NOTES: 11.3 - Exceptions to Mendelian Genetics! ● Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles OR multiple genes. ● Examples of genes that are different than being totally “Dominant” or “Recessive:” 1. Incomplete dominance 4. Polygenic Traits 2. Codominance 5. Environmental Influences 3. Multiple Alleles 6. Sex-Linked Inheritance 1) INCOMPLETE DOMINANCE: ● One allele is NOT completely dominant over another. -The heterozygous phenotype is somewhere between the 2 homozygous phenotypes . What does this mean? ● Mendel crossed a homozygous red plant with a homozygous white plant. ● What do you think would be the expected results?... R’ = R= ● P: RR x R’R’ ● F1: what is the F1 generation going to look like (phenotype)? ● F2: what is the F2 generation going to look like (phenotype)? ● F1: ● F2: **notice the ratio for incomplete dominance 1:2:1 2) CODOMINANCE: ● Definition: for a trait contribute to the phenotype of the organism. ● Examples: -The alleles for red (RR) and white (WW) hair in cattle are co-dominant. Cattle with both alleles have brown/white patterning or ( ). -In certain varieties of chickens the alleles for black and white feathers are co-dominant. Chickens with both alleles appear . What is the difference between incomplete dominance and codominance? • Incomplete dominance = heterozygous phenotype is somewhere in between the 2 homozygous phenotypes. • For example, in (RR’), the R’ allele is not active, but R cannot produce its full effect when it is combined with R’. RR’ = RR = R’R’ = • Codominance = heterozygous phenotype has characteristics of .… -BOTH alleles are active and are expressed together (both act like dominant genes). • For example, in a cross between red hair (RR) and white hair (WW), the calf will be roan (RW) both red and white hairs. RR = WW = RW= 3) MULTIPLE ALLELES: ● Definition: ● Remember: YOU only inherit TWO alleles (one from mom, one from dad) ● Example 1: -in rabbits coat color is determined by a single gene with four alleles. ● Example 2: Human Blood Types: ( , , ) -Phenotypically Type A Blood (genotype = IAIA or IAi) -Phenotypically Type B Blood (genotype = IBIB or IBi) -Phenotypically Type AB Blood (genotype = IAIB) -Phenotypically Type O Blood (genotype = i i) 4) POLYGENIC INHERITANCE: ● Traits that are ● Examples: - in some plants; - in fruit flies is controlled by three genes; - is controlled by more than 4 different genes; -Shows a wide range of phenotypes as result Example: STEM LENGTH ● suppose stem length in a plant is controlled by 3 different genes: A, B, and C ● each diploid plant has 2 alleles for each gene (e.g. AaBBcc OR aaBbCc, etc.) ● a plant homozygous for short alleles for all 3 genes ( ) might grow to 4 cm ● a plant homozygous for TALL alleles for all 3 genes ( ) might grow to 16 cm ● the difference in heights is 12 cm (or, 2 cm per each of the 6 tall alleles)… ● you could say that each “uppercase” allele contributes 2 cm to the total plant height… SO, predict the phenotypes for the following genotypes: AaBbCc: AabbCc: AABBCc: ● so, if you crossed a TALL 16 cm plant (AABBCC) with a short 4 cm plant (aabbcc), all of the F1 plants would be: Genotype: Phenotype: ● THEN, if you let 2 F1 plants cross, you would see a broad range of heights in the F2 ● if you counted the different phenotypes, they could be represented with a “bell curve” – a typical pattern see with POLYGENIC INHERITANCE! -Human skin color is controlled by -Dark skinned people have “uppercase” alleles that code for melanin at all gene positions for skin color. -Lighter skinned people have few gene positions with alleles that code for melanin (in other words, they have more “lower case” alleles for those genes) 5) Environmental Influences: ● as an organism develops, many factors can influence how the gene is expressed, OR even whether the gene is expressed at all ● influences can be or EXTERNAL INFLUENCES: Examples: Light (e.g. shade or sunlight for plant leaf size) Chemicals / pH Infectious agents INTERNAL INFLUENCES: ● the internal environments of males and females are different because of and structural differences ● Examples: -horn size in mountain sheep -feather color in peacocks ● could also include AGE (although the effects of age on gene expression are not well understood) SEX DETERMINATION: (CH 14) ● RECALL: in humans, the diploid # of chromosomes is 46 ( ● of the 23 pairs, 22 are ) , and the 23rd pair represents the ● human females: ● human males: ● Males (XY) can produce 2 kinds of gametes: sperm cells carrying X sperm cells carrying Y ● Females (XX) will only produce “ “ ● so the odds of having a boy or girl are always 50/50 6) SEX-LINKED INHERITANCE: (CH 14) ● SEX-LINKED TRAITS = ● the alleles are written as superscripts of the X and Y chromosome ● Y-linked traits are passed only from male to male ● since males only have 1 X chromosome, if there is a gene on the X chromosome, males only get 1 copy (see reverse for 2 examples of sex-linked crosses!) Example: eye color in fruit flies -the gene for eye color is on the X chromosome -RED eyes are dominant: -white eyes are recessive: CROSS #1: heterozygous red-eyed female X white-eyed male Female genotype: Punnett Square: Male genotype: Offspring genotype ratio: Offspring phenotype ratio: CROSS #2: heterozygous red-eyed female X red-eyed male Female genotype: Male genotype: Offspring genotype ratio: Offspring phenotype ratio: Punnett Square: