Mendelian Genetics, part II

From Section 11-3, 11-4, 14-1

Beyond simple dominant and

recessive alleles (other possibilities out there

in the real world).

The real world of genetics

• Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete dominance

• Incomplete dominance – neither allele is completely dominant over the other.

• Therefore, in this case, the heterozygote’s phenotype is in between the two homozygous phenotypes (intermediate phenotype).

Incomplete dominance - snapdragons

Incomplete dominance

?

?Allow these to self-pollinate

Incomplete dominance,

cont.A brown horse bred with a white horse produces…tan-colored foals

Incomplete dominance

• Make a Punnett square and give the genotypic and phenotypic ratios for a cross between two heterozygotes for a trait with incomplete dominance.

• (B allele for black fur, b allele for white fur)

Example: incomplete dominance in fur color

BB = Black FurBb = Gray FurBb = White Fur

Parents: Bb x BbBoth gray

B b

B BB Bb

b Bb bb

B b

B BB-Black Bb-Gray

b Bb-Gray bb-White

The heterozygous phenotype is in between the homozygous phenotypes. The alleles are incompletely dominant.

Codominance

• In codominance, both alleles contribute to the phenotype (neither allele is completely dominant nor completely hidden by the other-it’s a combination).

Codominance in Shorthorn cattle

white

roan

red×

Codominance in Rhododendrons

Codominance

• Make a Punnett square and give the genotypic and phenotypic ratios for a cross between two heterozygotes for a trait with Codominance.

• G allele for green leaves, g allele for yellow leaves)

Example: Codominance in leaf color

GG = GreenGg = BOTH green and yellowgg = yellow

Parents: Gg x GgBoth green and yellow leaves

G g

G GG Gg

g Gg gg

G g

G GG-Green Gg-yellow & green

g Gg- yellow and green

gg- yellow

The heterozygous phenotype Expresses both the green and yellow leaf color of the homozygotes. The alleles are codominant.

Multiple Alleles

When there are more than two alleles for a gene the trait is said to have multiple alleles.

Multiple Alleles

• Human blood type is controlled by multiple alleles.

• There are 3 alleles:

IA IB i

codominant

Recessive to

Two different things are going on—codominance and dominance.

• What are the possible combinations of the three alleles and what blood types result from each combination?

• IA IB i

Genotypes and Phenotypes for ABO blood groups (alleles IA,

IB, i)

• IA IA

• IA i• IB IB

• IB i• IA IB

• ii

• Type A blood

• Type B blood

• Type AB blood

• Type O blood

Genotypes

Phenotypes

Polygenic Traits

• Polygenic traits are those traits that are controlled by two or more genes (poly=many, genic=genes).

• Polygenic traits often show a wide range of phenotypes.

Multiple Gene (Polygenic) Inheritance of skin color

Gametes

 ABC  ABc

  AbC 

 Abc 

 aBC 

 aBc 

 abC 

 abc 

ABC 6 5 5 4 5 4 4 3

ABc 5 4 4 3 4 3 3 2

AbC 5 4 4 3 4 3 3 2

Abc 4 3 3 2 3 2 2 1

aBC 5 4 4 3 4 3 3 2

aBc 4 3 3 2 3 2 2 1

abC 4 3 3 2 3 2 2 1

abc 3 2 2 1 2 1 1 0

Table 1. Polygenic inheritance in people showing a cross between two mulatto parents (AaBbCc x AaBbCc). The offspring contain seven different shades of skin color based on the number of capital letters in each genotype.

Parent 1 genotype: AaBbCc

Parent 2 genotype:

AaBbCc

Each capital letter represents an allele for dark skin, each lower-case letter represents an allele for light skin. Count the number of capital letters each possible child has to determine the how dark or light their skin would be.

Sex Chromosomes

• Two of the 46 chromosomes in humans are known as the sex chromosomes.

• The remaining 44 chromosomes (22 pairs) are known as autosomes.

X and Y chromosomes

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