2) XXY were male (klinefelter) (figure 4.9)
which is opposite of what the sex balance theory would predict. All males
have at least one Y and all females have no Y's, regardless of
the number of X's. The reason is that on the Y chromosome, there is
a gene that causes an undifferentiated gonad to become a testis.
This gene is called the sex determining region Y (sry) (figure 4.10). Its mode
of action is basically to control a number of other genes that effect
the development of the sexual characteristics (figure 4.12).
In animals with XY sex determining mechanism, the X chromosome has
many loci, many that have nothing to do with sex as such. The Y is
usually smaller and possesses fewer loci that are not the same loci
as that on the X chromosome. Thus females that have the
same allele at a locus on the X chromosome are homozygous. Different
alleles would be heterozygous. Males, because they have only one X,
are hemizygous and can have
only one allele
at a locus. Because of this, one copy of a recessive allele will be
expressed in the phenotype in males.
In sex-linked inheritance, crosses are not reciprocal. The X-linked
pattern is called the criss-cross pattern of inheritance because
fathers pass the trait to daughters who pass it on to sons (figures
4.12, 4.13, and 4.15).
Sex-limited traits are traits that are autosomally inherited,
and they are expressed in one sex, but not in the other. Some
examples include sexually dimorphic plumage in birds, milk yield in mammals, antlers in deer, beards in humans (figure 5.14).
Sex-influenced traits appear in both sexes but more so in one sex
than another. Male pattern baldness in humans is an example.
The male hormone testosterone is needed for full expression of baldness.
Because of this hormone difference, the allele for baldness behaves as a dominant trait in males (expressed when heterozygous), but behaves as a recessive allele in females (must be homozygous to be expressed) (figure 5.12).
pleiotropy - a gene that causes or affects the phenotypic development
in two or more characteristics.
For example, the many symptoms that are seen in individuals who are
homozygous for cystic fibrosis - one trait is the build-up of thick
mucus in the lungs of affected individuals and a second trait is the
abnormal development of the vas deferens in males, which often leads
penetrance - an allele that produces the same effect in every individual
of the proper genotype is said to have complete penetrance. If this
is not the case, the allele is said to be incompletely penetrant )page 67).
# of individuals with correct phenotype
% penetrant = ----------------------------------------------------
# of individuals with genotype coding for phenotype
Most alleles show complete penetrance. Here are some examples of
alleles that do not.
1) the autosomal dominant allele that causes retinoblastoma
2) the autosomal dominant allele that causes polydactyly
3) the sex-linked dominant allele that causes rickets
4) the autosomal recessive allele that causes eyelessness in Drosophila
expressivity - once a gene is expressed, it may have different
degrees of expression (page 67).
We can now apply what we know about X-linked and autosomal inheritance
to determine the inheritance pattern of human traits (figure 6.2).
- eyeless in Drosophila. If it penetrates (85%) the
phenotype can range from totally eyeless to eyes barely reduced.
- polydactyly in humans. If it penetrates, it can range from
one extra finger or toe to several extra fingers or toes.
To do pedigree analysis, we must start with a family tree (figures 6.2 & 6.3
for definitions of symbols).
- We must use pedigree analysis because obviously we cannot do the
appropriate test crosses to determine inheritance pattern. This
often leads us into the situation where we can eliminate some
patterns and are left with a couple of possible patterns.
- Many times, we do not have enough progeny to make strong
statistically valid statements about the mode of inheritance.
- We must be concerned with the certainty of paternity. In many
studies in the U.S. and Europe, 15-25% of children do not have
the father of record.
Four Basic Patterns of Inheritance
Autosomal dominant inheritance -- (figure 6.5)
Autosomal recessive inheritance -- (figure 6.4)
- If the trait is rare, most matings are between heterozygotes
(affected) and homozygous (unaffected) recessives, which leads
to predominantly 1:1 phenotypic ratios for all children,
regardless of sex
- the dominant phenotype should appear in every generation,
unless there is reduced penetrance
Sex-linked dominance -- (figure 6.9)
- generations are often, but not always, skipped
- there should be equal distribution among the sexes
- if both parents are affected, all children will be affected
- often found in consanguineous marriages
- most affected children have normal parents
Sex-linked recessive -- (figures 6.7 & 6.9)
- no generations are skipped
- affected males must come from affected mothers
- all the daughters, but none of the sons, of an affected father
- approximately half of an affected female's sons and daughters
- males are most affected
- affected males have carrier mothers, who are known to have
affected brothers, fathers, or maternal uncles
- affected females come from affected fathers and carrier mothers
- affected female's sons must be affected
- approximately half the sons of carrier females should be affected
The stainable material in a chromosome is called chromatin. In a G1
stage-chromosome, it consists of a single strand of DNA, plus several
types of protein. There are two such types of chromatin.
There are two kinds of
- euchromatin is uncoiled or uncondensed during interphase
- heterochromatin remains coiled or condensed during interphase
and is replicated late during the S phase.
In males, the single X chromosome is euchromatic. In females, one of
the two X chromosomes becomes condensed heterochromatin on the 16th
day of embryonic development. In the interphase nucleus, this appears
as a dark spot near the edge of the nucleus.
- facultative heterochromatin - this heterochromatin may
revert to euchromatin depending upon the physiological or
developmental conditions of the cell. An example of this
is X-inactivation due to the formation of a Barr-body (figure 4.16).
- constitutive heterochromatin - is permanently condensed and
never euchromatic. The centromeric region is constitutive
heterochromatin, but there may be other regions as well.
A person will have n-1 Barr bodies where n equals the number of X
chromosomes (table 4.2). Thus. . .
The result of X-inactivation is to produce a mosaic of phenotypes
within females that are heterozygous at an X-linked locus. The
inactivated X remains inactivated through all subsequent mitotic
events within that cell line.
- a normal female (XX) will have one Barr body
- a normal male (XY) will have no Barr bodies
- a Turner female (XO) will have no Barr bodies
- a Klinefelter male (XXY) will have one Barr body
In human beings, females can be heterozygous for the enzyme
glucose-6-phosphate dehydrogenase. Skin cells taken from these
individuals can be grown in tissue culture, where each cell grows into
a cell line that expresses only one form (allele) of glucose-6-phosphate dehydrogenase. This also
occurs in cats, where the locus for coat color is found on the X
chromosomes. Males, because they are hemizygous, are either black or
orange. Females that are heterozygous the black and tan alleles produce a calico coat color
because of random X-inactivation during embryonic development (figure
4.17). Males that are calico, which are quite rare, always turn out
to be XXY and are thus heterozygous at the coat color locus and
produce the same pattern as females.
Last update on 31 January 2007
Provide comments to Dwight Moore at email@example.com
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