DNA and genes



DNA is a double
helix
discovered by
Watson and
Crick in 1953
they won a
Nobel prize for
their work
The structure of DNA
uncoiled diagram
diagram
spacefilling
model
Chromosome –
condensed DNA
Chromatin–
extended DNA
Chromosome –
condensed DNA
Chromatin–
extended DNA
The structure of DNA
S
S
P
P
G
C
S
S
P
T
P
A
S
S
P
P
The basic chemical makeup of DNA:
DNA is made of the same repeating
subunits called…… Nucleotides.
5 carbon sugar
deoxyribose
Nitrogenous
Base
Phosphate
Forms the “backbone” and
always the same
Variables:
A, T, G, C
The base pairs
Hydrogen bond
Guanine
G
Cytosine
C
Thymine
T
Adenine
A
Chargaff’s Rule
Adenine must pair with Thymine
 Guanine must pair with Cytosine
 because they always pair together, their
amounts in a DNA molecule are the same

T
A
G
C
Question:

If there is 30% Adenine in a strand of
DNA, how much Cytosine is present?
Answer:

there would be 20% Cytosine:
Adenine (30%) = Thymine (30%)
Guanine (20%) = Cytosine (20%)
(50%) = (50%)
Genes
DNA is not a sequence of random
information, it is broken up into areas with
different functions
 sections of DNA with particular functions
are called genes

These are all genes!
start of human chromosome 1
Alleles
different versions of the same gene are
called alleles
 e.g. the gene for eye pigment in fruit flies
(Drosophila) has a red allele, a white
allele, and a brown allele
 chromosomes are always paired, so it is
possible to have two different alleles for
any given gene

eye colour alleles
in Drosophila
Genes and alleles
You will need:
 1 pencil crayon or pen
(preferably purple)
Ishihara Colour
Blindness Test
everyone sees 12
normal = 8
red-green = 3
total = nothing
normal = 74
red-green = 21
total = nothing
normal = 6
red-green = nothing
total = nothing
Gregor Mendel: father of genetics


discovered heredity –
the passing on of
characteristics from
parent to offspring
bred pea plants until
they were true-breeding,
the offspring always
resembled the parents
Mendel’s experiment

he took one plant of each type, such as
purple and white flowers, and bred them
together:


all the offspring were purple
took two of these offspring and bred them
to make a second generation:

3 purple offspring / 1 white offspring
Parental
generation
cross-fertilise
true-breeding
purple flowers
First
generation
(F1)
all offspring purple
true-breeding
white flowers
First
generation
(F1)
all offspring purple
self-fertilise
Second
generation
(F2)
3/4 purple
1/4 white
Mendel’s Conclusions
1.
2.
3.
parents pass on ‘elements’ to their
offspring; these ‘elements’ remain
unchanged during life
offspring receive one ‘element’ from each
parent; therefore offspring have a pair
each parent can pass on a different
version of the ‘element’
Mendel’s ‘elements’ are now called GENES.
Number of genes
Carsonella ruddii (bacterium) 180
Streptococcus pneumoniae (bacterium) 2,300
Escherichia coli (bacterium) 4,400
Saccharomyces cerevisiae (yeast) 5,800
Drosophila melanogaster (fruit fly) 13,700
Caenorhabditis elegans (nematode) 19,000
Strongylocentrotus purpuratus (urchin) 23,300
Homo sapiens (human) 27,000
Mus musculus (mouse) 29,000
Oryza sativa (rice) 50,000
Dominant vs. recessive
dominant = when one allele overwrites the
expression of another allele
recessive = when one allele is masked by
the expression of another allele
 the dominant allele is given a capital letter
(P), the recessive allele a lower case letter
(p)
Dominant and
recessive traits
in peas
Human dominant/recessive traits:
Cleft in chin No cleft dominant, cleft recessive
Hairline Widow peak dominant, straight hairline recessive
Eyebrow size Broad dominant, slender recessive
Eyebrow shape Separated dominant, joined recessive
Eyelash length Long dominant, short recessive
Dimples Dimples dominant, no dimples recessive
Earlobes Free lobe dominant, attached recessive
Freckles Freckles dominant, no freckles recessive
Tongue rolling Roller dominant, nonroller recessive
Tongue folding Inability dominant, ability recessive
Finger mid-digital hair
Hair dominant, no hair recessive
Bent little finger Bent dominant, straight recessive
Interlaced fingers Left thumb over right dominant, right over left
recessive
Hair on back of hand Hair dominant, no hair recessive
Heterozygous vs. homozygous
Heterozygous = having two different alleles
for a particular gene (e.g. Pp)
Homozygous = having two of the same
allele for a particular gene (e.g. PP or pp)
Genotype vs. phenotype
genotype = the genetic
characteristics of an
organism
eg. PP or Pp
phenotype = the visible
physical features of an
organism
e.g. white flower
Phenotype
Genotype
purple
flower
PP or Pp
white
flower
pp
The Punnett Square
In three steps, it’s an easy way to determine
the probability of offspring:
Step 1: Make a 2 X 2 Square grid
Step 2: Put the alleles of each parent on the
outside of the square
Step 3: Combine alleles to make potential
offspring in the middle of the square
Sample Punnett problem
 The
gene for free ear-lobes is
dominant (E), and the gene for
attached earlobes is recessive (e). If
a heterozygous male mates with a
homozygous recessive female, what
is the probability that their child will
have attached earlobes?
Solution:
Male


Female
E
e
e
Ee
ee
e
Ee
ee
50% chance heterozygous (Ee – free lobes)
50% homozygous recessive (ee – attached lobes)
Fruit fly genetics lab
Drosophila chromosomes
Fruit fly genetics lab
the common fruit fly, Drosophila
melanogaster is a staple of genetics
research
 it reproduces quickly and is easy to keep
in the laboratory
 we know more about the genetics of this
organism than any other (even humans)

♀
♂
(female)
(male)
wildtype (normal) Drosophila
Vestigial wing mutant
Wing phenotype varies from almost absent
(left) to shriveled (right)
Today’s procedure in brief:
1.
2.
3.
4.
carefully pour out your bottle of flies
sort them by sex and by wing phenotype
(normal or vestigial) into four categories
count and record the number of flies in
each category
share and record the fly counts from the
other groups
How to tell males from females:
no sex combs
sex combs
large,
striped
abdomen
small,
dark
abdomen
♂
♀ (female)