Dihybrid Crosses and Gene Linkage

Topic 10.2

10.2.1: 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. Use the same steps as in monohybrid crosses to complete this problem. Test yourself: determine the genotypes and phenotypes of a dihybridhybrid cross between the F1 offspring of a wrinkled, yellow pea plant and a round, green pea plant. Placing your mouse pointer on the figure below will show the genotypes and phenotypes of the offspring of the F1 cross.

Sex Linkage explained

10.2.2: Autosomes vs sex chromosomes

Autosomes are numbered red sex chromosomes are lettered blue.

Autosomes vs sex chromosomes

10.2.3: Crossing over and exchange of alleles

Crossing over between non-sister chromatids of a homologous pair of chromosomes during prophase I can result in exchange of alleles.

Definition homologous chromosomes

10.2.4: What is a linkage group?

Meiosis and genetic variation

10.2.5: 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, as shown in Figure 5 below, 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. Note: In representing crosses with linkage, it is more common used to show alleles has vertical pairs as illustrated in the figure below.

Example of dihybrid cross with linked genes

10.2.6: Can you explain the results?

Have a look at the observed results of the dihybrid cross involving two heterozygotes shown below. The cross has produced recombinants of the parental types (i.e., parents of the parental cross). Which of the offspring are recombinants and why were they produced? Place mouse pointer on the figure to see the answer to the question.

Definition homologous chromosomes
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