* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Dihybrid Crosses and Linked Genes
Public health genomics wikipedia , lookup
Skewed X-inactivation wikipedia , lookup
Neocentromere wikipedia , lookup
Gene desert wikipedia , lookup
Y chromosome wikipedia , lookup
Essential gene wikipedia , lookup
Polycomb Group Proteins and Cancer wikipedia , lookup
Genome evolution wikipedia , lookup
Site-specific recombinase technology wikipedia , lookup
Gene expression programming wikipedia , lookup
Nutriepigenomics wikipedia , lookup
Minimal genome wikipedia , lookup
Ridge (biology) wikipedia , lookup
History of genetic engineering wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Dominance (genetics) wikipedia , lookup
Biology and consumer behaviour wikipedia , lookup
Epigenetics of human development wikipedia , lookup
X-inactivation wikipedia , lookup
Gene expression profiling wikipedia , lookup
Designer baby wikipedia , lookup
Genomic imprinting wikipedia , lookup
Genome (book) wikipedia , lookup
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. 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. 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. PG Pg pG pg PpGg pg PpGg Ppgg ppGg ppgg ppgg What’s the phenotypic ratio? Same as the parent combinations so it is referred to as parental phenotypes 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. Normal Normal skin, skin, normal grasshopper gait gait 47 Prune Skin, normal gait Prune skin, grasshopper gait 3 44 6 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 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 Normal skin, skin, normal grasshopper gait gait 47 Prune Skin, normal gait Prune skin, grasshopper gait 3 44 6 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) 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?