Phenotypic Ratios

How can the ratios of phenotypes that you would expect to see in the offspring produced from a dihybrid cross be calculated?

As long as you know which alleles are dominant and which are recessive. The actual numbers may vary from those expected to some extent because the process is random but the differences should not be large.

Why do the ratios observed in many dihybrid crosses differ significantly from those expected?

This is often due to linkage meaning that the genes are located on the same chromosome.

What is autosomal linkage?

When the genes that are linked are found on one of the other pairs of chromosomes.

How are linked genes inherited?

As one unit - there is no independent assortment during meiosis, unless the alleles are separated by chiasmata. They tend to be inherited together.

What are linked genes unable to undergo?

The normal random ‘shuffling‘ of alleles during meiosis, and the expected rations will not be produced in the offspring

What is the recombination frequency a measure of?

The amount of crossing over that has happened in meiosis.

What is the recombination frequency formula?

Number of recombinant offspring ÷ total number of offspring.

What does a recombination frequency of 50% indicate?

That there is no linkage, and the genes are on separate chromosomes.

What does a recombination frequency of less than 50% indicate?

That there is gene linkage and the random process of independent assortment has been hindered.

As the degree of crossing over reduces, what else reduces?

The renomination frequency.

What is the degree of crossing over determined by?

How close the genes are on a chromosome. The closer they are, the less likely they will be separated during cross over and vice versa.

How can the genes on a chromosome be mapped?

Looking at the recombination frequencies for a number of characteristics coded for by genes on the same chromosomes.

What does a recombination frequency of 1% relate to?

A distance of one map unity on a chromosome.

What is chi-squared?

A statistical test that measures the size of the difference between the results you actually get and those you expected to get. It helps you determine whether differences in the expected and observed results are significant or not.

What is the chi-squared conventionally used to test?

The null hypothesis, that states there is no significant difference between what we expect and what we observe - and that any differences we see are due to chance.

What do large chi-squared values indicate?

A statistically significant difference between the observed and expected results, and that the probability of these differences being due to chance is low.

What does it mean if the calculated x² value is greater than the critical value?

There is a significant difference between the observed and expected results, and we reject the null hypothesis.

What does it mean if the calculated x² value is less than the critical value?

We can accept the null hypothesis.

What is epistasis?

The interaction of genes at different loci.

What is gene regulation a form of?

Epistasis, with regulatory genes controlling the activity of structural genes like the lac operon. Gene interaction also occurs in biochemical pathways involving only structural genes.

Although it was originally thought that all genes were expressed independently, what is now known?

That many genes interact espistatically. It is the results of these interactions that we seen in phenotypes of living organims.

What do the characteristics of plants and animals that show continuous variation involve?

Multiple genes and epistasis occurs frequently.

What may an epistatic gene influence the activity of?

Other genes, as a result of the presence of dominant or recessive alleles.

What is recessive epistasis?

When the presence of two recessive alleles at a gene locus lead to the lack of an enzyme, for example.

What is dominant epistasis?

When a dominant allele results in a gene having an effect on another gene.

What is the colour of Labrador dogs produced as a result of?

The pigment melanin being deposited in the skin and fur.

What are the different genes that determine Labrador colour?

• One that codes for the production of the pigment and has two alleles, coding for either black or brown pigment.

• One that codes for where the pigment is deposited and has two alleles, coding for pigment to be deposited either in both the skin and fur, or just the skin.

What does the colour of a Labrador vary depending on?

Whuch alleles are present at each locus. The genes are not expressed independently and so this is an example of epistatis.