Modes of Inheritance Exceptions to Mendel’s Principles

Exceptions to Mendel’s principles: 1.Some alleles are neither dominant nor recessive. 2.Many traits are controlled by more than one gene (polygenic traits)

Incomplete Dominance  The phenotype of the heterozygote is in between those of the two homozygotes.  In other words, in heterozygotes neither allele is dominant. How to Spot these:  Look for a third phenotype that is in between in the heterozygote

red pigment  The R allele codes for a red pigment no pigment  The W allele codes for a defective enzyme that produces no pigment And Pink is Made.  The heterozygous cross will one R allele and thus produce only half of the red pigment. And Pink is Made. Incomplete Dominance

 A third (new) phenotype appears in the heterozygous condition.  Flower Color in 4 O’clocks (a type of flower0 RR = redWW= whiteRW = pink

RR = red flowerWW = white flower Ex. RR = red flower, WW = white flower. A cross between the red and white flowers pink flowers (RR x WW) yields all pink flowers (RW) RR W RW (pink) RW (pink) W RW (pink) RW (pink) Incomplete Dominance

Cross a pink flower and a white flower.  What is the genotype of the pink flower?  What is the genotype of the white flower?  Draw the punnett square.  What is the genotypic ratio (fraction or %)?  What is the phenotypic ratio (fraction or %)? Practice!

Genotypes: Pink Flower: RW White Flower: WW 2RW : 2WW 2pink : 2white R W WWWW WWRW WW RW Genotypic Ratio: Answer! Phenotypic Ratio:

Codominance  The phenotype of the heterozygote has two alleles that are expressed at the same time.  In other words, in heterozygotes neither allele is dominant. How to Spot these:  Look for a third phenotype that shows both of the homozygote traits in the heterozygote

 The heterozygous condition, both alleles are expressed equally  Sickle Cell Anemia in Humans NN = normal cells SS = sickle cells NS = some of each Codominance

 Black is dominant in roosters (B)  So is white (W)  The cross of a BB and WW results in a BW or black/white mix… however, it’s not grey Codominance

Another Similar Example… Color in Animals: partially Color in Animals: “Red” and “white” are both dominant and when crossed— both colors are partially expressed as in “roan”.

Practice! Cross an individual with sickle-cell anemia and another who is a carrier but not sick.  What is the genotype of the individual with sickle cell anemia?  What is the genotype of the carrier?  Draw the punnett square.  What is the genotypic ratio?  What is the phenotypic ratio?

Genotypes: Carrier: NS Sickle Cell: SS 2NS 2SS 2carrier 2sick Genotypic Ratio: Answer! Phenotypic Ratio: N S SSSS SS

Multiple Alleles  There are more than two alleles for a trait  Blood type in humans  Blood Types? Type A, Type B, Type AB, Type O  Blood Alleles? A, B, O (in book – I A, I B, I)

Rules for Blood Type  A and B are codominant AA = Type A BB = Type B AB = Type AB  A and B are dominant over O AO = type A BO = type B OO = type O

Practice! Cross a mother who is O with a father who is AB  What is the genotype of mother?  What is the genotype of the father?  Draw the Punnett square.  What is the genotypic ratio?  What is the phenotypic ratio?

Genotypes: Mother: OO Father: AB 2AO 2BO 2 type A 2 type B Genotypic Ratio: Answer! Phenotypic Ratio: O O ABAB AO BO AO BO

More Practice! Cross a mother who is heterozgous B with a father who is heterozygous A  What is the genotype of mother?  What is the genotype of the father?  Draw the Punnett square.  What is the genotypic ratio?  What is the phenotypic ratio?

Genotypes: Mother: BO Father: AO 1AB, 1BO,1AO, 1OO 1 type A: 1 type B: 1 type AB : 1 Type OO Genotypic Ratio: Answer! Phenotypic Ratio: A O BOBO AB OO BO AO

Polygenic traits  Traits controlled by two or more genes.  Examples: Human height Eye color Skin color