Transcript genetics - eduBuzz.org

GENETICS
• The nucleus of a cell carries the genetic
information which allows it to control all of
the activities of the cell and determines the
overall characteristics of the organism
nucleus
The nucleus of a living cell contains
threadlike structures called chromosomes.
A chromosome is a threadlike structure
which carries genetic information.
All the nuclei of the body cells of a living
organism contain identical copies of the
chromosomes.
chromosomes
nucleus
Chromosome complement
The number of chromosomes present in the
cells of a living organism is called the
chromosome complement and depends on
the species.
Humans have 46 chromosomes
in every body cell.
Species
Chromosome
complement
Horse
64
Sheep
54
Human
46
Mouse
40
Maize
20
Pea plant
14
Drosophila fruit fly
8
Chromosomes and Genes
Chromosomes are made from tightly coiled
molecules of DNA.
DNA is a long chain made up of a backbone
with bases attached.
DNA = deoxyribonucleic acid!!
Label the following diagram
DNA of 1 gene uncoiling
DNA backbone
Centromere
Positions of
individual
genes
bases
DNA coiled
into a
chromosome
= Adenine (A)
= Guanine (G)
= Thymine (T)
= Cytosine (C)
• There are 4 different types of base within a strand of
DNA.
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
The DNA carries pieces of coded genetic information.
An individual section of DNA with a single piece of genetic
information is a gene.
Chromosomes are thought of as lots of genes in a chain
The Function of DNA
DNA molecules carry genetic instructions
which allow the cell to make specific protein
molecules.
Proteins are made from amino acid units linked
together to form long chains.
The order of DNA bases encodes the
information for the sequence of amino acids in
proteins
Sets of 3
BASES on a
DNA strand
carry the codes
for………….
DNA backbone
AMINO
AMINO
AMINO
AMINO
ACID R
ACID S
ACID T
ACID S
Protein molecule
forming
An amino acid is a unit of protein structure.
A base is a part of DNA structure
….a chain of
amino acids
which join up to
make a protein
molecule.
DNA base
sequence
Amino acid
coded for
P
Q
R
S
T
This table shows the
information about the
base sequences for
some amino acids
It is your job to decode
the next few diagrams of
pieces of DNA and to
draw the chain of amino
acids they encode.
1.
2.
3.
4.
5.
1.
2.
P
Q
R
R
S
T
3.
T
S
4.
Q
Q
5.
T
R
P
P
P
Protein Structure & Function
• The chains of amino acids are folded and
twisted to give the molecules 3-D shapes.
• The sequence of amino acids is determined
by the sequence of the DNA bases.
• The sequence of the amino acids dictates the
structure and function of the protein
produced.
e.g. Enzymes
Enzymes are made of protein.
The folding of the chains of amino acids
allows the formation of the active sites which
makes the enzymes specific to their
substrate.
Relationship between proteins present in a
cell and the organisms characteristics
• Inherited characteristics are the result of many biochemical
processes controlled by enzymes (which are made of protein!)
• In humans, enzymes control the reactions that lead to the formation
of hair or a certain texture, eye irises of a certain colour etc..
• The protein haemoglobin gives red blood
cells their red colour.
• The body also possesses many hormones which are also made of
protein. Hormones are chemical messengers around our body.
MEIOSIS
• Meiosis is the name given to the process
which produce gametes (sex cells).
Chromosome revision
one
chromatid
centromere
single chromosome
Double chromosome
In cells that are not about to divide, chromosomes
are found as single chromosomes. In cells that are
about to divide, the DNA makes an extra copy of
itself (shown above as the dark strand) and the
chromosomes become double chromosomes.
Each strand is called a chromatid and the chromatids
are held together by a centromere.
meiosis
sperm cell
fertilisation
zygote
Sperm mother cell
meiosis
Egg mother cell
Egg cell
Zygote ready to divide
The Process of Meiosis
1) Gamete
mother cell
containing 4
double
chromosomes
2) Matching
chromosome pair
and line up across
the middle
3) The pairs
separate to either
end of the cell and
the cell divides
into 2
4) The
chromosomes turn
and line up. Each
cell then divides
again.
5) The centromeres split and the
chromatids are pulled apart. After
the chromatids are separated from
each other they are known as
single chromosomes
6) 4 gametes are produced, each
with only 1 set of single
chromosomes.
Meiosis reduces the total number of sets of
double chromosomes from 2 matching sets
in the gamete mother cell to 1 set of single
chromosomes in each gamete.
SO:
Gamete mother cell = 2 sets chromosomes
Gametes = 1 set chromosomes
The 2 sets of chromosomes are restored at
fertilisation.
Chromosome shuffling
• The different ways that the matching
chromosomes can pair increases the total
number of gamete varieties.
• Any process which increases the number
of different gametes must also increase
the variety of offspring.
• The random assortment of chromosomes
during meiosis leads to variation in
offspring.
So instead of this arrangement
You get this instead
Which results
in 4 gametes
like this
Because…..
Confused??!!!??!!
Sex Determination
In humans, each male gamete has an X or a
Y chromosome.
So males are XY.
Each female gamete has an X chromosome.
So females are XX.
The sex chromosomes of an individual determine
their sex.
Genetic Symbols
= male symbol
He’s a
man, man!
= female symbol
Stick in the ‘Sex chromosomes’ cut out
• All egg cells will contain an X
chromosome.
• Half of sperm cells will be X and half will
be Y.
• It is the sperm cells that determine the sex
of the baby – the egg will always be X but
the sperm will either be X or Y.
female
male
XY
X
Y
Baby girl
XX
XX
X
X
XY Baby boy
• Collect a sex determination grid and
complete the blank squares to show the 4
possible combinations in the offspring.
• Use a crayon lightly to shade the boxes to
show the male and female offspring.
• Complete the ratio information below the
grid
• The ratio of males to females is 1:1
• But the process of fertilisation is random
so it is a matter of chance which sperm will
fertilise the egg – X or Y.
• It is for this reason that there will be
roughly a 1:1 ratio of males to females.
Genetics for Beginners!
• Genes are parts of chromosomes
• Alleles are the different forms of a gene.
• Each gamete will carry one allele of the
gene.
• E.g. Gene for height in pea plants.
Pea Plants can be tall or dwarf.
Each plant will carry 2 copies of a gene, one
from each parent.
The alleles are represented by letters and will be T
for tall and t for dwarf.
A tall plant will either be TT or Tt
A dwarf plant will be tt.
• If a tall plant and a small plant cross, the
offspring are all tall.
• This means that ‘Tall’ is dominant.
• ‘Dwarf’ is recessive.
• The dominant form of the gene always
gets a capital letter e.g. T = tall.
• The recessive form of the gene always
gets the same letter but lower case e.g. t
= dwarf
X
dwarf
tall
All tall
Complete the Symbols for Alleles
Table
Symbols for Alleles
Organism
Gene
Dominant allele
Recessive allele
Word
Symbol
Word
Symbol
Pea plant
Height
Tall
T
Dwarf
t
Human
Eye colour
Brown
B
Blue
b
Drosophila
Wing length
Long
L
Short
l
Maize
Grain colour
Purple
P
Yellow
p
Guinea pig
Coat colour
Black
B
White
b
• An individual with 2 of the same allele is
said to be HOMOZYGOUS. (e.g. tt or TT)
• An individual with 2 different alleles of a
gene it is said to be HETEROZYGOUS.
(e.g. Tt)
• The genetic symbols an individual has is
its GENOTYPE, e.g. Tt
• The physical appearance an individual has
is its PHENOTYPE e.g. Tall
E.g. flower colour in pea plants
• A pea plant with lilac flowers was
crossed with a white flowered plant.
• All offspring were lilac.
x
All lilac
• Which is the dominant characteristic?
Lilac
• What letter would we give the dominant
allele?
L
• What letter would we give the recessive
allele?
l
• What is the recessive characteristic?
white
• If a pea plant had the alleles ll – would the
individual be homozygous or heterozygous?
homozygous
• What colour would it be? White
• If a plant had the alleles Ll – would the individual
be homozygous or heterozygous? heterozygous
• What colour would it be? Lilac
• If a plant had the alleles LL – would the
individual be homozygous or heterozygous?
homozygous
• What colour would it be? Lilac
Genotype and Phenotype
An organism can have the same phenotype
but have a different genotype.
Example: Organism: Pea plants
Gene
height
flower colour
TT = tall
LL = lilac
Tt
Ll
= tall
= lilac
True Breeding
True breeding lilac strain
Parents (P)
True breeding white strain
X
X
X
X
1st generation (F1)
Members
of F1 cross
2nd generation (F2)
All lilac
All white
• When 2 lilac parent plants cross, the
offspring are all lilac. When the lilac
offspring cross, all their offspring are lilac.
• When 2 white parent plants cross, the
offspring are all _______. When the
______ offspring cross, all their offspring
are _______.
• So, when the flower colour of the offspring is
identical to the parent flower colour, the
members of the strain are true breeding. (they
are always homozygous, e.g. LL or ll)
Terms for Monohybrid Crosses
P = parents
F1 = 1st filial generation.
F2 = 2nd filial generation
Monohybrid Crosses
A monohybrid cross is a cross that involves
only one difference between the original
parents, e.g. flower colour or height.
Parents in monohybrid crosses are usually
true breeding and show different phenotypes
How to do Monohybrid Crosses
Question: A true breeding black mouse
was crossed with a true breeding white
mouse. All of the offspring were black.
Show this as a monohybrid cross using
appropriate symbols right through to the F2
generation
X
P
Black mouse
White mouse
phenotype
Black
white
genotype
BB
F1
phenotype
genotype
bb
All Black
Bb
F2
phenotype
genotype
gametes
X
Black
Bb
B
b
X
Black
Bb
B
b
Punnet
Square
Sperm
Eggs
B
b
B
BB
Bb
b
Bb
bb
F2 genotypic ratio
1BB:2Bb:1bb
F2 phenotypic ratio
3 black: 1 white
Try for yourself….
A true breeding pea plant with round seeds
was crossed with a true breeding pea
plant with wrinkled seeds. All the F1
generation had round seeds. Show this as
a monohybrid cross using appropriate
symbols right through to the F2
generation.
X
P
phenotype
genotype
F1
phenotype
genotype
F2
phenotype
genotype
gametes
X
All ________
X
Punnet
Square
Sperm
Eggs
F2 genotypic ratio
F2 phenotypic ratio
:
:
:
P
Round seeds
Wrinkled seeds
phenotype
Round
wrinkled
genotype
RR
F1
phenotype
genotype
rr
All Round
Rr
F2
phenotype
genotype
gametes
X
Round
Rr
R
r
X
Round
Rr
R
r
Punnet
Square
Sperm
Eggs
R
r
R
RR
Rr
r
Rr
rr
F2 genotypic ratio
1RR:2Rr:1rr
F2 phenotypic ratio
3 round: 1 wrinkled
• Now do the Possible monohybrid crosses
in mice sheet.
Observed vs Predicted Ratios
Monohybrid crosses that we have seen so far, always
produce a 3:1 ratio in the F2 generation.
However, there is often a difference between the observed
and predicted numbers of the different types of offspring
as an exact 3:1 ratio rarely happens in nature as you can
see from the table below…(stick in table)
This is due to the fact that fertilisation is a random process
involving an element of chance.
We can show this by experiment…..
Bead Experiment
….flower colour in pea plants
PP
Pink
Pp
Pink
pp
yellow
Pick a female and a male gamete at random
and record your results in the table, then work out
the ratio – is it 3:1?
Co-dominance
When 2 alleles of a gene are codominant
this means that neither allele is
dominant to the other. Both alleles are
expressed equally in the phenotype of
an organism with the heterozygous
genotype.
e.g. Coat colour in horses and cattle,
feather colour in domestic fowl, flower
colour in carnations.
Black stallion
x
White Mare
All offspring
grey roan
Black coat is codominant to white coat. They are
expressed equally and so offspring have coats
with black and white hairs, these are called
grey roans.
Genotypes in Co-Dominance
• Neither allele in co-dominance is recessive so
neither symbol has a small letter. Both are
capital letters since both alleles are equally
dominant.
Phenotype
Black coat
White coat
Grey roan
Red coat
White coat
Red roan
Genotype
BB
WW
BW
RR
WW
RW
• Time to do some co-dominance
problems….
Remember co-dominant alleles
are expressed equally. They are
equally dominant. Neither is
recessive so both alleles have a
capital letter.
Polygenic Inheritance
• This is when characteristics are controlled
by the alleles of more than one gene.
• E.g. Skin colour in humans, seed mass in
plants.
• The characteristics arise due to the
interaction of the alleles of several genes.
• Remember back to continuous and
discontinuous variation?…….
Discontinuous variation is controlled by a single
gene and is an example of single-gene
inheritance.
Continuous variation is controlled by the alleles
of more than one gene and is an example of
polygenic inheritance.
Example of Polygenic
Inheritance
When a characteristic is controlled by 2
genes there may be 4 alleles working
together.
In maize, kernel colour is controlled by
several genes, we will say 2 genes for this
example. (Each gene will have 2 alleles)
• Each gene has a dominant allele giving a
red colour to the kernel and a recessive
allele giving a white colour to the kernel.
• R1 = red
• r1 = white
R2 = red
r2 = white
• If a maize inherits all dominant alleles,
(R1 R1 R2 R2), it will have very dark red
kernels
• If a maize inherits all recessive alleles
(r1 r1 r2 r2) then it will have white kernels
Parent Phenotypes
Very dark red
white
genotype
R1 R1 R2 R2
Gametes
R1 R2
F1 Genotype
F1 Phenotype
x
r1 r1 r2 r2
r1 r2
R1 r1 R2 r2
Medium red
If 2 of the F1 generation are crossed….
R1 r1 R2 r2
Medium red
x
R1 r1 R2 r2
Medium red
??
See your diagram of polygenic
inheritance in maize and
complete the missing genotypes
of the offspring.
So…..
The more genes there are for a particular
characteristic, the more different phenotypes
there are.
FAMILY TREES
• Family tree diagrams are set out in a standard way.
• Male =
• Female =
• The squares and symbols can be shaded in or left, depending on
the phenotype.
• Parents are joined by a horizontal line
• Offspring are connected by a branched line
• Parents are joined to offspring by a vertical line.
Brown-eyed male
Brown-eyed female
Blue-eyed male
Blue-eyed female
1
3
4
8
9
B = brown eyes
2
5
6
10
11
b = blue eyes
7
12
13
COMPLETE THESE STATEMENTS
a) The phenotype of person 2 is:- Blue-eyed female
b) The phenotype of person 3 is:-
Brown-eyed male
c) The genotype of person 1 is:-
Bb
d) The genotype of person 4 is:-
Bb
e) Person 7 is likely to be homozygous dominant
because…. All offspring are brown eyed.
f) The genotype of person 8 is….bb
g) The genotype of person 9 cannot be stated with
certainty because… Could be Bb or BB
Environmental Impact on Phenotype
• The final appearance of an organism is the result of its
genotype and the effects of the environment.
• If organisms of identical genotype are subject to
different environmental conditions they show
considerable variation (differences).
• These changes are not genetic so they are not passed
on from one generation to the next.
Twins reared together
Both twins fed same
healthy diet – both reach
full potential height.
Twins separated at birth
1 twin fed
healthy diet –
reached full
potential height
1 twin fed poor
diet – did not
reach full
potential height
So: The environment has an effect
on our overall phenotype…
Genotype + Environment = Phenotype