Download Mendelian Exceptions & Sex

Mendelian
Exceptions & Sexlinked Genetics
Objective: E4 - Predict possible
outcomes of non-Mendelian
inheritance & explain how sex is
determined
Mendelian
Exceptions
- Not all genes show simple patterns of
dominant and recessive alleles
• Because the majority of traits are
controlled by more than one gene/alleles
Codominance
• Cases in which both alleles are expressed
• White cow x Brown cow =
Speckled (white and brown) cow
Cow-1.au
Incomplete dominance
 Neither allele is
dominant
• Red flower x
White flower =
Pink flower
Practice Problems
• In a certain cactus, prickly spines can be two
pronged or one pronged. If a true breeding
(homozygous) one-pronged cactus is crossed with
a true breeding two-pronged cactus, the F1
generation has a mixture of spines, some are twopronged, some are one-pronged.
• Is this an example of codominance or incomplete
dominance?
codominance
• In snapdragons, the combined expression of both alleles for
flower color produces a new phenotype that is pink. This
illustrates incomplete dominance. The Punnett square above
shows that both the white and red snapdragons are
homozygous. Which of the following would be the correct
product from a cross between two heterozygous pink
snapdragons? [VA05 EOC]
• A 1 red, 2 pink, 1 white
• B 2 red, 2 white
• C 2 red, 1 pink, 1 white
• D 1 red, 1 pink, 2 white
Sex-linked Genetics
Sex Determination
• If you are female,
your 23rd pair of
chromosomes are
homologous, XX.
XY
Male
X
X
X
XX = Female
XX
Female
• If you are
male, your
23rd pair of
chromosomes
X
Y
XY, look
XY = Male different.
X
Y
XX
Female
XY
Male
XX
Female
XY
Male
X
The male determines
the sex of the offspring.
Sex-linked Inheritance
• Traits controlled by genes located
on sex chromosomes are called
sex-linked traits.
• The alleles for sex-linked traits
are written as superscripts of the
X or Y chromosomes.
Sex Linked Traits
• When genes are located on the X chromosome,
females receive two alleles for these genes, but
males only receive one.
• In males, the genotype is automatically known.
• Colorblind male - XbY (recessive)
• Normal male - XBY (dominate).
• A female can be:
• XBXB - normal
• XBXb - carrier
• XbXb - colorblind
• The following shows a
cross between a normal
man and a woman who is
a carrier.
• The mother always
passes sex-linked
traits on to her
son because he got
the X from his mom
& the Y from his
dad.