Transcript Chapter-11-Monohybrid-Cross

Higher Human Biology
Unit 1: Cell Function and
Inheritance
Chapter 11: Monohybrid
Cross
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Lesson Aims
• To revise and consolidate understanding
of monohybrid crosses
• To examine Rhesus and Rhesus- blood
groups
• To learn about different conditions caused
by genetic mutations
• To find out the difference between
incomplete dominance and co-dominance
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You need to know these words
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Things you need to know
Monohybrid inheritance
i
The pattern of inheritance of a pair of
alleles where one is dominant and one is
recessive.
ii The effects of alleles exhibiting dominance,
co-dominance and incomplete dominance.
iii Possible combinations of multiple alleles.
ALSO REMEMBER: Dominant and co-dominant alleles should be represented
by upper case letters and recessive alleles by lower case letters.
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History
Gregor Mendel - The Father of Genetics
1. Monk who used science and maths to
establish patterns in how traits were inherited
2. Year: 1857 – carried out early monohybrid cross.
3. He used the garden pea as his test subjects
Some Vocabulary
• Character - a heritable feature (e.g. flower colour)
• Trait - a variant of each character (e.g. purple or white)
• Cross Pollination - one plant fertilizes a different plant
• Self Pollination - a plant fertilizes itself
• True-Breeding - plants that over several generations only
produce plants like themselves
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Monohybrid cross.
• A cross between two parents who
possess different forms of a gene referred
to as a MONOHYBRID INHERITANCE.
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Mendel’s Experiments - Monohybrid
Cross (pea plant cross).
• Monohybrid Cross: involved plants that differed
for a single character: tall x short, purple flower x
white flower, round seed x wrinkled seed.
• P (Parental Generation): True breeding plants
• F1 (First Filial): The offspring of the P generation
--> they always displayed a single trait, the
dominant one.
• F2 (Second Filial): The offspring of the F1
generation, self fertilized --> always had a 3:1 ratio.
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Pea plant cross
Parent plant true
breeding for
round seeds
x
Parent plant true
breeding for
wrinkled
Cross-pollination
First filial
generation (F1 )–
ALL ROUND
SEEDS
Self-pollination
Second filial generation (F2) –
3 ROUND: 1 WRINKLED SEEDS
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• Since wrinkled seeds were
absent in the F1 and
reappears
in
the
F2,
‘something has to be
transmitted undetected in
the
gametes
from
generation to generation.
Today we call this a GENE.
In this case it is a gene for
seed shape, which has two
alleles, round and wrinkled.
• Since the presence of
round allele masks the
presence of the wrinkled
allele, round is said to be
DOMINANT and wrinkled
RESSESSIVE.
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Phenotypes and genotypes
• An organisms genotype is its genetic constitution (i.e.
Alleles of genes) that is inherited from parents.
• These instructions are intimately involved with all
aspects of the life of a cell or an organism
• An organisms phenotype is its appearance resulting
from this inherited information (Genotype).
• This is anything that is part of the observable structure,
function or behaviour of a living organism. e.g. Eye
colour
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Mendel’s Law of Segregation
• States…The alleles of a gene exist in pairs
but hen gametes are formed, the members
if each pair pass into different gametes.
Thus each gamete contains only one allele
of each gene.
– For example a Tt parent can produce both T
sperm, and t sperm.
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Locus - spot on the chromosome
where an allele (gene) is located.
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Punnet squares
A punnet square is a
representation of the
law of segregation,
showing how gametes
separate and then
come together during
fertilization.
ALSO REMEMBER: Dominant and co-dominant
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Monohybrid
alleles
should be represented by
case
letters
Cross.
and recessive alleles by lower case letters.
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Homozygous and Heterozygous
• When an individual possesses two similar alleles of
a gene (e.g. R and R or r and r), its genotype is
said to be HOMOZYGOUS (true-breeding) and all
of it’s gametes are identical with respect to that
characteristic.
• When an individual possesses two different alleles
of a gene (e.g. R and r), its genotype is said to be
HETEROZYGOUS. It produces two different types
of gamete with respect to that characteristic.
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Task: Torrance pg 83 Qu’s 1-4
CAN YOU ROLL YOUR TOUNGE?
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Monohybrid Inheritance in Humans
• Tongue rolling is inherited
as a simple Mendelian trait.
• R is the allele for roller
• r is the allele for non-roller.
R
R
Genetics of tongue rolling
r
RR
Rr
Rr
rr
r
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Monohybrid inheritance in
humans: Rhesus D Antigen
• In addition to the ABO system of
antigens, most people have a further
antigen on the surface of their red
cells. This is called Antigen D.
• Most people are Rh+ (rhesus
positive) as they posses this antigen
• A minority of people are Rh- (rhesus
negative) they do not possess this
antigen. But these people react to
the presence of antigen D by
forming anti-D antibodies
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Rhesus D Antigen Con’t
• If a Rh- person is given Rh+ red blood cells
during a transfusion the persons immune
system responds by producing anti-D
antibodies. This leaves the person
sensitised.
• If this person receives more Rh+ red blood
cells they suffer from severe or fatal
agglutination.
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Agglutination of Red Blood Cells
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• Presence of Antigen D is genetically
dominant (D)
• Lack of antigen D is due to a recessive
allele (d)
P DD x dd
(Rh+)(Rh-)
D
or
D
P dd x Dd
(Rh-) (Rh+)
D
d
d
Dd
Dd
d
Dd
dd
d
Dd
Dd
d
Dd
dd
F1: all Dd (Rh+)
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F1: Dd (Rh+) and dd (Rh-)
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Examples RECESSIVE
monohybrid inheritance in humans
• Albinism - inability of the body to make
melanin - inherited as simple Mendelian
recessive trait.
• Cystic Fibrosis - disorder of the mucus
secreting glands - simple Mendelian
recessive trait..
• PKU – inborn error of metabolism – simple
Mendelain recessive trait
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Example of a DOMINANT
monohybrid inheritance in humans
Huntingdon’s Chorea
• Degeneration of the nervous system which
leads to premature death.
• Determined by dominant allele.
• Allele not expressed in phenotype until about
38 years of age when sufferer will probably
have had a family and passed on the allele.
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Huntington’s Chorea – The genetics
• H = allele for Huntington's, h = allele for normal condition
• 5 combinations HH x HH, HH x Hh, Hh x Hh,
HH x hh, hh x hh.
• HH x HH all offspring HH – none survive
• HH x Hh offspring HH, HH, HH, Hh – None
survive
• Hh x Hh offspring HH, Hh, Hh, hh – 75% don’t
survive (hh lives)
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Huntington’s Chorea – The genetics
• H = allele for Huntington's, h = allele for normal condition
• Most likely combination Hh (but doesn’t know
yet: breeds with hh.......
• Potentially tragic situation 1 in 2 inherit
condition.
• Hh x hh - offspring = Hh, Hh, hh, hh – 50%
don’t survive (hh lives) – but no one will know
till mid thirties.
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Task: Torrance pg 85 Qu’s a-h
Incomplete
Dominance
• Sometimes one allele is not completely
dominant over the other,
• Occurs when the recessive allele has some
effect on the heterozygote.
• Here the heterozygote exhibits a phenotype
which is different from both of the hetrozygotes .
– e.g.
• Sickle Cell Anaemia
• Resistance to malaria
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Incomplete dominance –
Example: Sickle cell anaemia.
Can see
the cells
have the
typical
sickle cell
shape.
• An example of incomplete dominance is illustrated in the
condition known as sickle cell anaemia.
• Here one of the genes which codes for haemoglobin (Hb)
undergoes a mutation The Hb produced is an unusual type
called Hb- which is an inefficient carrier of oxygen.
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Homozygous for the mutant
allele: SS
Homozygous for the
mutant allele: SS
• Disastrous consequences,
sufferers SICKLE CELLED
ANAEMIA, they have the
abnormally shaped sickle cell
blood, RBC’s fail to perform
function well.
• Causes shortage of oxygen,
damage of internal organs and
in many cases death.
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Picture shows blood
containing only Haemoglobin
wit the Sickle shape.
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Heterozygous for the mutant allele: HS
(H=normal S=sickle both uppercase
because neither is dominant)
Heterozygous for the
mutant allele:
– Do not suffer from Sickle Cell
Anaemia,
– Instead RBC’s contain both
forms of Hb – giving a milder
condition called SICKLE CELL
TRAIT.
– Causes slight anaemia, which
does not prevent moderate
activity.
Picture shows blood containing both
forms of Haemoglobin (although the
mutant cells are not completely sickle)
This ‘in-between’ situation where the mutant allele is partially
expressed, neither allele is completely dominant over the other
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Resistance to malaria (HS genotype)
• The S is rare in most populations.
• However, in some parts of Africa up to 40% of
the population has the heterozygous genotype
HS.
• This is because the parasite cannon make use
of the RBC’s containing haemoglobin S.
• People with the normal homozygous genotype
HH are susceptible to malaria (and may die).
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Co-dominance
• Describes the situation where two alleles can
be expressed in the heterozygote, neither
suppressing the other, e.g. MN blood grouping.
• Blood groups are determined by the presence
of antigens on the surface of RBC’s.
• In addition to the ABO and Rhesus D-Antigen
system, a further example is the MN blood
group system.
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MN Blood Group
• Controlled by two alleles M and N which are
co-dominant (both alleles expressed in the
phenotype of the heterozygote).
• Heterozygous MN blood group have both M
and N antigens on rbc
• Homozygous MM blood group have M
antigens on rbc
• Homozygous NN blood group have N
antigens
on
rbc
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Cross.
Multiple Alleles
• Each of the genes considered so far has two alleles (
which display complete, incomplete or co-dominance).
• Some genes are found to possess 3 or more different
alleles for a certain characteristic.... It has multiple
alleles.
• If 3 alleles of a gene exist, and since a diploid individual
has 1 or 2 of these alleles, then there are 6 genotype
combinations possible.
• The phenotype depends on whether the alleles are
complete, incomplete or co-dominant.
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ABO Blood Group
Antigens coded by a gene that has three
alleles A, B and O.
6 possible genotypes: AA, AO, BB, BO, AB, OO
4 Phenotypes, A, B, A&B, or Neither A or B...
• Allele A produces antigen A.
• Allele B produces antigen B.
• Allele O produces no antigens.
• Alleles A and B are co-dominant to one another and
completely dominant over allele O.
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TASK: Complete Torrance TYK
questions on page 87
Essay Question
Guide to H essays – pg 58
• Discuss inheritance under the following
headings
– (a) Patterns of dominance
– (b) Multiple Alleles.
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(7)
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Essay Question –
Guide to H essays – pg 58
• Discuss monohybrid inheritance in
humans. (15)
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