AP Bio Unit 5 - Topics 4, 5, and 6

Non-Mendelian Genetics

  • Many traits do not follow the ratios predicted by Mendel’s laws. Why?

    • Varying degrees of dominance

    • Many traits are produced through multiple genes acting together

    • Some traits are determined by genes on the sex chromosomes

    • Some genes are adjacent or close to one another on the same chromosome and will segregate as a unit

    • Some traits are the result of non-nuclear inheritance (i.e. chloroplast and mitochondrial DNA)



Degrees of Dominance

  • Alleles can show varying degrees of dominance

  • In Mendel’s experiments, he worked with traits that showed complete dominance

    • Homozygous dominant and heterozygous individuals are phenotypically the same

  • Incomplete dominance: neither allele is fully dominant

    • F1 generation has a phenotype that is a mix of those of the parental generation

      • ex) red flowers x white flowers = pink flowers

  • Codominance: two alleles that affect phenotype are both expressed

    • ex) human blood group

      • Type AB blood: A and B are both expressed

  • Multiple alleles: genes that exist in forms with more than 2 alleles

    • ex) human blood group

      • Alleles: IA, IB, i

  • Human blood group


Phenotype


Genotype

A

= 

IA IA or IA i

B

= 

IB IB or IB i

AB

= 

IA IB

O

=

i i



Multiple Genes

  • In many cases, two or more genes are responsible in determining phenotypes

  • Epistasis: the phenotypic expression of a gene at one locus affects a gene at another locus

    • ex) coat color in labs and some mice

      • One gene codes for pigment and a second gene determines whether or not that pigment will be deposited in the hair

  • Polygenic inheritance: the effect of two or more genes acting on a single phenotype

    • ex) height, human skin color



Sex Chromosomes

  • Thomas Hunt Morgan experimented with fruit flies and determined that specific genes can be carried on sex chromosomes

    • Sex-linked genes: a gene located on either the X or the Y chromosome

      • Y-linked gene: genes specifically found on the Y chromosome

        • Very few Y-linked genes, so very few disorders

      • X-linked genes: genes found on the X chromosome



Inheritance of X-Linked Genes

  • Fathers can pass X-linked alleles to all of their daughters, but none of their sons

  • Mothers can pass X-linked alleles to both their daughters and sons

  • If an X-linked trait is due to a recessive allele:

    • Females will only express that trait if they are homozygous recessive

    • Because males only have one X chromosome, they will express the trait if they inherit it from the mother

      • They are called hemizygous

      • Due to this, males are much more likely to have an X-linked disorder



X-Linked Disorders

  • Duchenne muscular dystrophy: progressive weakening of muscles

  • Hemophilia: inability to properly clot blood

  • Color blindness: inability to see colors correctly



X-Inactivation

  • Females inherit two X chromosomes, which is double males

    • During development, most of the X chromosome in each cell becomes inactive

      • The inactive X in each cell of a female condenses into a Barr body

        • Helps to regulate gene dosage in females



Genetic Recombination

  • Genetic recombination: production of offspring with a new combination of genes from the parents

    • Parental types: offspring with the parental phenotype

    • Recombinants: offspring with phenotypes that are different from the parents

  • Mendel also observed recombinants during his crosses

    • ex) green wrinkled plant crossed with a yellow round plant (yyrr x YyRr)

    • 50% recombination, however, indicated that genes are unlinked, or on different chromosomes



Linked Genes

  • Linked genes: genes located near each other on the same chromosome that tend to be inherited together

    • Meiosis and random fertilization generate genetic variation in offspring due to:

      • Independent assortment of chromosomes

      • Crossing over in meiosis I

      • Any sperm can fertilize any egg



Linked Genes: Crossing Over

  • Linked genes show parental phenotypes in offspring higher than 50%

    • During crossing over, chromosomes form one paternal chromatid and one maternal chromatid which exchange corresponding segments

  • Crossing over helps to explain why some linked genes become separated during meiosis

  • The further apart two genes are on the same chromosome, the higher the probability that a crossing over event will occur between them and the higher the recombination frequency



Mapping Distance

  • Experiments performed by Sturtevant allowed scientists to map genes and their locations on chromosomes

    • Linkage map: genetic map that is based on recombination frequency

    • The distance between genes are map units

      • One map unit = 1% recombination frequency

        • Express the relative distance along chromosomes

        • 50% recombination means that the genes are far apart on the same chromosome or on two different chromosomes



Non-Nuclear DNA

  • Some traits are located on DNA found in the mitochondria or chloroplasts

    • Both chloroplasts and mitochondria are randomly assorted to gametes and daughter cells

      • In animals, mitochondria are transmitted by the egg, not sperm

        • Therefore, all mitochondrial DNA is maternally inherited

      • In plants, mitochondria and chloroplasts are transmitted in the ovule, not the pollen

        • Therefore, both mitochondrial and chloroplast determined traits are maternally inherited



Statistical Analysis: Chi Square

Goodness of Fit Test

  • Chi-square: a form of statistical analysis used to compare the actual results (observed) with the expected results

    • Helps to:

      • Determine whether the data obtained experimentally provides a “good fit” to the expected data

      • Determine if any deviations from the expected results are due to random chance alone or to other circumstances (ie data calculation error)

    • Designed to analyze categorical data



Chi Square (X^2)

  • Use the equation to test the null hypothesis

    • The prediction that data from the experiment will match the expected results

  • Formula: X^2 = ((O)(E))^2/E



Example: How to Solve



You are interested in examining the trait that allows people to roll their tongue.

  • Tongue rolling is dominant to non tongue rolling

  • You survey 100 people and you find:

    • 90 people were able to roll their tongue

    • 10 people could not roll their tongue

  • You also took genetic data from the parents of all 100 people

    • The parental genotypes for all 100 people was Rr

  • Null: any difference between the observed and expected data is due to chance



Step 1: Determine what your expected and observed values are

  • Observed values: the numbers that you get in your data

    • Usually no calculations

  • Expected values: based on probability

    • Need to do calculations

    • In this example, you know that all parents are Rr for the trait. So, set up a punnett square o determine the expected number of people who can/cannot roll their tongue

Step 2: make a table


Trait

Tongue roller

Non tongue roller

Total 

Observed - O

90

10

100

Expected - E

75

25

100

O-E

15

-15

0

(O-E)^2

225

225

450

(O-E)^2/E

3

9

12 = X^2



Step 3: determine the degrees of freedom for your experiment

  • With X^2, you must determine the probability that the difference between the observed and expected values occurred by chance

  • You need to compare your calculated value to the appropriate value in a degrees of freedom table

  • To calculate degrees of freedom:

    • Degrees of freedom = number of categories - 1

      • For this problem there are 2 categories

        • degrees of freedom = 2-1 = 1

P represents the confidence

  • 0.05 means you are 95% confident that your observed data fits your expected data

  • 0.01 means you are 99% confidence that you observed data fits your expected data

    • As a general rule of thumb, you look at the p = 0.05 row, unless instructed otherwise



Interpreting results and degrees of freedom chart:

  • If X^2 > critical value: there is a statistically significant difference between the actual and expected values

    • Reject null

  • If X^2 < critical value: there is not a statistically significant difference between the actual and expected values

    • Accept null

  • For this example: X^2 = 12

    • 12 > 3.84

    • statistical significant difference

      • Reject null

      • Difference is not due to chance



Environmental Effects on Phenotypes

Environmental Factors

  • Various environmental factors can influence gene expression and lead to phenotypic plasticity

    • Individuals with the same genotype exhibit different phenotypes in different environments

  • Examples:

    • Temp can change coat color in rabbits and siamese cats

    • Soil pH can affect flower color

    • UV exposure can increase melanin production in the skin



Chromosomal Inheritance Disorders

Genetic Disorders

  • Some genetic disorders can be linked to affected or mutated alleles or chromosomal changes



Mutated Alleles

  • Tay-Sachs disease:

    • Autosomal recessive disease

      • Mutated HEXA gene

        • Body fails to produce an enzyme that breaks down a particular lipid

      • Affects central nervous system and results in blindness

  • Sickle cell anemia:

    • Autosomal recessive disease

      • Mutated HBB gene

        • Sickle cells contain abnormal hemoglobin molecules



Chromosomal Changes

  • Nondisjunction: chromosomes fail to separate properly in meiosis 1 or meiosis 2

    • Karyotyping can detect nondisjunction

      • ex) down syndrome

        • 3 copies of chromosome 21