Transcript - MentorsProject

It’s All In Your Genes
Introduction to Genetics
and
Punnett Squares
Developed By:
Stephanie Shirley
Senior Graduate Student
MD Anderson Cancer Center
Science Park Research Division
Background
• Inherit - to receive from a parent or ancestor
by genetic transmission
• Genetics - the study of inheritance
• Gregor Mendel
– 1822-1884
– Pea plants
– “Father of Genetics”
Mendel’s Laws
•
The Law of Segregation
– Alternative versions of genes account for variations in traits
Mom
Dad
– For each trait, two copies of a gene are inherited;
one from each parent.
– The two copies of a gene segregate during reproduction
• Eggs have one copy of each gene
• Sperm have one copy of each gene
• Offspring have two copies of each gene
•
The Law of Independent Assortment
– The inheritance of one trait will not affect the inheritance of
another
• The genes that end up in gametes are randomly sorted from all
possible combinations of maternal and paternal genes
• Because the gametes end up with a random mix of genes, they
assort independently (any combination is possible)
Children
Definitions
• Genes - sequence of DNA coding for a protein
– units of inheritance
• Allele - different forms of a gene
• Genotype - the genetic makeup of an individual
• Phenotype - an observable trait in an organism
• Homozygous - having two copies of the same allele
• Heterozygous - having one copy of two different alleles
Dominant and Recessive
Alleles
• Remember that Mendel’s Laws state that some traits aren’t seen in
the offspring but can still be passed on to the next generation
• Traits that require two alleles to be seen are called recessive
– Phenotype of aa is different from both AA and Aa
• Traits that require only one copy to be seen are called dominant
– Phenotype of Aa is the same as AA
• Even one copy of a dominant allele will cover up a recessive trait
Can genetics be predicted?
• Probability - chance of something happening
• Punnett Square - diagram used to predict
probability of genotype
Gummy Bear Genetics
• Each bear has 2 genes
– one from Mom and one from Dad
• Possible Combinations:
– A and A = Red
Mom
– A and a = Red
a
a
– a and a = Yellow
Dad
A
A
a
A
a
A
A
a
A
a
Gummy Bear Genetics
Test Crosses
What Genotype is Dad?
a
a
A
A
a
A
a
a
a
a
a
a
AA or Aa ?
Harry Potter
and the
Recessive Allele
How Are Wizards Made?
• Wizarding ability and muggleness are observable traits i.e. phenotypes
• All humans including wizards and muggles receive one allele from each
parent
• So, being a wizard or a muggle is all decided by genetics i.e. a person’s
genotype
How Are Wizards Born?
• The allele for wizarding ability is m
• Wizards have the genotype mm
• The allele for muggleness is M
• Muggles have the genotype MM or Mm
The Malfoys
All their ancestors are wizards so they
must have the alleles mm
Narcissa Malfoy
(mm)
Lucius Malfoy (mm)
m
m
Draco Malfoy
m( m)
m
mm m m
m mm m m
4/4 = 100% mm
The Potters
Both Harry’s parents had magical ability so they must both have been mm
Lily Potter (mm)
James Potter (mm)
m
Harry Potter
( mm)
m
m
m
mm m m
mm m m
4/4 = 100% mm
Hermione is a witch so she must be mm
Both her parents are muggles so they must be Mm so they can give her a m allele
each
Mm
Mm
M
m
M
MM
Mm
m
Mm
mm
mm
1/4 MM
1/2 Mm
1/4 mm
3/4
Muggle
1/4
Magic
Tom Riddle is a ‘half blood’.
His mother was a witch (mm) and his father was a muggle
His father must have had the alleles Mm so he could give him the other m allele
mm
Mm
2/4 Mm - Muggle
M
m
m
Mm
mm
m
Mm
mm
2/4 mm - magic
mm
What wizarding alleles would Ron and Hermione’s children have?
Ron (mm)
Hermione
(mm)
Hermione X Ron
Hermione
m
m
m
m
m
m
m
m
m
m
m
m
Ron
All kids will be mm = wizard/witch
4/4 = 100 % mm homozygous
• Which allele is recessive?
Dominant and Recessive
Alleles
• Which allele is dominant?
Intermediate Concepts
•
Incomplete dominance
– The heterozygote (Aa) has a phenotype intermediate to that of
the two homozygotes (AA or aa)
• Orange Gummy Bears
• Pink snapdragn flowers
•
AA aa
Aa
Codominance
– The heterozygote has a phenotype distinguishable from both
homozygotes and both alleles are separately manifest in this
phenotype.
• Blood type in humans (proteins A and B are both expressed on
surface of blood cells in AB individuals)
•
Multiple alleles
– A group of individuals may have more than two different alleles
for a given gene (but only two are inherited, one from each
parent)
• ABO blood group system in humans
– 3 alleles: A, B or O. A and B are codominant and both A and B
are dominant to the O allele. So, there are 4 blood group
phenotypes:
»
»
»
»
A: AA and AO
B: BB and BO
AB: AB
O: OO
O
O
A
A
B
B
AB
AB
Intermediate Concepts (cont)
• Genetic Variation
– Phenotypic polymorphism
• Qualitative differences in phenotype as
a result of genetic variation
– Eye color variation in humans - two
major eye color genes with multiple
minor genes
– DNA polymorphisms
• Minisatellites and microsatellites variable number of tandem repeats
• SNP - single nucleotide polymorphisms
– Antigenic polymorphism
• Sequence differences in a molecule due
to gene rearrangement
– MHC - major histocompatibility
complexes
Intermediate Concepts (cont)
• Genetic variation continued:
– Mutation
Myostatin gene
Crustacyanin gene
AD genes
OB gene
Melanin gene
Fd gene
Summary
Mom
Dad
• Genes
• Genetics
• Genotype and phenotype
Children
• Predict Inheritance using Punnett Squares
Mom
• Dominant vs. recessive alleles
Example: A or a for bear color
Example: M or m for wizard or muggle
a
Dad
a
A
A
a
A
a
A
A
a
A
a
Short tail (S)
White coat (b)
Long tail (s)
Brown coat (B)
•Short tail with brown coat
•SS BB (1)
•Ss BB (2)
•SS Bb (2)
9
•Ss Bb (4)
•Short tail with white coat
•SS bb (1)
•Ss bb (2)
3
•Long tail with brown coat
•ss BB (1)
•ss Bb (2)
3
•Long tail with white coat
•ss bb (1)
1