Download Chromosomes

Chromosomes, genes & DNA
Chromosomes are X-shaped objects found in the
nucleus of most cells. They consist of long strands of a
substance called deoxyribonucleic acid, or DNA for
short. A section of DNA that has the genetic code for
making a particular protein is called a gene.
Genes
Our DNA contain two genes (one from our
mother and one from our father) for each
characteristic (eye colour, hair colour etc.)
Alleles
Different genes for the same characteristic (e.g.
eye colour) are called alleles.
Pron: “Al – eels”
For example, you all have two genes (alleles) for
eye colour
Which colour eyes?
Your eye colour depends on which genes
you have. Lets look at the genes for brown
and blue eyes.
The brown eye gene (B) is dominant and
the blue eye gene (b)is recessive
(heterozygous)
Which colour eyes?
If you have the alleles for eye colour of Bb,
you will have brown eyes (because the
brown gene is dominant).
(homozygous)
Which colour eyes?
If you have BB, you will also have brown
eyes.
(homozygous)
Which colour eyes?
Only if you have bb will your eyes be blue
(both genes are recessive)
Which colour eyes?
Your combination of genes (BB, Bb or bb)
is called your genotype.
What you lookin
at?
The colour of eyes you actually have is
called your phenotype
An example
Let’s imagine your mother has Bb
genotype and your father also has Bb.
Bb
Bb
An example
Half your mother’s eggs will have B genes
and half b (eggs are gametes and only
contain one of each pair of genes)
eggs
B
gamete cells
b
An example
Half your father’s sperm will have the B
gene and half the b gene too.
B
gamete cells
b
Sperm
Gene diagram (Punnett square)
We can look at the
possible combinations
on a gene diagram
Sperm Sperm
with B with b
Egg
with B
Egg
with b
Gene diagram (Punnett square)
Here are all the
possible combinations
for a fertilised egg
Sperm Sperm
with B with b
Egg
with B
BB
Bb
Egg
with b
Bb
bb
Gene diagram
This means that there
is a 75% chance the
baby will have brown
eyes (BB or Bb) and
only a 25% chance
the baby will have
blue eyes (bb)
Sperm Sperm
with B with b
Egg
with B
BB
BROWN
Egg
with b
Bb
BROWN
phenotype
genotype
Bb
BROWN
bb
BLUE
Pure breeding
If one parent has both
dominant genes(BB),
any baby will have to
have brown eyes. We
say the parent is pure
breeding.
Sperm Sperm
with B with B
Egg
with B
BB
BROWN
Egg
with b
Bb
BROWN
phenotype
BB
BROWN
Bb
BROWN
Codominance
Codominant characteristics
Some characteristics are the result of 2
types of alleles that are equally dominant.
Such characteristics are said to be
codominant.
Incomplete Dominance
Red and white Snapdragons
To summarize:
• Codominance: when both alleles are
dominant.
 The heterozygous individual shows both
alleles expressed fully in some way
 Normally a capital letter represents one of the
codominant alleles and different capital letter
represents the other codominant allele so that the
two do not get mixed up.
To summarize:
• Incomplete dominance: when both alleles are
dominant.
 The heterozygous individual shows both
alleles expressed in a combined way
 Normally a capital letter represents one of the
incompletely dominant alleles and different
capital letter represents the other incompletely
dominant allele
Chromosomes, genes & DNA
Chromosomes are X-shaped objects found in the
nucleus of most cells. They consist of long strands of a
substance called deoxyribonucleic acid, or DNA for
short. A section of DNA that has the genetic code for
making a particular protein is called a gene.
Human Chromosomes
23 pairs (46 in total)
Other animals
Fruit fly – 4 pairs
I like fruit.
Cat – 19 pairs
I just like
food!
Other animals
Dog – 39 pairs
Water rat – 46 pairs
I like cats
I don’t
Sex
• 22 pairs of human chromosomes are
matching, but the 23rd pair sometimes do
not match
Sex chromosomes
The 23rd pair are the sex chromosomes.
These determine whether you are a male
or a female
Male (XY) or female (XX)?
Why equal numbers of males or females?
Why equal numbers of males or females?
female
Why equal numbers of males or females?
female
male