Mendel and Genetics - Lake Stevens High School
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Transcript Mendel and Genetics - Lake Stevens High School
Chapter 14
In the 1800s the popular inheritance
theory was “blending”--offspring were a
mixture of their parents
◦ this suggests that organisms will
become uniform over time (we know this
isn’t true)
Mendel had a “particulate” theory (genes)
◦ this was observed through his
observations of pea plants
He carefully planned all his breeding experiments,
taking careful notes on the results.
his experiments started with true-breeding
varieties
◦ then followed the offspring for 2 generations.
(P, F1, and F2)
Through thousands of crosses, Mendel’s observations
led to 2 fundamental principles of heredity.
◦ Law of Segregation
two alleles separate during gamete formation
(meiosis) and end up in different gametes
dominant and recessive alleles
two heterozygous parents crossed always have a
phenotypic ratio of 3:1 (Punnett Squares)
◦ Law of Independent Assortment
each pair of alleles segregates independently of
each other pair of alleles during meiosis
the chance of inheriting one trait from either parent
is separate from all other traits
for typical Medelian inheritance only
Certain patterns of inheritance are more complex than those
discovered by Mendel (either controlled by one gene or 2+
genes)
When trait is controlled by a single gene...
◦ Complete Dominance--classic Mendelian patterns (strictly
dominant or recessive)
◦ Incomplete dominance--neither allele is completely dominant
(blending in heterozygous phenotype)
flower color
◦ Codominance--two alleles shown independently in
heterozygous phenotype
animal coloration
Codominance
Incomplete
Dominance
multiple alleles-when a gene for a specific trait
has more than two alleles. Results in multiple
phenotypes.
◦ This usually works in combination with incomplete
or codominance
Human ABO blood groups
Rabbit Fur Color
pleiotropy--when a gene has multiple
phenotypic effects.
◦ Single gene affects multiple things in an
organism.
◦ Most genetic diseases present this way
Cystic fibrosis and Sickle Cell anemia
Lethal Genes: a gene that leads to the death
of the organism when inherited in
homozygous genotype (either dominant or
recessive)
◦ Dwarfism in humans (dominant allele)
◦ Manx cats (recessive)
◦ Yellow coat color in mice (dominant)
When a trait is determined by two or more
genes...
◦ epistasis-the phenotype at one locus alters the
gene at a second locus
Interaction of two genes to control a single
phenotype, does not have an additive effect
Might mask another gene, or cause a completely
new phenotype
Labrador Retrievers and coat color
2 genes: E (pigment) e (no pigment) ; B (black), b
(brown)
◦ polygenic inheritance--an additive effect of two or
more genes on a single phenotypic character
Many genes working together to determine a
particular trait
skin color, height, weight, hair color, eye color in
humans
When inheritance depends on
chromosomes...
◦ sex-linked traits--specific
traits are carried on the X or Y
chromosome.
results in some traits affecting
boys more often than girls
X-linked traits: carried
on X chromosome
females carriers;
males have trait or not
Colorblindness, baldness, sickle-cell anemia,
hemophilia, Duchenne muscular dystrophy all
are examples of sex-linked traits.
If a normal-sighted woman whose father was
colorblind marries a colorblind man, what
percentage of their sons will be colorblind?
Daughters?
Chromosome Number
◦ During meiosis chromosomes can fail to split
evenly (nondisjunction)
Aneuploidy
◦ Results in severe phenotypic changes in an
individual
◦ Diagnosed via karyotype
◦ Down Syndrome (trisomy 21)
◦ Klinefelters Sydrome (XXY)
◦ Turner Syndrome (X)
Chromosome Structure
◦ Sometimes parts of chromosomes are altered
during cell division or altered due to environment
Deletion: missing piece
Duplication: extra piece
Inversion: attach upside down in homologous pair, or
within chromosme
Translocation: piece joins non-homologous
chromosome
Cri du chat: deletion chromosome 5
Leukemia: translocation (chromosome 9 attaches to
22) “Philadelphia Chromosome”
Fragile X: duplication (repeat at end of X)
Good Morning AP Bio!
Today we are going to discuss our last type of
inheritance pattern (linked genes)…then
practice solving some of those problems.
Reminder: Test corrections are due tomorrow!
You will have time tomorrow to work through
and finish your genetics practice problems
packet (due Monday)
Remember: crossing over occurs during
meiosis, when chromosomes trade alleles
◦ Produces “recombinant chromosomes”
Some genes are located very closely on a
chromosome, and are usually inherited
together.
They are called “linked genes”
linked genes: genes located near each
other on the same chromosome are often
inherited together
◦ genes do not assort independently, so
ratio of offspring varies depending on
location of genes
result in genetic recombination
(offspring with traits different from
parents)
This lack of independent assortment
indicates the genes are on the same
chromosome.
Thomas Morgan and his grad student first
discovered linked genes in drosophila (fruit
flies).
When crossing a heterozygous wild-type fly
(b+b vg+vg) to a black body, vestigial wings
(b vg) he discovered allele frequencies that
didn’t match the prediction
◦ 83% parental types, 17% recombinant types
◦ Identified that crossing over had occurred.
http://www.bozemanscience.com/geneticrecombination-gene-mapping/
The recombination frequency (%) is the same
as the map units (distance) between genes on
a chromosome
◦ Less than 50% recombination = same chromosome
We can use this information to map genes
Smaller number = closer together
◦ Greater than 50% recombination = different
chromosome
Not able to map
The crossover frequency (recombination)
between genes E and F is 6%, between E and G
is 10% and between F and G is 4%.
Determine the sequence of genes on the
chromosome.
Environmental
Influence
◦ Nature vs.
nurture
◦ Expression of
traits determined
by environmental
influences
Nonnuclear Inheritance (mitochondria and
chloroplasts)
◦ These organelles have their own DNA that
replicates separately from nuclear DNA
◦ Follows non-mendelian inheritance
◦ All your mitochondrial DNA (mDNA) is from your
mom!
“mitochondrial diseases”—result from mutations in
mDNA
Genomic Imprinting
◦ Phenotype depends on if allele is inherited from
mom or dad (autosomal)
Allele from either parent is “silenced” by the
presence of other allele
Example of epigenetics
◦ Affects very few genes, not common
Chimera
◦ Single organism
composed of
genetically distinct
traits
Two genomes, one
organism!
Results from multiple
fertilized eggs fusing
during development
Used to visually trace traits within human
families (helps identify inheritance patterns)
◦ Circle= female
◦ Square = male