Genetics
Quick Review of Grade 11
Sex Linked Traits
Sex Influenced Traits
Polygenic Traits
Epistatic Genes
How many chromosomes do
humans have?
• 46…or 23 homologous pairs
• 23 from your mother and 23 from
your father
• Pairs1 through 22 = autosomal
chromosomes
•Which chromosomes determine if
you’re a girl or boy?
Two Types of Cells
1. Somatic Cells – All body cells,
diploid (2n)
2. Sex Cells – Gametes, haploid (n)
Two Types of Chromosomes
1. Autosomal – All body cells
2. Sex Chromosomes – X and Y
The Sex chromosomes
Its all up to
Dad….
If you get a X
you’re a girl
The Sex chromosomes
But... If you get a
Y you’re a boy
Punnett Squares
X
Y
X X
XX XX
XY XY
Expressed trait
(physical appearance)
Different forms
of the same
gene
When two alleles
are identical
Ex. DD
The set of genes
that an organism
possesses
When the two alleles
are different
Ex. Dd
Allele which controls
the characteristic
whenever it is
present.
BB or Bb
Allele whose characteristics only show up
when it is present on both chromosomes
bb
Genetics
Quick Review of Grade 11
Sex Linked Traits
Sex Influenced Traits
Polygenic Traits
Epistatic Genes
Sex Linked Traits
• Sex-linked traits are traits that are
controlled by genes on the sex
chromosomes
• Examples:
– Colour Blindness
– Hemophilia
3 Different Forms of
Sex Linked Inheritance
1. X-linked recessive inheritance
2. X-linked dominant inheritance
3. Y-linked inheritance
Sex Linked Inheritance Notation
• X-linked recessive alleles are
represented by a X with a
superscript of either:
+ (dominant)
or
– (recessive)
Genotypes of Sex Linked Inheritance
• What are the possible genotypes of a sex linked
inheritance for a female?
X+X+
X+X -
X -X -
• What are the possible genotypes for sex linked
inheritance for a male?
X+Y
X-Y
1. X-linked Recessive Inheritance
X-linked recessive traits are traits
resulting from a recessive allele on
the X chromosome
Why do X-linked recessive traits show
up more often in men than women?
Are YOU Colorblind?
Normal Color Vision:
A: 29, B: 45, C: --, D: 26
Red-Green Color-Blind:
A: 70, B: --, C: 5, D: -- 3.
Red Color-blind:
A: 70, B: --, C: 5, D: 6 4.
Green Color-Blind:
A: 70, B: --, C: 5, D: 2
X-linked Recessive Inheritance
• Example #1: Colorblindness
• The allele that controls colorblindness is found on
the X chromosome and is recessive
• What genotype must a female have to be
diagnosed as colorblind?
X -X-
Possible Colorblindness Phenotypes
X+X+
Female with Normal Vision
X+X –
Female with Normal Vision (Carrier)
X -X –
Colour Blind Female
X+Y –
Male with Normal Vision
X–Y
Colour Blind Male
Let’s try a cross: X+X– • X+Y
• Phenotypes
– Females:
– Males:
Let’s try a cross: X+X– • X-Y
• Phenotypes
– Females:
– Males:
Let’s try a cross: X+X+ • X-Y
• Phenotypes
– Females:
– Males:
Let’s try a cross: X-X- • X+Y
• Phenotypes
– Females:
– Males:
Try these questions on your own:
1. A woman who is heterozygous (a carrier) for
colorblindness marries a man with normal vision.
What will be the possible phenotype ratio of their
children?
2. What is the probability that the sons of a
homozygous recessive mother would be colour
blind?
*Remember to represent colorblindness with a “-”
More examples of
X-linked Recessive Inheritance
• Example #2: Hemophilia
• Hemophilia is a serious ailment in which the blood
lacks a clotting factor, and therefore when an
individual is injured, they cannot stop bleeding
• About 1 in 4,000 males are born with disorder, much
lower incidence in females
Hemophilia Pedigree
Just to make sure you got it! ;)
A woman who is heterozygous for hemophilia
marries a normal man:
a. What are the genotypes of the
parents?
b. Make a Punnett square for the cross
c. What is the probability that a male
offspring will have hemophilia?
d. What is the probability of having a
hemophiliac female offspring?
2. X-linked Dominant Inheritance
• X-linked dominant traits are traits that
result from the presence of a dominant
allele on the X chromosome
• Unlike X-linked recessive traits,
females and males both require only
ONE dominant allele in order to
express the trait
X-linked Dominant Inheritance
• Example #1: Faulty Tooth Enamel and Dental
Discoloration
• Individuals who have an X chromosome that
carries a dominant allele for this trait will have
dental discoloration.
Possible Dental Disorder Phenotypes
X+X+
Female with Dental Discoloration
X+X –
Female with Dental Discoloration
X -X –
Normal Female
X+Y
Male with Dental Discoloration
X–Y
Normal Male
Let’s think about it...
1. Would a heterozygous woman for
dental discoloration display the trait?
– Yes, because dental discoloration is a
X-linked dominant trait, so only one
dominant allele is needed to express the
trait
2. What percentage of the children from
a heterozygous mother and an
affected father would have dental
discoloration?
3. Y-linked Inheritance
• Y-linked traits are controlled by alleles on the Y
chromosome
• Another word for Y-linked traits is holandric traits
(“wholly male”)
• Are females affected by Y-linked traits?
• Do the words homozygous or heterozygous apply to
Y-linked traits?
What do these 2 have in common?
• Example of Y-Linked Inheritance – HAIRY EARS!
• Anthony Victor (India) has hair sprouting from the
centre of his outer ears (middle of the pinna) that
measures 18.1 cm (7.12 in) at its longest point.
All of the sons of an affected male
will display this Y-linked trait
Last Sex-linked Trait Example
Eye color in fruit flies (Drosophilia melanogaster )
• Eye color is controlled by
the X chromosome
• Red eyes are dominant to
white
• White eyes are most
common in males
• Females only display white
eyes if they are homozygous
recessive for the trait
Fruit Flies Continued
• What type of X-linked inheritance is this?
X-linked recessive Inheritance because the
males show the trait more often that the
females. Also, the females must have two X
chromosomes, both of which carry the
recessive alleles for white eye color in order
to have white eyes!
• Try your new knowledge out on the handout you
are about to receive!
Genetics
Quick Review of Grade 11
Sex Linked Traits
Sex Influenced Traits
Polygenic Traits
Epistatic Genes
What’s the Difference?
• Sex-Linked Traits: alleles on sex
chromosomes
• Sex-Influenced Traits: alleles on
autosomal (1-22)
chromosomes
Sex-Influenced Traits
• Controlled by a pair of alleles on autosomal
chromosomes but its phenotypic expression is
influenced by the presence of certain hormones
Estrogen, Progesterone, Testosterone, etc.
• Can be seen in BOTH sexes, but will vary in
frequency between the sexes or in the degree
of the phenotypic expression
Sex-Influenced Traits
• Example #1: Pattern Baldness
• Pattern Baldness can occur in both males and
females, but is much more common in males….
Why?
– Because the pattern baldness trait is influenced by the
hormone testosterone
The combination of alleles for pattern baldness
will lead to different phenotypic expressions
depending on the sex of the individual
Unlike male-pattern
baldness, female-pattern
baldness is an over-all
thinning which maintains
the normal hairline
Possible Pattern Baldness Phenotypes
Example: Let B represent the non-bald allele
BB
Non-bald in both sexes
bb
Bald in both sexes
Bb
Bald in men
Non-bald in females
The “B” allele acts as a
dominant allele in the
heterozygous genotype in
females, but acts as a
recessive allele in the
heterozygous genotype of
the male
Let’s check your understanding…
• What would Locke’s
genotype be?
Bb or bb
• What about his ‘we’llassume-for-the-sake-ofthis-example’ balding
mother?
bb
To solve these questions, we can use
simple Monohybrid Crosses
Then able to make conclusions
regarding phenotypes based on the
sex of the individual
Now you try…
A heterozygous balding male
reproduces with a heterozygous
normal female.
– Do the cross and determine the phenotypic
ratios for males and females
Let’s make it personal…
What is the probability that YOU will be
bald if your father is homozygous and
balding, and your mother is
homozygous and not balding?
If only it were that simple!
• Hair is controlled by many different factors
including several alleles and environmental
factors (polygenic traits)
• So, we cannot fully explain pattern baldness
using sex-influenced inheritance.
• It is not easy to explain the diversity found in
different ages of onset, as well as severity of
baldness.
More Sex-Influenced Trait Examples
A male homozygous for clubfoot reproduces with a
normal homozygous female.
What are the genotypes and phenotypes of their
children if testosterone alters the phenotypic
expression of the trait in the heterozygous
expression?
Let F represent normal feet
Let f represent clubfoot.
More Sex-Influenced Trait Examples
Two heterozygous individuals get married and have
lots of children. The father suffers with a painful
condition called gout.
What are the parent’s genotypes?
What are the genotypes and phenotypes of the
children?
Let G represent no gout
Let g represent gout
More Sex-Influenced Trait Examples
Rheumatoid arthritis occurs more often in
females than males due to the presence of
estrogen. A heterozygous woman marries a
heterozygous male
RR would cause the
condition in both sexes.
A homozygous recessive,
rr, genotype would
prevent the disorder in
both sexes
More Sex-Influenced Trait Examples
Singing voice!
The genotype that
causes males to have a
high tenor voice causes
females to have a deep
contralto voice. The
genotype that causes
males to have deep bass
voices is the same
genotype that causes
females to have high
soprano voices!
Design a question
involving the SexInfluenced trait of singing
voice
Genetics
Quick Review of Grade 11
Sex Linked Traits
Sex Influenced Traits
Polygenic Traits
Epistatic Genes
Polygenic Traits
• Those traits that are determined by the
combined effect of 2 or more pairs of alleles
• Each pair of alleles adds something to the
resulting phenotype
• Other names for polygenic traits are:
multi-factorial traits or quantitative traits
Polygenic Traits
• Because so many alleles contribute to the final
phenotype, a variety of phenotypes can occur!
• For example, height is a polygenic trait
To solve polygenetic trait questions that
are only controlled by 2 pairs of alleles,
we can use a Dihybrid Cross
(Punnett Square)
Polygenic Traits
• Example #1: Pepper Colour
Gene 1: R = red
r = yellow
Gene 2: Y = absence of chlorophyll (no green)
y = presence of chlorophyll (green)
Possible Pepper Colour Phenotypes
R -Y –
Red (Red/No Chlorophyll)
R – yy
Brown/Orange (Red/Chlorophyll)
rrY –
Yellow (Yellow/No Chlorophyll)
rr/yy
Green (Yellow/Chlorophyll)
Pepper Colour Questions
• Try crossing a brown pepper (RRyy) with a
yellow pepper (rrYY).
• Which trait will your offspring (F1
generation) produce?
• What traits are produced when you cross
two of the peppers found in the F1
generation?
Polygenic Traits
• Example #2: Chicken Combs
Comb shape is controlled
by 2 genes found on 2
different pairs of
chromosomes
Gene 1: R
Gene 2: P
Possible Chicken Comb Phenotypes
R – pp
Rose Combs
R -P –
Walnut Combs
rrP –
Pea Combs
rr/pp
Single Combs
Practice Makes Perfect!
Indicate the phenotypes of the parents and
give the genotypic and phenotypic ratios of
their offspring for the following crosses:
1. rrPP x RRpp
2. RrPp x RrPp
**Refer to your phenotype chart for chickens
Polygenic Traits
• Example #3: Hair Colour
Hair colour is controlled by
alleles on chromosomes 3,
6, 10 & 18
The most dominant alleles
that appear in the genotype,
the darker the hair
Polygenics Plant Height Problem
The height of plants is controlled by 4 pairs of alleles.
Alleles A, B, and C contribute 3 cm to the plant's height.
Alleles that are recessive do not contribute to the height.
Gene L is always found in a homozygous dominant condition
and always contributes 40 cm to the height.
1.
What would be the height of a plant with the genotype
AABBCCLL?
2. What would be the height of a plant with a genotype
aabbccLL?
3. What would be the height of the offspring produced from a
cross between the plants in a) and b)?
4. What would be the heights of the offspring produced from a
cross between AaBbCcLL and AaBbCcLL? (now there is a
challenge!)
Genetics
Quick Review of Grade 11
Sex Linked Traits
Sex Influenced Traits
Polygenic Traits
Epistatic Genes
Epistasis
• Epistasis is a form of a gene
interaction in which one pair of
alleles (gene) masks the
phenotypic expression of another
• There are no new phenotypes
produced by this interaction
Epistatic versus Hypostatic
• Epistatic: the alleles that mask the
effect
• Hypostatic: the alleles whose effect
is being masked
Epistatic trait questions can be solved
like polygenic trait questions with a
Dihybrid Cross
The difference is that there is now one
pair of alleles masking the other