review what is an allele? what is a gene? define phenotype. define genotype. each person inherits...
TRANSCRIPT
Review Review
What is an allele? What is an allele?
What is a gene? What is a gene?
Define phenotype. Define phenotype.
Define genotype. Define genotype.
Each person inherits two Each person inherits two
alleles for each gene, one allele from each alleles for each gene, one allele from each
parent. parent.
The basic unit of heredity.The basic unit of heredity.
The physical appearance The physical appearance
or visible traits displayed by offspring.or visible traits displayed by offspring. The genetic makeup or The genetic makeup or allele combination present in offspring.allele combination present in offspring.
PunnettPunnett SquareSquareWhatWhat is it? is it?
A graphical grid showing the potential offspring allele combinations from two parents.
Punnett Squares are grids that show the potential offspring of two parents.
T T
t
t
Tt
Tt Tt
Tt
PARENT P
ARENT
OFFSPRING
KEY:T = Tallt = short
BackgroundBackground
Professor Professor Reginald PunnettReginald Punnett, (1875-1967) a , (1875-1967) a British geneticist created the Punnett square, a British geneticist created the Punnett square, a tool in genetics which is still used by biologists tool in genetics which is still used by biologists today to predict the probability of possible today to predict the probability of possible genotypes of offspring. genotypes of offspring.
Constructing a Punnett squareConstructing a Punnett square
Begin by drawing a Begin by drawing a grid.grid.Next, you put the Next, you put the genotype (allele genotype (allele combination) of one combination) of one parent across the top parent across the top and that of the other and that of the other parent down the left parent down the left side. side.
11stst parent: T T parent: T T 22ndnd parent: t t parent: t t
T T
t
t
TT x tt
Note: Capital letters denote dominance
KEY:T = Tallt = short
Constructing a Punnett squareConstructing a Punnett square
Next, all you have to Next, all you have to do is fill in the boxes do is fill in the boxes with the potential with the potential offspring offspring combinations. combinations. BUT HOW???BUT HOW???
Like a multiplication Like a multiplication table, combine each table, combine each column-head letter column-head letter and row letter into the and row letter into the empty squares. empty squares.
T T
t
t
1 2
4
3 1 & 3
2 & 41 & 4
2 & 3
Tt
TtTt
Tt
TT x tt
KEY:T = Tallt = short
Interpreting a Punnett SquareInterpreting a Punnett SquareWhat does it tell us?What does it tell us?
T T
t
t
Tt
TtTt
Tt
KEY:T = Tallt = short
1st parent: TTPhenotype?_________2nd parent: ttPhenotype?_________
Tall
Short
Offspring: ____Phenotype?_______All Tall
Tt
One parent dominant homozygous, other parent recessive homozygous.
TT x tt
TT x tt
New TermsNew Terms
Homozygous: Homozygous:
Heterozygous:Heterozygous:
Dominant:Dominant:
Recessive:Recessive:
having 2 identical alleles for a trait
having 2 different alleles for a trait
the term dominant gene refers to the allele that causes a phenotype that is seen
a gene that does not produce its effect when it occurs with a dominant gene, but produces its effect only when there are two copies of it