Download X-Linked Dominance

Beyond Mendel
1.Thomas Hunt Morgan (1866
- 1945) and Sex linkage
2.Co-Dominance
3.Environmental Influence
•Gregor Mendel provided for the Scientific community a base upon which to build a
new branch of Biology – namely Genetics. His elegant and meticulous
experimentation breeding common peas provided a standard law on how alleles
behave during reproduction, both at meiosis when the alleles segregate, and at
fertilization when they come together to determine the phenotype of the organism.
•An Englishman, William Bateson (who was a great advocate of Mendel’s work)
started much of genetics culture. He announced 1906 that the 3rd International
conference on Hybridization was actually the 3rd International Conference on
Genetics (even though there had not been a second or first!!) and hence a new field
was born.
•New terms introduced by Bateson were, ‘zygote’ from the Greek zugotos meaning
‘yolked’ to describe the fertilized egg; homo & heterozygote and ‘allelomorph’ from
allelon (of another) and morph (form) to describe different versions of a particular
trait. This turned the new field of genetics into a coherent discipline.
•Mendel became the emotional center – easy to be a hero as little was known
about him, and what was known was admirable in the sense he was disciplined,
modest, humorous, hard working and humble.
•1909 Wilhelm Johannsen (prof of Plant physiology) at Copenhagen University in
Denmark coined the word ‘gene’. Also introduced genotype – an organisms genetic
makeup, and phenotype – an organisms appearance. Nowadays a phenotype is best
described as the expression of a gene, as not all traits can be ‘seen’ with the naked
eye and some may develop until later in life.
•There was a combination of reasons why Mendel was not recognised for his
work within his own lifetime and one of those was that not all traits behaved in
the classic manner that Mendel described. Many traits did, for example egg
shell colour in poultry, wool/coat colour in sheep, mice and rabbits, and
various seed qualities in maize and wheat.
•In contrast, there were many more traits that did not follow classical
Mendelian inheritance patterns and two examples studied fell under the
category of sex linkage and incomplete or co-dominance.
•I for one was not initially convinced of Mendel’s Laws and based many of my
early experiments on these inconsistencies. In 1907 when I was working at
Columbia University, I first met Bateson when he came to the US to speak at a
Genetics conference. By then I was beginning to piece together my
understanding of Mendelian Genetics, although this did not stop Bateson from
treating my like his Scientific enemy. He accused me of deliberately trying to
prove Mendel wrong. We were both outspoken characters and he took it upon
himself to defend Mendel at all costs. Often in a new science, studying what
appears to be a contradiction helps shine a light on the theory being proposed
as it uncovers another version. My informative and significant studies did just
that.
•Like Mendel, I focused on one organism, the fruit fly or vinegar fly Drosophila
melanogaster. They were quick and easy to breed and didn’t take up much
space. I used empty milk bottles to breed them in and fed them overripe
bananas. Males and females were easy to tell apart and only weeks after
fertilization females produced hundreds of eggs.
•Drosophila usually have red eyes – this is called the ‘wild’ type characteristic
and is kind of like the default characteristic.
•One day in 1910 I saw a white eyed male and realized I had something
important. I kept the fly in a bottle for a few days and took it everywhere with me
(some history books even state I took the white eyed fly to the hospital to visit
my wife when she was having a baby. It has been told that my wife asked about
the fly before I asked about baby!!!)
•I crossed the white eyed male with a pure breeding red eyed female and in the
F1 generation got 100% red eyed progeny. This was consistent with Mendel’s
statistics. Red eye was clearly dominant.
•When I crossed this F1 generation they produced the typical monohybrid
Mendelian ratio of 3:1 red to white in the F2 generation. If I recalculated the
statistics according to sex, I found that all the females had red eyes and only half
the males had red eyes (the other half had white eyes). So only males had white
eyes. This was the beginning of sex linkage in genes.
http://pro.corbis.com/search/searchFrame.asp
Here are the Punnett Squares for the F1 and F2 generations
Parents: Red eye female (XX) x
White eyed male (XY)
Gametes: X X & X Y
Parents: Red eye female (XX) x
Red eyed male (XY)
Gametes: X X & X Y
F1
X
X
F2
X
X
X
XX XX
X
XX
XX
Y
XY
Y
XY XY
100% Red eyes
XY
Female: 100% red eyes/Male: 50% red
eyes & 50% White eyes
I concluded that the gene for eye colour was on the X chromosome and there was no
corresponding locus on the Y chromosome. Drosophila only have 4 pairs of
chromosomes anyway – 3 pairs of autosomes and a pair of sex chromosomes. Sex
linked genes can be on the X or the Y chromosome. Would it be possible to breed a
white eyed female? If so how?
This is how a sex linked trait is conventionally written
XR X R
X R Xr
XR Xr
X
X
Xr Y
XR Y
XR Y
F1
100% Red eyes
F2
X R XR
XR Xr
Red eye
XR Y
Xr Y
White eye
3:1 Red
eyes to
white eyes
Pedigree of the Drosophila experiment
X
F1
X
Red eyed female
White eyed male
100% Red eyed female
100% Red eyed male
3:1 Mendelian
ratio of red eye
to white eye in
F2 generation
F2
All females red eyed, 50%
homozygous for red eye and
50% heterozygous for red eye
50 % of males
white eyed
50 % of males
red eyed
Males cannot be heterozygous for red eye as they only have one X chromosome.
If they have the white eye allele then they have to show a white eye phenotype.
Not long after I discovered sex linkage I set out to find more X linked genes and
then created a map for chromosome X. When a locus for a gene lies on the X
chromosome, the disease is said to be X-linked. The inheritance pattern for Xlinked inheritance differs from autosomal inheritance only because the X
chromosome has no homologous chromosome in the male, the male has an X and
a Y chromosome. Very few genes have been discovered on the Y chromosome. In
humans X linked genes include colour-blindness, muscular dystrophy and
haemophilia.
http://www.people.virginia.edu/~rjh9u/xped.html
•As with any X-linked trait, the disease is never passed from father to son.
•Males are much more likely to be affected than females. If affected males
cannot reproduce, only males will be affected.
•All affected males in a family are related through their mothers.
•Trait or disease is typically passed from an affected grandfather, through his
carrier daughters, to half of his grandsons.
X-Linked
recessive –
two copies of
the gene is
required for
expression in
the phenotype
http://www.uic.edu/classes/bms/bms655/lesson6.html
•The trait is never passed from father to son.
•All daughters of an affected male and a normal female are affected. All sons
of an affected male and a normal female are normal.
•Matings of affected females and normal males produce 1/2 the sons affected
and 1/2 the daughters affected.
•Males are usually more severely affected than females. The trait may be lethal
in males.
•In the general population, females are more likely to be affected than males,
even if the disease is not lethal in males.
X-Linked
Dominance
– only one copy
of the gene is
required for
expression in
the phenotype
http://www.uic.edu/classes/bms/bms655/lesson6.html
•As well as sex linked genes, co-dominance is another variation from Mendel’s
classic simple dominance inheritance.
•I didn’t study co-dominance but when this takes place, the heterozygous
geneotype shares the effect of its alleles so that they are both expressed in the
phenotype. The heterozygote therefore has a different phenotype to both
homozygotes.
•In some breeds of cattle, if you cross a white (WW) coat with a red (RR) coat, all
the offspring with be roan (WR). Instead of being only white or red in colour, the
coat is a mixture of both colors, and gives a patched appearance.
•In humans the ABO blood group shows co-dominant inheritance.
Here are the crosses written out fully for the F1 and F2 generations
Parents: RR (Red) x WW (white)
Parents: RW (roan) x RW (roan)
Gametes: R
Gametes: R
F1
R & W
R
W
R
F2
W & R
R
W
W
W
RW RW
R
W
RW RW
W RW WW
100% RW/Roan (note all
capital letters are used to
designate shared dominance)
RR RW
25% RR/Red; 50%
RW/Roan; 25% WW/White
•As well as sex linked genes, incomplete dominance is another variation from
Mendel’s classic simple dominance inheritance.
•I didn’t study incomplete dominance but when this takes place, the heterozygous
geneotype shares the effect of its alleles so that they are both expressed in the
phenotype. The heterozygote therefore has a different phenotype to both
homozygotes.
•In snapdragons if you cross a red (RR) petal with a white (rr) petal, all the
offspring will have pink petals (Rr). Instead of being white or red in colour, the
phenotype of the petal is a blend of both colours, which actually produces another
colour.
http://www.people.virginia.edu/~rjh9u/snapdragon.html
•It is not only the genotype that has an effect on the phenotype. The environment
can also have an effect on the appearance of an individual.
•Identical twins don’t always look identical as the environment will have some
effect on how they look.
•One classic example is the hydrangea. Depending on the pH of the soil, the
flowers change the pigments in their petal and display different colours. If the soil
is acidic then the flower appears blue and when the soil is alkaline then the flower
is pink. The flowers contain a natural pH indicator called anthocyanin which reacts
with the pH in the soil.
http://pro.corbis.com/search/searchFrame.asp
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