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BIOL 1101 > Lecture 10: Genes, Chromosomes, and Human Genetics > Flashcards

Lecture 10: Genes, Chromosomes, and Human Genetics Flashcards

1
Q

What makes you who you are

A
  • your genes; found on chromosomes
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2
Q

Modifications to Mendelian Genetics

A
  • genetic linkage
  • sex-linked genes
  • chromosomal alterations
  • non traditional patterns of inheritance
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3
Q

1) Genetic Linkage

define genes

A

genes
- sequences of nucleotides in DNA
- arranged Linearly in chromosomes (we have thousands on 1 chromosome)

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4
Q

Do all genes assort independently

A
  • not necessarily, even though in mendels work all 7 characters did
  • some genes are inherited together because they’re on the same chromosome (2 diff genes on 1 chromosome)
  • Mendel never saw this because he didn’t pick Linked genes
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5
Q

Linked Genes

A
  • genes on 1 chromosome
  • linked during transmission from parents to offspring (passing entire chromosome to gamete)
  • inherited like single genes

Break Linkage:
- prophase 1 (meiosis 1)
crossing over: enzymatic cut and pasting can break them

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6
Q

Drosophila Melanogaster

A

Fruit Fly
- model organism for animal genetics
- compared to Mendel’s Peas
- Morgan: first genetic map
- Used to test linkage and recombination (how we unlink genes)

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7
Q

How far apart genes are on a chromosome will determine

A

degree of linkage if they’re linked

  • closer 2 genes are they more linked because the chance of crossing over decreases (crossing over unlinks)
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8
Q

Gene Symbolism

A

normal alleles (wild-type)
- usually most common allele
- designated by + symbol (wild-type common)
- usually dominant

FRUIT FLIES:
wild-type:
pr+=red eyes
vg+=normal wings

mutant:
pr=purple
vg=vestigial wings

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9
Q

Genetic Recombination

how will it occur?

A
  • alleles linked on same chromosome exchange segments between homologs chromosomes
  • exchanges occur while homologs chromosomes pair during Prophase 1
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10
Q

Experimental Evidence for Gene Linkage

A

Q: do purple-eye vestigial wings (pr vg) assort independently?

A: Morgan crossed wild breed with purple vestigial wings, F1 were all wild breed (dominant), then did a testcross with F1 generation and didn’t get 1:1:1:1
- HENCE NO INDEPENDENT ASSORTMENT, they’re linked
- principle of independent recombination

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11
Q

Recombination Frequency

A
  • amount of recombination between 2 genes that reflects the distinct between them
  • the greater the distance the greater the recombination frequency
    = greater chance of crossover btwn genes
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12
Q

Linkage maps of chromosomes

A
  • recombination frequencies used to determine relative borders on a chromosome
  • linkage map for genes a,b, and c

8% recombination f=8 map units apart between gene a and c

a (—-8%) c (—2%) b
total: 9.6%, because its an underestimation (because of double crossing over) the real f combined is 10%

therefore 1 map unit=1% recombination

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13
Q

Single vs Double Crossing Over

A

single: two chromosomes exchange info at 1 point, leading to swapping of genes
( chromosome A has genes A1 and A2 and B has B1 and B2
leads to A1B2 and A2B1)

double: two different exchange events occur at 2 points on chromosome, more variety and combos
(so we can get more variation A1B1 and A2B2)

BECAUSE OF DOUBLE CROSSING OVER:
= sometimes the chromosome can look the same as it did before crossing over so we think 0 crossing over happened or less crossing over happened=underestimation

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14
Q

Can genes on the same chromosome ever assort independently

A
  • widely separated linked genes often recombine
  • seem to assort independently
  • detected by testing linkage to genes between them
  • Mendel looked at genes on separate chromosomes OR they were linked but so far apart that they behaved separately *
    = at 50+ mpu the 2 genes behaved as if they were on 2 separate chrosomes
  • hence principle of independent assortment
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15
Q

Why only 50% or 50 MPU MAX?

A
  • only half of the offspring show new genetic combos because of crossing over, refers to MAX AMOUNT OF RECOMBINATION WE CAN OBSERVE BTWN 2 GENES
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16
Q

2) Sex Linked Genes

A
  • sex chromosomes determine sex
    X and Y chromosomes in many species
    XX: woman
    XY: man
  • other chromosomes are called autosomes
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17
Q

Human Sex chromosomes

A
  • Human X chromosome
    *large=2350 genes (some determine sex, most determine nonsexual traits)
    -many X-linked genes are nonsexual traits
  • Human Y chromosomes
  • small (few genes)
  • very little homology with X chromosome
  • contains SRY gene (8 weeks post-fertilization)
  • regulates expression of genes that trigger male development (females have 0 deviation)

OG DEVELOPMENT PATHWAY
- females continue
- males deviate (SRY gene)

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18
Q

Sex Linkage

A

Female (XX) 2 copies of X linked alleles
- heterozygote recessive: allele hidden (carrier)
- homozygote recessive: trait is expressed

Male (XY): 1 copy of X-linked allele, only males have Y-linked alleles
- 1 copy of a recessive allele results in expression of the trait

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19
Q

Eye Colour In fruit flies

A

-Morgans work
- Normal wild-type: RED
- Mutant: WHITE
(sex-linked)

how was the white-eye inherited?
P: RED (female) x WHITE (male)
F1: ALL RED (therefore, white is recessive)
- follows mendels laws so far…

F2: all red-eyed females,
1/2 red eyed and 1/2 white eyed males
=3/4 red;1/4 whites
= 3:1 RATIO
- female: 1:0
- male: 1:1

RECIPROCAL CROSS
- Morgan flips it to determine if sex has any influence!

P: WHITE (female) X RED (male)
F1: Females have red eyes, and males have white eyes (therefore, its X-Linked gene)

F2: he wasn’t seeing 3:1
- ratio amongst general population doesn’t line up with sex based ratio

NEW RATIO: female: 1:1
male: 1:1
1/2 red, 1/2 white

20
Q

Human Sex-Linked genes

whats more common 4 males

A
  • X-Linked recessive traits are more common in males
    1) red-green colour blindness
    2) hemophilia: defective blood clotting protein=mutation
  • Seen more commonly because they only have 1 X, females need both Xs to match to acquire a trait *
21
Q

Pedigree

A
  • show genotypes and phenoytpe
  • X-Linked
22
Q

1 X chromosome Inactivation

A
  • DOSAGE COMPENSATION
  • in female mammals, inactivation of 1 X chromosome in each cell makes the dosage of X-linked genes the same smalls
  • Occurs during embryonic development
  • because the condensation of chromosomes makes them so tight and inactivates them, its hard for them to be expressed because condensation prevents transcription/translations *
23
Q

Barr Body

A
  • Random Inactivation of either X chromosome by condensation
  • Attached to side of nucleus
  • Copies during mitosis but always inactive
  • Results in patches of cells with different active X chromosome
24
Q

Calico Cats

A
  • Heterozygote Female (RARE to be Male)
    if cat has xxi it can be male

Dominant O gene is expression turns B off
or!
Expressed; the O is mutant recessive, B is not turned off

Creates patches of orange and black fur on cat
orange: determined by inactivation of X
black: NOT determined by inactivation of X (determined by expression of O gene)

Therefore, O gene is EPISTATIC to B gene
- turning it (B) off prevents melanin deposition hence ORANGE FUR
- expression of the O gene will ctrl expression of the B gene to either produces melanin (black patch) or turning it off (orange fur)

25
Q
  1. Chromosomal Alteration
A
  • Deletion: broken segment lost from chromosome
  • Duplication: broken segment inserted into homologs chromosome
  • on exact same chromosome
  • Translocation: broken segment attached to nonhomologus chromosome
  • can also exchange DNA by accident
  • Inversion: broken segment re-attached in reversed orientation (put in backwards)
26
Q

When does chromosomal alteration occur

A

prophase 1
- to be inherited must occur in cells giving rise to sperm and egg

27
Q

Reciprocal Translocation of CML

A

CML- cancer of leukocytes
- translocation of chromosomes 9 and 22
- nonhomologus but they still recombined

ABL gene: next to BCR gene it becomes overactive
- codes for tyrosine kinase (add PO4 to Try a.a. to target protein)
- Cells grow out of ctrl
(cell response on chromosome a, the gene is very regulated but the gene is swapped to chromosome 22 causing deregulation = increased production of Tyr Kinase)

Gleevac- drug that inhibits tyrosine kinase
Inhibits Kinase-enzyme for tyrosine

1) a piece of chromosome 9=ABL fuses with a piece of chromosome 22=BCR, which will create the gene on chromosome 22

2) BCR-ABL on chromosome 22 will code for an abnormal tyrosine kinase, its always active leading to out of ctrl growth

3) gleevac target the BCR-ABL tyrosine kinase to block abnormal signalling of out of ctrl growth

28
Q

Nondisjunction

A
  • Failure of homologs pair separation during Meiosis 1
  • Failure of Chromatid separated during Meiosis 2 (mislivision)
    (NONDISJUNCTION)
  • Cells with too many or too little chromosomes deal with the failure to properly separate into hc in meiosis 1 and sc in meiosis 2)
29
Q

What does nondisjunction produce

A

MEIOSIS 1: 4 GENETICALLY ABNORMAL CELLS
- 1/4 too many
- 1/4 too little
- 1/2 just right

MEIOSIS 2: 2 GENETICALLY ABNORMAL CELLS

30
Q

Changes in Chromosome Number

A

euploids:
- normal number of chromosome

aneuploids:
- extra or missing chromosomes in a cell

polyploids:
- extra set of chromosomes (triploids)
- spindle fails during cell division

31
Q

Aneuploids

A
  • abnormalities usually prevent embryo development
  • in animals: lethal before birth
  • in humans: 25% of miscarriages during trimester 1 occur because of this

Exception in humans is Down syndrome:
- Three copies of chromosome 21
- Physical and learning difficulties (but usually long life, caused by failure to separate hc during gamete formation more common in females)
- frequency of nondisjunction increases as women age

32
Q

Down syndrome and genetic testing

A
  • genetic testing can tell us the likelihood of this happening
33
Q

Aneuploidy of Sex Chromosome

A

OO x Y sperm: YO (NOT VIABLE)
- will die bc w/o any X chromosome you’re missing over 2300 genes that determine important characteristics

XO x X sperm: XO (TURNER SYNDROME)
- 1 X
- Undeveloped female reproductive system

XX x Y sperm: XXY (KLINEFELTER SYNDROME)
- biologically male may have undeveloped or remnants of female reproductive parts
- male calcite cats would be XXY

XX x X sperm: XXX (TRIPLE X SYNDROME)
- biologically female
- no shown symptoms because 2/3 Xs will inactivate showing no phenotype of that, only way you’d know is via karyotype

34
Q

Polyploids

A

Common in plants
- polyploidy often hardier and more successful
- source of variability in plant evolution

Uncommon in animals
- usually has lethal effects during embryonic development
- won’t even get past early developmental stages
therefore, animals have a different system compared to plants

35
Q

Modes of Inheritance

A

1) autosomal recessive inheritance
2) autosomal dominant inheritance
3) X-linked recessive inheritance
4) Nontraditional patterns of inheritance

36
Q

1) Autosomal Recessive Inheritance

A
  • Males or females carry a recessive allele on an autosome
    Ex. Cystic Fibrosis, albinism, sickle cell

Heterozygote: don’t show trait, they can put it on though
- Carrier
- No symptoms

Homozygote Recessive
- shows symptoms of trait

37
Q

2) Autosomal Dominant Inheritance

A

Therefore, if you’re homozygous dominance or heterozygous you’ll show symptoms of traits if you’re homo recessive you’ll be normal

Dominant gene is carried on an autosome

Homozygote dominant (abnormal trait) or heterozygote
- show symptoms of the trait

Homozygote Recessive
-Normal

EX. ACHONDROPLASIA- type of dwarfism
-people who don’t have it are homo recessive, so to not be a dwarf you have the 2 recessive genes

38
Q

3) X-Linked Recessive Inheritance

A

Recessive allele carried on X chromosome
*Males
- recessive allele on X chromosome
- show symptoms
*Females
- heterozygous carriers, no symptoms
- homozygous, show symptoms

Ex. Colour blindness and Hemophilia

39
Q

Genetic Counselling Techniques

A

TRYING TO DETERMINE PROBABILITY OF OFFSPRING HAVING A PARTICULAR DISORDER

Identification of parental genotypes
- construction of family pedigrees

1) Parental Diagnosis: tests cells
Cells obtained from
- embryo
- amniotic fluid around the embryo
- placenta

2) Postnatal Genetic screening
- biochemical and molecular tests

40
Q

4) Nontraditional Patterns of Inheritance

A
  • goes against Mendel’s Law (he never saw it)

a) cytoplasmic inheritance: follows the pattern of inheritance of mitochondria or chloroplasts
b) in genomic imprinting the allele is inherited from one of the parents, is expressed while the other is silent

41
Q

Cytoplasmic Inheritance

A
  • genes carried on DNA in mitochondria or chloroplasts
  • cytoplasmic inheritance follows the maternal line
    *zygotes cytoplasm originates from egg (and everything in there including mitochondrial and chloroplast DNA and organelles are transferred to the zygote *

Mutant alleles in organelle DNA
- Mendelian inheritance not followed
- Uniparental inheritance from females

42
Q

Genomic Imprinting

A

Expression of an allele is determined by the parent that contributed it
- Only 1 allele (from either father or mother) is expressed
- Other allele is turned OFF (silenced) : IMPRINTED ALLELE

Often, result of methylation of region adjacent to gene is responsible for trait

= EPIGENETICS
-LOSS OF IMPRINTING LEADS TO DISORDERS

43
Q

IgF2 gene (and genomic imprinting on mice)

A

Igf2- codes for insulin-like growth factor

Parental copy of lgF2 gene is active (expressed)

Maternal copy of lgF2 gene is imprinted (inactive)

SMALL MOUSE
a) a heterozygote inheriting a deleted lgf2 gene from the male parent develops into a small mouse
- dad gene turned off growth factor=small mouse

NORMAL MOUSE
a) heterozygote inheriting a deleted lgf2 gene from mom (because its alr imprinted and silenced)

b) homozygous for normal allele, parental allele is active and maternal allele is silenced
as long as normal allele is inherited from male parent=normal size

44
Q

evolutionary explanation of IgF2 gene on mice

A
  • Mom will feed mouse, so we imprint to prevent huge babies to ration food
  • Doesn’t affect dad
  • They can feed all offspring if smaller and have enough energy to produce more litter
45
Q

Explain the processes of Prenatal Diagnosis

A

TWO WAYS:

1) Obtaining Cells from Amniotic Fluid
- centrifuged so cells are at bottom
- these are cells from the embryo (the baby’s cells)
- amniotic fluid is around the uterus
DANGER: CAN RUPTURE SAC (WATER BROKE) YOU DON’T WANNA DISRUPT BABY’S ENVIRONMENT

2) Placenta
- the placenta is an organ produced during pregnancy to help nourish baby made with its cells

Method you do depends on the mom’s age and how far along she is

Postnatal Genetic screening will only occur if the baby is already born (series of blood tests)

46
Q

Methylation and genomic imprinting

A
  • adding methyl groups will prevent expression
  • nucleotide sequences and their modifications (methylation) can be inherited, depends on whether its mom or dads gene *
47
Q

epigenetic

A
  • takes environment into account

BIOL 1101 (15 decks)

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