Types of Dominance

Complete Dominance Genetic crosses that result in the dominant phenotype being shown due to a dominant gene being present. Ex: Tallness is dominant. Crossing a homozygous dominant pea plant (TT) with a homozygous recessive pea plant (tt) results in F1 offspring all having a dominant gene (Tt) These problems use the same letter (T) T T Tt Tt t Tt Tt

Incomplete Dominance When some alleles are neither dominant nor recessive. One allele is not “stronger” than the other. Phenotypes are blended together. Uses TWO letters Crossing a red flower (RR) with a white flower (WW) creates pink flowers (RW). R R RW RW W RW RW

Co-dominance When both of the phenotypes are visibly seen. Phenotypes do NOT blend, they are both seen. Uses TWO letters Ex: In a breed of chickens, the allele for black feathers is co-dominant with the allele for white feathers. Black (BB) father and White (WW) mother produced Black AND white (speckled) baby chicks B B BW BW W BW BW

Example 1 In mice, the two most common colors seen are brown (BB) and white (WW). How would you cross a brown mouse with a white mouse to get a cream-colored mouse? What type of dominance is this? What is the probability of getting a cream colored mouse from this cross?

Example 2 In certain breeds of salmon, there are fish with the “A” allele for scales (Aa) and without scales (aa). When you cross the two breeds, some fish end up with scales, some don’t. What type of dominance? What’s the probability of getting a fish with scales?

Example 3 What type of dominance is this? You have identified a new species of grasshopper that come in two different colors: ones with red stripes (RR) and ones with yellow stripes (YY). When you cross them you get a grasshoppers with red and yellow stripes. What type of dominance is this? What will the offspring look like?

Complete a Punnett Square for this cross Example 4 A species of chicken’s color is determined by CODOMINANT genes. A Heterozygous (BW) chicken is crossed with a Homozygous (BB) chicken: Complete a Punnett Square for this cross What color is the Heterozygous (BW) parent chicken? What color is the Homozygous (BB) chicken? What is the probability that a black chick will be born? What is the probability that a white chick will be born? What is the probability that a black AND white chick will be born? What is the probability that a GREY chick will be born? What is the genotypic ratio for this cross? What is the phenotypic ratio for this cross?

Complete a Punnett Square for this cross Example 5 A species of flower’s color is determined by INCOMPLETELY DOMINANT genes. R is for Red and W is for White. A Heterozygous (RW) flower is crossed with a Homozygous (RR) flower: Complete a Punnett Square for this cross What color is the Heterozygous parent flower? What color is the Homozygous (RR) parent flower? What is the probability that a Red flower will be made? What is the probability that a White flower will be made? What is the probability that a Red AND White flower will be made? What is the probability that a PINK flower will be made? What is the genotypic ratio for this cross? What is the phenotypic ratio for this cross?

Complex Inheritance Polygenic Traits Multiple Alleles Sex-Linked Genes

Multiple Alleles A gene (characteristic) with more than two alleles is said to have multiple alleles. Ex: coat color in rabbits, blood types. Most characteristics we’ve dealt with so far have two alleles (R and r). Multiple Alleles have more than two (ex: A, B, O in blood types). ***There can be multiple alleles for a certain trait, but each individual STILL ONLY HAS 2  ONE FROM MOM, ONE FROM DAD.****

Polygenic Traits A trait that is controlled by two or more genes (each with two alleles) Ex: AaBbCc Polygenic inheritance usually shows up as a range of variation such has height, skin color or eye color.

Human Blood Types

Human Blood Types A B AB O There are 4 different blood phenotypes 3 alleles are in the pool instead of just two (A, B, O) You still only get ONE from Mom and ONE from Dad.

Genotypes for blood type ALLELE CODES FOR IA Type A Blood IB Type B Blood i Type O Blood Type O is the recessive blood type, which is why it gets a lowercase (i).

Genotypes & Phenotypes of Blood IAIA Type A - Homozygous IAi Type A - Heterozygous IAIB Type AB - Heterozygous IBIB Type B - Homozygous IBi Type B - Heterozygous ii Type O – Homozygous recessive

Example Problem: IB i IAIB IAi IA Dad is homozygous for Type A blood. Mom is heterozygous for Type B blood. Do a Punnett Square to find out the offspring. What are the new genotypes? Phenotypes? IAIB Type AB IAi Type A IAIB Type AB IB i IAIB IAi IA

Blood Donors & Receivers

Sex Linked Traits

Sex – Linked Genes We each have two sex chromosomes (one donated from mom, one donated from dad. Females: X X Males: X Y Genes on the first X chromosome are shared traits between the sexes, and is inherited from your mother. Genes on the Y chromosome contain very few instructions (primarily just for male development).

Sex-Linked Disorders Colorblindness Hemophilia Two of the most common sex-linked disorders are Colorblindness Hemophilia The genes that cause these disorders is located on the X chromosome (usually passed from mother to son) This makes it very rare for a colorblind father to pass the gene onto his son.

Sex-linked Genotypes Genotype Codes for: XRXR XRXr XrXr XRY XrY Female – Homozygous – Dominant XRXr Female –Heterozygous – Dominant XrXr Female – Homozygous – Recessive XRY Male – Dominant XrY Male – Recessive

Example Problem: Xr Y XRXr XRY XrXr XrY XR Xr Colorblindness is an X-linked recessive disorder. Show a cross between a heterozygous mother and a father who is colorblind. What are the new genotypes? Phenotypes? XRXr – (F) Not Colorblind XRY – (M) Not Colorblind XrXr – (F) Colorblind XrY – (M) Colorblind Xr Y XRXr XRY XrXr XrY XR Xr