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