Download Extending Mendel: X-linked genes

Extending Mendel: X-linked
genes
Morgan and the fruit fly
• Reciprocal crosses between white and red
eyed flies produced different results
depending on whether the white eye color
was in the female or the male parent.
Sex determination in Drosophila
•  XX/XY.
•  It is the balance between X
chromosomes and autosomes
that determines sex: females
with 2 X chromosomes; males
with one.
•  What sex would a fly with
XYY be? XXY?
•  How about X–?
SO what if 2 genes are on the
same chromosome: will they
assort independently?
Morgan s experiment–linked genes should be
inherited together
wy+/wy+
wy+
w+y/w+y
w+y
Results
wy+/Y : 4292
w+y/Y : 4605
wy/Y : 86
w+y+/Y : 44
How did we get recombinants
among progeny?
But why so few recombinants?
Genetic mapping
• Morgan reasoned that if the distance
between 2 genes determined the frequency
of crossing over and therefore of
recombinants showing up, then crosses
involving genes on the same chromosome
could be used to map relative position.
• A heterozygous, but phenotypically
wild-type fruit fly (gray body color
and normal wings) was mated to a
black fly with vestigial wings. The
offspring had the following phenotypic
distribution: wild type, 720; blackvestigial, 780; black-normal, 280; grayvestigial, 220. What conclusion(s) is
(are) likely from these results?
Can linked genes assort
independently?
Incomplete dominance
Another example of incomplete
dominance
Codominance
Glycophorin A gene in
humans spans the
plasma membrane of the
red blood cells. There
are 2 alleles, M and N
based on 2 aa
differences. But both
appear on the cell
surface in heterozygotes
So why the variety of dominance
relationships?
• Completely recessive alleles are usually non
functional.
• Complete dominance occurs when one allele
is enough.
• Incomplete dominance occurs when one
allele does not suffice.
• Codominance occurs when both alleles
combine to create the phenotype.
Are there always 2 alleles per
gene?
Human Leucocyte Antigen–HLA
•  Code for the MHC proteins that
occur on the cell surface.
•  Several loci exist each with many
codominant alleles.
•  Variation occurs particularly
commonly at the site where the
molecule binds to antigens.
MHC class I
Major Antigens
HLA A
HLA B
HLA C
Minor Antigens
HLA E
HLA F
HLA G
580
921
312
9
21
28
MHC function
• Non self recognition
• Antigen presentation
Gene Interactions
• When a product from a gene interacts with
products form other genes, particular
combinations of alleles of the two genes can
produce distinct effects. I.e., the phenotype
of one gene depends on the phenotype of
the other.
cat coat color
• A coat color gene B codes for black pigment
while another allele b codes for brown. But
the gene C (full color gene) determines
whether pigment can be produced
independent of color. So a cat that is BBcc
will be white even though it has a gene to
make pigment. Similarly bbcc and Bbcc are
white. The B gene is hypostatic to the C
gene; the c gene is epistatic to the b gene.
Pleiotropy
•  When a gene has more than one phenotypic effect.
•  E.g. Marfan syndrome (see text).
•  Full color gene (tyrosinase) in cats and other
mammals
• Because many gene products may interact
differently and play slightly different roles
depending on the cell they are in, pleiotropy
may be more the rule than the exception.
Nature versus Nurture
• Genes are not the sole determinants of
phenotype; depending on the environment a
gene may have very different implications.
• E.g., PKU, diabetes. Both are heritable
through a single gene but the phenotype
differs depending on environment (diet,
insulin injection)
Quantitative Traits
• Mendel studied genes that cause qualitative
difference (yellow vs green seeds). Many
traits are quantitative such that individuals
differ by degree, not by kind.
• E.g. height, skin color, etc
Pedigree analysis