Dihybrid-Cross Analysis of Two Genes

Dihybrid Crosses

• To study the simultaneous transmission of two traits

• The ration of the crosses is 9:3:3:1 (second law, law of independent assortment)

Test Cross

is the most powerful for detecting independent assortment

Meiotic crossover

A basis for evolution

diversity enhances the chance that under changed environmental conditions some individual will be better adapted and will survive

Seed Shape Gene R (Round and Wrinkled)

• R is now called Sbe1; it produces an enzyme that converts a linear starch molecule (amylose) into a complex, branched form (amylopectin)

What do plant seeds contain for wild-type? _{(Seed Shape Gene R (Round and Wrinkled) )}

Plants with one or two wild-type copies have seeds that contain mostly amylopectin

What does seeds homozygous for the mutant allele contain? _{(Seed Shape Gene R (Round and Wrinkled) )}

Seeds homozygous for the mutant allele contain mainly amylose, which attracts water that is lost as the seeds mature, leading to the wrinkled appearance.

Seed Color Sgr (Green and Yellow Seeds)

In yellow peas, one or two alleles of the functional Sgr gene produce an enzyme involved in the breakdown of chlorophyll so that green seeds lose chlorophyll and turn yellow as they mature

What is the Sgr Mutation?

Sgr mutation prevents production of a functional enzyme; seeds homozygous for this mutation remain green due to absence of the enzyme’s activity in chlorophyll breakdown.

Sex-linked inheritance

the gene for a particular phenotype is on the X-chromosome

Indication of sex-linked genes (genes on sex-chromosomes)

Reciprocal crosses give different results

Male and female progeny and ratios are different

Consequence of heterogametic sex chromosomes

The heterogametic sex displays the genotype

(heterogametic is forming two kinds of gametic of which one produces male offspring and the other female offspring)

Pedigrees

• Pedigrees, or family trees, are a way of tracing the inheritance of traits in humans and some animals

• Standard notation is used to indicate males and females, their relationships, and the individuals who show the trait and those who do not

• The generations are indicated by Roman numerals

The characteristics Autosomal Dominant Inheritance

• Each individual who has the disease has at least one affected parent

In Autosomal Dominant Inheritance how many males and females are affected?

Males and females are affected in equal numbers

Who can transmit the allele for the disease in Autosomal Dominant Inheritance?

Either sex can transmit the disease allele

In crosses where one parent is affected and the other is not, approximately what number of offspring express the disease in Autosomal Dominant Inheritance

Approximately half the offspring express the disease

In Autosomal Dominant Inheritance what does two unaffected parent produce?

Two unaffected parent will not have any children with disease

In Autosomal Dominant Inheritance what does affected parent produce?

Two affected parents may produce unaffected children

In Autosomal Recessive Inheritance does individuals express the disease?

Individuals who have the disease are often born to parents who do not.

In Autosomal Recessive Inheritance if only one parent has the disorder what is the risk the child will have the disorder?

If only one parent has the disorder, the risk that a child will have it depends on the genotype of the other parent

In Autosomal Recessive Inheritance if both parent have the disorder will the children have it?

If both parents have the disorder, all children will have it

What is the sex ration of affected offspring in Autosomal Recessive Inheritance?

The sec ration of affected offspring is expected to be equal

In Autosomal Recessive Inheritance is the disease seen in each generations?

The disease is not usually seen in each generation, but if an affected child is produced by unaffected parents, the risk to subsequent children is 1/4

In Autosomal Recessive Inheritance if the disease is rare in the population why are children affected to an unaffected parent?

If the disease is rare in the population, unaffected parents of affected children are more likely to be related to one another

What is Hemophilia A?

Hemophilia A is an X-Linked Recessive trait

What is Hemophilia A caused by?

Hemophilia A is caused by a mutation in the factor VIII gene on the X chromosome

What dies the mutant allele in Hemophilia A produce?

The mutant allele produces a nonfunctional blood-clotting protein

Expression of X-linked recessive traits

A hallmark of recessive X-linked inheritance is the expression of the trait much more often in males than in females

X-linked recessive traits in humans

Humans have numerous X-linked recessive traits