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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
Mende l’s Hypo theses: As rephrased by C ampbe ll p158
1. There are alternative forms of genes, the units that determine
heritable traits.
2. For each inhe rited characteristic, an organism has two genes,
one from each parent. These genes may both be the same allele, or
they may be different alleles.
3. A sperm or egg carries only one allele for each inhe rited trait,
because allele pai rs separate (segregate) from each other during the
production of gametes.
4. When the two gene s of a pair are different alleles and one is
fully expressed while the other has no noticeable effect on the
organism’s appe arance, the al leles 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 causes cystic fibrosis. ?
Go to:
http://genome.nhg ri.nih.gov/clone/
What are some diseases with simple dominant or recessive
inheritance?
Recessive disorders
Albinism
cystic fibrosis
galactosemi a
pheny lketonuria (PKU)
sic kle-cell disease
Tay- Sachs di sease
Dominant disorders
Achond roplasia
Alzhe imer’s disease (One type )
Hun tington’ s dis ease
Hype rcho lesterole mia (some types )
Fetal testing can spot many inherited disorders early in pregnancy.
amniocentesis-- between 14th and 16th w eek, wit hdr aw some fluid through ne edle
inserted through abdomen-- the test fluid wit hdrawn also has some cells .
Grow cell s in cult ure and do a kayo type -- test for Down ’ syndro me (tris omy of
chromosome 21); test for Tay s Sachs dis ease.
1% risk of compli cation.
chorionic villus sampli ng (CVS) insert tube through vag ina into cervix and it no u terus
and suc tion off a small amount of fetal ti ssue (cho rionic villi ) from placenta. The se cell s
are rapidly grow ing and thu s allow a rapid test-- i.e. wit h in a few hour s.
Second advan tage is that CVS can be per formed earli er--between 8th and 10 th week.
2% risk of compli cation.
Ultra sound-- non- invasive, has no known risk.
fetoscopy. Tube inserted like in CVS but carries 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 p articular event is
unaffected by what has happen ed on previous event s
(i.e. for a coin toss, each toss the probability of getting a head is
1/2). Thu s each even t in an independent event.
2. The rule of multiplicity relates to the out come o f two
independen t events (i.e. the toss of two coins).
The probability of a compound event (i.e. getting t wo heads) is the
product of separate probabilities of the independ ent event s (i.e. 1/2
X 1/2 = 1/4).
A biological example is the probability of getting a r ecessive
homozygote o ffspring from two heterozygo te parents.
3. The rule of addition relates to an event that could occu r in two
or more alternative ways (i.e. the p robability of getting a head and
a tail from the toss of two coins). The probability of such an event
is the su m of the probabilities of the different ways. (1/4 + 1/4 =
1/2)
A biolog ical example is the probability o f getting a hete rozygote
offspring from two heterozygo te pa rents.