Exceptions to Single Trait Crosses

Rules to follow genetics 1. Dominant trait will normally be expressed over recessive (few exceptions). 2. Each offspring will get one allele from each parent for each trait. 3. Offspring can only get an allele that at least one of the parents have to give. Offspring cannot have an allele that one of the parents did not give them.

Quick Quiz…. True or False: 1. You have the trait of hair color. 2. You have the trait of polydactylism. 3. You have the trait of albinism.

Answers to the Quiz: 1. True

Answers to the Quiz: 1. True 2. True

Answers to the Quiz: 1. True 2. True 3. True You have the traits, but you may not have the alleles for that specific phenotype… Example: Ablinism… you are likely to be AA (normal) versus aa (being albino)… you may be a carrier, Aa, and not know until you have a child with albinism…

EXTENSION OF MENDEL’S RULES Phenotypic variation is not always due to simple dominance/recessive-ness of genes

EXTENSION OF MENDEL’S RULES Some Traits Deviate from Mendelian Patterns: Incomplete Dominance: Co Dominance: Environmental Effects: Quantitative Traits:

INCOMPLETE DOMINANCE (Blend) No Allele Is Completely Dominant Neither red nor white allele is completely dominant (Snap Dragon Flowers)

Incomplete dominance in flower color Parental generation RR rr X RR rr F1 generation: All Heterozygotes Rr Self-fertilization Figure: 10.17b Caption: (b)The hypothesis illustrated here is that flower color is controlled by a single gene with two alleles, symbolized R and r. Heterozygotes have an intermediate phenotype because of a phenomenon known as incomplete dominance. F2 generation 1/4 RR 1/4 Rr 1/4 Rr 1/4 rr Red Pink White

INCOMPLETE DOMINANCE Red allele = red pigment White allele = no pigment In heterozygous form, neither allele dominates

CODOMINANCE (Both Shown) Two alleles specify different phenotypes, but one does not mask expression of the other

CODOMINANCE (Both Shown) Two alleles specify different phenotypes, but one does not mask expression of the other

CODOMINANCE (Both Shown) Two alleles specify different phenotypes, but one does not mask expression of the other

CODOMINANCE (Both Shown) Two alleles specify different phenotypes, but one does not mask expression of the other

CODOMINANCE Determined by type of glycoprotein on RBC membrane Blood type: Determined by type of glycoprotein on RBC membrane 3 possible alleles: The existence of more than two alleles at a gene locus is called “multiple allelism (polymorphism)

CODOMINANCE ABO Blood System: Phenotypes of Blood A B O AB

CODOMINANCE Blood Type Genotypes With alleles: AA or AO With alleles: BB or BO With alleles: OO With alleles: AB

ENVIRONMENTAL EFFECTS Environmental conditions help determine expression of genotype Both carry allele specifying heat-sensitive enzyme for melanin production

ENVIRONMENTAL EFFECTS Temp. sensitive enzyme acts at surface of body:

QUANTITIVE TRAITS The greater the # of genes influencing a trait, A result of more than one gene contributing to phenotype Some traits show continuous variation: The greater the # of genes influencing a trait, the greater the variation in traits expressed Figure 10.19  (a) This photo was published in 1914 and shows male undergraduates at Connecticut Agricultural College sorted by height. (b) This histogram plots the heights of the students in part (a). The distribution of height in human populations forms a bell-shaped curve. This distribution is observed so frequently that it is often called a normal distribution.  Question The shortest student in the photo is 4’10” (147 cm) and the tallest is 6’2” (188 cm). How would a histogram of men in your class compare to the distribution shown here?

Multiple Alleles Blood types: Multiple Alleles (Choices) Examples: Blood, Skin Color, Hair Color and Eye Color Combinations of any two alleles produces different phenotypes.

Normal Karyotype

Down’s Syndrome or Trisomy 21