Other Patterns of Inheritance

Exceptions to Mendel’s Rules There are exceptions to every rule, and exceptions to the exceptions. Question: What happens if one allele is not completely dominant over another? What if a gene has several alleles?

Incomplete Dominance Incomplete Dominance: when neither allele is completely dominant over the other. Results in the heterozygous phenotype being a blending of the two homozygous phenotypes. Incomplete = Blended appearance

Incomplete Dominance A capital letter represents the trait of each of the extremes: R for red and W for white. RR RWWW

Incomplete Dominance In snapdragons, red flower color (RR) is incompletely dominant to white (WW) flower color. A heterozygous snapdragon is pink (RW). Cross a pink snapdragon with a pink snapdragon.

RW x RW

Codominance both alleles are dominant! Heterozygous individual (RW) shows both alleles expressed. WW

Codominance A capital letter represents one of the codominant alleles (B for black). A different capital letter (W = white) represents the other codominant allele The two alleles

Punnett Squares Make sure to label whether your Punnett Square is Incomplete Dominance or Codominance!

Codominance In horses, gray horses (GG) are codominant to white horses (WW). The heterozygous horses(GW) is an appaloosa horse (a white horse with gray spots on the rump and loins). Cross a white horse with an appaloosa horse.

GW x WW

Codominant and Multiple Alleles The alleles for many human genes display codominant inheritance. One example is the ABO blood group, determined by a gene with three alleles: I A, I B, and i. I A and I B are both dominant blood types and therefore codominant. i is recessive

Codominant and Multiple Alleles This table shows the relationship between genotype and phenotype for the ABO blood groups. It also shows which blood types can safely be transfused into people with other blood types.

Codominant and Multiple Alleles The i allele is recessive. Individuals with alleles I A I A or I A i produce only the A antigen, making them blood type A. Those with I B I B or I B i alleles are type B. Those homozygous for the i allele (ii) produce no antigen and are said to have blood type O.

Blood Type Video _XQ _XQ

Autosomal vs. Sex Chromosomes Autosomal Chromosomes (Autosomes) – chromosomes that do not determine sex Sex Chromosomes – chromosomes that determine sex In humans – X and Y chromosomes

Sex-Linked Inheritance Know the difference between autosomal chromosomes and sex chromosomes. Understand sex-linked inheritance. Be able to create a family pedigree. Solve sex-linked problems using a Punnett square

Gender Determination in Humans Humans have 23 pairs of chromosomes. 22 of them are called autosomal. The 23rd pair are the sex chromosomes. XX = Female XY = Male

Pedigree XY XX Father Mother DaughterSon Marriage Children OldestYoungest

Red/Green Colorblindness The gene which allows us to distinguish between red and green is on the X chromosome. = Female with normal vision X C X C X C X c = Female carrier X c X c = Female who is colorblind

Red/Green Colorblindness The gene which allows us to distinguish between red and green is on the X chromosome. = Male with normal vision = Male who is colorblind X C Y X c Y

Pedigree ?? Father Mother DaughterSon XY c XX CC

Pedigree XY XX Father Mother DaughterSon c CC XX cC XY C C

Using a Punnett Square XX Cc XY c Y X c X c X C X c X C X c X c X c X C Y Y

Genotype & Phenotype XX Cc XY c Y X c X c X C X c X C X c X c X c X C Y Y Female CarrierFemale Colorblind Male NormalMale Colorblind

Hemophilia XX XY H HH H h hh h = Female normal = Female carrier = Female with hemophilia = Male normal = Male with hemophilia

Review Autosomal?? Pedigree - Male? Female? Marriage? Children? Siblings? Why do sex-linked traits affect males more frequently than females?

Warm Up4/26/12 What is the phenotypic ratio for a cross between a mom with type AB blood and a dad with type O blood? (Genotypes: _____ x ______)