Inheritance (B)
DNA, Genes
and Chromosomses
Learning Intention
• Identify factors that are due to genes and
those due to environmental influences.
Genetics
The study of inherited
characteristics and of
those factors that
determine or affect
inheritance.
Meet our Mr Men
And our Little Misses
Now lets meet their children!
Who do you think
are his parents?
Write the answers in
your book.
Who do you think
are her parents?
Write the answers in
your book.
Who do you think
are his parents?
Write the answers in
your book.
Who do you think
are his parents?
Write the answers in
your book.
Who do you think
are her parents?
Write the answers in
your book.
Who do you think
are her parents?
Write the answers in
your book.
What did we learn from this?
• Children inherit features from their parents
• If two parents have a certain characteristic
then their child may show it even more (e.g. Mr
Small + Little Miss Tiny = Mr Very Small!)
• Some things such as glasses, scars and muscles
we get from our environment, they are not
inherited.
Nucleus
• The nucleus carries
genetic information
which controls the
characteristics of the
organism.
• The structures which
carry this information
are called
chromosomes.
Chromosomes
• The gene is the unit of inheritance, and each
chromosome may have several thousand genes.
• We inherit particular chromosomes through the
egg of our mother and sperm of our father.
• The genes on those chromosomes carry the code
that determines our physical characteristics.
Physical Characteristics
• Some physical characteristics, like eye
colour, are controlled by genes.
• Other physical characteristics are
controlled by the environment.
• Can you think of a physical characteristic
that isn’t controlled by genes?
Learning Intention
• Define the term genotype and
phenotype.
Phenotypes
• Each characteristic may display several
possible traits or versions.
e.g. hair colour can be brown, black or red
• Each of these is called a PHENOTYPE
Phenotype
• Leaf shape in plants, coat colour in guinea
pigs, wing type in fruit flies and eye colour
in humans are all examples of physical
characteristics.
• These physical characteristics make up
the organisms phenotype.
Genotype
• The phenotype (physical appearance)
that you have is controlled by the
genes that you inherit from your
parents.
• The combination of genes that you
have is called your genotype.
Genes
• We have two copies of every chromosome,
this means we have two copies of every gene.
1st gene
for eye
colour
Chromosome 1
2nd gene
for eye
colour
Chromosome 1
Alleles
• The different forms of a gene are called
alleles.
• For example the gene that controls eye colour
has different alleles blue, green or brown.
Blue form
eye gene
Green form
eye gene
Brown form
eye gene
Alleles
• If you inherit a blue allele from your
mum and a brown allele from your dad
then you will have brown eyes.
• This is because some alleles are
DOMINANT over others.
Dominant or recessive
• Alleles can be dominant or recessive.
• Alleles which show up in the appearance of
an organism are said to be dominant.
• Alleles which do not show up in the
appearance of an organism are said to be
recessive.
Dominant or recessive?
Allele for
white fur
Allele for
brown fur
Which allele is dominant white fur or brown fur?
Dominant or recessive?
Allele for
green body
Allele for
yellow body
Which allele is dominant green body or yellow body?
Learning Intention
• Be able to solve simple genetic
problems.
Some definitions
• Genes: Sections of DNA in a chromosome that
control an inherited characteristic, such as,
hair colour, flower colour, height, etc.
• Phenotype:
• Genotype:
The observable characteristics
(appearance) of an organism.
The complete set of genes possessed
by an organism.
Alleles
• When describing an allele, we give it a letter.
• If it is dominant it gets a capital letter and if
it is recessive it gets lower case of the same
letter.
• e.g. in pea plants tall is dominant over dwarf,
and so the tall allele is given the letter T and
the dwarf allele is given t.
Alleles
BB
This mouse has two
dominant brown
alleles
bb
This mouse has two
recessive white
alleles
Alleles
• The genotype of the organism is the alleles it
carries for that particular characteristic.
• The genotype for this fly is Rr, this tells us
that it has one red eye allele and one white
eye allele.
Alleles
• TT is a genotype and the plant will be tall.
• Tt is a genotype and the plant will be tall
as T is dominant over t.
• tt is a genotype and the plant will be
dwarf.
• What the plant actually looks like is called
the phenotype.
Monohybrid cross
• A monohybrid cross is
the study of the
inheritance of one
characteristic.
• Monohybrid crosses
were first carried out
by a monk called Gregor
Mendel who worked on
pea plants.
Mendel’s Pea Plant Crosses
Tall
plant
Dwarf
plant
X
Parent
Offspring ALL Tall
X
F1
Phenotypic
Ratio
F2
3 Tall:1 Dwarf
Tall
Tall
Tall
Dwarf
parents (P)
true breeding
tall strain
true breeding
dwarf strain
X
first filial
generation (F1)
all tall
members of F1
self-pollinated
X
second filial
generation (F2)
F2 phenotypic ratio
3 tall
:
This type of
experimental
cross is called a
monohybrid
cross as it
follows the
inheritance of
only one
characteristic at
a time.
1 dwarf
Gregor Mendel
X
RR
rr
P
X
rr
RR
F1
Rr
Rr
Rr
F1 X
X
F1
Rr
Rr
F2
Red 3
White 1
Punnet square
• Punnet squares allow us to figure out
the genotypes and phenotypes of the
offspring in the F2.
F1
X
Bb
Bb
??
F1
X
Bb
Bb
B
b
B
BB
Bb
b
Bb
bb
F2 genotype – BB, Bb, Bb, bb
F2 phenotype – 3 brown mice and one white mouse
F1
X
Rr
F2 genotype –
F2 phenotype –
Rr
Ratio
• In a cross where the 2 parents are
true-breeding, the expected
phenotype ratio in the F2 generation
is:
3:1
Dominant
Recessive
Build a baby
• Head: RR= Purple, Rr= Purple, rr= Dark Pink
• Body: BB= Bright pink, Bb= Bright pink, bb= green
• Arms: AA= Dark pink, Aa= Dark pink, aa= purple
• Legs: HH= green, Hh= green, hh= bright pink
• Eyes: EE= dark pink, Ee= dark pink, ee=purple
• Nose: NN= Bright pink, Nn= Bright Pink, nn=green
• Mouth: PP= Green, Pp= green, pp=bright pink
Build a baby
• What body colour is dominant, what letter is
this represented by?
• Now take your baby and find it a partner.
• If they were to have babies of their own in
the future, what colour of eyes might they
have? Calculate the possible ratios.
• Now do the same for body colour and arm
colour.