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CBG Genetics > Genetics Lectures 11-13 Mendelian Genetics + Meiosis + Linkage > Flashcards

Genetics Lectures 11-13 Mendelian Genetics + Meiosis + Linkage Flashcards

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1
Q

what was mendels first law

A

law of segregation: when gametes are forming, the paired hereditary factors segregate equally

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2
Q

whats mendels second law

A

during gamete formation, the paired hereditary factors segregate independently: independent assortment genes are unlinked

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3
Q

for N segregating genes, how many phenotypes and genotypes in F2?

A

2^N phenotypes, 3^N genotypes

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4
Q

whats medels third law

A

the product of law probability: multiply the probabilities of independent events to get the probability of the combined event: eg. for a dihybrid cross do a combo of two monohybrids giving a 3:1 phenotypic ratio

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5
Q

what patterns are there for autosomal recessive inheritance

A

males and females equally affected. can only express trait if homozygous recessive. if parents have, then children will have. if children have but parents dont, then parents are heterozygotes. can skip generations

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6
Q

what patterns exist for X-linked dominant inheritance

A
  1. all daughters of affected male will have trait
  2. no male to male transmission
  3. sons can have trait only if mother has
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7
Q

what patterns are there for autosomal dominant traits

A
  1. equally likely in males and females
  2. male to male transmission
  3. traits dont tend to skip gens
  4. if parents have trait, children have
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8
Q

what inheritance patterns for X-linked Recessive inheritance

A
  1. more likely in males
  2. if mother has trait, all sons have
  3. no male to male transmission
  4. son of female carrier has fifty % chance of being affected
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9
Q

give an example of recessive lethal genes

A

manx phenotype in cats. homozygous recessive: lethal, heterozygote: tailless

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10
Q

give an example of incomplete/partial dominance –> what is it?
what is genotype/phenotype ratio?

A

eg. flower colour of Snapdragon plant (Antirrhinum). cross red x white –> 1:2:1 ratio of red:pink:white BLENDING of dominant and recessive allele. Heterozygote has an intermediate phenotype to parents due to haploinsufficiency? PHENOTYPE RATIO = GENOTYPE RATIO

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11
Q

give an example of codominance –> what is it?

what is genotype/phenotype ratio?

A

eg. human MN blood group.
2 alleles encode different gene products, homozygote expresses both alleles
2 forms of antigenic glycoprotein M and N on surface of RBC
3 genotypes: L^M/L^M; L^M/L^N ; L^N/L^N
3 phenotypes: L^M, L^MN, L^N
1:2:1 ratio
the recessive and dominant traits appear together, eg. in patches/splodges

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12
Q

what is the inheritance of a single gene affected by? (Two main points)

A
  1. intra - locus gene action: how alleles interact (complete/incomplete/co dominance)
  2. inter-locus gene action: no interaction, epistasis, additive gene action)
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13
Q

give an example of where two genes affect a single trait

A

skin pigmentation in corn snakes: one gene determines orange pigment and other determines black pigment. they act independently. P: OObb (orange) x ooBB (black) –> OoBb (camouflage)

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14
Q

give example of when multiple alleles exist in a popn

how do pops vary in the frequency of these alleles?

A 
human ABO blood system
a-->21%
b-16%
O-->63% recessive therefore no antigens
3 alleles, 6 combos/genotypes/4 phenotypes (AB,A,B,O)
I^A/I^B are codominant
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15
Q

what is epistasis

what is general overall result

A

an interaction between two genes in a pathway whereby one (epistatic) gene interferes with the phenotypic expression of the other hypostatic gene
result in fewer phenotypic classes than expected

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16
Q

what does a ratio totalling 16 suggest

A

dihybrid: 2 genes

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17
Q

give ratio for both dominant and recessive epistasis

A

DOMINANT: 12:3:1
RECESSIVE: 9:3:4

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18
Q

give example of dominant epistasis

A

12:3:1 fruit colour in summer squash:Curcurbita
W is dominant at its own locus and masks the expression of alleles @ a 2nd locus. W inhibits gene Y, if no W (ie.ww), Y not inhibited
pathway: white–>yellow–>green

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19
Q

give an example of polygenic inheritance
what ratio can exist?
how does it occur?
how many phenotypic classes do 2 polygenes give?

A

2 genes each with 2 alleles with additive gene action
eg. Kernal colour in wheat (F2 ranges from white to red –> 4 colours and white)
2 genes each with 2 alleles with additive gene action
each A or B adds 1 unit of colour, each a or b adds 0 units
2 polygenes give 5 phenotypic classes
1:4:6:4:1

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20
Q

define penetrance (in a popn)

A

the percentage of individuals exhibiting the phenotype associated with their allele

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21
Q

define expressivity (wrt. alleles)

A

the degree to which a given allele is expressed at the phenotypic level

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22
Q

what is variable expressivity

A

all individual express the phenotype but to different degrees

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23
Q

what is variable penetrance

A

not all individuals exhibit the phenotype associated with their allele

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24
Q

what are derivations away from the expected Mendelian phenotypic ratios caused by

A 
sex linkage
incomplete dominance
codominance
multiple alleles
lethal genes
variable expressivity
variable penetrance
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25
Q

give an example of an autosomal recessive disease

A

PKU: phenylketonuria

albinism

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26
Q

give example of autosomla dominant

A

achnodroplasia: dwarfism

huntingtons (HH is v. v rare)`

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27
Q

give an example of incomplete penetrance of an autosomal dominant trait

A

polydactyly

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28
Q

give an example of variable penetrance of an autosomal dominant trait

A

type 1 osteogenesis imperfecta: brittle bone disease
varying levels of disease include: mild short stature/>10 fractures in childhood/premature hearing loss/reduced bone density

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29
Q

outline s ymptoms of PKU

what type of disease is it, how is it caused, incident rate

A

autosomal recessive
inborn errors of the metabolism
1:10000-25000
symptoms: irreversible mental retardation, smelly odour, poor feeding, eczema, reduction in melanin: pale skin/fair hair/blue eyes
caused by a mutation in a gene encoding phenylalanine hydroxylase enzyme, so phenylalanine cant convert to tyrosine, instead get phenylpyruvate

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30
Q

what are the main outcomes of meiosis

A

reduces chrom number
diploids form haploid gametic cells which are n (haploid number of chromosomes) and c (haploid DNA content)
this is so that fertilisation can restore diploidy (2n+2c)

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31
Q

how does meiosis restore haploidy in yeast/alga/fungus

A

meiosis restores haploidy in some haploids after diploid zygote is formed

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32
Q

how are variant gametes produced?

A

due to independent assortment of non homologous chromosomes and crossing over of homologous chromosomes

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33
Q

what similarities exist between spermatogenesis and oogenesis

A

both involve meiosis

both produce 4 haploid daughter cells

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34
Q

what differences exist between spermatogenesis and oogenesis

A
  1. TIME: sperm process begins at puberty and ends at death, ovum process begins before birth and ends at menopause
  2. FORMATION: sperm: each daughter cell develops into sperm, ovum: one daughter cell produces the ovum, the 3 other polar bodies die
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35
Q

when does male/female meiosis start?

A

female meiosis starts in fetus and stops at diplotene before birth

male meiosis starts at puberty

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36
Q

what happens to menstruated oocytes

A

they continue meiosis 1 but stop in meiosis 2 unless fertilised

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37
Q 
compare meiosis and mitosis for the following:
1. occurs in which cells
2. no. divisions
3.what happens to chromosome number
4. is there an S phase?
5.does synapsis occur?
6, do crossovers occur?
7.does the centromere divide?
8. does it cause variation?
9. what is the ploidy level of daughter cell?
A

meiosis: ME mitosis:MI
1. MI: somatic ME:sex
2. MI:1 cell divides into 2 daughter cells ME: 2 cell divisions and 4 daughter cells
3. MI: maintain ME:halved
4. MI: one premiotic S phase per cell division ME: one premiotic S phase for both cell divisions
5. MI: normally no pairing of homologues ME: Prophase 1: full synapsis of homologues
6. MI: NO ME:Yes at least one per homologous pair
7. MI:centromeres divide at anaphase ME: centormeres dont divide at anaphase1 but do at anaphase 2
8. MI:NO ME:YES
9. MI:Diploid ME:Haploid

38
Q

what is the synaptonemal complex? when does it form?

A

protein lattice resembling railroad tracks and connects paired homologous chroms
mediates chromosome pairing, synapsis, and recombination
there is no SC in male Drosophila as no recombination

forms during prophase 1 in zygotene

39
Q

what happens prior to meiosis 1

A

interphase: DNA replication produces sister chromatids

40
Q

what happens overall in meiosis 1

A

separation of homologous chromosomes

41
Q

what stages are there in meiosis 1

A

Prophase (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis), Metaphase, Anaphase, Telophase

42
Q

what are the stages in prophase1, what happens? generally

A

pairing of homologous chromosomes and crossing over of non sister chromatids at chiasma
Leptotene, Zygotene, Pahcytene, Diplotene, Diakinesis)

43
Q

what happens in metaphase 1?

A

tetrads migrate to metaphase plate - after spindles formed. random alignment of tetrad

44
Q

what happens in anaphase 1

A

segregation of homologous chroms and separate by moving to opposite sides of cell

45
Q

what happens in telophase 1

A

cytoplasmic cleavage and cell division. chromosomes move to opposite sides of cell adn then cell divide

46
Q

what happens overall in meiosis 2

A

segregation of sister chromatids then cell division

47
Q

what happens generally in prophase 2

A

centromeres replicate, spindle apparatus forms

48
Q

what happens in metaphase2

A

chromosomes line up along metaphase plate in middle

49
Q

what happens in anaphase 2

A

sister chromatids separate and begin moving to opposite sides of cell

50
Q

what happens in telophase 2

A

chromosomes move to opposite sides of cell and then cell divides

51
Q

discuss karyotype of grasshoppers

A

have 8 pairs autosomal chroms and XX/XO sex chroms

2n=16 autosomes + XX/XO

52
Q

what chroms do female grasshoppers have? how many

A

2n=16+XX chromosomes therefore 18 chromosomes

53
Q

what chroms do male grasshoppers have? how many

A

2n=16 +X chromosomes therefore 17 chromosomes

54
Q

discuss sperm formation in grasshopper

A

autosomes: 2 x 8+X (primary spermatocyte), goes through meiosis 1, to form 8 and 8 + X (secondary spermatocyte) then meiosis 2 to form 2 times 8 (male determining sperm) and two of 8+X spermatozoa which are female determining

55
Q

what does the male grasshopper karyotype consist of?

A

3 large metacentrics (L1,L2,L3)
1 X chromosome acrocentrics (M4,M5,M6,M7)
1 pair short acrocentrics (S8)

56
Q

what happens in leptotene 1

A

condensing and coiling of chromatin and chromosome becoming visible
“homology search” which precedes homologous pairing

57
Q

what happens in zygotene 1

A

shortening/thickening and initial rough pairing ie. alignment of homologues
SC synaptometal complex begins to form
paired homologues called bivalents

58
Q

why is there no SC in male drosophila?

A

no recombination

59
Q

why can only females be used for Drosophila gene mapping

A

because gene mapping relies on recombination and mala drosophila dont undergo recombination

60
Q

what happens in pachytene 1

A

condensation continues
chromosomes in the homologous pair become visible
chromatids may be visible
pairing is stronger now in SC
crossing over (physical exchange of non-sister chromatids)

61
Q

define crossing over (in meiosis 1)

A

physical exchange of non- sister chromatids

62
Q

what occurs in early diplotene 1?

A

SC synaptometal complex starts to break down
repulsion of homologous chromosomes extends along the length except @ chiasmata
chromatids clearly evident

63
Q

what occurs mid diplotene 1

A

repulsion of homologues

chiasmata (interchange points)

64
Q

when does human female meiosis start and stop?

A

in foetus and stops at diplotene before birth

65
Q

what happens to menstruated oocytes

A

they continue meiosis 1 but stop in meiosis 2 unless fertilised

66
Q

what happens in diakinesis 1

A

chiasmata near the ends
at least 1 chiasma holds tetrad together
spindle formation occurs
nucleolus and nuclear envelope breaks down
two centromeres of tetrad attach to spindle

67
Q

what happens in metaphase 1

A

chromosomes of tetrad are their shortest
tetrads move onto the equator of spindle
centromeres orientated to the poles
non homologous pairs align randomly on spindle

68
Q

what happens in early anaphase 1

A

contraction of spindle fibres drags apart the homologous chromosomes
four arms of metacentric L3 visibleX chrom is undivided and moves to one of the daughter cells

69
Q

what happens in telophase 2

A

spindle plane occurs at a 90degree angle to first division

70
Q

what is the chromosomal theory of inheritance

A

parallel behaviour of chromosomes and genes they contain links the process and pattern of inheritance
independent assortment of genes on different chromosomes

71
Q

how are different meiotic products produced

A

due to random alignments of bivalents at metaphase 1 resulting in parental or recombinant genotypes

72
Q

how do you work out RF values (recombination frequency)

A

%RF = [(no. recombinants)/total number]*100

73
Q

how do you know that 2 genes are on different chromosomes (wrt. RF values)

A

Rf=50%

no preferential inheritance for parental arrangement of alleles

74
Q

what did TH Morgan discover, how did he do this

A

sex linkage
tested male offspring; akin to a testcross as male offspring only inherit mums X
so looking at genes on the X chromosome

75
Q

when does crossing over occur

A

during pachytene 1 of meiosis

76
Q

what is recombinance a function of (wrt. genes)

A

the distance between genes

77
Q

what RF values are expected for incomplete linkage

A

0-50%

usually

78
Q

define syntenic

A

when genetic loci are on same chromosome

79
Q

define non-syntenic

A

when genetic loci are on different chromosomes

80
Q

are syntenic genes linked

A

not always - syntenic genes may or may not be linked

81
Q

give an example of Y linked inheritance

A

hairy ears

82
Q

give an example of X linked inheritance

A

X - linked recessive Ichthyosis

83
Q

discuss cytoplasmic inheritance

A

mitochondrial genes and chloroplast genes are linked and show maternal inheritance

84
Q

how does recombination happen for non-syntenic genes

A

for genes on different chromosomes the random alignment of tetrads in metaphase 1 and segregation of homologous chroms results in recombination through independent assortment

85
Q

how does recombination happen in syntenic genes

A

for genes on same chromosome, crossing over of the homologous chroms during pachytene 1 (visualised as chiasmata) results in recombination

86
Q

why dont syntenic genes obey Mendels law? which law do they not obey

A

genes showing linkage (syntenic) are inherited together thus they disobey Mendels second law of independent assortment - unless crossovers occur

87
Q

why were Mendels experiments successful in showing that traits are passed from parent to offspring in a predictable way

A
  1. chose suitable organism (small,easy to grow/short gen time)
  2. pea varieties available with lots of contrasting traits (7)
  3. he could ‘cross’ pea plants (usually self fertilise, but he could cross-fertilise/pollinate varieties with contrasting traits
  4. he showed good experimental technique and scientific rigour
  5. the genetics were the simplest (discrete traits, determined by single genes, with two alleles at each locus)
88
Q

what is complementary gene action and its ratio

A 
• A pair of genes can often work together to create a specific phenotype. 
 2 different phenotypes
instead of the 4.
 must have a dominant
allele in both genes to result in
the purple flower phenotype
9:7 RATIO
89
Q

what is a proband

A

the affected individual under investigation in a human pedigree analysis

90
Q

What were Morgans main insights``

A

1st: crossing over - visualized by chiasmata
2nd: genes are arranged linearly on chromosomes

91
Q

what RF value expected for complete linkagw

A

50%

CBG Genetics (4 decks)

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