Download 3.4 Inheritance

Punnet squares
Steps to remember…
1.Choose a letter
2.Identify parents genotypes
3.Identify gametes parents could produce
4.Draw punnet square
5.Calculate chances
These are all mono-hybrid crosses. They
show one trait.
A = dominant allele for albinism
a = recessive allele for fewer pigments/albinism
What is the probability of the offspring developing
albinism?
The six genotypes are:
IAIA, IAi = blood type A
IBIB,IBi = blood type B
IAIB = blood type AB
ii = blood type O
Multiple alleles
With two alleles, three different genotypes are possible
producing two different phenotypes. This is the simplified
version.
In blood types, there are three alleles for the same gene –
ABO
There are four possible Human blood types – A, B, AB, O.
These can produce six genotypes
The gene for blood type is I (or Ii) The four blood types
are written IA, IB, i. IAB
A and B are co-dominant
Phenotype
Group A
Genotype
I AI A
Gametes
X
Group B
IBIB
IA
IB
F1 genotype
IA IB
F1 phenotype
Group AB
Phenotype
Group A
Genotype
I Ai
IA
Gametes
F1
genotype?
F1 phenotype?
Group B
X
IBi
i
IB
IB
i
IA
I AI B
I Ai
i
IBi
ii
i
How to test for blood types?
All three genes (A, B and O) cause the production of glycoprotein
(inside the membrane of RBC)
IA adds acetyl-galactosamine to the glycoprotein. Only people with A
blood have this protein, therefore if someone who does not have IA is
exposed to the altered glycoprotein, they produce anti-A antibodies.
IB adds galactose, Only people with B blood have this protein. therefore
if someone who does not have IB is exposed to the altered
glycoprotein, they produce anti-B antibodies.
IAIB causes both proteins to be produced. As a results neither
antibodies are produced when someone with IAIB blood is exposed to
either protein (their bodies recognise the proteins).
i is recessive, basic glycoprotein is produced.
Sex chromosomes
XX or XY
X is longer than Y. All female eggs contain one X
chromosome. In sperm, 50% contain X, 50% contain
Y.
Alleles which have their loci on the X or Y
chromosomes are sex linked. E.g. colour blindness &
haemophilia
What are the main effects of these two conditions?
Alleles & genotypes
Xb – recessive colour blindness
XB – colour vision
Xh - haemophilia
XH – blood clotting
Construct a monohybrid cross for each
Assume each genotype is heterozygous
XBXB – phenotype, non-affected female
XBXb – phenotype, carrier female
XbXb – phenotype, affected female
XBY – phenotype, non-affect male
XbY – phenotype, affected male
Xb alleles are rare in human population, the likely
hood of having two recessive alleles is very small.
This is why colour blindness mainly affects males.
Females can carry the allele – XBXb
Males cannot be carriers – XbY
This is the same for haemophillia – it affects the X
chromosome
Queen Victoria’s Pedigree Diagram
Analysis of pedigree charts
Dihybrid crosses & gene linkage
Mendel (yes again!) experimented with these traits in his pea experiments:
- Seed shape
- Seed colour
F1 = first generation
True breeding = homozygous for desired traits.
R = round peas
r = wrinkled peas
Y = yellow peas
y = green peas
What would the
offspring of the F1
generation look like?
Complete the following table to show the genotypes:
Parents phenotypes
Round yellow
Green wrinkled
Parents genotypes
RRYY
rryy
Parent gametes
RY
ry
F1 genotypes
RrYy
F1 phenotype
Round yellow
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Round yellow peas x9
Round green peas x3
Wrinkled yellow peas x3
Wrinkled green peas x1
Mendels conclusions from his
pea experiments were very
close to these theoretical
probabilities.
This is unusual.
Statisticians are convinced that Mendel’s results are too close to
exact ratios to be genuine. We shall never know how this came
about, but it offers an opportunity to discuss the need for
scientists to be truthful about their results, whether it is right to
discard results that do not fit a theory as Louis Pasteur is known
to have done, and the danger of publishing results only when
they show statistically significant differences.
TOK: Reasons for Mendel’s theories not being accepted by the
scientific community for a long time could be considered. Other
cases of paradigm shifts taking a long time to be accepted could
be considered. Ways in which individual scientists are most likely
to be able to convince the scientific community could be
considered, and also the need always to consider the evidence
rather than the views of individual scientists, however
distinguished.
All chromosome which are NOT sex chromosomes are
called Autosomes
Humans have 22 pairs of autosomes and one pair of sex
chromosomes
Some genes can be linked (found on the same
chromosome).
Groups of inherited genes are called linkage groups.
Applies to autosomes & sex chromosomes.
Linked genes are the exception to the law of independent
assortment. Linked groups can give a much wider variety
of ratios - not just 9:3:3:1