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MED-109 Biology II > topic 2: chromosomal inheritance > Flashcards

topic 2: chromosomal inheritance Flashcards

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1
Q

what is a gene locus?

A

the location of a particular gene on a chromosome

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2
Q

what are some characteristics of homologous chromosomes?

A
  • they pair up during meiosis I
  • they carry the same genes, but not necessarily the same alleles
  • in each pair, one chromosome is inherited from the mother and the other is inherited from the father
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3
Q

what does the chromosomal theory of inheritance state?

A
  1. genes have a specific loci (positions) on chromosomes
  2. chromosomes undergo segregation and independent assortment
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4
Q

Mendel’s laws of segregation and independent assortment depend on one thing, which is?

A

the separation of chromosomes during meiosis

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5
Q

what does the law of segregation state?

A

the two alleles for each gene separate during gamete formation

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6
Q

what does the law of independent assortment?

A

alleles of genes on non-homologous chromosomes assort independently during gamete formation

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7
Q

what is the difference in phenotypic ratios of (1) genes crossed on the same chromosome and (2) genes crossed that are on different chromosomes?

A

3:1 (dependent assortment)
9:3:3:1 (independent assortment)

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8
Q

who developed the chromosomal theory of inheritance? how did he develop this theory?

A
  • Thomas Hunt Morgan
  • his experiments with fruit flies provided that genes are located on chromosomes
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9
Q

what characteristics of fruit flies made them convenient organisms for genetic study?

A
  • they breed at high rate
  • a generation can be bred every 2 weeks
  • they have only 4 pairs of chromosomes
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10
Q

what phenotypes did Morgan observe?

A
  • wild type: (normal) phenotypes such as red eyes in fly populations
  • mutant phenotypes: alternative traits
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11
Q

what did Morgan observe in his experiment? explain his results

A
  • he mated male flies with white eyes (mutant) with females flies with red eyes (normal)
  • the F1 generation all had red eyes
  • the F2 generation had 3:1 red:white ratio but ONLY MALES had white eyes
  • he determined that the white-eyed mutant allele must be located on the X chromosome (X-linked)
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12
Q

how is the white-eyed mutant allele an X-linked trait?

A

since only males had white eyes in the F2 generation and females all had normal phenotypes, the logical explanation would be that some females were heterozygous to the condition, which means only one X chromosome carried the white eyes allele, but did not affect the overall phenotype

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13
Q

how much of the X and Y chromosomes are homologous?

A

only the ends of the Y chromosome (about 18 genes in common)

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14
Q

what is a sex-linked gene?

A

a gene located on either sex chromosome (X or Y)

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15
Q

why does “sex-linked gene” in humans usually refer to X-linked genes?

A

since the Y chromosome is much shorter and carries few genes, there are very few Y-linked genes

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16
Q

the X chromosome has about ___ genes, while the Y chromosome has ___ genes

A
  • 1100
  • 78
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17
Q

what are the requirements for a recessive sex-linked trait to be expressed (for females and males each)?

A
  • a female needs 2 copies of the allele
  • a male needs only 1 copy
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18
Q

where are sex-linked recessive disorders more common?

A

in males

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19
Q

what are examples of X-linked recessive disorders?

A
  • hemophilia
  • color blindness
  • Duchenne muscular dystrophy
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20
Q

what is an example of a Y chromosome gene?

A

the SRY gene (Sex-determining Region Y) on the Y chromosome codes for the development of the testes

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21
Q

what happens if the SRY gene in females is absent?

A

the absence of the SRY gene leads to development of gonads into overies

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22
Q

rare abnormalities in the SRY gene can lead to?

A
  • XY developing into females
  • XX developing into males
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23
Q

what are examples of Y-linked genetic disorders?

A
  • Swyer syndrome
  • XX male syndrome
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24
Q

what is Swyer syndrome?

A
  • also known as XY gonadal dysgenesis
  • mutations in SRY gene give rise to XY females with gonadal dysgenesis
  • SRY gene is essential for “maleness”, its inactivation means XY individuals that are normally male will have female characteristics
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25
Q

what is XX male syndrome?

A
  • translocation of part of the Y chromosome, carrying the SRY gene, goes to the X chromosome
  • X chromosome carries the normally male SRY gene
  • females have male characteristics
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26
Q

what is the difference between translocation and genetic recombination?

A
  • translocation: exchange of chromosomal fragments between non-homologous chromosomes (ABNORMAL)
  • genetic recombination: exchange of chromosomal fragments between homologous chromosomes (NORMAL)
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27
Q

what are the types of sex-linked genetic disorders?

A
  • X-linked (dominant)
  • X-linked (recessive)
  • Y-linked
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28
Q

what is color blindness?

A

the inability or decreased ability to see or perceive color differences

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29
Q

what is Duchenne muscular dystrophy?

A
  • progressive weakening of the muscles and loss of coordination
  • affected individuals rarely pass the age of 20 (short life span)
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30
Q

what is hemophilia?

A

a disease in which progressive bleeding is prolonged, following an injury, in the affected individual

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31
Q

what is X inactivation in female mammals?

A
  • 1 of the 2 X chromosomes in each somatic cell is randomly inactivated during embryonic development
  • the inactive X condenses into a Barr body (which lies inside the nuclear envelope)
  • in the ovaries, the Barr body chromosomes are reactivated in the cells that give rise to eggs, so every female gamete has an active X
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32
Q

what happens if a female is heterozygous for a particular gene located on the X chromosome?

A
  • she will be a mosaic for that character
  • MOSAICISM: when some somatic cells will express the phenotype of one X-linked gene and some cells will express the phenotype of the other
    (since one X is randomly inactivated)
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33
Q

what are examples of mosaicism in humans?

A
  1. hypohidrotic ectodermal dysplasia: a condition caused by a mutation that prevents the development of sweat glands
  2. heterochromia: having eyes that are different colors or color variations within the same eye
34
Q

what happens when a woman is heterozygous to hypohidrotic ectodermal dysplasia?

A

the woman would have patches of normal skin and patches of skin lacking sweat glands

35
Q

what are linked genes?

A

genes located near each other on the same chromosome, and tend to be inherited together

36
Q

what law is no longer valid when applied to linked genes?

A

Mendel’s law of independent assortment

37
Q

what was Morgan’s second experiment that determined the effect of gene linkage on inheritance of 2 characteristics?

A
  • Morgan crossed flies that differed in traits of (1) body color and (2) wing size
  • he discovered that these traits are usually inherited together in specific combinations because they are on the same chromosome
38
Q

how did Morgan’s predictions differ from the actual results of his second experiment? explain the results

A
  • he predicted that there would be 25% probability of appearance of each phenotype (according to law of independent assortment)
  • in reality, 80% had parental phenotypes while 20% had non-parental phenotypes
  • this is because the genes (G/W and g/w) were linked
    (traits were inherited together)
39
Q

since the genes in Morgan’s experiment were linked, explain how there were non-parental phenotypes in the offspring?

A
  • complete linkage of genes should have resulted in 2 phenotypes (50% GW and 50% gw)
  • however, non-parental phenotypes were also produced due to genetic recombination
40
Q

what is genetic recombination?

A
  • crossing over of non-sister chromatids of homologous chromosomes during Meiosis I
  • produces offspring with combinations of traits differing from either parent
41
Q

why did Morgan determine the linkage, in the case of the experiment, to be incomplete?

A

due to recombinant phenotypes

42
Q

what did Morgan propose regarding incomplete linkage?

A

he proposed that genetic recombination can sometimes break the physical connection between genes on the same chromosome
(unlinks/separates genes)

43
Q

Morgan’s findings conclude that parental phenotypes are the result of _____, while non-parental phenotypes are the result of _____?

A

parental - linked genes
non-parental - genetic recombination

44
Q

how is recombination frequency calculated?

A

number of offspring with recombinant (non-parental) phenotypes / total number of offspring

45
Q

the further apart the genes are, the more likely what happens?

A
  • the higher the probability they crossover, the higher chance they unlink (separate)
  • genes far apart on same chromosome are physically linked but genetically unlinked and have a recombination frequency near 50%
46
Q

what is a genetic map? what is a linkage map?

A
  • genetic map: an ordered list of the genetic loci along a particular chromosome
  • linkage map: a genetic map of a chromosome based on recombination frequencies
47
Q

1 map unit = ?

A

1 % recombination frequency

48
Q

what are the different probabilities of recombination frequency for completely, partially linked, and ulinked genes?

A
  • completely linked genes: 0% RF
  • incompletely linked genes: 0-50% RF
  • physically linked but genetically unlinked: RF close to 50%
  • unlinked genes: 50 % (law of independent assortment)
49
Q

what could large-scale chromosomal alteration lead to?

A
  • spontaneous abortions (miscarriages)
  • a variety of developmental disorders
50
Q

chromosomal abnormalities can be in either (1) chromosome _____ or (2) chromosome _____

A

(1) number
(2) structure

51
Q

what is aneuploidy?

A
  • the presence of abnormal chromosome number
  • results from the fertilization of gametes in which nondisjunction occurred
  • offspring with this condition have an abnormal number of a particular chromosome (abnormal karyotype)
52
Q

what is nondisjunction?

A

abnormal separation of homologous chromosome pairs during meiosis I OR sister chromatids during meiosis II

53
Q

describe nondisjunction in meiosis I and meiosis II

A
  • nondisjunction of homologous chromosomes in meiosis I: one gamete receives both homologous chromosomes from a pair, and another gamete receives none (0)
  • nondisjunction of sister chromatids in meiosis II: half the gametes have 1 more or 1 less chromosome, the other half are normal
54
Q

what are two types of anueploidy?

A
  • monosomy: a missing copy of a chromosome
  • trisomy: an extra copy of a chromosome
55
Q

what are monosomic and trisomic zygotes?

A
  • monosomic zygote has only 1 copy of a particular chromosome
  • trisomic zygote has 3 copies of a particular chromosome
56
Q

what is polyploidy?

A

a condition in which an organism has more than 2 complete sets of chromosomes

ex: triploidy (3n) / tetraploidy (4n)

57
Q

is polyploidy more common in plants or animals?

A

plants

58
Q

are aneuploids or polyploids more normal in appearance? which is a more serious condition and why?

A
  • polyploids are more normal in appearance
  • aneuploidy is a more serious condition since:
    polyploidy refers to whole genome duplications whereas aneuploidy refers to unbalanced losses and/or gains of individual chromosomes, or parts of chromosomes, from the basic chromosome set
59
Q

what types of changes in chromosome structure can the breakage of a chromosome lead to?

A
  1. deletion: removal of a chromosome segment
  2. duplication: repetition of a segment
  3. inversion: reversal of a segment within a chromosome
  4. translocation: exchange of segments between non-homologous chromsomes
60
Q

what is meant by a syndrome?

A

a condition characterized by a group of symptoms which consistently occur together

61
Q

what are some common types of human autosomal trisomies?

A
  • trisomy 21 (Down’s syndrome)
  • trisomy 18 (Edwards syndrome)
  • trisomy 13 (Patau syndrome)
62
Q

what are the autosomal trisomy symptoms?

A
  • birth defects
  • intellectual disability (mental retardation)
  • shortened life expectancy
63
Q

describe Down syndrome

A
  • an aneuploid condition that results from 3 copies of chromosome 21
  • its frequency increases with the age of the mother (due to the quality of her eggs)
64
Q

how can aneuploidy of sex chromosomes occur? what is the result of that?

A
  • through the non-disjunction of sex chromosomes
  • a variety of aneuploid conditions are produced such as:
  • XXY or XYY males
  • XXX, XXXX, or XO females
65
Q

what are two examples of conditions caused by aneuploidy of sex chromosomes?

A
  1. Klinefelter syndrome: XXY individuals
    - males with an extra X
    - males have female characteristics and developmental abnormalities (ex: gynecomastia)
  2. Turner syndrome (monosomy X):
    - XO females
    - sterile (infertile)
    - only known viable monosomy in humans
66
Q

what are examples of disorders caused by structurally altered chromosomes?

A

Syndrome Cri du chat:
- results from the deletion of part of chromosome 5
- children born with this syndrome are mentally retarded, have a cat-like cry, usually die in infancy or early childhood

Certain cancers are caused by translocations of chromosomes
- ex: chronic myelogenous leukemia (CML) is caused by translocations between chromosome 9 and 22

BOTH are AUTOSOMAL chromosomal abnormalities

67
Q

what is an example of a SEX chromosome structural abnormality?

A

XX male syndrome (SRY gene disorders)

68
Q

what are the 2 exceptions to standard chromosomal theory of inheritance (Mendelian genetics)?

A
  • inheritance of nuclear genes (genomic imprinting)
  • inheritance of genes located outside the nucleus (extranuclear/cytoplasmic genes ex: organellar genes)
69
Q

what is genomic imprinting?

A

silencing (inactivation) of either maternal or paternal alleles of certain genes at the beginning of development

70
Q

how is silencing of certain genes done?

A
  • it involves “stamping” with an imprint (methylation) during gamete production
  • imprinting is the result of DNA METHYLATION (adding of CH3)
  • only affects a small fraction of mammalian genes (1%)
  • most imprinted genes are critical for embryonic development
71
Q

what is an example of genomic imprinting?

A
  • genomic imprinting of the mouse Igf2 gene
  • if mouse carries the mutant Igf2 allele from the mother, it will express a normal size phenotype, but if mutant allele is inherited from father, it expressed the mutant phenotype (dwarf)
72
Q

what is Igf2?

A
  • insulin growth factor 2
  • a growth factor essential for embryonic/fetal development
  • it is imprinted (inactivated) in humans -> maternal allele is normally silenced by methylation
73
Q

what condition is related to the Igf2 allele?

A

Beckwith-Wiedemann syndrome (BWS)

74
Q

describe Beckwith-Wiedemann syndrome (BWS)?

A
  • abnormal activation of maternal Igf2 allele during egg formation/early development where double the amount of proteins are produced
  • symptoms: overgrowth and increased risk of childhood cancer
75
Q

what are extranuclear genes?

A
  • genes found in organelles of the cytoplasm
  • genes of mitochondria, chloroplasts, and other plant plastids
76
Q

how are extranuclear genes inherited and why?

A

extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg

77
Q

describe the inheritance pattern of mitochondrial disorders from affected mothers and fathers

A

(1) children of affected mothers will ALL be affected
(2) children of an affected father will be unaffected

78
Q

what do some defects in mitochondrial genes cause?

A

they prevent cells from making enough ATP (ATP synthase disorders) and result in serious neuromuscular disorders

79
Q

what are examples of neuromuscular disorders related to ATP synthase disorders?

A
  • mitochondrial myopathy
  • Leber’s hereditary optic neuropathy
80
Q

what is meant by myopathy and neuropathy?

A

myopathy = muscle deterioration

neuropathy = nerve deterioration

81
Q

what is a method that can prevent mitochondrial disorders?

A

three-person IVF procedure

82
Q

describe three-person IVF procedure

A
  • IVF with healthy embryo from “2 mothers” (3 parents)
  • 2 eggs enables women with mitochondrial DNA (mtDNA) disorders to have healthy children

MED-109 Biology II (10 decks)

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