Lecture 4

Question 1

Why was Mendel able to show independent assortment

Answer 1

He succeeded by coincidence because the traits he evaluated were on different chromosomes, and they were all monogenic, so they did inherit separately but this isn’t the case for all genes, as seen in Morgans fly experiment

Question 2

Considering the rule on independent assortment, what gametes could an individual of genotype AaBb produce and in what ratios

Answer 2

1/4 chance of the following, so equal chance, due to the nature of meiosis
AB, Ab, aB, ab
Refer to slide 4

Question 3

How many different genotypes of gamete can a human produce solely via independent assortment

Answer 3

2^23, about 8 million
Why 2: because in meiosis we split up homologous chromosomes, mom and dad chromosomes so to speak, so that give us two options at every chromosome
Why 23: there are 23 chromosomes

Question 4

What’s mendels famous ratio for dihybrid crosses

Answer 4

So crossing plant individuals but only looking at 2 genes that follow independent assortment
Famous ratio is 9:3:3:1 after crossing heterozygous parents, 9 phenotypically dominant, 3 phenotypically dominant in one trait and recessive in the other, 3 the with the same situation but opposite trait, and one fully recessive, this out of 16

Question 5

Compare what happens when you do a test cross with heterozygous individuals who either have these traits on the same chromosome or different ones

Answer 5

Separate chromosomes: we can still follow the laws of independent assortment, so you’ll get a phenotypic ratio of 1:1:1:1
Same chromosome: we cannot follow the laws in independent assortment because these traits have to be inherited together, so instead well get the parental alleles, phenotypically one heterozygous dominant and the other recessive
Note: this does not account for crossing over

Question 6

What were Morgans findings where he did his dihybrid drosophila experiment

Answer 6

He crossed a heterozygous individual for 2 traits with an individual recessive for those traits (tester), and his results greatly favoured the parental phenotype, which makes sense if were saying the traits exist on the same chromosome, BUT there was also some recombinant offspring.
This meant that there was some mechanism by which sometimes the genes were on the same chromosome, sometimes they were not. This is now crossing over

Question 7

Chi-square test

Answer 7

Test we do to determine whether the differences we see in our ratios are small enough to be due to random chance (as it so often is in genetics) or if its due to poor experiementation
Formula: x2= sum of (O-E)^2 all over E
Where O is the observed value and E is the expected value

Question 8

If your critical value falls in with a P value of less than 0.05, what does that mean for you hypothesis

Answer 8

It means that you can reject the hypothesis (in this case of Mendelian independent assortment) because the odds of it being true while having those ratios are less than 5%

Question 9

If your critical value falls in with a P value of above 0.05, what does that mean for you hypothesis

Answer 9

It means you fail to reject the hypothesis because theres more than a 5% chance that it could still be accurate, even given that data

Question 10

How do you get the degree of freedom

Answer 10

Df is the number of values -1