Dihybrid Crosses and Linked Genes Chapter 10 Pages 312 - 316

Grasshopper Phenotype Found on chromosome 14 Autosomal recessive Alleles: G – normal gait g – grasshopper gait To have this trait the mouse would have to be homozygous recessive (gg) for the allele. Happens with mice, bounding gait. They have an autosomal recessive phenotype.

Prune Phenotype Found on chromosome 19 Autosomal recessive Alleles P – normal skin p – prune skin To have this trait the mouse would have to be homozygous recessive (pp) for the allele. On a separate chromosome. Not linked – it is independently assorted

Same as the parent combinations so it is referred to as Dihybrid test cross Take a mouse that is heterozygous for both traits PpGg and cross it with a mouse that is homozygous recessive ppgg for both gene loci. What’s the phenotypic ratio? PG Pg pG pg PpGg pg PpGg Ppgg ppGg ppgg Draw the Punnett square on the board and complete it. Simulation of meiosis and fertilization – when meiosis occurs the chromosomes line up randomly from one another. Big B could end up with big L: or could end up with little l, that’s just part of independent assortment. This occurs between nobn-homologous chromosomes. Use FOIL to complete. You would expect 4 different phenotypes in equal proportions. Due to the 1:1:1:1 ratio it tells us that the traits are independently assorted. Same as the parent combinations so it is referred to as parental phenotypes ppgg Different to the parent combinations so are referred to as recombinant phenotypes

Lets say that a mouse breeder actually performed the test cross we just did and got the following results. 1) What do the results indicate about the loci of the gene for skin type and the gene for gait? The genes are linked 2) What was the genotype of the heterozygous normal skin, normal gait mouse? PG/pg Normal skin, normal gait Normal skin, grasshopper gait Prune Skin, normal gait Prune skin, grasshopper gait 47 6 3 44 The genes are linked PpGg but written as PG/pg – we can see that normal skin and normal gait seem to be inherited together and prune skin and grasshopper gait seem to be inherited together. So instead we tend to write which genes are more likely to be inherited together (as a package).

Why does that happen? We have already looked at inheritance of unlinked genes which are usually located on non-homologous chromosomes. Their alleles sort independently into gametes. Linked genes are located close together on a chromosome.

The combinations of their alleles on homologous chromosomes tend to stay together, but they can, on occasions, be separated by crossing over during meiosis. Genes that are closer together on a chromosome are less likely to be separated during crossing over. Genes that are further apart on a chromosome are more likely to be separated during crossing over. Yes, crossing over No crossing over

Detecting linkage. If two gene loci are linked, the outcome of the test cross can reveal that linkage. There will be four classes of offspring but the proportions of these will not be equal. Instead, there will be an excess of offspring from parental gametes and a deficiency of offspring from recombinant gametes. This is all because the location of the gene loci are close together.

Normal skin, normal gait Normal skin, grasshopper gait Prune Skin, normal gait Prune skin, grasshopper gait 47 6 3 44 You can also estimate the distance of the linked gene loci from one another. This estimate is based on the percentage of recombinant offspring. In this case: No. of recombinant phenotype offspring = 6 + 3 = 9 Total no. of offspring = 100 9/100 = 9% This 9% of recombinant offspring corresponds to the number of map units separating the two genes. So in this case the loci of the two genes are about 9 map units apart (which is roughly 9 million base pairs apart) Map units is a measure of recombination

Predicting outcomes of crosses for linked genes When a test cross is carried out with two genes that are known to be linked and are separated by a known number of map units (but fewer than 40), the outcome of the cross can be predicted. e.g. If two linked genes are separated by 8 map units, the a test cross involving these genes would produce about 8% of the recombinant type offspring and about 92% of the parental type offspring.

Identify whether each of the following statements is true or false. An organism that is heterozygous at two gene loci will produce two kinds of gametes. An offspring from a dihybrid cross of two heterozygotes has a 1 in 16 chance of showing both recessive traits. The expected outcome from a dihybrid test cross is four phenotypes in the ratio of 1 : 1 : 1 : 1 What is the difference between a parental and a recombinant gamete? No, it will produce 4 from a dihybrid cross True Parental are identical to the original allele combinations. Recombinant gametes occur from crossing over and independent assortment during meiosis.