Genetics, inherited disorders
Drag and Drop
Heterozygous
Recessive
Dominant
Homozygous
Phenotype
Genotype
Allele
Lesduur is: 30 minOnderdelen in deze les
Drag and Drop
Heterozygous
Recessive
Dominant
Homozygous
Phenotype
Genotype
Allele
What? – You will have an understanding on how recessive and dominant diseases are inherited.
How? – You will be analysing pedigree diagrams and producing genetic crosses to show how diseases are inherited.
Why? – To understand the importance of knowing family history and the causes of genetic diseases before starting a family.
New Learning
Some disorders are inherited.
These disorders are caused by the inheritance of certain alleles.
Some disorders are described as dominant if only one allele of that gene is needed.
Other disorders are described as recessive if both alleles must be of the recessive type for the person to have the disease.
Recessive disorders are not seen if there are any copies of the Dominant allele present.
What type of allele is needed for dominant disorders?
Both alleles must be dominant
One recessive allele
Two recessive alleles
Only one dominant allele
What do you think the problem might be with sickle cell anemia?
New Learning
The Cause: The HBB Gene The instructions for making haemoglobin (the protein in red blood cells that carries oxygen) are found in the HBB gene.
Normal Red Blood Cells: Contain Haemoglobin A.
Sickle Cells: A mutation in the HBB gene causes the body to produce Haemoglobin S.
This makes the cells "sticky" and prone to clumping, or clotting.
These red blood cells carry less oxygen too
New Learning
How is it inherited -
Sickle cell anaemia is a recessive condition.
This means a person must inherit two copies of the "sickle" allele to have the disease.
We can represent the alleles like this:
A: The dominant allele for normal HBA haemoglobin.
a: The recessive allele for sickle HBS haemoglobin.
How Many Genotypes are there for sickle cell Anaemia and will the person be affected or not?
Using B for dominant allele and b for recessive, write down the homozygous dominant combination
BB
Write down the homozygous recessive combination
bb
Write down the homozygous recessive combination
bb
Punnett square
If a father who is a carrier for Sickle cell anaemia (Aa) and an unaffected homozygous dominant mother (AA) have a child, we can predict the likelihood (%) of their offspring being unaffected (AA)carriers (Aa) or having the condition (aa).
A
A
a
A
AA
Aa
AA
Aa
Phenotype : 1 100% the child will not be affected by sickle cell anaemia but 50% chance they are a carrier of the allele.
Not affected
Not affected
NA - carrier
NA - carrier
Drag and Drop
AA
Aa
AA
aa
What is the percentage probability that their child will be affected by sickle cell anaemia (genotype aa?)
0%
25%
50%
75%
What is the probabilty that their child will be a carrier (Aa)?
25%
50%
75%
100%
It is possible because sickle cell anaemia is caused by a recessive allele. Two parents who do not have the condition can both be heterozygous, meaning they have one normal allele and one recessive allele. (Aa)
If both parents pass the recessive allele to their child, the child will inherit two recessive alleles and will develop sickle cell anaemia.(aa)
A heterozygous person is called a carrier because they carry the recessive allele but do not show the disease. (Aa)
New Learning
Dominant genetic conditions
Polydactyl (‘many toes’) is caused by a Dominant allele.
Only one copy of this allele is needed for the disease to condition to be expressed and the phenoytype observed.
Polydactyl involves the individual having an extra digit.
This cat has been born with more than the usual number of toes.
Punnett square
If 2 people with polydactyl (Pp) have a child, we can predict the likelihood (%) of their offspring being affected (Pp or PP) or not having the condition.
Genotypes :1 PP = homozygous Dominant 2 Pp heterozygous, 1 pp homozygous recessive
Phenotype : 1 unaffected (pp) 25% 3 have polydactyl (1PP & 2Pp) 75%
Punnett square
If a person homozygous Dominant(PP) for the polydactyl condition has a child with a person with homozygous recessive (pp), we can predict the likelihood (%) of their offspring being affected (Pp or PP) or not having the condition (pp).
Genotypes :4 Pp =heterozygous = 100%
Phenotype : 4 have polydactyl 100%
What are the two possible genotypes for the children?
Pp and pp
PP and Pp
What is the ratio of affected to unaffected children?
2:1 (67% affected to 33% unaffected)
1:1 (50% affected to 50% unaffected)
3:1 (75% affected to 25% unaffected
1:0 (100% affected
Can a person be a 'carrier' for polydactyly without having the condition? Explain why or why not?
Pedigree diagrams (Family tree history)
Dominant inherited disease
PP & Pp have it
pp don’t have it
Success criteria
Identify the recessive alleles (p) normal.
If a child has two recessive alleles they must get one from each parent.
The use the key to work out the second allele that the child did not inherit – based on the disease.
p
p
p
p
P
p
p
DOMINANT INHERITED DISEASE
p
p
P
P
P
P
p
p
p
Pedigree diagrams (Family tree history)
AA & Aa Don’t have it
aa have it
Success criteria
Identify the recessive alleles (p) normal.
If a child has two recessive alleles they must get one from each parent.
The use the key to work out the second allele that the child did not inherit – based on the disease.
RESSESSIVE DISEASE
Challenge
Is this condition dominant or recessive?
Click to zoom in
Dominant
Recessive
Explain your reasoning. What specificevidence in the pedigree chart proves that whether the condition is dominant or recessive?
Explain your reasoning. What specific evidence in the pedigree chart proves that whether the condition is dominant or recessive?
Step 3: Justification
Explain why the parents in Generation 2 MUST be heterozygous (Bb) for an affected child (bb) to be born
