Download DNA, Genes and inheritance

DNA, Genes and
inheritance
I got it from my mama!
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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
The structure of DNA
S
S
P
P
G
C
S
P
S
T
P
A
S
S
P
P
The base pairs
Hydrogen bond
Guanine
G
Thymine
T
Cytosine
C
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
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)
Second
generation
(F2)
all offspring purple
self-fertilize
3/4 purple
1/4 white
Mendel’s Conclusions
1.parents pass on ‘elements’ to their offspring; these ‘elements’ remain
unchanged during life
2.offspring receive one ‘element’ from each parent; therefore offspring
have a pair
3.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
Cleft in chin No cleft dominant, cleft recessive
traits:
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:
Female
Male
E
e
e
Ee
ee
e
Ee
ee
• 50% chance heterozygous (Ee – free lobes)
• 50% homozygous recessive (ee – attached lobes)
Types of Evolution and Genetic Change
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
1. Divergent Evolution
When a population of a species become different enough in
their genotype and phenotype to be considered a separate
species
Factors that lead to Divergent Evolution
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Can happen by:
Genetic drift (population changes due to migration or
adaptation
Reproductive isolation (isolation—ie cut off lake from
another, mountain ranges)
2. Convergent Evolution
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
When two species develop similar characteristics based on
environmental conditions.
Organisms are not directly evolutionarily related