Genetics and Inheritance

Dihybrid Cross Phenotypic Ratio

  • The phenotypic ratio in the F2 generation of a dihybrid cross is 9:3:3:1, totaling 16.
  • 9/16 are yellow and round.
  • 3/16 are green and round.
  • 3/16 are yellow and wrinkled.
  • 1/16 are green and wrinkled.
  • Different characters are inherited independently, meaning being yellow and being round aren't a single unit passed on together.

Law of Independent Assortment

  • Each pair of alleles segregates or separates and is passed on independently.
  • This holds true as long as the different characters (like shape and color) are located on different chromosomes or are really far apart on the same chromosome.
  • If they are close together on the same chromosome, they will be inherited as a unit.

Laws of Probability

  • The probability of two or more independent events occurring together is the product of their individual probabilities (multiplication rule).
  • The probability that any one of two or more mutually exclusive events will occur is calculated by simply adding their probabilities (addition rule).
  • If the problem is a monohybrid cross, it uses one letter (e.g., R).
  • If the problem is a dihybrid cross, it uses two letters (e.g., Y and R).

Multiplication Rule in Monohybrid Crosses

*It is a cross between two heterozygous individuals (Rr x Rr).
*Each parent has a 50% or 1/2 probability of passing on either the dominant (R) or recessive (r) allele.
*When determining the probability of offspring genotypes, you multiply the individual probabilities.

Dihybrid Crosses

  • Instead of using a large Punnett square with 16 fields, separate the dihybrid cross into two monohybrid crosses.
  • Multiply the individual probabilities to find the probability of specific genotypes.
Example
  • Heterozygous for Y (Yellow) and homozygous dominant for R (Round): YyRr
  • Separate the crosses: Yy x Yy and RR x RR
  • Solve each monohybrid cross and multiply the probabilities.
  • Probability of being heterozygous for Y (Yy) is 2/4 or 1/2.
  • Probability of being homozygous dominant for R (RR) is 1/4.
  • Multiply: 12×14=18\frac{1}{2} \times \frac{1}{4} = \frac{1}{8}

Trihybrid Crosses

  • Trihybrid crosses involve three different genes (e.g., P, Y, and R).
  • Separate the trihybrid cross into three monohybrid crosses and multiply the individual probabilities.
  • Addressing more elaborate problems requires mindful consideration.
Example Problem
  • Determine the fraction of offspring exhibiting a homozygous recessive genotype for at least two of the three characters.
  • Identify the genotypes that fit the criteria (e.g., ppyyrr, ppyyRr, ppYyrr, Ppyyrr, etc.).
  • Calculate the probability for each genotype by multiplying the probabilities from the monohybrid crosses.
  • Sum the probabilities to find the total fraction of offspring fitting the description.

Practice Problems

  • Practice simple monohybrid crosses.
  • Work on dihybrid crosses.
  • Trihybrid crosses for solid understanding.
  • Online search for "DieHireCross practice" can get you more.

Deviations from Mendelian Rules

  • Many characters don't follow simple Mendelian rules of one dominant and one recessive trait.
  • Complicating factors include:
    • Degrees of dominance
    • Multiple alleles
    • One genotype producing different phenotypes

Degrees of Dominance

  • Complete dominance: One allele is dominant, the other recessive.
  • Incomplete dominance: Heterozygous phenotype is an intermediate between the two homozygous phenotypes.
    • Example: Snapdragons, where red (CRCR) and white (CWCW) flowers produce pink (CRCW) flowers.
    • Use superfixes instead of capital and lowercase letters to denote alleles (e.g., CR for red, CW for white).
  • Codominance: Both alleles are fully expressed in the heterozygote.

Common Misconceptions about Dominant Alleles

  • Dominant does not mean chemically stronger.
  • Dominant alleles are not necessarily more common.
  • Example: Polydactyly (extra fingers/toes) is a dominant trait but less common than having the typical number of digits.

Founder Effect

  • Example: Amish population has a higher rate of the polydactyly due to the small group of founders that had a higher than average chance of having that recessive allele.

Multiple Alleles

  • More than two alleles exist in the population for a particular gene.

ABO Blood Type System

  • Over 30 different blood type systems in humans, but the ABO system is the main one.
  • Determines which carbohydrate is attached to the outside of red blood cells.
  • Three alleles: A, B, and O.
  • A and B are codominant: If you inherit both, you express both carbohydrates (blood type AB).
  • O allele does not code for any carbohydrate and is recessive.
  • Blood type A: Inherit the A allele and produce carbohydrate A.
  • Blood type B: Inherit the B allele and produce carbohydrate B.
  • Blood type O: Inherit two copies of the O allele and produce no carbohydrates.
  • O and A blood types tend to be the most common, with AB being very rare.
Example
  • In blood transfusions, the body recognizes only what it is familiar with.
  • If you're blood type O, you can only receive blood type O.
  • AB positive can receive any blood type since the body is familiar with carbohydrate a, carbohydrate b, and the risk protein.
  • O negative is the universal donor because anybody can receive O negative but it also the pickiest one.

Rhesus System

  • Codes for a particular protein (not a carbohydrate).
  • If you're positive, you produce the protein.
  • If you're negative, you do not.
  • Combined with the ABO system (e.g., O positive means you have two O alleles and produce the Rhesus protein).

Pleiotropy

  • One gene affects multiple phenotypes.
  • Common in genetic diseases, where one gene affects multiple traits or symptoms.

Epistasis

  • A gene at one location affects the expression of a gene at a different location.
  • Example: Labrador coat color.
    • One gene (B) determines black (B) or brown (b) color.
    • A second gene (E) determines whether the pigment is deposited.
    • If the second gene is homozygous recessive (ee), the pigment is never deposited, resulting in a yellow lab, regardless of the B gene.
  • Genetic mechanisms for color are the same in every dog, only difference is determined by breed.

Polygenic Inheritance

  • One trait is affected by multiple genes.
  • Also hair color and hair texture.
  • Examples: Eye color and skin color.
  • Skin color: Multiple genes control melanin production, with each allele having an additive effect.
  • Blending hypothesis is not as accurate, instead depends on additive effect alleles.

Norm of Reaction

  • One genotype can produce multiple phenotypes depending on environmental factors.
  • Example: Flower color varying based on soil acidity.
  • Genetic lottery is a mix of DNA and environment.

Human Genetics

  • Ethical issues prevent Mendelian research projects on humans.
  • Practicality is an issue as gestation period of some animals are long.
  • Clinical trials has significant evidence to work and not be harmful.
  • Studies of pedigrees can reveal inheritance patterns.
    • Males are represented by squares, females by circles.
    • Filled shapes indicate the presence of the trait.

Widow's Peak Example

  • Having a widow's peak is dominant.
  • The heterozygote shows it (express the dominant allele).

Earlobes Example

  • Unattached earlobes (hanging free) is dominant.
  • Attached earlobes is recessive.

Genetic Disorders

  • Recessively inherited disorders: Need two copies of the recessive allele to express the disorder.
    • Carriers are heterozygous and do not have the disorder, but can pass on the allele.
    • Example: Albinism refers to significant reduction all the way up to the lack of a particular type of pigment.

Albinism Example

*If both parents are carriers (heterozygous), there is a 25% chance of having a child with albinism (homozygous recessive).
*First child being albino does not change the probabilities for subsequent children.
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*### Conscientious Matings
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*Conscientious matings are fancy term for inbreeding, the probability of a higher rate of individuals become carriers.
*Inbreeding increases the chance of offspring being homozygous recessive for harmful alleles.
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*### Conservation Biology
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*High inbreeding coefficient means limited amount of genetic diversity, which would result to high susceptibility to extinction.
*

Specific Recessively Inherited Diseases

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Cystic Fibrosis

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*Affects more white people, some of the chloride transport channels are not functioning very well. Get layers of thick mucus. Has particular sub group.
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Sickle cell disease

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*Affects more African Americans, gives you partial immunity to malaria. The shape of the red blood cells is abnormal.
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Dominantly Inherited Disorders

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*Need one copy of the allele to express the disease.
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Examples:

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Achondroplasia

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*Genetic condition that leads to dwarfism. Has to do with the development of your cartilage.
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Huntington’s disease

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*Nerudogenerative disease, analogous to Parkinson's disease and Alzeheimer's.