10.2 Dihybrid Cross

Vocabulary Dihybrid crosses involve two genes which control two characteristics. There are complications of these patterns as illustrated in the calculations that follow. Unlinked genes are found on different chromosomes and can be segregated by random assortment of meiosis/ metaphase II Autosomal are those chromosomes other than the XY gender determining chromosomes (not sex linked).

IB Assessment Statement Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

IB ASSESSMENT STATEMENT Distinguish between autosomes and sex chromosomes.

19

10.2.2 Distinguish between autosomes and sex chromosome. In some species there is one homologous pair which carry genes that determine gender. THis work has already been covered in the section on sex chromosomes. There are other genetic conditions (also covered in topic 4) associated with the so called 'sex linked" conditions.

IB ASSESSMENT STATEMENT Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.

Crossover The diagram shows the loci of genes A and B. The alleles A is linked to B, a is linked to b. The homologous pairs are held together like this during prophase I.

Crossover The crossover point occurs above the loci for gene A and below the gene loci for A. The position at which the exchange occurs is called the chiasma. The recombinants will form between non-sister chromatids which are crossing over. The homologous pairs remain attached at the chiasma until anaphase I when they are pulled apart.

Crossover After anaphase II, the chromatids are separated. The linked genes are: A with B a with b recombinant a with B recombinant A with b

Define linkage group.

Linked Genes Any two genes that are found on the same chromosome are said to be linked. These two genes are actually on the same chromosome. The A allele is linked to the B allele along the length of DNA. The a allele is linked to the b allele along the length of DNA. Linked genes do not follow Mendel’s law of independent inheritance

IB ASSESSMENT STATEMENT Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

Recombination A recombinant refers to offspring that do NOT have the same phenotypes as either of their parents. Unless the IB questions says genotypic recombinant , then it means a different genotype than its parents.

Unlinked vs. Linked Unlinked Genes on a different chromosomes 2 genes assort into gametes into gametes independently of each other Follows expected ratios phenotype ratios 9:3:3:1 Linked Genes On the same chromosomes. Do not assort into gametes independently of each other. Unexpected phenotypic ratios

Practice A. SsYy and Ssyy B. SsYy and ssYy C. SsYy and ssyy In garden peas, the pairs of alleles coding for seed shape and seed colour are unlinked. The allele for smooth seeds (S) is dominant over the allele for wrinkled seeds (s). The allele for yellow seeds (Y) is dominant over the allele for green seeds (y). If a plant of genotype Ssyy is crossed with a plant of genotype ssYy, which offspring are recombinants? A. SsYy and Ssyy B. SsYy and ssYy C. SsYy and ssyy D. Ssyy and ssYy

RECOMBINANT ANIMATION http://bcs.whfreeman.com/thelifewire/content /chp10/1002s.swf

Practice Question How does recombination normally occur for unlinked genes? A. Crossing-over in Prophase I B. Random chromosome assortment C. Failure of spindles to form D. Random gene mutations

Practice Question Two genes A and B are linked together as shown below. If the genes are far enough apart such that crossing over between the alleles occurs occasionally, which statement is true of the gametes? A. All of the gametes will be Ab and aB. B. There will be 25% Ab, 25% aB, 25% ab and 25% AB. C. There will be approximately equal numbers of Ab and ab gametes. D. The number of Ab gametes will be greater than the number of ab gametes.

Practice Question What constitutes a linkage group? A. Genes carried on the same chromosome B. Genes whose loci are on different autosomes C. Genes controlling a polygenic characteristic D. Alleles for the inheritance of ABO blood groups

Linked Genes Practice Problems http://www.ksu.edu/biology/pob/genetics/linke d.htm

IB ASSESSMENT STATEMTN Explain an example of a cross between two linked genes.

Example of Linked Genes Using an example you have studied, explain a cross between two linked genes, including the way in which recombinants are produced.(Total 9 marks) Go to the below website to help you answer this question: http://click4biology.info/c4b/10/gene10.2.htm#5 linked genes occur on the same chromosome / chromatid;
genes (tend to be) inherited together / 
not separated / do not segregate independently;
non- Mendelian ratio / not 9:3:3:1 / 1:1:1:1; real example of two linked genes; Award [1] for each of the following examples of a
 cross between two linked genes.
key for alleles involved in the example of a cross;
homozygous parental genotypes and phenotypes shown;
F1 genotype and phenotype shown / 
double heterozygote genotype and phenotype;
possible F2 genotypes and phenotypes shown;
recombinants identified; recombinants due to crossing over;
in prophase I of meiosis;
diagram / explanation of mutual exchange of parts
of chromatids during crossing over;