Phenotypic Ratios 20.4

Question 1

Sex linkage

Answer 1

When a particular gene is located on the sex chromosome and the effects of this are seen in conditions like haemophilia

Question 2

Autosomal linkage

Answer 2

When linked genes are located on the other chromosomes (not the sex-linked ones) and are inherited as one unit.

Question 3

Why are linked genes inherited together?

Answer 3

Because they’re on the same chromosome, independent assortment doesn’t take place so will be inherited together. Only if they are on either end of the chiasmata will there be two alleles.

Question 4

How can linked heterozygous genes make aB and Ab

Answer 4

Because the homologous chromosomes cross over and if the genes are on either side of the chiasmata, any part below the chiasmata will break and reform to make recombinant alleles and that is how they make those combinations.

Question 5

Recombinant offspring

Answer 5

Offspring that has a different combination of alleles than either of its parents

Question 6

How do you reduce recombinant offspring?

Answer 6

If the genes are closer together, it is more likely that they won’t be separated during crossing over and will therefore produce fewer recombinant offspring.

Question 7

How to calculate the recombinant frequency

Answer 7

Number of recombinant offspring / total number of offspring

Question 8

What is the recombination frequency?

Answer 8

The amount of crossing over that has taken place in meiosis.

Question 9

What does it mean if your recomb freq is 50% or higher

Answer 9

The genes aren’t linked as crossing over has obviously occurred and independent assortment has also taken place

Question 10

What does it mean if your recomb freq is less than 50%

Answer 10

The genes will most likely be linked as the smaller the recomb freq is, the closer together the genes will be. This is because there is less chance that crossing over will separate them

Question 11

What is the chi-squared test used for?

Answer 11

Its used to compare the difference between the observed and expected ratio to see if the difference is because of chance or because of a significant reason. you will normally have to accept or reject a null hypothesis which will state that there is no significant difference so it is all up to chance.

Question 12

What is the chi-squared formula and what does it mean?

Answer 12

x^2 = sum of (O-E)^2 / E

x^2 = test statistic
O = observed 
E = expected

Question 13

What do you do once you’ve found the chi-squared value?

Answer 13

You compare the value to a table of data. You need to also find out the degrees of freedom and read across to find your critical value

Question 14

How do you calculate degrees of freedom?

Answer 14

(The number of categories/phenotypes) -1

Question 15

What if your chi-squared value is less than your critical value?

Answer 15

This means there is no significant difference so you can accept the null hypothesis

Question 16

What if your chi-squared value is more than your critical value?

Answer 16

This means there is a significant difference so you reject the null hypothesis

Question 17

What is epistasis?

Answer 17

The effect of one gene on another and how they interact at different locations on the chromosome. Gene regulation is an example of this because regulatory genes control the activity of structural genes.

Question 18

Why can we actually see phenotypes?

Answer 18

Because our genes interact epistatically in order to make the phenotype.

Question 19

Hypostatic gene

Answer 19

A gene that is affected by another gene

Question 20

Epistatic gene

Answer 20

A gene that affects the expression of other genes.

Question 21

Dominant Epistasis

Answer 21

This is when an epistatic gene has dominant alleles that code for the modification of an enzyme for example and therefore would inhibit it from binding to the next substrate in the pathway so would inhibit all the genes in the sequence.

Question 22

Recessive Epistasis

Answer 22

This is when an epistatic gene e.g one that would code for an enzyme has recessive alleles which means that enzyme can’t be coded for.