterozygous for a particular trait?
In the homozygous plant the two alleles are identical.
In the heterozygous plant the two alleles are different.
Which is true-breeding?
Homozygous
Which is hybrid?
Heterozygous
Some definitions:
What’s a phenotype?
The expressed or physical traits.
What’s a genotype?
The genetic makeup leading to
these traits.
What’a an allele?
An alternate form of a gene on the
homologous chromosomes
Mendel’s Hypo theses: As rephrased by Campbell p158
1. There are alternative forms of genes, the unit s that determine
heritable traits.
2. For each inherited characteristic, an organism has two genes,
one from each parent. These genes may both be the same alle le, or
they may be different alleles.
3. A sperm or egg carries only one alle le for each inherited trait,
because allele pai rs separate (segregate) from each other during the
production of gametes.
4. When the two genes of a pair are different alleles and one is
fully expressed while the other has no noticeable e ffect on the
organism’s appearance, the alleles are called the dominant allele
and the recessive allele, respectively
Variability and the Environment.
Consider musical ability.
Is it determined by genetics? By environment?
Environmental influences can act at many levels.
Gene expression can be affected by many variables including:
Nutrients, toxins, hormones, even temperature.
Some traits are determined entirely by the environment
with no basis in the genome.
Social traits that show the highest correlation between parent
and offspring are politics and religion.
Neither is genetically determined but is inherited if we simply
measure correlation.
Distinguish:
Familial traits
Shared by relatives for whatever reason.
Heritable traits
Shared by relatives because they have the
same genes
Which is easy to establish?
Which is hard?
It is hard to establish that a trait is heritable.
What is the gen e that caus es cys tic fibrosis. ?
Go to:
http://genome.nhg ri.nih.gov/clone/
What are some diseases with simple dominant or recessive
inheritance?
Re cessive disorders
Albinism
cystic fibrosis
galactosemia
phenylketonuria (PKU)
sickle-cell disease
Tay-Sachs disease
Dom inant disorde rs
Achondroplasia
Alzheimer’s disease (One type)
Huntington’s disease
Hypercholesterolemia (some types)
Fetal testing can spot many inherited disorders early in preg nancy.
amniocentesis-- between 14th and 16th week, withdraw some fluid through needle
inserted through abdo men-- the test fluid withdrawn also has some cells.
Grow cells in culture and do a kayotype-- test for Down’ syndrome (trisomy of
chromosome 21); test for T ays Sachs disease.
1% risk of complication.
chorionic villus samplin g (CVS) insert tube through vagina into cervix and itno uterus
and suction off a small amount of fetal tissue (chorionic villi) from placenta. These cells
are rapidly growing and thus allow a rapid test-- i.e. with in a few hours.
Second advantage is that CVS can be performed earlier--between 8th and 10th week.
2% risk of complication.
Ultra sound-- non- invasive, has no known risk.
fetoscopy. Tube inserted like in CVS but carri es a camera-- allows phys ician to look at
growing fetus. 10% risk of complication.
What determines which tests are used?
Consider the inheritance of two traits at once.
Round yellow peas
RRYY
Gametes: RY
X
wrinkled green peas
rryy
X
ry
Round yellow peas
RrYy
Gametes: RY, Ry, rY, ry
X
RY, Ry, rY, ry
Round yellow, green round, yellow wrinkled, green wrinkled
RRYY 1
rrYY 1
RRyy 1
RRYy 2
rryy 1
rrYy
2
Rryy 2
RrYY 2
RrYy
4
Total:
9
3
3
1
There are lots of different genotypes that could give a plant
with the phenotype of round and yellow peas.
If you had such a plant, how could you determine what
genotype it had?
Cross it against a double recessive plant, i.e. in this case a
plant with green wrinkled peas.
Parental?
RRYY
RRYy X ry
RrYY
RrYy
F1 Genotypes
RrYr
RrYy and Rryy
RrYr and rrYy
RrYy, rrYy, Rryy, rryy
F1 Phenotypes
All round and yellow
All round, both green and
yellow
All yellow, both round and
wrinkled
Both yellow and green, both
round and wrinkled
The rules of probability (statistics) can predict the outcome of a
particular cross.
If AaBb is crossed with AaBb, what is the probability of
having an aa offspring?
1/2 X 1/2 = 1/4
What is the chance of having a bb offspring?
1/2 X 1/2 = 1/4
What is the chance of having either an aa or a bb offspring?
1/4 + 1/4 = 1/2
What is the chance of having an offspring with both aa and bb?
1/4 X 1/4 = 1/16
Rules of probability (Campbell p 162)
1. Probability scale is from 0 to 1.
An event certain to occur has probability = 1
An event certain Not to occur has probability = 0
Consider a situation where the outcome of any particular event is
unaffected by what has happened on previou s events
(i.e. for a coin toss, each toss the probability of getting a head is
1/2). Thus each event in an independent event.
2. The rule of multiplicity relates to the out come of two
independent events (i.e. the toss of two coins).
The probability of a compound event (i.e. getting two heads) is the
product of separate probabilities of the independent events (i.e. 1/2
X 1/2 = 1/4).
A biologic al exa mple i s the probability of getting a recessive
homozygote offspring from two heterozygote parents.
3. The rule of addition relates to an event that could occur in two
or more alternative ways (i.e. the probability of getting a head and
a tail from the toss of two coins). The probability of such an event
is the sum of the probabilities of the different ways. (1/4 + 1/4 =
1/2)
A biolog ical example is the probability of getting a hete rozygote
offspring from two heterozygote parents.