Guided notes covering material from Topic 3.4 of the updated IB Biology syllabus for 2016 exams. Notes sequence and prompts are based on the Oxford IB Biology textbook by Allott and Mindorff.

1. IB Biology Chapter 3 Notes: Genetic Inheritance (3.4) NAME:
Word Definition
Fertilization The fusion of male and female gametes
Zygote Diploid cell formed by the fusion of male and female gametes
Genotype The combination of genes for a trait—represented by two letters
Phenotype The traits/characteristics of an organism
Dominant allele The allele that only needs one copy to be expressed.
Recessive allele An allele that needs two copies to be expressed
Codominant allele Pairs of alleles that both affect the phenotype when present in an individual
Homozygous Having two identical alleles for a gene
Heterozygous Having two different alleles for a gene
Carrier An individual that has a recessive allele but does not express it
Test cross Testing to determine the genotype of an unknown individual that has a dominant
phenotype. The unknown individual is crossed with a recessive individual—if any
offspring are recessive then the unknown is heterozygous
Punnett grid A way of determining the phenotype and genotype of the offspring
Monohybrid cross Crossing only one trait at a time
Sex-linked genes Genes that are found on the sex chromosomes (usually the X chromosome)
Multiple alleles When more than two alleles determine a trait
Pedigree A diagram that shows how a trait is passed through many generations

2. 3.4.1 Mendel discovered
the principles of inheritance
with experiments in which
large numbers of pea plants
were crossed.
3.4.2 Gametes are haploid
so contain one allele of
each gene.
3.4.4 Fusion of gametes
results in diploid zygotes
with two alleles of each
gene that may be the same
allele or different alleles.
3.4.3 The two alleles of
each gene separate into
different haploid daughter
nuclei during meiosis.
3.4.5 Dominant alleles
mask the effects of
recessive alleles but co-
dominant alleles have joint
effects.
3.4.15 Construct Punnett
grids (squares)for
predicting the outcomes of
monohybrid genetic
crosses.
Describe Mendel’s experiments:
Purebred Parent Plant Cross Offspring Phenotypes Ratio
Tall stem x Dwarf stem
Round seed x Wrinkled seed
Yellow peas x Green peas
Purple flowers x White flowers
Describe how alleles segregate into gametes:
Dominant alleles:
Example:
Recessive alleles:
Example:
Co-Dominant alleles:
Example:
Cross two heterozygous tall pea plants (T = tall allele, t = short allele):
Parent Genotypes: Tt x Tt
Parent Phenotypes: Tall x Dwarf
Punnett Grid:
Offspring Genotype Ratio: ______ : ______ : ______
Offspring Phenotype Ratio: ______ : ______

3. 3.4.11 Inheritance of ABO
blood groups.
3.4.16 Comparison of
predicted and actual
outcomes of genetic crosses
using real data.
3.4.6 Many genetic diseases
in humans are due to
recessive alleles of
autosomal genes.
3.4.7 Some genetic diseases
are sex-linked and some are
due to dominant or co-
dominant alleles.
3.4.13 Inheritance of cystic
fibrosis and Huntington’s
disease.
3.4.8 The pattern of
inheritance is different with
sex-linked genes due to
their location on sex
chromosomes.
3.4.12 Red-green color-
blindness and hemophilia
as examples of sex-linked
inheritance.
Describe the phenotypes produced by each of the three blood group alleles:
IA :
IB :
i :
Which blood group alleles are co-dominant?
Why aren’t the outcomes of actual genetics crosses always the same as the predicted outcomes?
How can an offspring inherit a recessive genetic disorder?
What does it mean if someone is a ‘carrier?’
Example of a recessive genetic disorder:
Example of a sex-linked genetic disorder:
Example of a dominant genetic disorder:
Example of a co-dominant genetic disorder:
Cystic Fibrosis:
What type of allele causes it? ______________________
Describe the disease:
Huntington’s Disease:
What type of allele causes it? ______________________
Describe the disease:
What makes a gene ‘sex-linked?’
Color-blindness:
Cross a color-blind male and a carrier female (XN = normal vision allele, Xn = color-blind allele):
Parent Genotypes: XnY x XNXn
Punnett Grid:
What are the chances of a color-blind boy? ________
What are the chances of a color-blind girl? ________

4. 3.4.17 Analysis of pedigree
charts to deduce the pattern
of inheritance of genetic
diseases.
3.4.9 Many genetic diseases
have been identified in
humans but most are very
rare.
3.4.10 Radiation and
mutagenic chemicals
increase the mutation rate
and can cause genetic
disease and cancer.
3.4.14 Consequence of
radiation after nuclear
bombing of Hiroshima and
Nagasaki and the nuclear
accident at Chernobyl.
Hemophilia:
Cross a color-blind male and a carrier female (XH = normal clotting allele, Xh = hemophilia allele):
Parent Genotypes: XHY x XhXh
Punnett Grid:
What are the chances of a hemophiliac boy? ________
What are the chances of a hemophiliac girl? ________
Squares =
Circles =
Label each pedigree as one of the following inheritance patterns: autosomal dominant,autosomal
recessive, and sex-linked recessive:
How many alleles in the human genome are estimated to cause disease?
Why are genetic diseases very rare?
Radiation:
Mutagenic chemicals:
How could a mutation end up being passed on to an offspring?
Describe consequences/ effects ofradiation in Hiroshima and Nagasaki:
Describe consequences/ effects ofradiation in Chernobyl: