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Genetics of Reproduction
Notes
The Genetics of Reproduction
• Organisms can either reproduce
asexually or sexually
– Asexual reproduction = 1 parent
• Since there is only 1 parent, the offspring
is genetically identical to the parent (no
new DNA combinations)
– Sexual reproduction = 2 parents
• “Males” produce sex cells called sperm
– Half of the “father’s” DNA is in this sperm
• “Females” produce sex cells called eggs
– Half of the “mother’s” DNA is in this egg
• Eggs and sperm combine in an event
called fertilization which creates the
new offspring organism
– The offspring is gets half its DNA from its
“mother” and half from its “father”
– This results in offspring that are
genetically similar to the parents, but not
identical (new DNA combination)
DNA in a normal cell
• In a normal cell, humans will have 46 chromosomes
– Chromosomes are long, continuous strands of DNA that
contain both genes and introns
– This number is different for organisms that belong to different
species
• 23 of those chromosomes came from the mother’s egg
– 1 of each numbered chromosome and 1 sex chromosome
• 23 of those chromosomes came from the father’s sperm
– 1 of each numbered chromosome and 1 sex chromosome
•
These pictures are called karyotypes and are used for finding problems with the
number or shape of chromosomes and for determining sex.
Making sperm and egg
•
•
The process of making sperm or egg cells is called meiosis
Which copy of each numbered chromosome and the sex chromosome goes into
each egg or sperm during meiosis is random
–
•
For example: your father could give the chromosome number 7 he got from his mother or the
chromosome number 7 he got from his father
Whichever chromosome gets donated, you get all the versions of the genes (alleles) on that
chromosome
Sorting it out…
• You get 2 copies of each chromosome
– 1 from mom and 1 from dad
• Each chromosome has genes on it
• There are 2 basic types of genes
– Dominant = makes a working protein
– Recessive = makes a non-working
protein
• This means there are 3 basic
combinations of genes you could get
from your parents for each trait
– Both dominant
• This is called homozygous dominant
– One dominant & one recessive
• This is called heterozygous
– Both recessive
• This is called homozygous recessive
Genotypes and Phenotypes
• The combination of genes you get from the chromosomes your parents
give you is called your genotype
– Dominant genes are written as capital letters
• Different letters are used to represent different genes
• Usually the first letter of the dominant trait is used (not always)
– Recessive genes are written as lower case letters
– Example: for the height of a pea plant,
•
•
•
•
•
T = dominant gene, which makes a protein that makes plants tall
t = recessive gene, which makes a protein that makes plants short
TT = homozygous dominant genotype
Tt = heterozygous genotype
tt = homozygous recessive genotype
• The actual traits you get are called your phenotype
– Tall plant = dominant phenotype
– Short plant = recessive phenotype
Using genotypes to figure out phenotypes
• If an organism is homozygous dominant:
–
–
–
–
They got 2 copies of the dominant gene
They make 2 working proteins
They get the dominant phenotype
Example: TT = tall plant
• If an organism is heterozygous:
– They got 1 copy of the dominant gene and 1
copy of the recessive gene
– They make 1 working protein and 1 nonworking protein
– They get the dominant phenotype
– Example: Tt = tall plant
• If an organism is homozygous recessive:
–
–
–
–
They got 2 copies of the recessive gene
They make 2 non-working proteins
They get the recessive phenotype
Example: tt = short plant
Pedigrees
• Carriers will not always be marked.
Sometimes a fully colored in symbol
will actually be a carrier.
• Divorces are indicated by putting a diagonal slash through the horizontal line
between a husband and wife
• A dead person is indicated putting a diagonal slash through the circle or square
• Diamond shapes are used if the person’s gender is not known
Tracking traits on pedigrees
• R = dominant gene = can roll tongue
• r = recessive gene = cannot roll tongue
• Possible genotypes & phenotypes:
• RR = homozygous dominant =
can roll tongue
Rr
rr
Rr
Rr
• Rr = heterozygous = can roll
tongue
• rr = homozygous recessive =
cannot roll tongue
R-
R-
rr
Rr
rr
TIPS:
• Fill in the known genotypes first.
rr
rr
rr
• Look at the kids to figure out what the parents might be for the unknowns.
Mendel’s Experiments
•
Gregor Mendel was an Austrian monk whose experiments with pea plants helped us
understand how traits are inherited
•
He crossed purebred plants with similar and different traits to see what traits the offspring
received
–
–
•
He then crossed the offspring with each other to see what traits they would pass on
–
•
We now know that purebred means homozygous (carrier means heterozygous)
Mendel did not know anything about DNA, genes or genotypes when he was doing his work
Plant brothers and sisters are allowed to make offspring
In the bottom row of crosses, the short trait “skipped a generation”. This is
caused by recessive genes.
Punnett Squares
• To figure out what traits the offspring will get, we have to
account for all the possible combinations of sperm and egg
that could fertilize each other
– We can do this with a tool called a Punnett Square
– Punnett Squares do NOT show you exactly what will happen
• They show the possible genotypes and phenotypes of the offspring
• They can show what genotypes and phenotypes are not possible in the
offspring
• They can tell you the probability of each of genotype and phenotype
happening
Steps for filling in Punnett Squares
Let’s cross a pea plant heterozygous for its yellow peas with a plant homozygous
recessive for its green peas
1.
Figure out what letter is being used to
represent the gene (see chart from
earlier in notes)
y
y
Y
Yy yy
y
Yy yy
Seed color = Y or y
2.
Figure out what the dominant trait is and
what the recessive trait is
Y = yellow
3.
y = green
Figure out the parent genotypes
Yy x yy
4.
Draw the Punnett Square
5.
Fill in the parent genotypes
6.
Fill in the possible offspring genotypes
7.
Answer the question being asked
see next slide
Probability
• Probability is the likelihood, odds, or chance
of an event happening
– This can be represented as a fraction,
decimal, or a percent
• In order to calculate the probability of a
certain type of offspring happening from a set
of parents:
– Count the number of offspring that have the
genotype or phenotype you’re looking for
2/4 = 50%
• Put this number in the top (numerator) of a
fraction
– Count the number of total offspring in the
Punnett square
• Put this number in the bottom (denominator) of
a fraction
• To figure out the probability of multiple events
happening, multiply their probabilities
together
2/4
1/4
2/16
1/4 = 25%
Explaining Mendel’s Results
•
•
•
•
Let’s cross the purebred tall
plant with the purebred
short plant
Results = 4/4 Tt = Tall
Let’s cross the offspring
from the first Punnett
square with each other
Results = 1/4 TT = Tall, 2/4
Tt = Tall, 1/4 tt = short
t
T
Tt
T
Tt
t
Tt
Tt
T
T TT
t
Tt
t
tt
Tt
Polygenic Traits
• Polygenic traits are characteristics of organisms that are
controlled by more than one gene
– This term is also (sometimes) applied to genetics problems where
people are studying more than one trait at a time
• We can still use Punnett Squares to see how these might be
passed on
– There are 2 methods:
• One method is a little longer, but the math is easier
• The other method is faster, but requires more math
• Let’s solve the same problem both ways and see which you
prefer:
– What is the probability of getting a plant with purple, terminal flowers
from a cross between two parents that are both heterozygous for their
purple, axial flowers?
More boxes, less math method
What is the probability of getting a plant with purple, terminal flowers from a cross between two parents that
are both heterozygous for their purple, axial flowers?
1.
Figure out what letter is being used
to represent the gene
P = flower color
A = flower position
2.
Figure out what the dominant trait is
and what the recessive trait is
P = purple
A = axial
3.
p = white
a = terminal
PA
Pa
pA
pa
PA
PPAA PPAa PpAA PpAa
Pa
PPAa
PpAa
Ppaa
pA
PpAA PpAa ppAA
ppAa
pa
PpAa
Ppaa ppAa
ppaa
PPaa
Figure out the parent genotypes
PpAa x PpAa
4.
Draw the Punnett Square
5.
Fill in the parent genotypes
6.
Fill in the possible offspring
genotypes
7.
Answer the question being asked
3/16 = 37.5%
Fewer boxes, more math method
What is the probability of getting a plant with purple, terminal flowers from a cross between two parents that
are both heterozygous for their purple, axial flowers?
1.
Figure out what letter is being used to
represent the gene
P = flower color
A = flower position
2.
Figure out what the dominant trait is and
what the recessive trait is
P = purple
A = axial
3.
p = white
a = terminal
P
p
P
PP
Pp
p
Pp
pp
A
a
A
AA
Aa
a
Aa
aa
Figure out the parent genotypes
PpAa x PpAa
4.
Draw the Punnett Square
5.
Fill in the parent genotypes
6.
Fill in the possible offspring genotypes
7.
Answer the question being asked
Purple = 3/4
Terminal = 1/4 Both = 3/16 or 37.5%