Transcript Multiple alleles

11- 3: Exploring Mendelian Genetics
&
11-5: Linkage and Gene Maps
What are the chances?
• Do people with brown hair always have
brown eyes?
• Do people with brown hair have a higher
chance of having brown eyes?
• Is a round pea seed always yellow? Can
a short plant have purple flowers?
INDEPENDENT ASSORTMENT
• The inheritance of one gene does not
influence the inheritance of another.
• Mendel named this:
The Law of Independent Assortment
• In meiosis, the chromosomes line up
randomly on the equator to be separated.
• If your parents are heterozygous for any
traits, this leads to lots of possibilities!
How did Mendel figure this out?
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Through experiments…
He crossed two different plants
Each was true-breeding for 2 different traits
P gen: rryy (wrinkled & green)
x RRYY (ROUND & YELLOW)
• F1 gen: RrYy
• F2: a bunch of possibilities!
Segregation of Chromosomes Video
Aligning the Punnett Square
• F1 gen now has: RrYy
• Distribute the 1st letter of the 1st set to
each of the 2 letters in the 2nd set.
• Distribute the 2nd letter of the 1st set to
each of the 2 letters in the 2nd set
RrYy
• 4 possibilities: RY, Ry, rY, ry
Patterns
A Het x Het dihybrid cross yields the typical
phenotypic ratio of:
9: 3: 3: 1
Now You Try!
In humans, the gene that causes a unibrow
(u) is recessive to not connected eyebrows
(U); the gene for thick lips (T) is dominant
over the gene for thin lips (t). If a male
that is homozygous for not connected
eyebrows and heterozygous for thick lips
mates with a woman who has a unibrow
and is heterozygous for thick lips, what is
the phenotypic ratio of the offspring?
How do you set it up?
• Genotypes
U = not connected eyebrows
--Dad = UUTt
--Mom = uuTt
u = unibrow
T = thick lips
T = thin lips
• Possible Different Combinations of Alleles
--Dad = UT or Ut
--Mom = uT or ut
dad
UT
2. Make one row for
each possible
different
combination of
alleles from mom
m
o
m
Ut
1. Make one
column for each
possible different
combination of
alleles from dad
uT
3. Fill in the boxes
ut
Results of the Cross
UT
uT
Only include the
phenotypes
present in the
results.
ut
UuTT
UuTt
Ut
UuTt
Uutt
Phenotypic Ratio
3 not connected eyebrows & thick lips : 1 not connected eyebrows & thin lips
Summing It Up: Mendel’s Principles
1. Parents pass on characteristics, sexually, through
genes to their offspring
2. When there are multiple alleles (appearances) for
one gene, some are dominant & some are recessive
3. During formation of parental gametes, alleles are
segregated into separate gametes. Each parent is
then able to pass ONE allele to the child. The child
therefore gets ONE allele from EACH parent
4. The chromosomes (and therefore alleles) from each
parent arrange themselves independently during
meiosis
11-3 Exploring Mendelian Genetics
Going Beyond Simple Dominance:
1.
2.
3.
4.
5.
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits
X- linked (or sex linked) Traits
Incomplete Dominance
• In some cases, neither allele truly
dominates over the other.
• No allele is really dominant or recessive
• The heterozygous genotype shows a MIX
of the two traits.
• Example- Four O’Clocks
– R- gene for red flowers, W- gene for white
flowers:
– RR- red, WW- white, RW- pink
Example: Snapdragons
RR = Red
WW = White
All RW = Pink
Codominance
• In some cases, both alleles are dominant.
– No allele is really recessive.
• The heterozygous genotype shows BOTH
of the two traits.
• Example- Chicken feathers
– B- gene for black feathers, W- gene for white
feathers:
– BB- black, WW- white,
BW- “erminette” Black and White!
Multiple Alleles
• Many genes have more than just two
alleles for a trait
– Remember, you can still only have 2 alleles at
a time.
– It is still just ONE gene, but lots of possibilities
• Example: Alleles for rabbit fur
– C-full color, dominates over cch, ch, c
– cch – chinchilla, dominates over ch and c
– ch- himalayan, dominates over c
– c- albino, recessive to all
Practice Cross
cchch x cc
• What are the phenotypes of
the parents?
– light gray x albino
• Fill in the Punnett Square.
• What is the probability that
an offspring will be albino
(albino = cc)?
– 0 out of 4 = 0%
• What is the probability that
an offspring will be
himalayan (himalayan =
chch, chc)?
– 2 out of 4 = 50%
cch
c
Light gray
cchc
Light gray
c
cchc
ch
Himalayan
chc
Himalayan
chc
Polygenic Traits
• Many traits result from the interaction of
several genes.
– Multiple genes, perhaps on different
chromosomes even,
– produce one phenotype
• Polygenic traits can produce a large range
of phenotypes
• Examples: human skin color (at least 4
genes), human eye color, human height
Capital letters = dark; small letters = light;
more dark alleles = darker!!
Gradation of Human Skin Color
Height in Humans
• Range of phenotypes resulting from
polygenic trait
Human Blood Types
• Exhibit 3 special situations
• Multiple alleles - A, B, O
• Polygenic - one gene controls type,
another gene controls rH factor (+, -)
• Codominance - A and B are codominant
but both dominate over O
What does your blood type actually mean?
Linkage
• Really it is the chromosomes that are
segregated independently, not necessarily
individual genes.
• Some genes are LINKED if they are on the
same chromosome
– Ex: you get all of the genes on chromosome
1 from your mom if you get her chr.1
• Is that always the case though? What do
you know might happen?
Crossing Over
• Depending on how FAR
APART genes are on
chromosomes, they
may be switched during
meiosis
– Occurs during Prophase I
– Must be homologous
chromosomes
Crossing Over Video
Gene Maps
• Where actual genes are located on
chromosomes.
• Discovered by a student working in
molecular lab at Columbia in 1931.
• By looking at statistical inheritance patterns,
you can calculate the “recombination
frequency” of alleles.
• If genes are far apart, crossing over is more
likely
Sex-linked Traits
• Often called X- linked traits
• Trait can be dominant or recessive
• Probability of inheritance is altered
because the trait is on the X chromosome
• Females- XX Males- XY
Examples of X-linked traits
• Colorblindness
• Hemophilia
• Duchenne
Female: Carries
Muscular
hemophilia gene
Dystrophy
Red = hemophiliac gene
Male: Has
hemophilia