Review of Patterns of Inheritance Gregor Mendel’s Principles 1. Principle of Dominance 2. Law of Segregation – each parent has 2 alleles for a trait but these are separated (segregated) during the formation of gametes (meiosis) 3. Law of Independent Assortment – gene pairs separately independently of each other (unless they are closely linked on the same chromosome) 6/13/2016

Monohybrid (one trait) crosses Homozygous Dominant x Homozygous Dominant TT x TT T T T 100% Tall: 0% Short 4:0 100% TT: 0% Tt: 0% tt 4:0 6/13/2016

Monohybrid (one trait) crosses Homozygous Dominant x Homozygous Recessive TT x tt T T t 100% Tall: 0% Short 4:0 0% TT: 100% Tt: 0% tt 0: 4:0 6/13/2016

Monohybrid (one trait) crosses Homozygous Dominant x Heterozygous TT x Tt T T T t Genotypic Ratio 50% TT: 50% Tt: 0% tt Phenotypic Ratio 100% Tall: 0% Short 4:0 6/13/2016 TT TT Tt

Monohybrid (one trait) crosses Heterozygous x Heterozygous Tt x Tt T t T t Genotypic Ratio: 25% TT: 75% Tt: 25% tt 1:2:1 Phenotypic Ratio: 75% Tall: 25% Short 3:1 6/13/2016 TT Tt tt

Monohybrid (one trait) crosses Heterozygous x Homozygous recessive Tt x tt T t t Genotypic Ratio: 0% TT: 50% Tt: 50% tt 0:1:1 Phenotypic Ratio: 50% Tall: 50% Short 1:1 6/13/2016 Tt tt Tt tt

Variations to Mendelian Genetics

Not all genetic traits strictly follow the laws discovered by Gregor Mendel. Five types of non-Mendelian inheritance: –Incomplete dominance –Codominance –Multiple alleles –Polygenic inheritance –Sex linked traits and/or Sex Influenced 6/13/2016

Variation #1: Incomplete Dominance Dominant allele does not completely “mask” the recessive allele. Results in a “BLENDING” of the two forms of the trait. This intermediate phenotype is expressed in the heterozygous condition (Bb) Example: B = black and W = white forms of hair color trait BB= black WW= white BW= gray - blended form of the 2 trait forms

6/13/2016 Incomplete Dominance Example 1 In rabbits black fur is incompletely dominant over white fur. Cross a black fur male with a white fur female Black= BB White=WW

Incomplete Dominance Example 2 In carnations, red is incompletely dominant over white. Blended inheritance results in pink flowers. RR RR BB RW WW 6/13/2016

Variation #2: Codominance A condition in which both alleles are expressed equally because they have equal dominance The two alleles don’t blend but are present and can be seen in full form at the same time in the offspring Ex. Checkered chickens, variegated clover, roan horses and cows 6/13/2016

Variation #2: Codominance 6/13/2016 Example: B = black and W = white BB= Black WW= White BW= Black and White Both forms of trait are expressed

Variation #2: Codominance 6/13/2016 In Rhododendrons, pink is codominant with white. This plant displays codominance. Cross a pink rhododendron P with a white rhododendron W. X=

Example 3: Codominance In cattle brown B is codominant over white W. – BB – WW – BW 6/13/2016

Cross two roan cattle BW x BW 6/13/2016

Incomplete Dominance vs. Codominance Incomplete dominance – Heterozygote’s traits are a blend of the two alleles Codominance – Both alleles for gene are equally strong and are both seen

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a blending of the parental traits. Ex. Red X White flowers > Pink flowers With codominance, a cross between organisms with two different phenotypes produces offspring with phenotype in which both of the parental traits appear together. Ex. Red x White flowers > Both colors seen

Which inheritance pattern does each cross represent? Codominance or Incomplete Dominance 6/13/2016 X = 100% X =

Variation #3 Multiple Alleles A gene may have more than just two allele versions. Ex: Human Blood Type 3 alleles 4 phenotypes Type A Type B Type AB Type O 6/13/2016

Variation #4: Polygenic Inheritance Poly= Many Genic= Genes Polygenic inheritance is when many genes work together to produce one trait. Polygenic traits usually show a lot of variation. Examples: Hair color, Skin color, Eye color

Hair color is controlled by three sets of genes all working together to create various hair colors. aabbcc AaBbCcAABBCC BlondestMedium Brown Blackest hair color hair colorhair color (3 dominant/3 recessive)

Skin tone is controlled by four sets of genes all working together to create one shade of skin tone. aabbccdd AaBbCcDd AABBCCDD palest skin medium skin darkest skin tone tone tone (4 dominant/4 recessive)

AABbCc X AaBBCc 6/13/2016 AABBCcAaBbcc

Example : In snapdragons color of the blossoms displays polygenic inheritance. AABB-red AABb-peach AaBb and AaBB –pink Aabb – white All other genotypes - yellow 6/13/2016 How many genes control the snapdragon color trait?

Variation #5: Sex Linked Sex linked traits are controlled by a gene located only on the X or Y sex chromosomes These genes are “linked” on the sex chromosomes Most of these traits are found only on the X sex chromosome which makes them specifically “X linked” traits 6/13/2016

Females sex chromosomes: XX Males sex chromosomes: XY Females get sex linked diseases less often than males because it is less likely to inherit two “bad” genes than it is to inherit one. Females must inherit one gene from her father and one from her mother. Males only inherit one X linked gene which comes from his mother. (

6/13/2016 Ex. Sex linked traits – Red Green Colorblindess

Ex. Sex linked trait – Hemophilia Blood clotting disorder 6/13/2016

Ex of Sex linked trait Muscular Dystrophy 6/13/2016

: Carrier: a person who is does not express a phenotype but “carries” the allele for it and may pass that gene on to their child. Is heterozygous. 6/13/2016

Pedigree Charts: 6/13/2016

Which inheritance pattern does each cross represent? Codominance or Incomplete Dominance 6/13/2016 X = 100% X =