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Genes and Heredity
Update:
• UNIT TEST ON THURSDAY.
– Chapters 11, 12, 13
• Suggested Review:
Page 514, #1-11, 14, 16-17, 23, 25-27, 2932.
Page 515, # 33-40, 42, 44, 49-53.
(Look over your notes!)
Theories of Heredity
•
•
17th century - the
homunculus, a
miniature human being,
in human sperm cells
17th century - the egg
contains an entire
human in miniature and
that semen only
stimulates its growth.
Gregor Mendel (1822-1884)
- was an Austrian monk
- during his childhood,
he worked as a
gardener, studied
beekeeping
- between 1856 and
1863 tested 29,000
pea plants
Why Garden Peas?
- He noticed
that there were
a number of
distinct traits
that were only
one of two
ways
Why Garden Peas?
- Secondly, pea plants can
reproduce through selffertilizing or crossfertilizing
- Pollen produced from the
male stamen (anther and
filament) fertilizes the
female ovary
- He cut the stamens off
and used pure breeding
plants only
Mendel’s Experiments
- before Mendel, people thought mixing traits would create
a blend (e.g smooth peas bred with wrinkled peas
produces slightly wrinkled peas)
- he took pollen from round seed plants and crossed it
with the egg of a wrinkled seed plants and vice versa
- ALL THE SEEDS CAME OUT ROUND. The trait
seemed to dominate the other trait
- He repeated this for other characteristics of the pea
plant, and realized that one trait dominated the other
Results
-
Mendel hypothesized that factors controlled the traits of a plant
We now know these factors as genes – segments of DNA on a
chromosome that code for a certain trait (hair colour, eye colour etc)
Two or more alternate forms of a gene are called alleles, which are either:
Dominant, meaning they are always expressed when they are genetically
present, or they are:
Recessive, meaning they are overruled by dominant alleles, and are
expressed only when they are the sole allele present
Hybrid crosses
- When Mendel crossed a pure breeding
round seed pea plant with pure breeding
wrinkled seed plants, he produced
hybrids, meaning each plant had one
allele different from their parent for a
specific trait
-
The parent generation were
pure breeding (RR and rr for
seed shape) producing all
round peas (Rr)
-
He crossed the plants of this
first filial generation (F1) (Rr X
Rr) which make R and r
gametes
-
75% were round, and 25%
were wrinkled in the second
filial generation (F2)
In summary
1. Inherited characteristics are controlled by
factors, known as genes, that occur in pairs.
2. One factor, or allele, masks the expression of
another. This is the principle of dominance.
3. A pair of factors, or alleles, separate or
segregate during the formation of sex cells
(meiosis). This is known as the law of
segregation. Mendel predicted this without
knowing about meiosis!
Patience is a Virtue
- Mendel’s paper was referenced three times in
thirty five years and basically forgotten
- Darwin (1809 –1882) didn’t know of it. He
believed in:
- Pangenesis, which holds that body cells shed
gemmules, which enter the blood stream and
collect in the reproductive organs prior to
fertilization. Thus every cell in the body has a
'vote' in the constitution of the offspring
- Mendel’s work was rediscovered in 1900
4.2 – The Inheritance of One
Trait
Human Genetics
• Each human cell has 46 chromosomes in
23 pairs, half donated from your mother
(M) and half donated from your father, (F).
Homologous Chromosomes
• Again, one chromosome from the pair is
donated from the mother, one from the
father.
• Homologous: chromosomes which are the
same size and shape, and code for the
same traits (genes).
• How many pairs of homologous
chromosomes would a woman have? A
man? Why?
For a certain gene (like eye colour), the
location of where it is coded for is the
same on both homologous chromosomes.
B
b
For a certain trait (like eye colour), there is variation (brown
or blue) which we attribute as either dominant or
recessive based on how strongly they are expressed.
Single-Trait Inheritance Terms
• ______________: refers to the alleles that
an organism contains for a particular trait.
One allele for each trait is inherited from
each parent, which is why they occur in
pairs.
• E.g. Tall (T): is the
___________________ for pea plants and
short (t) is the ____________________ for
the height gene.
• _________________: refers to the
observable traits of an individual. In short,
what we see (not always... Exceptions?).
Genotype vs. Phenotype
• How many genotypes for seed colour
does a pea plant have?
• How many phenotypes for seed colour
does a pea plant have?
• Review
– All recessive PHENOTYPES must have a
____________________ genotype.
– Dominant PHENOTYPES can have
___________________ or
____________________ genotypes.
Punnett Squares
• A diagram used to predict the outcome of
a particular cross or breeding experiment.
– Determines the PROBABILITY of an offspring
having a particular genotype.
Punnett Squares Example
• The allele for blue eyes (b) is recessive, whereas the allele for
brown eyes (B) is dominant. If a homozygous brown eyed man
conceives a child with a blue eyed woman, what is the probability of
the child having blue eyes?
Monohybrid Cross
• Deals with ONE trait, and is the cross or
breeding of TWO different, contrasting
alleles.
• Eg. Tt x Tt
Mendelian Ratios
• Mendelian Ratio for Phenotype:
• Mendelian Ratio For Genotype:
Practice Problems
• Let’s do questions 1-3 on page 130!
Pedigrees
• Crosses can be done easily with plants,
such as the common garden pea, in order
to determine the inheritance of traits.
• Experimental crosses are not possible with
humans.
• Human geneticists use medical, historical,
and family records to study crosses that
have already occurred.
Simple Dominant Traits
• A simple dominant trait has only two
possible alleles – dominant or recessive.
• Examples of simple dominant traits:
widow’s peak hairline, ability to roll the
tongue, strait thumb, and detached
earlobe.
Back to Pedegrees
• Squares generally used to represent
males and circles represent females.
• Colour/shading represents individuals who
are recessive or dominant for a single trait.
• Generations represented by roman
numerals.
• What do you think a HALF SHADED
square or circle means?:
__________________________________
Determining Genotypes
• How can you determine if a particular
organism is homozygous dominant or
heterozygous?
• TEST CROSSES!
Test Cross
• When we know the dominant phenotype of
an organism but we want to know it’s
genotype.
• We always test it with the recessive
phenotype.
Test Cross Example
• We have two parental sheep, one black
and one white. White is dominant and
black is recessive. How do we know the
genotype of the white sheep?
• If the phenotype of the offspring were 50%
white and 50% black, we would know that
the unknown allele is actually the
recessive allele, w. If all the offspring were
white, then we would know that the
unknown allele is actually the dominant
allele, W.
Test Crossing to determine the
Number of Alleles
• Using test crosses, we can determine
whether a particular trait is passed by one,
two, or more pairs of alleles.
Fun Stuff
• Page 132, Investigation.
Section Review
• Page 135, #1,2,3,4,5,6,7,10,11,12,13-16
(look in section 4.2 for answers to questions
not covered in the ppt).
4.3 – The Inheritance of Two
Traits
• Were there any problems with the
homework?
POP QUIZ!
• On the board, define the following terms:
– Dominant Allele
– Recessive Allele
– Homozygous
– Heterozygous
– Genotype
– Phenotype
– Trait
– Homologous Chromosomes
RECAP ON MENDEL
How did Mendel determine the concept of
dominant and recessive ‘factors?’
(What was his experiment)?
Question...
• Does one trait affect another?
(Does pea colour influence pea shape?)
Mendel’s Dihybrid Cross
Experiments
• First produced plants that were purebred
for 2 traits that he wanted to examine.
– Ex// Plant with round yellow peas & Plant with
wrinkled, green peas.
These plants were HOMOZYGOUS for both traits.
Mendel’s Dihybrid Cross
Experiments (Example)
• Mendel bred plants until the offspring
always had round, yellow seeds. (RR YY)
– Homozygous dominant
• He also bred plants with wrinkled, green
seeds. (rr yy)
– Homozygous recessive
Dihybrid Cross!
• F1 generation all had round, yellow seeds.
– Why is this?
Dihybrid Cross!
• F1 generation all had round, yellow seeds.
– Why is this?
– The offspring are all heterozygous for the two
traits. (RrYy)
RRYY
RRYY
rryy
RRYY
rryy
RrYy
RrYy
• Mendel allowed the F1 generation to selfpollinate. Of 551 plants in the F2
generation:
– 320 round
– 104 round green
– 101 wrinkled yellow
– 26 wrinkled green
FIND THE RATIO OF PHENOTYPES!
Law of Independent Assortment
• What explains the ratio?
– If the alleles from one trait were inherited
independently of the alleles for another trait.
• LAW OF INDEPENDANT ASSORTMENT
Law of Independent Assortment
• What explains the ratio?
– If the alleles from one trait were inherited
independently of the alleles for another trait.
• LAW OF INDEPENDANT ASSORTMENT
• Different pairs of alleles are passed to the offspring
independently from each other.
WHAT DOES THIS MEAN!?
• Offspring may have new combinations of
alleles that are not present in either
parent.
Dihybrid Test Crossing
• Two-trait test cross involves crossing an individual that
shows the dominant phenotype for the two traits with an
indivudual that is homozygous recessive for the same
two traits.
• The individual showing the dominant phenotype for both
traits may be either heterozygous or homozygous
dominant.
• Use a homozygous recessive individual to cross
because better chance of producing an offxpring that is
homozygous recessive for both traits.
Example:
• Ex//
– Pea plant homozygous dominant for purple
flower colour (PP) and round for pea shape
(RR), then the F1 generation will all have
__________________ phenotypes.
– If heterozygous for both traits (PpRr), then
25% chance the F1 generation will show the
recessive condition for one or both of the
traits.
Sample Problem
• A male and female guinae pig are both heterozygous for
fur colour and fur texture. Both dark fur (D) and rough fur
(R) are dominant traits.
a)
b)
c)
d)
e)
f)
What are the recessive traits and what variables would you
use for them?
What is are the parent’s phenotypes?
How many different gametes are formed and what are they?
Determine the frequency of offspring that are homozygous for
both traits.
Determine the frequency of offspring that have rough, dark fur.
Determine the frequency of offspring that express both
recessive traits.
• ICW: Practice problem, Page 140
• Section review, page 141: #1-9
4.4 – Beyond Mendel’s Laws
• Some organisms show different patterns
of inheritance other than dominant and
recessive.
INCOMPLETE DOMINANCE
• In some cases, neither of the alleles is
dominant.
• Blending of traits can occur: incomplete
dominance.
• Occurs in heterozygotes.
Example
• Snapdragons
– White and red flowers are
homozygous.
- Pink flowers are
Heterozygotes.
• The variables R and R’ are used to
indicate alleles.
– Two R (R red) alleles are necessary to
make a red flower  individuals with only one
R allele are unable to make enough red
pigment to produce red flowers.
– Two R’ (R’  white) alleles are necessary to
make a white flower.
• If RR and R’R’ (P) flowers were to be
crossed, what would be the genotypes of
the offspring?
• If RR and R’R’ (P) flowers were to be
crossed, what would be phenotypes of the
offspring?