Review of Patterns of Inheritance Gregor Mendels Principles

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) 9/12/2021

Monohybrid (one trait) crosses Homozygous Dominant x Homozygous Dominant TT x TT • T • 100% Tall: 0% Short 4: 0 • 100% TT: 0% Tt: 0% tt 4: 0 9/12/2021

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 9/12/2021

Monohybrid (one trait) crosses Homozygous Dominant x Heterozygous TT x Tt • T TT • t Genotypic Ratio Phenotypic Ratio 9/12/2021 Tt TT Tt 50% TT: 50% Tt: 0% tt 100% Tall: 0% Short 4: 0

Monohybrid (one trait) crosses Heterozygous x Heterozygous Tt x Tt • T • t TT Tt Tt tt Genotypic Ratio: 25% TT: 75% Tt: 25% tt Phenotypic Ratio: 75% Tall: 25% Short 9/12/2021 1: 2: 1 3: 1

Monohybrid (one trait) crosses Heterozygous x Homozygous recessive Tt x tt • T t • t Tt • t Tt tt tt Genotypic Ratio: 0% TT: 50% Tt: 50% tt Phenotypic Ratio: 50% Tall: 50% Short 9/12/2021 0: 1: 1

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 9/12/2021

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= WW= BW= 9/12/2021 black white gray - blended form of the 2 trait forms

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 9/12/2021 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 9/12/2021

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 9/12/2021

Variation #2: Codominance Example: B = black and W = white BB= Black WW= White BW= Black and White Both forms of trait are expressed 9/12/2021

Variation #2: Codominance • In Rhododendrons, pink is codominant with white. This plant displays codominance. Cross a pink rhododendron P with a white rhododendron W. X 9/12/2021 =

Example 3: Codominance • In cattle brown B is codominant over white W. 9/12/2021 – BB – WW – BW

• Cross two roan cattle BW x BW 9/12/2021

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 9/12/2021 X = 100%

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 9/12/2021

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 Blondest hair color Aa. Bb. Cc Medium Brown hair color (3 dominant/3 recessive) AABBCC Blackest hair color

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

AABb. Cc X Aa. BBCc AABBCc 9/12/2021 Aa. Bbcc

Example : • In snapdragons color of the blossoms displays polygenic inheritance. How many genes control AABB-red the snapdragon color trait? AABb-peach Aa. Bb and Aa. BB –pink Aabb – white All other genotypes - yellow 9/12/2021

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 9/12/2021

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. ( 9/12/2021

Ex. Sex linked traits – Red Green Colorblindess 9/12/2021

Ex. Sex linked trait – Hemophilia Blood clotting disorder 9/12/2021

Ex of Sex linked trait Muscular Dystrophy 9/12/2021

: • 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. 9/12/2021

Pedigree Charts: 9/12/2021

Which inheritance pattern does each cross represent? Codominance or Incomplete Dominance 9/12/2021 X = 100%