Transcript probability & genetics

Probability & Genetics
What are the chances of flipping a
head?
tail?
3 heads in a row?
probability
• What is probability?
– The chance an event occurs
• Coin flipping
– Chances of heads? Tails?
– Two coins
• Chances of two heads? Two tails? Heads & tails?
• Rule of Multiplication
– “And” Rule
• Past events do not affect future probabilities
• INDEPENDENT!
11-2 Probability and
Punnett Squares
• Punnett squares can be
used to show the
possible outcomes for a
trait according to the
traits of the parents
• Also show the
probability of the
outcome
E
e
Ee x Ee
• Squares represent possible offspring
• Each offspring gets one gene from
“dad” (from the top) and one gene
from “Mom” (from the side)
• What % of the offspring will have
free earlobes?
• What % will have attached ears?
• What % will be heterozygous for the
trait?
E
E
e
e
EE
Ee
Ee
ee
Fast Patterns to Know…
Practicing with Punnett Squares
Together, then assigned one to
share!
Monohybrid Cross
TT x tt
Tt x TT
TT x TT
tt x tt
Tt x Tt
tT x tT
Genotype Ratio
Phenotype Ratio
Dom.Hom : Het : DomRec
Dom: Rec
• So what about your “kids”?
• Complete yesterday’s analysis.
• What are the chances your “kid” will have a
widow’s peak? Curly blond hair? Attached
earlobes?
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 doesn’t influence
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!
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
RY, Ry, rY, ry
Patterns
A Het x Het dihybrid cross yields the typical
genotypic 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 normal eyebrows and
heterozygous for thick lips mates with a
woman who is unibrowed and heterozygous
for thick lips, what is the genotypic and
phenotypic ratio of the offspring?
How do you set it up?
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Dad’s phenotype:
Mom’s phenotype
UUTt
uuTt
Distribution of alleles
UT – Ut – UT – Ut
uT – ut – uT – ut
Genotype Ratio (should add up to 16)
UuTT : UuTt : Uutt =
4 : 8 : 4
Phenotypic Ratio =
12 : 4 : 0 : 0
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.
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4.
5.
6.
Independent Assortment
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits
X- linked (or sex linked) Traits
• In some cases, neither allele
truly dominates over the other.
• No allele is really dom. or rec.
• 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
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 – in the population.
– Remember, you can still only have 2 alleles at a
time, in a single organism
– 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
C chCh x cc
• What are the phenotypes
of the parents?
• Fill in the Punnett Square.
• What is the probability
that an offspring will be
albino?
• What is the probability
that an offspring will be
himalayan?
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
• Multiple alleles, Polygenic and Codominance!
• 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
–Prophase I
–Must be homologous
Actual location of genes on
Gene
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
Maps
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
Muscular
Dystrophy