Question

How to write the Cross for genes that are Linked together? for example a female heterozygous and male that is homozygous recessive. Based on the Chi square I found that there is no independent assortment of genes therefore they are linked. How can I determine correct genotype notation and the cross for linked gene ?

Answer 1

Dihybrid cross of unlinked autosomal genes:

A cross between two organisms involving two genes is called a dihybrid cross. A greater number of gamete types (four) is produced when two genes are considered. Note the genes described are carried on separate chromosomes, the genes are unlinked, and will therefore sort independently of each other during meiosis. Determine the genotypes and phenotypes of the offspring of a dihybrid cross between a homozygous, dominant round, yellow pea plant with a homozygous, recessive wrinkled,green pea plant.

Parent Cross rryy (wrinkled, yellow) X RRYY (round, green) Gametes rY Ry rY Fi generation Ry RrYY + phenotypic ratio (round, yellow) genotypic ratio RrYY F2 generation F1 Cross RrYY RrYY X Gametes RY RY ry ry RY Ry ry ry RY Ry ry rY RRYY RrYY RrYY RRYY To RY 16 16 RRYY RrYY RRYY to Rryy To Ry 16 Round, yellow Round, green Wrinkled, yellow Wrinkled, green Rryy RrYY To rryy i rryy to ry 16 RrYY RrYY rryy rryy to rY 16 16 phenotypic ratio 9 (round, yellow) :3 :3 :1 (round, (wrinkled, (wrinkled, green) yellow) green) :3 R-YY :3 rry :1 rryy genotypic ratio 9 R-Y-

Linkage group

gene 1 gene 2 gene 3 gene 4 Linkage group A set of genes at different loci on the same gene that except for crossing over tend to be inherited together. Unlinked genes are genes located on different chromosomes.

Dihybrid Cross with Linkage:

Linked genes occur on the same chromosome, therefore, tend to be inherited together (i.e., do not segregate independently). When two heterozygotes are mated in a normal dihybrid cross with independent assortment of alleles, the expected ratio in the offspring is 9:3:3:1. However in cases of dihybrid crosses involving linkage, the ratio of the offspring produced is 3:1 and only the parental types with no recombinants are observed.

X Parental cross LLRR long wings, red eyes ||rr short wings, white eyes LR linkage notation CJE LR L = long wings | = short wings R = red eyes r = white eyes — r mejosis gametes LR Fi generation L R 100% long wings, red eyes LR I R F1 Cross х r Ir long wings, red eyes long wings, red eyes mejosis gametes L R and īr L R and r - r expected F2 generation 1 L R LR 1 2 LR r long wings, red eyes r short wings, white eyes