IB DP Biology Topic 3.4 Inheritance

1. Topic 3.4 Inheritance
IB Biology
R. Price
v. 1 2015

2. Allott 168

3. #1: Mendel discovered the principles of inheritance with experiments in
which large numbers of pea plants were crossed
Gregor Mendel
• 1866: “Experiments in Plant
Hydridization”
• Pea Plants: cross pollinate or self-
pollinate
• Quantitative results
• Large numbers of replicates
• Seven different cross experiments
• One father of genetics
• Pioneer in research methods in Bio

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7. #2: Gametes are haploid so contain one allele of each gene
• Gametes = sex cells
• Male gamete + female gamete =
zygote
• Male gamete: generally smaller,
mobile
• Contain one chromosome of each
type (haploid)
• Therefore, only one allele of each
gene

8. #3: The two alleles of each gene separate into different haploid
daughter nuclei during meiosis
• During meiosis a
diploid nucleus
divides twice to
produce four haploid
nuclei.
• The diploid nucleus
contains two copies
of each genes, but
the haploid nuclei
contain only one.

9. #4: Fusion of gametes results in diploid zygotes with two alleles of each
gene that may be the same allele or different alleles
Example
Two alleles possible: A or a
Zygote has 3 possibilities: AA, Aa, aa
More than one allele
ABO blood type: IA IB i
Six possibilities: IA IA IB IB ii
Iai IB i IA IB

10. #5: Dominant alleles mask the effects of recessive alleles but co-
dominant alleles have joint effects
• The usual reason for dominance of
one allele is that this allele codes for a
protein that is active and carries out a
function, whereas the recessive allele
codes for a non-functional protein.
• Some genes have pairs of alleles
where both have an effect when
they are present together. They
are called co-dominant alleles.

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20. #6: Many genetic diseases in humans are due to recessive alleles of
autosomal genes
• Most genetic diseases are caused by a
recessive allele of a gene
• Most genetic diseases must have
*two* copies of an allele to develop
• If someone has one dominant allele
and one allele for the genetic disease
= “carrier”
• If two carriers have a child, the
probability of their child to develop
the disease = 25%
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21. #7: Some genetic diseases are sex-linked and some are due to dominant
or co-dominant alleles
Sex-linked
• Red-green color blindness
• Hemophilia
• Much more common in males than
females

22. #7: Some genetic diseases are sex-linked and some are due to dominant
or co-dominant alleles
Dominant: Huntington’s Disease
• Dominant allele
• Located on chromosome 4. Gene
produces protein named huntingtin
• Degenerative changes in the brain
• Symptoms start between ages 30-50.
• Severe changes in behavior, thinking,
emotions. Will eventually need full
nursing care.

23. #7: Some genetic diseases are sex-linked and some are due to dominant
or co-dominant alleles
Co-Dominant: Sickle Cell Anemia
• Normal hemoglobin: HbA
• Sickle cell hemoglobin: HbS
• If HbA HbS then mild anemia, but
increased resistance to malaria

24. #8: The pattern of
inheritance is different
with sex-linked genes
due to their location on
sex chromosomes
• Location on X-
chromosome
• Males: inherit X
from mother, can’t
be “carriers”
• Females: father
must have disorder,
mother must pass
on allele

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32. #9: Many genetic diseases have been
identified in human but most are very
rare
• More than 4,000 genetic diseases
have been identified
• Most caused by very rare recessive
alleles
• Would have to inherit *two* very rare
recessive alleles

33. #10: Radiation and mutagenic chemicals increase
the mutation rate and can cause genetic disease and
cancer
Chemical Changes in DNA
• Gamma rays, alpha
particles, UV, X-rays
• Tobacco smoke, mustard
gas
• Mutation of cell division
gene = cancer
• Mutation of gamete DNA
= possible genetic
disease for offspring

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39. Sources
Content
Allott, Andrew, and David Mindorff. Biology: Course Companion. 2014
ed. Oxford: Oxford UP, 2014. Print. Oxford IB Diploma Programme.
Walpole, Brenda. Biology for the IB Diploma. 2nd ed. Cambridge:
Cambridge UP, 2014. Print.
Images
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