INHERITANCE
TOPIC 3 - 2015
INHERITANCE
Things to cover

Review of Topics 1 & 2

Genes & alleles
Heterozygous & homozygous
Genotypes & phenotypes
Dominant traits & recessive traits
Autosomal & sex-linked traits
Punnett Squares & pedigrees
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REVIEW
REVIEW – TOPIC 1




cells
nucleotides
phosphate
double
helix
covalent
nitrate
4
pentose
hexose
phosphorus
linear
hydrogen
nitrogen
2
DNA is composed of _______________.
Each nucleotide consists of:
◦ A _______________ sugar
◦ A _______________ group
◦ A _______________ base (_____ types)
The subunits form chains. Two chains twist to form the
_______________ shape of the DNA molecule.
Chemical bonds hold the DNA molecule together. Strong
_______________ bonds hold the nucleotides together, while weak
_______________ bonds form between the bases.
REVIEW – TOPIC 1

DNA is composed of nucleotides.

Each nucleotide consists of:
◦ A pentose sugar
◦ A phosphate group
◦ A nitrogen base (4 types)

The subunits form chains. Two chains twist to form the
double helix shape of the DNA molecule.

Chemical bonds hold the DNA molecule together. Strong
covalent bonds hold the nucleotides together, while weak
hydrogen bonds form between the bases.
REVIEW – TOPIC 2
autosomes
2 pairs
chromosomes
centrosomes
thymine
sex chromosomes
cytosine
23 pairs

DNA is coiled up tightly to form _______________

There are _______________ of chromosomes in every cell.
o Most of these are _______________ , controlling general
characteristics.
o One pair are the _______________ , controlling both sexual
and general characteristics.
REVIEW – TOPIC 2
autosomes
2 pairs
chromosomes
centrosomes
thymine
sex chromosomes
cytosine
23 pairs
DNA is coiled up tightly to form chromosomes
 There are 23 pairs of chromosomes in every cell.
o Most of these are autosomes, controlling general
characteristics.
o One pair are the sex chromosomes, controlling both sexual
and general characteristics.

REVIEW – TOPIC 2
growth
mitosis
gametes
asexual
identical
46
23
differentiation
meiosis
sexual
larger
repair
smaller
4
2
non-identical

Cell division is necessary for _______________ , _______________ and
reproduction.

One type of cell division, _______________ , is used in the formation of
new body cells. It is also used by unicellular organisms for
_______________ reproduction. The ______ daughter cells formed are
_______________ to the parent cell and have ______ chromosomes.

The other type of cell division, _______________ is used to form sex
cells. Another name for the products of this type of division are
_______________. This process takes place in the reproductive organs.
The ______ daughter cells formed are _______________ to the parent
cell and have ______ chromosomes. They are also _______________ in
size.
REVIEW – TOPIC 2
replicate
mutants
trisomy
monosomy
mutations
chromosomes
genes
an extra



differentiate
a missing
In order for cell division to occur, the DNA must
____________________ and the chromosomes must divide
equally between the cells.
____________________ can occur if this does not happen
correctly. For example, disorders involving additional or missing
____________________.
Down Syndrome is one example. This disorder results from the
presence of ____________________ Chromosome 21. This is
called ____________________ 21.
REVIEW – TOPIC 2

Cell division is necessary for repair, growth and reproduction.

One type of cell division, mitosis, is used in the formation of new
body cells. It is also used by unicellular organisms for asexual
reproduction. The 2 daughter cells formed are identical to the
parent cell and have 46 chromosomes.

The other type of cell division, meiosis, is used to form sex cells.
Another name for the products of this type of division are
gametes. This process takes place in the reproductive organs. The
4 daughter cells formed are non-identical to the parent cell and
have 23 chromosomes. They are also smaller in size.
REVIEW – TOPIC 2

In order for cell division to occur, the DNA must replicate and the
chromosomes must divide equally between the cells.

Mutations can occur if this does not happen correctly. For example,
disorders involving additional or missing chromosomes.

Down Syndrome is one example. This disorder results from the
presence of an extra Chromosome 21. This is called Trisomy 21.
VARIATION
VARIATION
The population of the Earth is more than 6
billion people, and no two individuals (apart
from identical twins) are genetically the same.
Why?

People are different because they inherit different
characteristics (or traits) from their parents.

Children carry a unique set of genes; half from
their mother and half from their father.
VARIATION

Some characteristics, such as eye colour and earlobe
shape, are only determined by genes. These are called
inherited characteristics.

Other types of characteristics, such as scars and hair
length, are not inherited but depend on environmental
factors. These are called acquired characteristics.

In some cases, it can be difficult to say how much
influence the environment has over the expression of a
trait.
VARIATION

All the observable characteristics of an
organism are called its phenotype.

The full set of genes of an organism is called its
genotype.

An organism’s phenotype therefore depends on
its genotype plus environmental conditions.
VARIATION

Sexual reproduction is the most important cause of
genetic variation because it mixes up genetic material.
◦ Meiosis creates a variety of gametes
◦ Any male gamete can combine with
any female gamete during fertilisation.
◦ Mate selection is random.

All these events occur randomly and
create new combinations of genetic
material.
GENES
VS
ALLELES
GENES

The basic unit of inheritance

A segment of DNA

Control one specific characteristic.

Contain the code needed to produce a protein.

Located in the same position on the
same chromosome. This position is
called its locus.
ALLELES

Different versions of a gene

Code may only differ by a few bases:
◦ eg. The base eye colour gene has
2 options: blue and brown
Chromosome
Gene
 AGTACGGTACG = blue eyes allele (b)
 AGTCAGGTACG = brown eyes allele (B)

This diagram shows one chromosome,
with ten genes. There are 20 alleles shown
(two for each gene) in various combinations.
a
A
B
b
c
C
D
D
e
E
f
f
G
G
h
H
i
i
J
j
EXPRESSION

All alleles contain a base sequence (code) that is used
by the cell to synthesise (build) a protein.
◦ eg. eye colour pigment

The allele that will have its code used is determined by
its dominance.
DNA  protein  gene expression
DOMINANCE

Most genes are controlled by 2 alleles.

Some alleles are dominant over others, so some traits
can be hidden (= recessive) by others (eg. blue eyes)
DOMINANCE

Dominant alleles:
◦ always expressed in a cell’s phenotype
◦ only one copy of the dominant allele needs to be inherited in
order for it to be expressed
◦ represented by an upper case letter (eg. B)

Recessive alleles:
◦ only expressed in a cell’s phenotype if two copies are present
◦ if only one copy is present, its effect is “masked” by the
dominant allele
◦ represented by an lower case letter (eg. b)
HOMOZYGOUS

If the alleles for a characteristic are the same, the
organism is said to be homozygous for that trait.

If there are two dominant alleles,
the organism is said to be
homozygous dominant
for that gene.

If there are two recessive alleles,
the organism is said to be
homozygous recessive
for that gene.
HETEROZYGOUS

If the alleles for a characteristic are different, the
organism is said to be heterozygous for that trait.

The protein made (and trait expressed)
will depend on which allele is dominant
and which allele is recessive.

The allele for brown eyes is dominant over the allele for
blue eyes.

The individual will have brown eyes,
because the allele for brown eyes masks
the allele for blue eyes.
TYPES OF
INHERITANCE
TYPES OF INHERITANCE
1) Complete dominance
2) Co-dominance
COMPLETE DOMINANCE
Complete dominance = NORMAL!!!!!
If the dominant allele is present, it will be
expressed
 eg. Brown eyed allele (B) always
expressed when present (BB or Bb)


Notation used:
◦ Choose 1 letter
◦ Use lowercase (recessive) and uppercase (dominant)
◦ eg. BB, bb or Bb  only B trait shown
CODOMINANCE


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There is no dominant allele!
The phenotype is a blend of both alleles
present
eg. red allele + white allele = pink phenotype
eg. Type AB blood from combination of
A and B alleles
Notation used:
◦ Choose 2 letters
◦ Use uppercase for both
◦ eg. RR, WW or RW  mixture of R & W traits is
shown
AWESOME EXAMPLE:
ABO BLOOD GROUPING

To determine your blood type, there are three alleles:
◦ A – IA
◦ B - IB and
◦ O-ί


The alleles IA and IB are codominant
However, both of these alleles are completely dominant
over ί

This results in four different phenotypes.
ABO BLOOD GROUPING
Phenotype
or blood group
Type A
Type B
Type AB
Type O
Genotypes
IAIA or IAί
IBIB or IBί
IAIB
ίί
TYPES OF INHERITANCE
3) Autosomal inheritance
4) Sex-linked inheritance
AUTOSOMAL INHERTIANCE
Autosomal inheritance = NORMAL!!!!!
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
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The gene is located on an autosome
Traits can be:
◦ autosomal dominant
– meaning that a trait/disorder is determined by the
presence of a dominant allele
◦ autosomal recessive
–determined by the presence of two recessive alleles
Notation used:
◦ BB, bb, Bb
SEX-LINKED INHERTIANCE




The gene is located on a sex chromosome
Can be either the X chromosome or the Y chromosome
(but is usually X)
eg. haemophilia, colour blindness
Traits can be:
◦ sex-linked dominant
– meaning that a trait/disorder is determined by the
presence of the dominant allele on the X chromosomes
◦ sex-linked recessive
– meaning that a trait/disorder is determined by the
presence of one or two recessive alleles on the X
chromosomes
SEX-LINKED INHERTIANCE

Notation used:
◦ Females:
 Homozygous XNXN or XnXn
 Heterozygous XNXn (carrier)
◦ Males (only one X chromosome):
 Unaffected
XNY
 Affected
XnY
 Why are there no male
heterozygotes??
CARRIERS
Someone who is heterozygous for a genetic disorder
 They do not have the disorder themselves
 However, the disorder can be passed on to the next
generation


eg.
◦ CC = no Cystic Fibrosis
◦ cc
= Cystic Fibrosis
◦ Cc
= carrier of CF
PUNNETT
SQUARES
PUNNETT SQUARES

Also called monohybrid crosses

Method used to find the expected genotype and
phenotype ratios of offspring when the parental
genotypes and/or phenotypes are known.
PUNNETT SQUARES
Steps:
1. Write down the parental genotypes
You may need to choose letter to represent your trait
– choose easy ones like T, G, H not C, S, Y
2. Write down the possible alleles that they are able to
3.
4.
5.
6.
pass on in their gametes (sex cells)
Construct the Punnett square
Fill in the potential genotypes of the offspring
Determine the genotype ratio of the offspring
Determine the phenotype ratio of the offspring
PUNNETT SQUARES
Example:
 An alien that is homozygous dominant for green skin
has chosen an alien who has purple skin for
his bride.
 Determine the likelihood
that their children will have
purple skin.

Like in Maths – you need to state what symbols you will
be using to represent your traits:
eg. Let G = green; g = purple
PUNNETT SQUARES
Answer:
 Parents:
GG
and
 Gametes: G
and
 First generation (F1):


gg
g
G
G
g
Gg
Gg
g
Gg
Gg
F1 genotype ratio: 100% Gg
F1 phenotype ratio: 100% green skin
PUNNETT SQUARES
Example:
 One of the couples sons (named Neyp) married a
female alien (named Gjup) who was homozygous
recessive for the skin colour trait.
 Work out the genotype and phenotype percentage of
their potential offspring.
PUNNETT SQUARES
Answer:
 Parents:
Gg
and
 Gametes: G,g
and
 Second generation (F2):
g


gg
g
G
g
Gg
gg
F2 genotype ratio: 50% Gg : 50% gg
F2 phenotype ratio: 50% green skin : 50% purple skin
PUNNETT SQUARES
Example:
 One of Neyp’s sons married a female alien who was
heterozygous for haemophilia. Neyp did not have
haemophilia.
 Calculate the likelihood that they could have a child
with haemophilia.



Again – specify your symbols!
It is a sex-linked recessive disorder!!!
So:
Let H = normal; h = haemophilia
PUNNETT SQUARES
Answer:
 Parents:
XHXh and XHY
 Gametes: XH, Xh and XH, Y
 First generation (F1):


XH
Xh
XH
XH X H
XH Xh
Y
XH Y
XhY
F1 genotype ratio: 25% XHXH : 25% XHXh : 25% XHY : 25% XhY
F1 phenotype ratio: 75% normal: 25% haemophilia
PEDIGREE
CHARTS
PEDIGREE CHARTS

Shows the members of a family and how they are
related to each other.

A genetic family tree!

Pedigree charts can also be used
to study the inheritance of a
characteristic.
PEDIGREE CHARTS
Circles = females
 Squares = males
 Shading = affected individuals
 Parents = linked by a horizontal line
 Children = vertical lines running down from parents
 Siblings = linked by
a horizontal line
above them

PEDIGREE CHARTS
Example:
 Draw a pedigree chart for Neyp and his wife Gjup,
including their parents and children.
 They had 3 children, 2 boys and a girl (in order of age):
◦ Their youngest son (Guol) had green skin and was
also married with one purple daughter (Zcug).
◦ Their daughter (Hefg) had purple skin and
married an alien who also had purple skin.
They had one son (Yerg).

NB. Being purple is the recessive trait!
PEDIGREE CHARTS
?
?
I
gg
GG
Gg/gg
Gg/gg
II
gg
Gg
III
?
Gg/gg
?
Gg/gg
Gg
gg
gg
IV
gg
gg
PEDIGREE CHARTS

Pedigree charts can also be used to determine if:
◦ The characteristic is dominant or recessive.
◦ The characteristic is sex-linked or autosomal.

Things to look for:
◦ Gender bias of males:females
more males indicates sex linkage; balanced ratio indicates autosomal
◦ Affected offspring from unaffected parents
indicates a recessive condition
◦ Affected sons from affected fathers
indicates an autosomal condition