Non-Mendelian Genetics Notes

Non-Mendelian Genetics

Inheritance Concepts

  • Mendelian Genetics (Complete Dominance)
    • Only two possible Phenotypes: either dominant or recessive.
    • Example: Tall plant X Short plant = Tall plant
      • (TT) x (tt) = (Tt)

Introduction to Non-Mendelian Genetics

  • Mendelian Genetics describes inheritance patterns based on Complete Dominance or Recessiveness.
  • There are other types of inheritance that Mendel never considered:
    • Incomplete Inheritance
    • Codominance
    • Multiple Alleles
    • Polygenic Traits
    • Sex-linked

Incomplete Inheritance

  • The hybrid (heterozygous) offspring displays a THIRD Phenotype.
  • Neither trait is completely dominant, resulting in a blending phenotype.
  • Example: Red Flower X White Flower = Pink Flower
    • (RR) x (WW) = (RW)

Incomplete Inheritance Problem

  • What is the probability of pink flowers if pink flowers are bred with red flowers?
    • 50% chance of Pink Flowers

Incomplete Inheritance Problem 2

  • What is the probability of white flowers if pink flowers are bred with pink flowers?

CoDominance

  • Both traits are dominant and show up in the phenotype together.
    • Co means “together.”
  • Example: Black Cow X White Cow = Spotted Cow
    • (BB) x (WW) = (BW)

CoDominance Problem

  • What are all the possible phenotypes when two spotted cows are bred?
    • Possible phenotypes are a black cow, two spotted cows, and a white cow.

Inheritance Type Determination

  • Examples with imaginary organisms noting horn shape, leg length, fur color, and tail shape to determine the type of inheritance.

Examples:

  • Horn Shape
    • Alleles: H (Branched), h (Simple)
    • Phenotypes: Branched, Simple
    • Genotypes:
      • Parents: Hh x Hh
      • Offspring: hh
    • Type of inheritance: complete dominance
  • Leg Length
    • Alleles: L (Long), l (Short)
    • Phenotypes: Long, Short
    • Genotypes:
      • Parents: Ll x Ll
      • Offspring: LL
    • Type of inheritance: complete dominance
  • Fur Color
    • Alleles: F^B (Brown), F^Y (Yellow)
    • Phenotypes: Brown, Yellow, Striped
    • Genotypes:
      • Parents: F^BF^B x F^YF^Y
      • Offspring: F^BF^Y
    • Type of inheritance: codominance
  • Tail Shape
    • Alleles: T^S (Short), T^C (Curled)
    • Phenotypes: Short, Curled, Corkscrew
    • Genotypes:
      • Parents: T^ST^S x T^CT^C
      • Offspring: T^ST^C
    • Type of inheritance: intermediate inheritance

Blood Type

  • Blood type displays both codominance and complete dominance.
  • Red Blood cells can either have a carbohydrate on their surface or not.
  • The presence of a carbohydrate (I) is dominant to the absence of a carb (i).
  • Additionally, there are two types of carbs that may exist on the surface of RBCs called A (I^A) and B (I^B).
  • Cell surface carbs A and B are codominant, which means they could also show up at the same time on an RBC.

Blood Type Inheritance Rules

  • A person with the I^A allele will have A carbohydrates.
  • A person with the I^B allele will have B carbohydrates.
  • A person with the recessive i allele will have no carbohydrates.
  • A person with both I^A and I^B alleles will have both A and B carbohydrates on their cells.

Blood Type Phenotypes and Genotypes

  • Genotypes I^Ai, I^AI^A: Type A blood
  • Genotypes I^Bi, I^BI^B: Type B blood
  • Genotype I^AI^B: Type AB blood
  • Genotype ii: Type O blood

Blood Type Problems

  • If a woman with AB blood has children with a man who has type O, what will be the possible genotypes of their children? What will be their blood types?
    • Possible genotypes: I^Ai, I^Bi
    • Possible blood types: Type A, Type B
  • Blood Type Problem 2: A woman with type B blood has a child with type O blood. How is this possible if her husband has type A blood?
    • Both the woman and the man must be heterozygous

Multiple Alleles

  • When more than 2 varieties exist in a trait.
  • Many animals have a variety of coat colors.

Polygenic Traits

  • Require more than one gene (allele) to determine trait.
  • Skin tone is determined by 4-6 genes—that means that there may be six different chromosomes involved!

Sex-linked Inheritance

  • Genes for some traits are found on the sex chromosomes (X or y).
  • Most of these traits are recessive, and the normal gene is dominant.
  • Heterozygous Females (X X^c) are carriers. They do not show the trait but carry a gene for the trait.
  • Homozygous Females (X^c X^c) have the trait.
  • Males with the gene (X^c Y) have the trait—They do not have another X to counterbalance the affected gene.

Color Blindness Example

  • Color blindness is a sex-linked recessive trait. The gene for this trait is inherited through the X chromosome.
  • If a woman with normal vision has children with a man who is colorblind, what are the chances that their children will be colorblind? Will any children be carriers of the trait?
    • Genotypes: X^c Y, XX^c, XY

Non-Mendelian Genetics Summary

  • What are the similarities and differences between each of these forms of inheritance?
  • How will you remember?
  • What are examples of each type?
    • Incomplete inheritance
    • CoDominance
    • Multiple Alleles
    • Polygenic Traits
    • Sex-linked inheritance