Transcript Mendel/Punnet/pedigrees powerpoint mendel.punnett

 There
have been two main hypotheses to how
traits are passed onto offspring. The first is the
“blending” hypothesis, where traits are mixed
to form an intermediate trait. An example would
be red and white flowers mate to form pink
offspring. The other hypothesis is the
“particulate” hypothesis where parents pass on
discrete heritable units, an example of this
would be if red and white flowers mate to make
some offspring that are red and some that are
white.
• Which do you think is correct? Support your
argument with evidence.
 Define
in your own words
• Homozygous
• Heterozygous
• Phenotype
• Genotype
 Who
is Mendel?
 Know how to put together a punnett
square with a monohybrid and dihybrid
cross.
 Know what is meant by the law of
segregation and the law of independent
assortment.
 Know why you would use a testcross.
A
monk living in the
1800’s
 Interested in the
causes of variation in
plants – used peas to
study inheritance
 Why Peas?
• Short generation time
• Large number of
offspring
• Could control mating
 How
did he discover “genes” when there
was not the technology to look at
chromosomes under the microscope?
 How did he know that we get one
“chromosome” from each of our parents?
• Note: he did not call them genes or
chromosomes, but rather, a heritable factor
 1. Alternative
versions of genes
account for variations
• Ex. Two versions of
flower color – purple
and white
 2. Organisms
inherit
two alleles – one from
each parent
 3. For
one trait, the
dominant allele
determines the
appearance over the
recessive allele
 4. Law of segregation:
two alleles separate
during gamete
formation
 Red
flowers are dominant over white
flowers. What is the probability that two
flowers, both heterozygous for flower
color, will have heterozygous offspring?
 What is the probability of a homozygous
dominant and a heterozygous flower
having offspring that is white?
 Testcross
– use a
homozygous
recessive individual
to determine if an
individual is
homozygous
dominant or
heterozygous.
 Monohybrid
cross – a
cross that creates
heterozygosity for
one character
• EX round seeds RR x rr
 Dihybrid
cross – a
cross that creates
heterozygosity at two
characters
• Ex round seeds and
seed color YYRR x yyrr
 Genes
segregate
independent of one
another (as long as
they are on different
chromosomes)
A
pea plant heterozygous for flower
position and stem length (AaTt) are
allowed to self pollinate. Axial is
dominant over terminal and tall is
dominant over dwarf. Draw a punnett
square for this cross. How many offspring
would be predicted to have terminal
flowers and be dwarf?
 If
a pea plant heterozygous for flower
color and seed shape PpRr is crossed
with a pea plant heterozygous for flower
color and homozygous recessive for seed
shape (Pprr), what are the phenotypic
ratios?
 Simply
multiply the chances
 Example
with the lottery:
 Winning the lottery
• 1/18 million
 Winning
the lottery twice
• 1/18 million x 1/18 million = 1/324 trillion
A
pea plant heterozygous for flower
position and stem length (AaTt) are
allowed to self pollinate. Axial is
dominant over terminal and tall is
dominant over dwarf. How many
offspring would be predicted to have
terminal flowers and be dwarf?
¼
x ¼ = 1/16
 The
genotype of F1 individuals in a
tetrahybrid cross is AaBbCcDd.
Assuming independent assortment of
these four genes, what are the
probabilities that F2 offspring will have
the following genotypes? (Hint: use the
probabilities in a monohybrid cross and
then multiply them. )
 A. aabbccdd
 B. AaBbCcDd
 C. AABBCCDD
 Incomplete
dominance – neither
allele is completely
dominant over the
other allele
• Ex. Red snapdragons
crossed with white
snapdragons make
pink offspring
 Codominance
– Both
alleles affect the
phenotype
• Ex. ABO blood typing
 Pleiotropy
– One gene having many
phenotypic effects
• Ex. Multiple symptoms for one genetic disease
 Epistasis
– A gene at one locus affects a
gene at another locus
• Ex. Black coat color in mice is dominant to brown
(Bb)
A second gene deterimes whether or not
pigment will be deposited into the hair – the
dominant allele is coat color (Cc)
 Polygenic
inheritance – Many genes
contribute to the phenotype
• Ex. Skin color in humans
 Disorders “live” in
a population because
they are carried by heterozygotes
• Ex. Cystic fibrosis
 Normal allele codes for a membrane protein that
transports chloride ions
 If there is not a normal allele, causes mucus to build
up in the lungs, pancreas, digestive tract and other
organs
 One out of every 2,500people of European descent
have it and one out of 25 people of European descent
carry it
 Disorders
can be due to dominant alleles.
• Ex. Achondroplasia
• If two people, heterozygous for Acondroplasia
have children, what is the probability that they
would be homozygous recessive (Normal size)
 Autosomal
recessive
 Autosomal dominant
 Sex linked
• X-linked recessive
• X-linked dominant
 Mitochondrial
 Males
– XY
 Females – XX
 Y chromosome
carries genes for sex
determination
 X chromosome
carries many other
genes besides sex
determination (such
as colorblindness)
 Affected
males transmit the gene to all
daughters but not to sons
 Affected females will always produce
affected sons
 Incidence in males higher than in
females
 Affected
males produce all affected
daughters and no affected sons
X
linked recessive
 Autosomal recessive
 Autosomal dominant
 Mitochondrial
 Nondisjunction
– homologous
chromosomes or sister chromatids fail to
separate during meiosis
 Aneuploidy
– abnormal number of
chromosomes
 Monosomic – missing a chromosome
 Trisomic – an extra chromosome
• Ex. Down Syndrome (Trisomy 21)
• Trisomy 13
 Cleft palat
 Extra toes and fingers
 Decreased muscle tone
 Developmentally delayed
 Polyploidy
– more than two sets of
chromosomes
• Ex. Bananas are triploid (set of three
chromosomes) and wheat is hexaploid (set of 6
chromosomes)