IB SL/HL Genetics 4.3
Classic Mendelian Genetics
‘Like begets like’…
Like Begets Like
Think about
physical
similarities
between you and
your ancestors
(mother, father,
grandparents,
siblings,cousins)
We inherit ‘traits’ from our parents…
• How do we inherit
traits?
• What are ‘traits’?
• Tasting for inherited
traits….
Pre-Mendel Belief of inheritance:
It was believed that offspring would have traits intermediate
between those of the parents (Blending Concept)
Mendel’s Law of Segregation:
Each organism contains two factors for each trait, and the factors
segregate during the formation of gametes so that each gamete
contains only one factor for each trait
The history of genetics…
• Meet Gregor Mendel...
• Timeline of genetics I
What Mendel didn’t know,
was that genetic information
is found on our
chromosomes
Essential vocabulary of genetics:
• Gene: A length of DNA that is the unit of
heredity
• Genotype: The genetic make-up of an
organism, in terms of the allels present
• Allele: Any of two or more alternative forms of
a gene
• Phenotype (trait): Physical or other features of
an organism. Caused by combination of
genotype AND environment
• Autosome: Any chromosome which is not a
sex chromosome
Each cell has (at least) 2 genes for
any characteristic
Homozygous
Homologous chromosomes
have identical alleles of a
particular gene.
Two
identical
homozygous
individuals
that
breed
together will always have
the same characteristics
‘Pure breeding’/’wild type’
Heterozygous:
Chromosomes have two
different alleles of a
particular gene (Ff).
•‘not ‘’pure breeding’…
Dominant and recessive alleles
• A dominant allele (e.g.
F) will be expressed if it is
present
• A recessive allele (e.g. f)
will only be expressed
when there is no
dominant allele of the
gene present
• Codominant alleles (eg IA
and IB) are both equally
expressed
Working out mono-hybrid crosses
• Is the gene located on an autosome
(chromosome 1 – 22 in humans) or on a sex
chromosome (X/Y- in which case inheritance is
sex-linked)
• Does the gene have 2 or more than two
alleles?
• Is the pattern of gene expression
dominant/recessive or co-dominant?
One-Trait
Inheritance
Monohybrid
cross
Working out inheritance: Genetic
Diagrams
Genetic diagrams
Working out inheritance: Punnet
Squares
• Introduction to Punnett
Squares
• Meet Mr Punnett
A chart that shows all
the possible
combinations of alleles
that can result from a
genetic cross
Steps in Making a
Punnett Square
I can only give 1/2 of my
DNA to my offspring, so I
will either give my baby plant
a T OR a t.
Tt
1. Draw a square and
divide it into four.
2. The alleles for one
parent go on the top.
The alleles for the other
parent go down the
side.
3. The boxes are like a
multiplication problem,
with one allele
contributed by each
parent.
One Trait Inheritance
• T = tall, t = short
• Complete the punnet
square in your notes.
• Label the P gametes
• Label the F1
generation
• What is the
phenotypic ratio?
• What is the
geneotypic ratio?
One Trait Inheritance
P gametes
F1 generation
Phenotypic Ratio = 3:1
Genotypic Ratio = 1:2:1
TT
Tt
Tt
tt
Cystic fibrosis is disease associated with
expression of a recessive gene
Cystic fibrosis Punnet Squares (1)
Cystic fibrosis Punnet Squares (2)
CODOMINANCE
Some genes have alleles that are
both expressed in the heterozygote
individuals
Codominance
• Both alleles are
equally dominant!
• Both alleles are
expressed fully,
resulting in a new
phenotype!
• e.g. Roan
horses/cattle – red
and white hairs are
intermingled
Codominant alleles have three
phenotypes
• Sickle cell anaemia is a genetic disease
affecting the haemoglobin of red blood
cells
• Haemoglobin is normally a ball-shaped
molecule
• The sickle cell allele makes it form a long
strands
• The red blood cell carrying these
molecules distorts into characteristic long
shape
Blood cells
Sickle cell anemia
http://www.netwellness.org/ency/imagepages/1223.htm
Blood smear (normal)
http://137.222.110.150/calnet/cellbio/cellbio.htm
© 2007 Paul Billiet ODWS
The genetics of sickle cell anaemia
The shape of the haemoglobin molecule is controlled by two alleles
•
Normal Haemoglobin allele
•
Sickle Cell Haemoglobin allele
There are three phenotypes
Normal
Normal individuals have two normal haemoglobin alleles
Sickle cell anaemia, a severe form where all the red blood cells are affected.
Sickle cell anaemia patients have two sickle cell alleles in their genotype
Sickle cell trait, a mild condition where 50% of the red blood cells are affected.
Sickle cell trait individuals are heterozygotes, having one of each allele
Symbols for codominant alleles
• Both alleles are expressed
in the heterozygote
• So both take a CAPITAL
CASE letter
• An index letter identifies the
allele
Therefore:
• Normal haemoglobin
allele is HbA
• Sickle cell allele is HbS
Genotypes
Phenotypes
HbAHbA
Normal haemoglobin
HbAHbS
Sickle cell trait
HbSHbS
Sickle cell anaemia
Unusual proportions
• In codominance, heterozygotes have their own phenotype
• This gives rise to different proportions amongst to
offspring of some genetic crosses
Phenotypes
Sickle cell
trait
Genotypes
HbAHbS
Gametes
HbA
x
Sickle cell trait
HbAHbS
HbS
HbA
HbA
Offspring
Proportions
HbS
HbS
HbA
HbAHbA HbAHbS
HbS
HbNHbS
HbSHbS
Normal
Sickle cell trait
Sickle cell anaemia
25%
50%
25%
Multiple
alleles
Multiple alleles
• When Mendel studied pea ‘traits’, there were
only 2 alleles for each of the traits he studied:
(round/wrinkled, tall/short, yellow/green)
• In fact, you CAN have > 2 alleles for any trait
• Human blood types are an example of a trait
with multiple alleles (A, B, O)
• Human blood types are also an example of
codominance
Human Blood types
• Type O is recessive
• Types A and B are
codominant
• Both Types A and B
are dominant to Type
O
What does ‘blood type’ mean?
We have both antigens (markers) and
antibodies in our blood
• 45% are Type O
• 42% Type A
• 10% are type B
• 3% are type AB
What happens if you receive the wrong type
of blood in a transfusion?
Sex linked genes
Why are some characteristics ‘sex
linked’?
• The X-chromosome (23)
carries 1098 genes
• Over 100(recessive) genes
for genetic disorders have
now been mapped to the
X-chromosome 23.
• The human Ychromosome is much
smaller, and only carries a
few genes
Diseases linked to the 23rd chromosome
•
•
•
•
•
Haemophilia A and B
Duchenne Muscular dystrophy
Red-green Colour blindness
Melanoma
X-linked severe combined immunodeficiency
(SCIDs)
• X-inactivation syndrome
Why are sex-linked diseases more
common in males?
1. Males have only 1 X chromosome; thus all X-linked
alleles will be phenotypically expressed in males,
even if they are recessive…
2. Females must have BOTH copies of the allele to be
recessive, in order to express the gene phenotypically
3. Thus the recessive phenotype of a sex-linked genetic
disorder is much more common in males
4. Sex-linked genes can move from fathers to their
daughter, where the daughters will be unaffected
carriers, but pass the gene to their sons…
Sex-linked inheritance
Red-green colour blindness
• test......
Genetics of colour blindness
• 0.4% of women are colour
blind
• 7% of men are colour blind
• In colour blindness, green
or red cones work poorly, or
not at all…
• Genes for green and red
cones are close to each
other on the X-chromosome
• Why is ‘blue blindness’
rare?
Haemophilia
• Recessive, sex linked
disease which causes
abnormal blood clotting
• Individuals are at risk of
severe bleeding
• Many die from brain
haemorrhages
• Type A: Lack of Factor VII:
80% of cases of haemophilia
• Type B: Lack of Factor IX:
20% of cases
Haemophilia in the Royal families of
Europe
• Queen Victoria has 9 children (4 sons, 2 daughters). She
was a carrier for the Haemophilia gene
• 1 of 4 sons developed haemohilia (died at age 30)
• 2 of 5 daughers were carriers (married and moved to
Spain, Russia, Germany)
How did Mendel determine the
genotype of an ‘unknown’ individual
plant?
Testing inheritance: The F1 cross…
• To test for recessive genes, Mendel
allowed all 7 kinds of F1 hybrid plants to
produce a second (F2, second filial)
generation by self-pollination
• He crossed the F1 generation with itself to
produce the F2 offspring
• Roughly 25% of the F2 plants
demonstrated the recessive allele!
Testing inheritance: The F1 cross…
• To test for recessive genes, Mendel
allowed all 7 kinds of F1 hybrid plants to
produce a second (F2, second filial)
generation by self-pollination
• He crossed the F1 generation with itself to
produce the F2 offspring
• Roughly 25% of the F2 plants
demonstrated the recessive allele!
Working out inheritance: Pedigree
Charts
Pedigree Charts
• A simple introduction