Lec 19

Question 1

Karyotype

Answer 1

an organized visual display of chromosomes as seen by microscopy; it may be used to identify abnormalities in chromosome # or structure

Question 2

Centromeres

Answer 2

• divide chromosomes into chromosome arms, segments of unequal length
• short arm- P arm
• long arm - Q arm

Question 3

How are chromosome shapes named?

Answer 3

based on centromere position, which determines the relative lengths of the arms

Question 4

Metacentric

Answer 4

the centromere is near the middle of the chromosome

Question 5

Submetacentric

Answer 5

the centromere is between the center and the tip

Question 6

Acrocentric

Answer 6

the centromere is close to one end

Question 7

Telocentric

Answer 7

the centromere is at the tip of the chromosome and there is no p arm

Question 8

Fluorescent in situ hybridization (FISH)

Answer 8

• uses molecular probes to detect a target sequence; sequences may be gene specific
• such probes are labeled with compounds that emit fluorescent light
• various labels that emit light of different wavelengths can be used simultaneously

Question 9

Chromosome banding

Answer 9

allows identification of each chromosome in a karyotype based on size, shape and banding pattern

Question 10

Somatic cells are…

Answer 10

diploid, two copies of each chromosome. One maternal and one paternal.

Question 11

Gamete cells are…

Answer 11

haploid, one chromosome per gamete. Maternal or paternal copy.

Question 12

Formation of haploid gametes

Answer 12

• meiosis I: reduction of ploidy

- meiosis II: similar to mitotic division without DNA duplication

Question 13

Nondisjunction

Answer 13

• the failure of chromosome or sister chromatids to properly separate during cell division. It can result in abnormalities in chromosome #. These abnormalities almost always alter the phenotype in animals, but less so in plants.

Question 14

Chromosome condensation reaches a max at

Answer 14

metaphase

Question 15

Two ways of doing FISH

Answer 15

1) multiple probes: chromosomes painting

2) Two-probe: identify position of specific genes

Question 16

Euploid

Answer 16

of chromosomes of an individual is the # of complete sets

Question 17

Aneuploid

Answer 17

if cells contain a # of chromosomes that are not euploid

Question 18

What is the result of nondisjunction in germ-line cells

Answer 18

aneuploid gametes that can produce aneuploid zygotes

Question 19

Nondisjunction in meiosis I results from…

Answer 19

• failure of homologs to separate; the gametes produced are either n+1 or n-1
• fusion of these gametes with normal (n) gametes produces trisomic 2n+1 or monosomic (2n-1) offspring

Question 20

The result of nondisjunction in meiosis II is…

Answer 20

• the failure of sister chromatids to separate normally. Among the four gametes produced, only two will be affected in this case.
• Two of the gametes will be normal; of the remaining two, one will be n+1 and the other will be n-1

Question 21

Gene dosage alteration

Answer 21

aneuploidy alters dosage of all the genes on the affected chromosome. Changes in gene dosage lead to an imbalance of gene products from the affected chromosome relative to the unaffected chromosomes.
- most animals are highly sensitive to changes in gene dosage. In contrast, plants tolerate gene dosage changes more readily.

Question 22

Only trisomies that happen frequently in humans

Answer 22

trisomies of chromosomes 13,18, and 21 are seen in newborn infants, and no autosomal monosomies are observed

Question 23

Trisomy 21

Answer 23

down syndrome

• research has identified a link between the risk of trisomy 21 & maternal age
• a small # of genes on chromosome 21 are responsible for the cognitive disabilities and heart abnormalities that are the principle symptoms in individuals affected with down syndrome.

Question 24

Down syndrome critical region (DSCR)

Answer 24

• a portion of the chromosome that can be correlated with the majority of the down syndrome symptoms
• A candidate gene called DYRK (a kinase), with a homolog known to produce dosage-sensitive learning defects in mice, makes a major contribution to down syndrome
• the gene DSCAM ( a cell adhesion molecule) has a homolog in mice and fruit flies that is associated with formation of the heart and nervous system

Question 25

Meiosis I in a trisomic cell

Answer 25

• any two chromosome combination will segregate together in meiosis I: there has to be engagement from both poles in prophase I
• 50% of the gametes produces are normal

Question 26

Turner syndrome

Answer 26

• a monosomy of the X chromosome, with no second sex chromosome (XO)
• in XO embryos, the single copy of the gene SHOX (a TF) is insufficient to direct normal development
• the haploinsufficiency of this gene plays a central role in producing the symptoms of the syndrome

Question 27

What is another way turner syndrome can come about?

Answer 27

• Mosaicism, which can develop as a result of mitotic nondisjunction early in embryogenesis
• 25-30% of cases occur in females who are mosaic, with some cell carrying:
  
  • 45 chromosomes (XO)
  • 46 chromosomes (XX)
  • 47 chromosomes (XXX)

Question 28

Uniparental disony

Answer 28

• a rare abnormality that occurs in humans. This happens when both copies of a homologous chromosome pair arise from the same parent.
• in very rare cases, the same chromosome undergoes nondisjunction in both sperm and egg. Fertilization occurs with one of the gametes contributing two copies of chromosome while the other gamete does not contribute a copy of the chromosome

Question 29

What are angelman syndrome and prader-willi syndrome caused by?

Answer 29

deletion of a small region of chromosome 15

Question 30

mechanism for uniparental disomy

Answer 30

• more often, nondisjunction occurs in one parent, resulting in a gamete with two copies of the chromosome (n+1). This unites with a normal gamete to produce a trisomic zygote
• in a process called trisomy rescue, one copy of the trisomy chromosome is randomly ejected in one of the first mitotic divisions after fertilization
• if two copies retained by the zygote are from the same parent, uniparental disomy results

Question 31

Polyploidy

Answer 31

• the presence of 3 or more sets of chromosomes in the nucleus of an organism
• it can result from duplication of chromosome sets within a species (autopolyploidy)
• it can also occur from combining the chromosomes sets of different but closely related species (allopolyploidy)
• many types of polyploidy are possible

Question 32

2 mechanisms most commonly cause autopolyploidy:

Answer 32

1. Meiotic nondisjunction leads to a diploid rather than haploid gamete
2. Mitotic nondisjunction that doubles chromosome #

• the two mechanisms can also combine

Question 33

Causees of allopolyploidy

Answer 33

• allopolyploidy carry multiple sets of chromosomes that originate in different species, resulting in infertility due to nonhomologous chromosomes that may have difficulty pairing in meiosis
• chromosome nondisjunction in these hybrids leads to cells with double the # of chromosomes so that each chromosomes has a homolog for pairing, and the hybrid is fertile

Question 34

consequences of polyploidy

Answer 34

• fruit and flower size is increased in polyploids
• hybrid vigor: more rapid growth, increased fruit and flower production, and improved resistance to disease that occurs in heterozygous progeny of inbred lines
• fertility is decreased, particularly in odd-numbered polyploids in which almost all of the resulting gametes are nonviable
• polyploid and evolution: allopolyploidy between two plants that yields an odd-numbered polyploidy will reduce fertility; self-fertilization of the hybrid can produce viable progeny