Lecture 18 Guiding Questions
What are autosomes? What are sex chromosomes? How does inheritance of genes located on sex chromosomes differ from inheritance of genes on autosomes?
-Autosomes: any chromosome that is not a sex chromosome -Sex chromosomes: a chromosome involved with determining the sex of an organism, typically one of two kinds. -When two true-breeds mate, if autosomal it would be the same outcome when doing reciprocal crosses. If X-linked, it would have different outcomes, depending on who has which genotype. See slide 1809, pg. 62.
What are some advantages of using fruit flies as a model organism in genetics?
-Short generation times -Easier to raise in the lab -Have a lot of genetic variation apparent in the phenotypes
How do you know which gametes should be designated as parental in which gametes should be designated as recombinant?
-parental would be AB or ab (no recombination) -recombinant would be aB or Ab
Describe a breeding experiments you could do to determine if two genes are linked.
1. Cross a homozygous dominant (purple flowers and round pollen) with a homozygous recessive (red flowers and oblong pollen) for two traits... PPRRxpprr=all PpRr 2. Test cross: Cross F1 with homozygous recessive... PpRrxpprr=???? 3. If expected offspring proportions do not match observed, then you can indie the two genes are located on the same chromosomes
Why would a geneticist do a reciprocal cross? What does it tell us?
A reciprocal cross can tell us whether the cross is autosomal or X-linked.
Why does a ratio of something other than 1:1:1:1 in a test cross of a double heterozygous when tracing the inheritance of two genes indicate linkage?
Because if unlinked... the offspring genotypes would be a probability of 1/4 each (1:1:1:1) but in 3:1:1:3, there are more parental genotypes than recombinant. Note: If super close, genotypes would be mostly parental... 20:1:1:20
Why do males (individuals who have XY sex chromosomes) exhibit X linked recessive diseases at a much higher frequency than females (individuals with 2X chromosomes)?
Because in a female, that other X chromosome would mask the effect of the recessive allele
Describe how TH Morgan figured out the white genes (that codes for eyecolor in fruit flies) was located in the X chromosome. To do this, trace the logic of the crosses he did.
He did two crosses. -First cross: with true-breeding red-eyed and white-eyed flies... White eyes reappeared in the next generation, but only in males. All females have red-eyes, and among the males, the ratio of red: white whites eyes is 1:1 -Second cross: with red female and white eyed male flies. Appear in both sexes in the ratio red: white of 1:1
If two genes are linked and are very close together on the same chromosome, they exhibit dependent assortment. How does dependent assortment affect the gamete genotypes, compared to independent assortment?
In dependent assortment (assuming no recombination): -Prop. of AB gametes: 1/2 -Prop of ab gametes: 1/2 -Prop of Ab gametes: 0 -Prop of aB gametes: 0 In independent assortment: -Prop. of AB gametes: 1/4 -Prop of ab gametes: 1/4 -Prop of Ab gametes: 1/4 -Prop of aB gametes: 1/4
How do we know the human X and Y chromosomes are a homologous pair?
Some sequence similarity between X and Y chromosomes (PAR 1 and XTR) -Nettie Stevens
Which parts of a homologous pair engage in crossing over?
The chiasma forms between the two inside non-sister chromatids -Also, sister chromatids cross over as well BUT it doesn't make a difference because they're identical
How do we use information on a genetic map?
The distance between adjacent genes is given in map units, which is equal to the frequency of recombination when expressed as a percentage. -Across sufficiently short distances, the map units are additive. Ex: the expected frequency of recombination between r and cv is 4.0+2.0+6.2=12.2 slide 1826, pg.66
Why is 50cM the largest distance we can calculate between two genes?
This would imply that crossover occurs in every cell and the genes would behave as if they were on separate chromosomes
How do we know how likely it is or how often that two genes are inherited together as a single unit? How do we know how far apart two genes are on a chromosome?
We use Recombinant Frequency (RF) to map relative positions of genes along a chromosome -The greater the physical distance, he greater the frequency of crossing over
Explain why linkage is a violation of independent assortment, and how that affects the resulting gamete genotypes.
When genes are on the same chromosomes, all gametes look like parental chromosomes... -Prop. of AB gametes: 1/2 -Prop of ab gametes: 1/2 -Prop of Ab gametes: 0 -Prop of aB gametes: 0
How does linkage notation work? Why do we use a different notation with linked genes?
With a slash that separates the two chromosomes where one side came from one parent while the other side came from another parent. -We use this notation to differentiate which genes came from which parent
What is the notation for 2 genes both located on the X chromosome?
X^Ab
Can two genes be linked and sex linked at the same time?
Yes, X^Ab X^aB // X^Ab or XaBY
