Topics 3 & 10 Genetics
3.4 & 10.2 Inheritance
• How many alleles of each gene does offspring
receive? Where do the alleles come from?
1 – Understanding Genes
Read & Consider Understandings 3.4.1-3.4.4 & 10.2.1-10.2.4
Key Terms
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Genotype – the alleles (genes)
Phenotype – the characteristics of an organism
Dominant allele – the allele that will be expressed
Recessive allele – expressed if inherited from both parents
Codominant allele –alleles that will both have an equal effect on
phenotype
Locus – the particular location of a gene on a chromosome
Homozygous – identical alleles were inherited from both parents
Heterozygous – different alleles were inherited from each parent
Carrier – an individual that has a recessive allele for some trait but is
not expressing it, though it could be passed on.
Test cross – testing a suspected heterozygote by crossing it with a
known homozygous recessive.
Allele Source
 The two alleles for each gene separate into different gametes during
meiosis.
 Gametes are haploid and contain only one allele for each gene they
contribute.
Things to remember 
 Fertilization is a random process with each allele from one
parent having an equal chance of fertilizing any other gamete
of the other parent.
 One allele for the offspring comes from each parent.
 One allele will be dominant over the alternative factor.
Allele Loci
 The genes carried on a particular chromosome are called a
linkage group.
 Chromosomes of a linkage group will tend to be inherited
together, barring crossing over of an allele.
 Unlinked genes segregate independently during meiosis
Mendel’s Study
 Gregor Mendel discovered the principles of inheritance by
carefully observing and recording hundreds of crosses between
pea plants.
Polygenic Traits
 All of Mendel’s observed traits were controlled by one gene
so had discrete variation.
 Polygenic traits are controlled by multiple genes, so have
continuous outcomes when observing phenotype.
• Describe dominant and recessive allele
behavior.
2 – Punnett Squares & Pedigrees
Read & Consider Understandings 3.4.5-3.4.8
Monohybrid Cross
T = tall; t = dwarf
 Parent 1: genotype Tt; phenotype Tall
 Parent 2: genotype Tt; phenotype Tall
T
T
t
t
Genotypes 
Genotype Ratios 
Phenotype 
Phenotype Ratios 
Dihybrid Cross
 Dihybrid cross – the
simultaneous inheritance of two
pairs of contrasting
characteristics, each
characteristic is inherited
independently. Example: pea
plants
R = round; r = wrinkled
Y = yellow; y = green
Parent 1 = RRYY; Parent 2 = rryy
RY
RY
RY
RY
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
Generation #2 
 Phenotypes 
RY
 Phenotype ratio 
 Genotypes 
RY
Ry
rY
ry
Ry
rY
ry
Co-Dominance
 Both alleles are expressed equally and simultaneously.
 represented by one capital letter to represent the gene locus
and additional letters, as superscripts, to represent the codominant alleles
FRFR = red; FWFW = white; FRFW = pink
FR
FR
FW
FW
Genotypes 
Genotype Ratios 
Phenotype 
Phenotype Ratios 
Multiple Alleles
 Multiple alleles – most
genes have more than two
trait possibilities.
 One capital letter represents
the gene.
 Individual alleles are
represented by letters as
superscripts.
Multiple alleles & Co-dominance –
 Human blood belongs to one of four groups: A, B, AB, or O.
Phenotype
O
A
B
AB
Genotype
IiIi
IAIA or IAIi
IBIB or IBIi
IAIB
Genotypes 
Genotype Ratios 
Phenotype 
Phenotype Ratios 
Parent 1 = heterozygous A (IAIi)
Parent 2 = heterozygous B (IBIi)
IA
IB
Ii
Ii
Sex Chromosomes
X
X
Y
X
Genotypes 
Genotype Ratios 
Phenotype 
Phenotype Ratios 
Sex Linkage
 Sex linkage is a special case of linkage occurring when a gene
is located on a sex chromosome (usually X) and is therefore
inherited with the sex of the individual.
 Most are carried on the X chromosome.
Sex-Linked Disease
XB = normal vision; Xb = color blind
XH = normal clotting; Xh = hemophilia
Color Blindness
Female
Male
Hemophilia
XBXB = normal
XHXH = normal
XBXb = carrier
XHXh = carrier
XbXb = severely colorblind XhXh = hemophiliac (fatal)
YBY = normal
XbY = color blind
XHY = normal
XhY = hemophiliac
Create Punnett grids to determine the genotype / phenotype
frequencies for a female carrier and a normal male. Repeat this for
each disease.
Predict the genotypic and phenotypic ratios of
offspring crosses of the above patterns.
Complete the practice worksheet of genetic crosses.
Autosomal Genetic Disease
 Most often a recessive trait that impacts a single gene allele.
There are more than 6000 known diseases caused by single
gene mutations and pass throughout families.
 Cystic Fibrosis
(Recessive)
 Huntington’s Disease
(Dominant)
Pedigree Chart
 Circles are female, squares are males
 Filled in shapes indicate that the individual is affected by the condition
 Empty shapes indicates that the individual has an unaffected phenotype
 Mating is indicated by a horizontal line from male to female
 Offspring branch vertically from the mating parents.
For the following pedigree, affected individuals are
homozygous for a recessive allele. Deduce the
genotypes of all individuals in the pedigree.
Dominant = D; Recessive = d
Crossing Over
 Crossing over is a process by
which a section of DNA is
exchanged between homologous
pair through the formation of
chiasmata.
Linked Genes
G = normal body color; g = ebony body color
S = straight wings; s = curled wings
Parent 1:
g
s
------------g
s
g
s
----------------------------------g
s
G
S
------------
Parent 2:
G
S
------------------------g
s
g
s
------------
Recombinants
Using the example from 10.2.4 there are also some
recombination possibilities:
G
s
G
S
These may be made as additions to the table or considered
separately as they are possibilities that may occur for linked
genes during meiosis.
 Normal body; curled wings
 Ebony body; straight wings, respectively.
Polygenic Inheritance
 Discontinuous variation
 Polygenes
 Polygenic inheritance
Continuous Variation
Parent 1: AaBb
Parent 2: AaBb
R = red; r = white
Parent 1: RrRrRr
Parent 2: RrRrRr
Polygenic Outcomes -
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
• What could cause observed genotypes to differ
greatly from the expected outcome?
3 - Significance
Read & Consider
Understandings 3.4.9 & 10.2.5
Mutation Rates
 Mutations are spontaneous permanent changes to base
sequences that can occur at any time in an organism. Some
environmental factors increase the number and frequency of
mutation.
Chi-Square Test
See totals for the following crosses from your notes. Use the
following observed outcomes to determine if there is a
significant difference from what is expected.
 Monohybrid Cross – Tall: 180, Dwarf: 50
 Dihybrid Cross – round/yellow: 213, round/green:78,
wrinkled/yellow: 70, wrinkled/green: 39
 Linked Gene – normal body/curled wings: 61, ebony
body/straight wings 49
 NOTE: parent 1 = normal body/straight wind, parent 2 =
ebony body/curled wings
Works Cited
 Diaz, Hortensia Jimenez. "How Mendel's Pea Plants Helped
Us Understand Genetics." YouTube. TED-Ed, 12 Mar. 2013.
Web. 15 Aug. 2015.
 "Drosophilab - A Free Genetics Simulator." Drosophilab. N.p.,
n.d.Web. 17 Aug. 2015.
 Walpole, Brenda. Biology Guide - First Assessment 2016.
International Baccalaureate Diploma Program. N.p., Dec.
2013. Web. Dec. 2013.
 "What Are Dominant and Recessive Alleles?" Your Genome.
N.p., n.d.Web. 15 Aug. 2015.