Incomplete Dominance

• One allele is not completely dominant over another

Incomplete Dominance

• A flower has genes for white petals (w) and red petals (r).  When you mix white flowers with red flowers you get pink flowers.  How is this possible?  The w gene does not dominate over the r gene and vice versa. Both genes express themselves equally so you get a pink color in the petals. 

w w

r wr wr

r wr wr

Codominance

• Both alleles contribute to the phenotype

Polygenetic Traits

• Traits controlled by two or more genes

Incomplete and Co-Dominance Punnett Squares

Incomplete Dominance

• RR x WW where RR is red and WW is white

R R

W

W

Incomplete Dominance

• RR x WW where RR is red and WW is white• Offspring RW are pink

R R

W

W

RW RW

RW RW

Co-Dominance

• RR x WW where RR is red and WW is white

R R

W

W

Co-Dominance

• RR x WW where RR is red and WW is white• Offspring RW show red and white: ex. Red Roan

Horse

R R

W

W

RW RW

RW RW

Karyotypes and Sex-Linked Traits

• There are traits that are controlled by one gene with 2 alleles. Often, one is dominant and the other is recessive

• Example:

widow’s peaks and dimples.

Some traits are controlled by a gene with multiple alleles – 3 or more for a single

trait.

For example: blood types and skin color in humans.

• There are 44 chromosomes that we call autosomal chromosomes.

• However, there are 2 chromosomes that determine our sex and we call them sex chromosomes.

• These 46 chromosomes all carrier genes on them that determine our traits.

• Out of our 23 pairs of chromosomes, 1 pair is the sex chromosomes (X and Y).

• Female = XX

• Male = XY

• Question: What is the probability that your parents will have a boy or girl?

• XY (dad) x XX (mom)

X Y

X

X

XX XY

XX XY

Phenotype:

50% boy

50% girl

Genotype:

50% XX

50% XY

Question?

If my parents have 5 boys in a row, what is the chance they will have a girl the next time?

50%

Sex-linked gene:• Some traits are carried on the sex

chromosomes. Genes on the X or Y chromosomes are sex-linked genes.

• These traits are passes on from parent to child. Sex- linked genes can be recessive or dominant.

• MALES are more likely to have a sex-linked trait because they only have ONE X and Y. The allele is USUALLY on the X chromosome.

• Ex. colorblindness, hemophilia, hairy ears, muscular dystrophy

Are you colorblind?

What numbers do you see?

Carrier – person who has one recessive allele and one dominant allele for a trait or heterozygous for that trait (only women can be carriers).

Example

Hemophiliac carrier XHXh

Colorblind carrier XBXb

Sex linked Punnett Squares:Question: What is the probability that a carrier female and a

colorblind male will have a girl who is colorblind (b = colorblind, B = normal)?

YXb

XB

Xb

XBXb

XbXb XbY

XBY

Phenotype:

25% normal boy

25% colorblind boy

25% normal girl

25% colorblind girl

Try this one on your own

Question:

What is the probability that a homozygous (normal vision) female and a colorblind male will have a girl who is colorblind (b = colorblind, B = normal)?

XB

XB

XBXb

XBXb XBY

XBY

Xb YPhenotype:

50% normal girls

50% normal boys

Parents: XBXB x XbY