Transcript 10.2 Genetics 2 - Mendel, etc Higher level only

Genetics 2
Genetic Crosses
Contents
Gregor Mendel
Mendel’s experiments
Mendel’s results
Mendel’s conclusions
Law of segregation
Dihybrid crosses
Law of Independent
Assortment
Linked genes
Crosses with linked genes
Sex-linked genes
Inheritance of a sex-linked
trait in humans
Non-nuclear inheritance
DNA – more information
A nucleotide
The four Possible Bases
Base Pairing
The DNA double helix
Protein synthesis – more
information
How mRNA is made
A tRNA molecule
Translation
The production of protein2
molecules from DNA
Gregor Mendel
The Father of genetics.
A monk, mathematician & scientist.
Did experiments using pea plants in the
monestery garden.
Published his findings in 1860.
First to approach genetics in a scientific
manner.
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Mendel’s experiments
He noticed that pea plants had contrasting
traits e.g. tall versus short, yellow veersus
green seed, etc.
He used pure breeding plants i.e. when
crossed with one another they always
produced offspring identical to themselves.
He called these plants the P (parental)
generation.
He examined only one contrasting trait at a
time.
This is a monohybrid cross.
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He crossed pure breeding tall with pure
breeding short (dwarf) plants.
He removed anthers from tall plants and
dusted the stigma of these tall plants with
pollen from the short plants. (see drawing
on next slide)
This prevented self-pollination (the usual
method in pea plants).
He collected the seeds produced – planted
them – and examined their size.
All plants were tall.
He called this the F1 generation.
The result was the same whether the male or
female plant was tall or short.
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The structure of a typical flower
Back to previous slide
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This was not the expected result.
He then looked to see if the information for
short plants had disappeared. How?
He let the F1 plants self-fertilise.
He collected the seeds produced – planted
them – and examined their size.
He called this the F2 generation.
He got 787 tall and 277 short plants.
Analysed these and results from other crosses
of contrasting traits and noticed that there
was an approximate 3:1 ratio.
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Mendel’s results
Trait
Flower
colour
Seed
colour
Seed
shape
Pod
colour
Dominant V Recessive
Purple
X
White
Yellow
X
Green
Round
X Wrinkled
Green
X
Yellow
(1/2)
F2 Results
Dom Rec
Ratio
705
3.15:1
224
6,022 2,001 3.01:1
5,474 1,850 2.96:1
428
152
2.82:1
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Mendel’s results
Trait
Pod
colour
Pod
shape
Flower
position
Plant
height
Dominant V Recessive
Yellow
(2/2)
F2 Results
Dom Rec
Ratio
428
152
2.82:1
Green
X
Round
X Constricted
882
299
2.95:1
Axial
X
Terminal
651
207
3.14:1
Tall
X
Dwarf
787
277
2.84:1
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Mendel’s conclusions
Each plant contains two ‘factors’ that control each
trait.
There are two alternative forms of each factor
- one is dominant (tall) & the other recessive
(small).
The dominant factor is always expressed, when
present, whether there is one or two copies of it
in the organism.
The recessive factor is only expressed when there
are two copies present in the organism.
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Could you rewrite these conclusions using modern terms?
Mendel suggested that
factors are transmitted from parent to
offspring via the gametes.
the F1 plants had one copy of each factor (one
factor coming from each parent).
This led to his first law, the Law of
Segregation
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The Law of Segregation
Mendel stated that
organisms contain two factors for every trait
which separate during gamete formation
producing gametes with only one copy of each
factor.
 The modern definition states that
characters (traits) are controlled by pairs of
genes (e.g. Tt) that
separate (segregate) at gamete formation, and
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each gamete carries only one gene for the trait.

The modern
explanation
for
Mendel’s F1
cross
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The modern
explanation for
Mendel’s F2
cross
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Dihybrid crosses
These involve the study of the inheritance of
two pairs of contrasting traits e.g. he crossed
a pure breeding tall plant with purple flowers
with a pure breeding small plant with white
flowers.
All the F1 plants were tall with purple flowers.
These were allowed to self fertilise.
The F2 produced four different phenotypes 15
The phenotypes produced
Tall purple 96
- Tall white
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- Short purple 34
- Short white
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This gives a ratio of 9:3:3:1
From this mendel formulated his second law,
the Law of Independent Assortment.
-
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The explanation
(1/2)
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The explanation
(2/2)
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Law of Independent Assortment
Mendel stated that
alleles of any one gene are transmitted
independently of any other pair of alleles.
 The modern definition states that
during the formation of gametes
each member of a pair of genes
may combine randomly with either of another
pair.

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Worked example
In pea plants yellow seed colour is dominant
to green and round seed shape is dominant
to wrinkled.
What results would be expected in a cross
between a pea plant heterozygous for seed
colour and seed shape and a plant
homozygous recessive for both traits?
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Parents
Heterozygous
Yellow & round
Green+
wrinkled
Parental
genotype
YyRr
yyrr
gametes
YR
Yr
yR
yr
All yr
F1 genotype
YyRr
Yyrr
yyRr
yyrr
F1 phenotype
Yellow
round
Yellow
wrinkled
Green
round
Green
wrinkled
Ratio
1
1
1
1
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Linked genes
(1/2)
An example of this is seen in the fruit fly.
They can have long (L) or vestigial / short
(l) wings and broad (B) or narrow (b)
abdomens.
If a heterozygous long winged, broad
abdomen is crossed with another similar fly
all the F1 offspring would be long winged
with broad abdomen or vestigial winged
with narrow abdomen.
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No other types of offspring are produced.
Linked genes
(2/2)
The reason for this is that the genes for both traits
are on the same chromosome and will be
inherited together.
These are linked genes.
Definition:
Linked genes are genes on the same chromosome
that are not separated at gamete formation and
are inherited together.
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A cross
demonstrating
linked genes
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Another example
(1/2)
In the fruit fly, normal antennae and grey body
are linked genes. The recessive alleles are
also linked and produce twisted antennae and
black bodied flies.
A grey fly with normal antennae was crossed
with a black bodied fly with twisted antennae.
All the resultant flies were grey with normal
antennae.
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Another example
(2/2)
When these F1 flies were crossed with black
flies with twisted antennae, an equal
number of black, twisted antennae flies and
grey, normal antennae flies were produced.
Explain these results.
The genes are linked.
Use a chromosome diagram.
The parental genotypes are:
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Diagram
representing
F1 cross
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Explanation
(1/2)
This fits in with Mendel’s expected results.
But, when we cross these F1 flies with the
black, twisted antennae flies, Mendel would
have expected four possible results.
This does not happen as the genes do not
assort independently of each other into the
gametes.
N and G travel together as do n and g.
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Explanation
(2/2)
There are only two types of gamete from the
F1 flies.
What are they?
As a result there are only two possible types
of flies in the F2 generation (not four as
would be predicted by Mendel).
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Diagram
representing
F2 cross
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Sex-linked Genes
XX – Female
XY – Male
The X chromosome carries more genes than
the Y chromosome.
A male has only one copy of many genes on
his X chromosome.
He has no matching gene (allele) on the Y
chromosome.
These genes are sex-linked or X-linked.
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Sex-linked genes – definition &
examples
Are genes found on the X chromosome with
no corresponding gene (allele) on the Y
chromosome.
Examples of sex-linked traits are:  Haemophilia, and
 Red/green colour blindness in humans.
 Eye colour in Drosophila melonagaster
(Fruit fly).
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Sex-linked genes – some facts
If a male has just one recessive gene for a sexlinked trait he will express the phenotype of
that trait.
A male can only pass this gene on to his
daughters. There is no male to male
transmission of sex-linked traits.
Males with a sex-linked condition got the
recessive gene from their mother.
Females with one recessive gene for the trait
are carriers of the condition and are
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phenotypically normal.
Inheritance of a sex-linked trait in
humans
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Question
Can females be colour blind? Explain your
answer using chromosome diagrams.
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Non-nuclear inheritance
(1/2)
A male gamete (sperm) is little more than a
motile nucleus.
A female gamete (egg) contains a cell as well
as a nucleus. The new individual inherits
this cell also at fertilisation.
DNA is found in cellular organelles other than
the nucleus e.g. mitochondria.
These structures are inherited from the female
only.
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Non-nuclear inheritance
(2/2)
When the cell divides the mitochondrial DNA
is replicated and passed on to the next
generation.
Non-nuclear DNA does not undergo meiosis
or fertilisation during sexual reproduction.
so some parts of the offspring’s cells get all
of their genetic information from the mother
only.
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DNA – more information
DNA is composed of nucleotides.
A nucleotide is composed of three basic
chemicals
 A five carbon sugar – deoxyribose
 A phosphate group, and
 One of four possible nitrogenous bases
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A nucleotide
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The four Possible Bases
Adenine
A
Purine bases – Double ringed
Guanine
G
Cytosine
C
Thymine
T
Pyramidine basses – Single
ringed
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A double
strand of
DNA
showing
bonding
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Base pairing
Because of the structure of each base, bonding
between bases is specific
i.e. A only with T and G only with C.
These are known as complimentary base
pairs.
The double strand of DNA coils around to
form a double helix.
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The DNA double helix
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Protein Synthesis – more info.
It is the order of the bases in the DNA that
determine the order of the amino acids in a
protein.
 Each group of three bases is a triplet.
 Each triplet codes for a particular amino
acid.
 DNA is found in the nucleus only.
 Proteins made at ribosomes in cytoplasm.
 Another type of nucleic acid, messenger
RNA (mRNA), carries the message
(instructions) from the nucleus to the
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ribosomes.

How is mRNA made?
Enzymes unwind or unzip the part of the DNA
molecule containing the information needed to
make the protein.
 Transcription occurs next i.e. RNA nucleotide
bases (A, G, C & U) bond with one strand of
exposed DNA.
 The enzyme RNA polymerase assembles these
bases to form mRNA.
 mRNA, therefore, has a series of bases that are
complimentary to those in DNA.
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
What happens next?
mRNA leaves the nucleus via the nuclear
pores and travels to and attaches itself to the
ribosomes (made of ribosomal RNA - rRNA)
 At the ribosome the mRNA code is matched
by nucleotides of transfer RNA (tRNA).
 Each tRNA carries a specific amino acid in
the correct sequence to the ribosome.
 They are attached by their ‘binding site’ to
complementary mRNA already attached to
the ribosome.
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
A tRNA nucleotide
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Translation
This ensures the amino acids are aligned in
the sequence determined by the codons of
the mRNA.
 The amino acids are then bonded together to
form the new protein molecule.
 This process of manufacture of the proteins
is called translation.
 tRNAs continue to move to the ribosome,
until a stop codon on the mRNA is reached.
 The protein is released when the mRNA
code sequence is complete and the protein is
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synthesised.

The production of protein
molecules from DNA
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END
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