Section 5Mendelian Genetics (The Inheritance of Many Traits)

Independent Assortment (Mendel second law)The law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the resulting gametes.

This law is dependent on the random arrangement of homologous pairs at metaphase.

Mendel Law of segregation:1- One trait is controlled by one gene.2- Each gene is found in two alleles3- During Gamete formation each allele is segregated in one gamete

A) Dominancy (Co-dominance (Complete Dominance), Incomplete Dominance)B) Multiple AllelesC) Lethal genes (Dominant Lethal, Recessive Lethal)

Exceptions to Mendel 1:

In two-trait crosses, genotypes of the parents require four letters because there is an allelic pair for each trait.

Gametes will contain one letter of each kind in every possible combination. (why?)

Crossing individuals who are heterozygous for two traits can produces four phenotypes.

The ratio of these four phenotypes will be in a 9:3:3:1.

Two-Trait Crosses

Slide Key:2 trait -- > 4 letters?

Gamete -- > 1 letter/Comb.Cross. 2 indiv. Heter. --> 9:3:3:1

Segregation and independent assortment

Independent Assortment (Mendel second law)The law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the resulting gametes.

This law is dependent on the random arrangement of homologous pairs at metaphase.

Repeated!

Independent Assortment (Mendel second law)The law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the resulting gametes.

This law is dependent on the random arrangement of homologous pairs at metaphase.

Modifying the Dihybrid RatioCombinations of two gene pairs involving two modes of

inheritance modify the 9:3:3:1 ratio

Mendel’s principle of independent assortment applies to situations in which two modes of inheritance occur

simultaneously, provided that the genes controlling each character are not linked on the same chromosome.

The probability of each phenotype arising in a cross can be determined by the forked-line method or by Punnett

square assuming that the genes under consideration undergo independent assortment.

Consider a cross involving two independently assorting genes with recessive alleles:

Aa Bb x Aa BbF2 ratios: 9/16 A_B_ 3/16 A_ bb 3/16 aa Bb 1/16 aa bb

What happens if both A and B affect the same trait? (Gene Interactions)

If both genes affect the same trait, segregation of genes is the same, but can get new phenotypic patterns

Modifying the Dihybrid Ratio

Exception to Mendel second lawA) Gene interaction.

B) Complementary Genes.C) Epistasis (dominant – Recessive).D) Recessive phenotype.E) Novel phenotype.

A. Four phenotypes, all affecting the same structure or process Ex.- Inheritance of Comb color in check

Rose, pea, walnut, and single combs

Try this!

RRWW x rrwwP1:G1: F1:

P2 (F1) x P2 (F1)P2G2:F2

B. Complementary Genes

(Recessive allele of either of two genes causes phenotype) 9/16 A_ B_ (Dominant Phenotype) 3/16 A_ bb 7/16 3/16 aa B _ (Recessive Phenotype) 1/16 aa bb

Example: Inheritance of flowers color in pea

C- Epistasis

Occurs when one gene masks (hide) the effect of another gene or when two

gene pairs complement each other such that one dominant allele is

required at each locus to express a certain phenotype.

Dominant Epistasis

• Dominant allele at one genetic locus masks the expression of the alleles of a second locusOccurs when the dominant allele at one genetic locus masks or hide the effect of the other allele at the other

locus.EX: Summer squash color

White yellow Green 12 : 3 : 1

WWYY x wwyyG1F1

WwYy x WwYyG2F2

Punnett square Analysis?W-Y-W-yy

Ww-Y-wwyy

Recessive Epistasis

• Occurs when the homozygous recessive genotype masks or suppresses expression of another gene

• Ex: Inheritance of coat color in mouse

Agouti Black albino 9 : 3 : 4

If You know that (A) is a recessive allele that masks the expression of allele (B) in the trait describing the coast color of mice, and (A) is responsible for Agouti colour, while

(B) will is responsible for the black coat colour.Find the expected ratio of the resulting phenotypes in the second filial generation

If you know that one dominant allele is enough for a triangular seedling shape in the capsule of sepherd’s purse.

Find the expected ratio of the resulting phenotypes in the second filial generation

D- Recessive Phenotype

Ex: Seed Capsule in Shepherd”s purse

15 (Triangular ) : 1 (Ovoid)

E-Novel Phenotypes

• In addition to modifying the dihybrid ratio, gene interaction may introduce new phenotypes which will be observed among heterozygotes

• Ex: Squash fruit shape

Disc (9) Sphere (6) Long (1)

If you know that the shapes of the squash fruit are: disc, sphere, and long, known that interaction between dominant and recessive alleles of the two genes results in a novel

phenotype which is the sphere shape fruitFind the expected ratio of the resulting phenotypes in the second filial generation