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