theme 5: modules 3 and 4

Question 1

Genes are in a linear sequence on a chromosome…

Answer 1

a chromosome is a DNA molecule wound around proteins.
* They are only ever condensed like this during mitosis or meiosis

Question 2

capital letters and lower case letters

Answer 2

Capital letter are dominant alleles
Small letters are recessive alleles

Genes: Alleles:
Dystrophin D and d
HPRT1 H and h
Colour-blindness CB and cb
Haemophilia HM and hm

Question 3

Pedigree chart (visual representation of segregation of the specific trait of interest) legend:

Answer 3

• circles represent females
• squares represent males
• a line between a male and a female represents a mating couple
• 2 lines between a male and female means that the male and female mating couple are related

Question 4

What did Mendel not observe in his dihybrid crosses?

Answer 4

recombinant versions of genotypes (eg. Seed shape and seed colour)

• He was lucky! Because he was
  looking at genes on completely different chromosomes
  -Remember, recombination only occurs between homologous chromosomes during prophase I

Question 5

Recombination occurs
anytime you are making

Answer 5

germ (egg or sperm) cells

End result: the haploid germ cells,have the potential to have one of the 2 non-recombined X chromosomes (the paternal or maternal chromosomes), or one of the 2 recombinant chromosomes that are produced as a result of crossing over

Question 6

Condensed chromosomes are:

Answer 6

DOUBLE STRANDED!

Question 7

Outer chromatids…

Answer 7

that are not crossed over stay in their paternal or maternal form.

The ones that crossed over are called recombinant or recombined chromosomes.

Question 8

Recombination occurs between chromatids on opposite, paired homologues = non-sister chromatids

2 possible scenarios:

Answer 8

A: 2 genes are far apart

B: 2 genes are very close together (alleles of these genes are more likely to be inherited together in parental form) - linked

Question 9

What are the possible egg genotypes if: no recombination?

Answer 9

Mother’s genotype (remember, she has two X-chromosomes!): D CB / d cb

If there is no recombination between the chromosomes that carry genes D and CB during meiosis I that occurs during egg formation:

-Genotype is D CB because alleles are always together = linked
-d cb because alleles are always together = linked

Question 10

What are the possible egg genotypes if: always have recombination?

Answer 10

If there is always recombination between the chromosomes that carry genes D and CB during meiosis I that occurs during egg formation:

-D and CB alleles are separated = unlinked
-d and cb alleles are separated = unlinked

• Still produce eggs with Maternal/parental genotype: D CB or d cb
• But also have eggs with
  recombinant genotype: D cb or d CB

Question 11

The probability of a crossover occurring within a region will increase:

Answer 11

with the increasing size of the region

or

The distance between two genes is proportional to the frequency of genetic recombination events

Question 12

Linkage

Answer 12

physical proximity along a chromosome that makes recombination less likely

Linked genes sit close together on
chromosome, so that two alleles on one chromosome more likely to be inherited together

Question 13

Association

Answer 13

increased probability of seeing alleles of two genes or two markers together due to linkage

Question 14

To map genes today:

Answer 14

-we can sequence the DNA…
This matches well with relative distances you get using recombination frequencies for linkage mapping (can also show order of genes along the chromosome)

Question 15

X and Y chromosomes

Answer 15

*tips of X and Y chromosomes share a small region of homology (can pair and segregate like homologous chromosomes)

Y: 78 genes that code for 25 proteins (half of which determine sex)
X: 1000 genes, other functions not related to sex determination

Question 16

Colour Blindness

Answer 16

X-linked trait is recessive, meaning women heterozygous for the allele will not show the phenotype, homozygous women will be colour blind

-heterozygous mother is still a carrier and can pass irtdown to offspring

Male that receives the recessive colour blindness allele from his mother will be colour blind (since males will only have one locus for this allele as they have one X chromosome)

Question 17

Hemizygous

Answer 17

rule of dominance and recessiveness no longer apply (male will show phenotype of the one allele they carry on their X-chromosome

50% chance of passing on color blindness allele if you have it

Question 18

Haemophilia (X-linked) - blood doesn’t clot (HM -dont have it, hm - have it)

Answer 18

hard for girls to get cause both parents need to be carriers

Question 19

will colour blindness and hemophilia genes always be inherited together

Answer 19

No, linkage can be broken
-linkage between neighbouring genes is broken during prophase of meiosis when all homologous chromosomes line up
-what were once linked genes are now inherited independently in subsequent offspring (as a result, some offspring may inherit only one of the genes due to this recombination)

Question 20

High density linakge maps

Answer 20

identify genetic loci that are a few thousand base pairs apart

Question 21

Linkage maps w/SNPs

Answer 21

used to map human genomes that determine various characteristics.
-we look at frequency of recombination to determine relative distance between 2 genetic loci, whether they are genes, markers or both.

(SNPs allow to look at unrelated individuals in populations to map the location of genes (looks for genes contributing to disease..))

Question 22

GWAS (genome wide association study)

Answer 22

looks across entire SNP linkage map for association between a particular phenotype and a mapped SNP

ex: HMGA2 gene - demonstrating association between height and a particular marker allele
-ppl w/ 2 C alleles are 0.8 cm taller than ppl w/ 2 T alleles
-ppl heterozygous for C and T alleles are only 0.4cm taller than ppl with two C alleles

Question 23

Sickle cell anemia

Answer 23

single nucleotide polymorphism leads to an altered red blood cell shape mutation and subsequent complications related to anemia and severe pain throughout the body

Question 24

do all variations in protein coding regions of our genomes affect cell functions?

Answer 24

No. (ex ABO blood typing system)

A and B: alleles code for a specific glucanotransferase enzyme that catalyzes the formation of specific A or B agglutinogens that are expressed on the cell surface. Several SNP polymorphisms that lead to formation of slightly diff transferases

O: encodes inactive glycosyltransferase

2 allele copies of the ABO genes lead to our specific blood type

Question 25

HIV (human immunodeficiency virus)

Answer 25

virus that invades T-cells by interacting with two proteins on the surface of the cell - the T-cell surface CD4 receptor and the CCR5 co-receptor.

Once attached the virus is engulfed by the cell and infection begins, leading to the death of the T-cell and compromised immune system

Question 26

mutations in CCRF gene (gives immunity to HIV infection) and history behind it

Answer 26

-a 32 base pair deletion within the CCR5 gene. it is not in a sequence of 3 thus it shifts the reading frame such that a stop codon terminates translation early and the result is production of a partial and inactive CCR5 protein.

-in Europe it conferred resistance to bubonic plague and small pox

Question 27

Gut Microbiomes

Answer 27

differ in each region and every person that help produce vitamins, enzyme and disease susceptibility

-associated w/recent dietary patterns
-responding to rapid changes in environment and diet