Download Lesson 4 Finishing genetics

Read pages 6 to 9 and write 10 multiple choice questions (each with 4
possible answers) on the subject of species, breeds, varieties, selective
breeding and cross-breeding.
FINISH for HOMEWORK due next Friday 23rd October
We will also be having a genetics (8A) test that day.
Mr Porter apologises that
he has not marked all
your books (IB evening
Wednesday night). He’ll
collect them in again
Monday and mark them
Monday night.
Last lesson
• How genetic information is transferred
• How genes work
Genes
Our DNA contain two genes (one from our
mother and one from our father) for each
characteristic (eye colour, hair colour etc.)
Obviously the best looking
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?
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.
Cats don’t eat
their own poo,
dogs do!
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
b
An example
Half your father’s sperm will have the B
gene and half the b gene too.
B
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
WARNING!
WARNING!
This is a simplified explanation. In reality
eye colour is a little more complex than
this. Please do not go home and accuse
your mother of sleeping with the cable guy
if both your parents have blue eyes and
you have green or brown!
Summary and word sheets
Let’s try some questions!