BIOL 226 MIDTERM

Question 1

Define and explain what P0 is

Answer 1

P0 is the parental cross
will be homozygotic for traits
known as true breeding

Question 2

Define and explain F1

Answer 2

First generation of offspring from P0, will be all heterozygotic
shows the dominant phenotypes
phenotypes will not equal genotypes

F1 generation suggests dominance

Question 3

Define and explain F2

Answer 3

Second generation of offspring/ hybrids
will show all possible phenotypes and can infer all genotypes

F2 generation shows evidence for the independent segregation of alleles

Question 4

What is a test cross?

Answer 4

Used to determine an unknown genotype by crossing with a homozygous recessive

genotypic and phenotypic frequencies will be identical

Question 5

Define and explain the first mendelian law

Answer 5

The law of segregation –> alleles of a gene separate independently from each other during transmission from parent to offspring
In a basic monohybrid cross the dominant phenotype will appear 100% of the time in the F1
F2 will show a 3:1 ratio for phenotype and a 1:2:1 ratio for genotype

P0:(AAxaa) = F1(all Aa) F2(1AA, 2Aa, 1aa)

In a dihybrid cross of F1 will show 9331 in F2

Question 6

Probability of independent events rule

Answer 6

Multiplicitive rule
If a and b are are independent p(a and B) = Pa x Pb

Additive rule
if a and b are independent probability of only one occurring p(a or b) is
Pa + Pb -(Pa xPb)

However, if two events do NOT overlap they are mutually exclusive
Pa x Pb = 0 and P(a or b) = Pa + Pb

Question 7

Define hetero and homozygote

Answer 7

Heterozygote –> two different alleles at the same locus
Homozygote –> two copies of the same allele at the same locus

Question 8

Define and explain the second mendelian law

Answer 8

Law of independent assortment
Alleles of two or more genes (loci) segregate independently during transmission from parent to offspring
the two dominant phenotypes appear at 100% in F1
In F2, 4 phenotypes are present —> two parental, two recombinant
expected frequency of phenotypes is 9331

ex) if YyRr x YyRr would have 3/4 Y, 1/4y , 3/4R, 1/4r = 3/4Y x 3/4R = 9/16 YR

Question 9

Explain how to calculate the # of phenotypes for nth hybrid crosses

Answer 9

dihybrid - 2 loci = 2^2 =4 –> 9:3:3:1
trihybrid - 3 loci = 2^3 =8 –> 27:9:9:9:3:3:3:1

Question 10

How many gametes (egg/sperm = haploid) can form from x genotype?

Answer 10

total possible allele combinations = 2^n where n= number of segregating loci

segregating being key term here, if homozygous for any allele that allele effectively doesn’t segregate because there is only one possible outcome

Question 11

wtf is a chi^2

Answer 11

Value testing to determine if data is consistent
The sum of observed - expected^2 / expected
Then take the df (#of categories -1)
If the p value is larger,than 0.05 accept the null hypothesis, the data is not strange and makes sense, there is no significant difference
However if p value is smaller than 0.05 then there IS a significant difference and the data cannot be explained by the null hypothesis (like cannot be explained by mendelian cross etc/)

Question 12

Explain sex ratios in drosophila

Answer 12

Sex determination is based on the ratio of x chromosomes to sets of autosomal (A) chromosomes
if X/A = 1 then female
if X/A = 0.5 then male
In between 1 and 0.5 is intersex
greater than one is metafemale
less than 0.5 is meta male

Question 13

How can you tell if a trait is x linked recessive?

Answer 13

Nearly all affected people are male
all his daughters are carriers, no sons will be affected

Carrier females are phenotypically normal but heterozygous for the allele
half her sons are affected
half her daughters are carriers

all sons of an affected female will be affected

not always seen in every generation

Question 14

How can you tell if a trait is x linked dominant?

Answer 14

affected males transmit the trait to all of their daughters but none of their sons

affected heterozygous females transmit the trait to half of their children regardless of sex (like autosomal dominant)

affected homozygous females transmit the trait to all of their children

because females can be hetero or homo, more females than males will have the trait

unaffected individuals cannot be carriers

Question 15

define Karyotype

Answer 15

A collection of chromosomes with specific number and structure of a species / individual in a condensed state

Question 16

Define diploid

Answer 16

2n
normal for most eukaryotes

Question 17

Define Monoploid

Answer 17

1n
the number of unique chromosomes in a set

Question 18

Define Haploid

Answer 18

1n
only one set of chromosomes - no homologous chromosomes
male bees, gametes

Question 19

Define Euploid

Answer 19

having a ‘normal’ number of each chromosome

Question 20

Define polyploid

Answer 20

having extra copies of each chromosome
common in plants and certain tissues
causes larger cells and increases cellular metabolism
Triploid = 3n

Only possible because gene dosage is preserved
Plants look the same but polyploid are bigger - same number of cells, but each cell is bigger
Lowers chances of inbreeding risk - buffered against deleterious alleles
Lessen selection on individual gene copies
Associated with asexual reproduction

Some tissues
Liver
Heart
bone marrow

Question 21

Define allo and autopolyplopid

Answer 21

Allopolyploid – mixing two uneven genomes via hybridization
combination of alloploidy and chromosome loss or gain creates new species
Usually sterile

Autopolyploid – multiplication of chromosomes from the same organism
Bigger plants = bigger fruits

Question 22

Define Aneuploid

Answer 22

does not equal 2n – abnormal number of one or more chromosomes but not all
Caused by nondisjunction - homologs don’t separate

Monosomic –> one missing chromosome - 2n-1
XO sex determination – bees, bats, snails
All lethal in humans except XO (turner syndrome)

Trisomic – one extra chromosome compared to wold type - 2n+1
Most are lethal in humans except
Kleinfelter syndrome (XXY)
Jacob syndrome (XYY)
Trisomy X (XXX)
Trisomy 21 - Down syndrome

Question 23

Define nondisjunction

Answer 23

Gametes have an incorrect number of chromosomes (either carrying both homologs or none)
Can happen in meiosis1 or 2
Rare and often lethal

Caused by lack of crossing over in meiosis 1
Caused by defect in chromosome cohesion in meiosis 2 – sister chromatids don’t separate

Question 24

What is the difference between intraallelic and interallelic effects?

Answer 24

Intraallelic effects are the affects of different alleles on the proteins produced
Interallelic affects are how different proteins interact

Question 25

Define pseudodominance

Answer 25

If chromosome deletion removes haplosufficient genes, any homologous haploinsufficient genes will have a phenotypic effect
Pseudodominance is when recessive alleles uncovered by the deletion will appear dominant in pedigree

Question 26

Define hypomorphic

Answer 26

An allele in the middle of the dominance hierarchy of multiple allels
Has partial function

Question 27

Define incomplete dominance

Answer 27

Appearance of a third phenotype that blends the parental phenotypes
no clear dominance in the heterozygote
new phenotype not present in parents
Non mendelian relationship
skin pigmentation in humans
intraallelic

Question 28

Define codominance

Answer 28

More than one allele is dominant
Heterozygote displays both parental phenotypes
variegation
Non mendelian relationship
found in ABO blood type system
intraallelic

Question 29

Define pleiotropy

Answer 29

one allele that affects two or more phenotypes

Example of coat color in mice, expected ratio is 3 yellow - 1 brown (yellow dominant over WT) but is actually 2-1 because AyAy is lethal, because gene codes for both fur color and lipid metabolism
This resulted due to chimerism where a piece of another gene was merged with the WT coat color gene

Question 30

Explain variable penetrance and expressivity

Answer 30

Variable penetrance is when individuals with the same genotype might not always express the phenotype, same genotype, different phenotypes
Polydactyly - 50-80% of people with dominant mutant allele develop malformations
Determined by modifier genes, environmental factors and complex interactions
All or none

Variable expressivity is the degree or intensity with which a genotype is expressed – all individuals express the mutation but the ‘severity’ is variable
ex Marfan syndrome
All or some

Can be both
All, some, or none

Question 31

Define polygenic inheritance

Answer 31

Where many genes / alleles affect the same phenotype
Interallelic affect

Question 32

Explain the complementation test

Answer 32

crossing homozygous recessive mutants together to hide mutations and show wild type
Used to determine which alleles causing the phenotype are on the same gene

If cross between two homozygous recessive mutants does not result in the WT and F1 are parental then there is no complementation, meaning the mutations are on the same gene – monogenic, allelic mutation

opposite will be polygenic, F1 is WT, complementation, nonallelic mutation

Often deals with the lack of production of substrates or enzymes in an earlier step

Question 33

Define additive gene action

Answer 33

alleles of two genes generate four phenotypes in a single trait
2 loci, 1 trait, 4 phenotypes

ex a 2 colour snake, can have both pigments present, one or the other, or neither

Cannot be codominance because there is four phenotypes not 3
Non mendelian

Still produces 9331 ratio in phenotypes, but only deals with one trait instead of 2 (dihybrid)

Question 34

Define complimentary gene action

Answer 34

2 loci, 1 trait, 2 phenotypes
homozygosity in mutations of either gene will lead to an identical mutant phenotype
Has a modified phenotypic ratio of 9:7

Question 35

Define redundant / duplicate gene action

Answer 35

2 loci, 1 trait, 2 phenotypes
Only two genes
Mutant only appears ion double homozygotes aabb 1/16
Dominant alleles of both genes overpower recessive alleles
modified ratio of 15:1
called pseudoalleles

Overlapping roles for two genes which independently can make up for the absence of function of the other

Question 36

Define epistasis

Answer 36

the phenotype of a gene is masked by alleles of a separate gene
One gene overrides the phenotypic effect of an allele of another gene when both are present in the same phenotype

The phenotype that is present over the other is epistatic to the other
the phenotype that is hidden is hypostatic to the other

interallelic
different locus (unlike standard dominance which is at the same locus)

Recessive epistasis – when the recessive allele of one gene masks the effect of the second gene (either dominant or recessive)

Dominant epistasis – when the dominant allele of one gene masks the effect of the second gene (dominant or recessive)

Question 37

Explain recessive epistasis - supplementary interaction

Answer 37

a recessive allele of a gene overpowers the effect of an allele of a second locus
2 loci, 1 trait, 3 phenotypes
Modified ratio of 9:3:4

Any genotype with a homozygous recessive epistatic gene shows the epistatic phenotype but a dominant copy of epistatic gene allows the phenotype of the hypostatic gene to appear – in coat color, think of the double recessive allele as blocking any pigment at all, resulting in yellow, but if its not double recessive than the other two colours can show through

Question 38

Explain dominant epistasis

Answer 38

The dominant allele of one gene hides the effect of the other gene
2 loci, 1 trait, 3 phenotypes
Modified ratio of 12:3:1

Question 39

What is crossing over and where does it occur?

Answer 39

Crossing over is the cellular mechanism that generates recombination
Chiasmata are the sites of crossing over
Happens in the prophase of meiosis 1
half parental gametes, half recombinant

Question 40

What is recombination frequency?

Answer 40

The distance between two markers on a chromosome is directly related to how often crossover events occur between them
= the percentage of offspring that is recombinant

Better to use a test cross, less of an underestimation of recombination

Max Rf is 50%, because the likelihood for crossing over is 50%
Min RF is 0%

For finding the trihybrid RF, least common genotypes are recombinant double crossover, the gene that is ‘switched’ compared to the parental(F1) will be the middle gene, because of this, multiply the RF X2 for the gene pair that DOES NOT include the middle gene

**Genes further apart are more likely to recombine

Question 41

Quick way to tell if genes are linked or unlinked?

Answer 41

Look at which non parental genotypes are common, then look for the two genes that “move” together

Question 42

What are Barr Bodies?

Answer 42

dense chromatin that acts as a marker for silenced X chromosome in cells, have to have equal gene dosage across sex chromosomes, for humans, male X cells is always active, whereas one of the X in female cells will be silenced so both sexes contain the same amount of X chromosome –> bc they are already compensating, makes trisomies or monosomies less deleterious in sex chromosomes

SRY gene –> without SRY female is default

Question 43

when is multiplication rule used, when is addition?

Answer 43

Multiplication for independent events

Addition for mutually exclusive events

Question 44

Define syntenic genes

Answer 44

syntenic genes are genes on the same chromosome
Linked genes are always syntenic
Max rf of syntenic genes is 50%, but they can be further apart

Question 45

Define sister chromatids

Answer 45

Have identical copies
Same genes in same order
Same alleles in same order

Question 46

Define homologous chromosomes

Answer 46

Same chromosomes from different parents
Have genes in same order
Homozygous - same alleles in same order
Heterozygous - different alleles in same order

Note that an allele is a different version of the same gene

Question 47

How does a deletion occur

Answer 47

Repair errors after breakage

Question 48

How does a translocation occur?

Answer 48

Caused by a recombination mistake
Crossover between repeated sequences that are not homologous

3 kinds
Non viable - lacks correct number of centromeres or arrangement of telomeres - nonfunctional
Balanced - reciprocal - does not change gene or centromere number can often go undetected
Unbalanced - non reciprocal - change in gene or centromere mumber

Question 49

How does gene duplication occur?

Answer 49

Naturally, creates duplicates for evolution to act on

Question 50

What happens when genes are inverted?

Answer 50

Depends on breakpoint of inverted section — if genes are truncated or not

Inversions can distrupt chromosome pairing in meiosis