Dihybrid (or greater) Crosses: Review

For either genotype or phenotype, the expected outcomes of a particular cross can be calculated by multiplying the probabilities of the individual outcomes.

Crosses: Review

For example, the ratios of an F1 resulting from the mating of a homozygous dominant (AA) to a homozygous recessive (aa) are

Phenotype = 3:1 dominant : recessive

Genotype = 1:2:1 AA:Aa:aa

Crosses: Review

The probabilities of all expected outcomes must add up to 1.

So, if the ratio is 3:1, the probabilities of the two phenotypes are 0.75 or ¾ and 0.25 or ¼

For the genotypes, when the ratio is 1:2:1, the probabilities are 0.25 or ¼ AA, 0.5 or 2/4 Aa and 0.25 or ¼ aa

Crosses: Review

If two traits are both inherited according to Mendelian principles, one can calculate the probabilities of all the individual outcomes for either phenotype or genotype.

If two heterozygotes are crossed in a dihybrid cross

AaBb x AaBb

Crosses: Review

You can either make a Punnett Square and count them all up, or you can multiply the probabilities of the individual probabilities.

Punnett Square of Dihybrid Cross: F2

AB Ab aB ab

AB

Ab

aB

ab

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBB AaBb aaBB aaBb

AaBb Aabb aaBb aabb

Crosses: Review

You can calculate the expected phenotype ratio

Dominant at both: ¾ x ¾ = 9/16

1st locus dominant, 2nd recessive: ¾ x ¼ = 3/16

1st recessive, 2nd dominant: ¼ x ¾ = 3/16

Recessive at both: ¼ x ¼ = 1/16

So the phenotype ratio is 9:3:3:1

Crosses: Review

How are the genotypes of the 9 dominant phenotypes assorted among heterozygous and homozygous?

Crosses: Review

Of the 9 dominant phenotypes,

¼ x ¼ = 1/16 will be homozygous dominant (AABB)

2/4 x 2/4 = 4/16 will be heterozygous dominant (AaBb)

¼ x 2/4 = 2/16 will be AABb

2/4 x ¼ = 2/16 will be AaBB

Extensions of Mendelian Genetics

Designations

Genotype is determined by the presence of alleles at a given locus.

Each individual can have only two alleles, one on one homologous chromosome and a second at the same locus on the homologous chromosome.

DesignationsIf an allele is dominant, the second allele may be indicated

by a dash, A-, which indicates that the second allele may be another dominant allele or a recessive allele that does not contribute to the phenotype.

So, going back to our original dihybrid cross, the results would be

9 A-/B-

3 A-/bb

3 aa/B-

1 aa/bb

Designations

Wild-type is used to indicate the normal state, and is often designated as a superscript wt or +.

A loss of function mutation may be designated with a superscript -.

Remember the Ames test? The strain of Salmonella used was his-, indicating a mutation in the histidine biosynthesis pathway that resulted in the inability to synthesize histidine. The Ames test looks for reverse mutation to the wild type, his+.

Designations

By convention, genes are referred to in italics, while gene products (proteins) are written in plain text.

For example, a gene that has been associated with breast cancer in women is BRCA1, and the protein it encodes is BRCA1.

Mendel

Mendel was careful to focus on traits that were influenced by a single locus and for which there were only two alleles possible at that locus, e.g., plants either inherited the tall phenotype (DD or Dd) or the dwarf phenotype (dd).

Only one locus influenced plant height, and the only two alleles D and d, were possible.

Gene InteractionSeveral genes may influence a particular

characteristic. 1. One gene may affect another. 2. Cellular function of numerous gene products

contribute to the development of the common phenotype (e.g. combine to form a structure of specific size, shape, color, texture, activity)

Think of development as a process of many steps, or a cascade of events

Epigenesis

Epigenesis Epigenesis is the idea that development occurs as new structures appear and grow.

This is in contrast to the idea of preformation, which holds that development is simply a continuation of growth of structures that were already present in the egg (nothing new is formed in the offspring)

Gene Interaction

EpistasisEpistasis: Expression of genes at one locus may affect expression of genes at another locus.

For example, one gene or gene pair may modify or mask the expression of a second gene or gene pair at another locus.

Epistasis is derived from the Greek word “to stop”

EpistasisThe homozygous presence of a recessive allele may

prevent or override the expression of other alleles at a second locus.

In this case, the alleles at the first locus are epistatic epistatic to those at the second locus, and the alleles at the second locus are hypostatichypostatic to those at the first allele.

(To help you remember, the locus that is epistatic does the stopping.)

Coat color in mice

Coat color options:

1. Agouti: a grayish coat characterized by bands of black and white pigment on individual hairs

2. Black

3. White or albino

Coat color in mice

Agouti is the dominant to black (non-agouti), which results from homozygous expression of a mutation, a.

Therefore, if a mouse is A- (AA or Aa), the color will be agouti, while an aa mouse is black.

Coat color in mice

Homozygous inheritance of a recessive mutation bb, at a second locus eliminates pigment formation altogether and results in albino mice, regardless of genotype at the a locus.

The presence of at least one B allele allows pigment to form.

Therefore, the B locus is epistatic to the A locus, and the A locus is hypostatic to the B locus.

Coat color in mice

Pigment formation follows a pathway like this:

Gene B Gene A

Precursor Black pigment Agouti

(colorless) B- A- pattern

Incomplete Dominance

Incomplete or partial dominanceIncomplete or partial dominance exists when one allele does not have clear dominance over the other.

The phenotype of the heterozygote is intermediate between the homozygous of either allele.

For example, mating homozygous white snapdragons to homozygous red snapdragons yields F1 offspring with pink flowers.

Incomplete Dominance

In this example, the gene dosage for red in the F1 plants is half that of the homozygous red parent, and half as much gene product is made in the offspring.

In the F1, the red gene is present, so some red pigment is made (but not as much as in the red parent, which has two red genes) and the flowers are pink.

Codominance

When to alleles of a single gene encode two distinct and detectable gene products, the joint expression of both alleles in the heterozygote is called

codominancecodominance.

Multiple Alleles

There may be several alleles possible at a given locus.

An individual only inherits two, one from each parent.

When three or more alleles are availaible, they can only be studied in populations.