theme 5: modules 3 and 4 Flashcards
Genes are in a linear sequence on a chromosome…
a chromosome is a DNA molecule wound around proteins.
* They are only ever condensed like this during mitosis or meiosis
capital letters and lower case letters
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
Pedigree chart (visual representation of segregation of the specific trait of interest) legend:
- 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
What did Mendel not observe in his dihybrid crosses?
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
Recombination occurs
anytime you are making
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
Condensed chromosomes are:
DOUBLE STRANDED!
Outer chromatids…
that are not crossed over stay in their paternal or maternal form.
The ones that crossed over are called recombinant or recombined chromosomes.
Recombination occurs between chromatids on opposite, paired homologues = non-sister chromatids
2 possible scenarios:
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
What are the possible egg genotypes if: no recombination?
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
What are the possible egg genotypes if: always have recombination?
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
The probability of a crossover occurring within a region will increase:
with the increasing size of the region
or
The distance between two genes is proportional to the frequency of genetic recombination events
Linkage
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
Association
increased probability of seeing alleles of two genes or two markers together due to linkage
To map genes today:
-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)
X and Y chromosomes
*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
Colour Blindness
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)
Hemizygous
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
Haemophilia (X-linked) - blood doesn’t clot (HM -dont have it, hm - have it)
hard for girls to get cause both parents need to be carriers
will colour blindness and hemophilia genes always be inherited together
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)
High density linakge maps
identify genetic loci that are a few thousand base pairs apart
Linkage maps w/SNPs
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..))
GWAS (genome wide association study)
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
Sickle cell anemia
single nucleotide polymorphism leads to an altered red blood cell shape mutation and subsequent complications related to anemia and severe pain throughout the body
do all variations in protein coding regions of our genomes affect cell functions?
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
HIV (human immunodeficiency virus)
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
mutations in CCRF gene (gives immunity to HIV infection) and history behind it
-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
Gut Microbiomes
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
