chapter 24
explain how it is possible for a person with a translocation or an inversion to be phenotypically normal
if in the process of translocation the allele of interest remains whole, then it is possible there will be no loss of function. if, after an inversion, the alignment of the two homologous chromosomes can be achieved by forming a loop, it is possible the individual can be phenotypically normal
explain why the chances of survival are greater for a trisomy or monosomy of the sex chromosomes than for autosomes
in normal XX females, one of the X chromosomes becomes inactive and forms a Barr body, which indicates that only one functional X chromosome is needed in females, just like in males. extra X chromosomes also become Barr bodies. extra Y chromosomes do not seem to interfere with normal development, perhaps because there are so few genes on the Y chromosome
the heterozygous female...
is referred to as a carrier female
karyotype
A display of the chromosome pairs of a cell arranged by size and shape any cell in the body except red blood cells, which lack a nucleus, can be a source of chromosomes for examination
cri du chat
"cats cry" when chromosome 5 is missing an end piece small head, mentally impaired, facial abnormalities
hemophilia B
"the bleeder's disease" the affected person's blood either does not clot of clots very slowly
sex chromsomes
1 pair differ between the sexes
autosomes
22 pairs
males and females have...
23 pairs of chromosomes
describe the phenotypic ratio that is expected for a cross in which both parents have one X-linked recessive allele
50% of female offspring and 50% of male offspring would express the recessive allele
explain the nondisjunction event that would cause a Turner or Klinefelter syndrome individual
Turner syndrome and Klinefelter syndrome can be caused by nondisjunction during oogenesis at meiosis I or at meiosis II
Jacobs Syndrome
XYY males, can only result from nondisjunction during spermatogenesis a nondisjunction event during meiosis II in males - two Ys are present only during meiosis II in males taller than average, persistent acne, tend to have speech and reading problems no difference in behaviors between XYY and XY males
duplication
a chromosomal segment is repeated in the same chromosome or in a non-homologous chromosome has more than two alleles for certain traits
nondisjunction
a failure of the chromosomes or sister chromatids to separate during meiosis
X-linked
a gene on the X chromosome
Y-linked
a gene on the Y chromosome
down syndrome
a genetic disorder that occurs when an individual inherits an extra copy of chromosome 21 typically inherited from the egg, but 23% of cases acquire the extra copy from the sperm most common autosomal trisomy older women are more likely to have a child with Down syndrome - risk increases rapidly at age 40 life expectancy is over 60 years
chromosomes contai...
a long series of genes in a definite sequence
Poly-X Females
a poly-X female has more than two X chromosomes and extra Barr bodies in the nucleus females with three X chromosomes (triplo-X), have no distinctive phenotype - tend to be tall and thin some have delayed motor and language development, most are not mentally impaired some have menstrual difficulties; many menstruate regularly and are fertile offspring usually have a normal karyotype
monosomy
a zygote that is missing a chromosome (2n - 1), one type of chromosome is present in a single copy
trisomy
a zygote with an extra chromosome (2n + 1), one type of chromosome is present in three copies
hemophilia
an X-linked disorder with two common types bleed externally and internally after an injury, particularly around joints
Duchenne Muscular Dystrophy
an X-linked recessive disorder characterized by wasting away of the muscles muscle weakness intensifies until the individual is confined to a wheelchair death usually occurs around age 20 recessive allele remain in the population by passing from carrier mother to carrier daughter the absence of a protein, dystrophin, causes calcium to leak into the cell, promoting action of an enzyme that dissolves muscle fibers
barr body
an inactive X chromosome in normal XX females, one X chromosome becomes a darkly stained mass of chromatin
inv dup 15
an inverted duplication occurs in chromosome 15 poor muscle tone, mental impairment, seizures, curved spine autistic characteristics: poor speech, hand flapping, lack of eye contact
color-blind males...
are more common they only need one recessive allele to be color-blind
many different types of genes...
are on a single chromosome
color-blind females...
are rare they must receive the allele from both parents
Y chromosomes contain...
around 200 genes
colorblindness
associated with recessive alleles on the X chromosome most common is red-green 7%-10% of males are affected
during meiosis I...
both members of a homologous pair go into the same daughter cell
environmental agents...
can cause chromosomes to break apart radiation, certain organic chemicals, viruses
XBXb
carrier female with normal vision
Fragile X Syndrome
caused by an abnormal number of repeat sequences in the genome and is the most common cause of inherited mental impairment - ranges from mild learning disabilities to more severe intellectual disabilities associated with some forms of autism
XbXb
color-blind female
XbY
color-blind male
during meiosis...
crossing-over sometimes occurs between the non-sister chromatids of a tetrad
types of mutations
deletion, duplication, translocation, inversion
hemophilia A
due to the absence or minimal presence of a clotting factor, factor VIII
normal development depends on the presence of...
exactly two of each kind of chromosome
XBXB
female with normal color vision
Turner Syndrome
females with Turner syndrome only have one sex chromosome, X many have congenital heart and kidney defects most have ovarian failure and do not undergo puberty or menstruate have a normal range of intelligence, but often have a nonverbal learning disability
carrier female
has normal vision but is capable of passing the allele for color blindness
explain why linked alleles do not show independent assortment
linked allele are all on one chromosome and they tend to be inherited all together, thus they do not exhibit independent assortment
XBY
male with normal vision
Klinefelter Syndrome
males born with two X chromosomes and one Y chromosome, XXY only 25% are ever diagnosed, very subtle usually not found until age 15 speech and language delays require assisted reproduction
symptoms of fragile x syndrome
males: long face, prominent jaw, large ears, excessively flexible joints and genital abnormalities females have most of the same symptoms, but milder and present a lower frequency
chromosomal mutations
occur when breaks occur in the backbone of the DNA molecule within a chromosome
nondisjunction during _____ or _____ results in an abnormal sex chromosome number
oogenesis or spermatogenesis
X chromosomes contain...
over 2,000 genes
explain how sex-linked patterns of inheritance differ from autosomal inheritance
sex-linked alleles appear on the X chromosome and more rarely on the Y chromosome. their inheritance is dependent on the distribution of the X and Y chromosomes in males and females. recessive alleles on the X chromosome are expressed in males, while they are not in heterozygous females
X-linked traits
the allele on the X chromosome is shown as a letter attached to the X chromosome
during crossing-over...
the chromatids exchange genetic material
translocation
the exchange of chromosomal segments between two non homologous chromosomes a person who has both of the involved chromosomes has the normal amount of genetic material and is healthy, unless the chromosome exchange breaks an allele into two pieces
an abnormal sex chromosome number is...
the result of inheriting too many or too few X or Y chromosomes
during meiosis II...
the sister chromatids fail to separate and both daughter chromosomes go into the same gamete
too many chromosomes are...
tolerated better than a deficiency of chromosomes
sex-linked genes
traits controlled by genes on the sex chromosomes
explain what a distance of 10 map units tells you about the distance between two genes
two genes that are ten map units apart exhibit a 10% chance of crossing-over
symptoms of Down syndrome
upward slant to the eyes, low muscle tone, a deep crease across the center of the palm (simian line), short stature, eyelid fold, flat face, stubby fingers, wide gap between first and second toes, large and fissured tongue, round head, increased risk of cardiac defects some degree of intellectual disability, most have IQs in the mild to moderate disability range
symptoms of muscular dystrophy
waddling gait, toe walking, frequent falls, difficulty in rising, may appear as soon as the child starts to walk
inversion
when a segment of a chromosome is turned 180 degrees reversed sequences of alleles can lead to altered gene activity
deletion
when a single break causes a chromosome to lose an end piece or when two simultaneous breaks lead to the loss of an internal chromosomal segment
Williams Syndrome
when chromosome 7 loses a tiny end piece look like pixies - turned up noses, wide mouths, small chins, large ears poor academic skills excellent verbal and musical abilities
explain why a duplication on one chromosome is usually associated with a deletion on the corresponding homologous chromosome
when chromosomes break and do not rejoin, the deletion from one chromosome can be added to the corresponding homologous chromosome, creating a duplication