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Chapter 11 & 14:
Fundamentals of Genetics & Human Genetics
Section 1: History
Genetics: The scientific study of heredity
Trait: A characteristic that can be passed from parent to offspring
How does this relate to our last chapter?
Chromosomes carry the traits and meiosis determines
which traits will be carried by the gametes (sex cells)!
History
Gregor Medel: Austrian Monk who, in the 1860’s, used pea plants
and statistics to study inheritance
Why Peas?
1. Ideal Flower Structure
2. Presence of distinctive traits
3. Rapid Reproduction Cycle (90 days)
How?
1. 1st grew 7 traits "PURE" through self-pollination
example: plant height, seed color, seed shape, pod
shape, pod color, flower type...etc
2. He crossbred (hybridized) different plants to see what the
offspring would look like...
i) tall x short
ii) yellow x green seeds
iii) wrinkled seeds x smooth seeds
...and found he got predictable outcomes
Self-fertilization: Pollen from anther fertilizing the pistil of the same
flower
Purebred: Organism that receives the same genetic traits from both
of it’s parents
Hybrid: Organism that receives different forms of a genetic trait from
each parent
Denoting Generations
P = Parents
F1 = First generation of offspring
F2 = Second generation of offspring
F3 = Third generation of offspring
*** For every trait, a plant must carry a pair of factors which affect each other***
***When a trait is inherited, the offspring receives 1 factor from each parent***
Factor = Gene: Section of a chromosome that code for a trait
Allele: Different and distinct forms of genes
Dominant: A form of gene that is fully expressed when two different
alleles are present
Recessive: A form of gene that is not expressed when paired with
a dominant allele
***Two recessives must be present for the recessive gene to be
expressed!***
Section 2: Principles of Inheritance
Walter S. Sutton (1903): Chromosome Theory of Heredity: The material
of inheritance is carried by the genes in chromosomes.
Representing Alleles
(Pea color is controlled by a single gene with 2 alleles)
Dominant = Y = yellow
Uppercase = Dominant
Recessive = y = green
lowercase = recessive
Genotype: The genetic make-up of an organism (both genes in a
homologous pair of chromosomes) (ex: YY or Yy or yy)
Phenotype: The outward expression of the trait (ex: yellow or green)
Homozygous: When two alleles in a gene pair are identical
(ex: YY or yy)
Heterozygous: When two alleles in a gene pair are different
(ex: Yy)
Mendel’s Laws (Basic Rules of Inheritance)
1. Law of Segregation: A pair of factors is segregated, or separated,
during the formation of gametes (each reproductive
cell receives only one of a pair of alleles- to be
passed on to offspring)
2. Law of Independent Assortment: Factors (genes) separated and are
distributed to gametes completely separate from
other genes(example: tall (plant height) gene is not
connected to pea color gene)
Ex:
P
(YY, RR)
F1
X
(yy, rr)
(Yy , Rr)
3. Law of Dominance: One factor in a pair may mask the other,
preventing it from having an effect (only the
dominant allele of a gene is expressed, although a
recessive allele may be carried)
Section 3: Genetics and Predictions
Probability: The likelihood of an occurrence or event (ex: coin flip, weather)
Represented using:
1. Fractions (1/2)
2. Percentages (50%)
3. Ratio (1:1)
***Used to predict phenotypes/genotypes***
Punnett Squares: Used for predicting the crossing of two organisms,
showing the probability of certain phenotypes/genotypes
(not actual results!)
Monohybrid: Looks at 1 trait at a time
Dihybrid: Looks at 2 traits at a time
Trihybrid: Looks at 3 traits at a time (extra credit!)
Tetrahybrid: Looks at 4 traits at a time (even more extra credit!)
How to make a Punnett Square:
1. Make a grid
2. Place alleles of 1 parent on the top of grid, alleles of other parent
along left side
3. Fill in the grid
a. combine parent alleles inside the boxes (letters show
genotypes)
b. determine the genotypic ratios
c. determine the phenotypic ratios
Ex:
***Dihybrid & Trihybrid crosses use the same method as monohybrids, but
use a larger grid to accommodate ALL of the possible parental
genotype combinations***
Ex:
Test Cross: Used to distinguish between homozygous dominant and
heterozygous organisms
How is it done? Breed an unknown with a Homozygous Recessive
***If unknown is heterozygous, then 1/2 should show recessive
***If unknown is homozygous dominant, then all of offspring show dominant
trait
Section 4: Predictions and People
**Humans & higher organism traits are more complex
**Longer time between generations
**Fewer offspring
Pedigree: Chart that shows how a trait and the genes that control it are
inherited within a family
**Information is key!
**Often blood (DNA) collected (if possible)
**Mormons & Ice landers often used in studies due to records
Carrier: An individual who carries a recessive trait that is not expressed
Example to Charting (for a recessive disorder):
Homozygous Recessives
I
II
III
Homozygous Dominant
Heterozygous
= Female showing trait
= Normal Female
= Female Carrier
= Male showing trait
= Normal Male
= Male Carrier
***Recessive allele can be carried/passed along without affecting carriers
health!
Ex: Tay-Sachs disease
Recessive allele causes disease
Homozygous recessive person can not metabolize a certain
lipid, which then accumulates in the brain
Affected people do not live for more that five years
Carriers can pass allele along
Other examples: Cystic Fibrosis, Albanism (multiple allele disorder)...
Dominant Disorders: Very Rare! Why?
Disorders often cause death, so individual dies before procreating
Exception: HD (Huntingtons Disorder): Fatal, but onset occurs later
in life
Genetic Counseling
Ethics
Down Syndrome
PKU (Phenylketonuria)
Section 5: Difficult Predictions (Exceptions to the Rules)
Intermediate Inheritance: Offspring have trait that is not exactly like that
of either parent
Three Forms:
1. Incomplete Dominance
2. Codominance
3. Polygenic Inheritance
1. Incomplete Dominance:
a. When 2 different alleles for the same trait combine, but neither
allele “wins” expression over the other
b. Heterozygous offspring show a phenotype that is in between the
phenotypes of the 2 homozygous parents
Ex: Snapdragons
RR (red)
X
rr (white)
Rr (pink)
Other examples: Roan Cows
Palomino Horses
Hypercholesterolemia (humans)
HH = Normal
Hh = 2X higher cholesterol levels (1 in 500)
hh = 5X higher cholesterol levels (1 in 1,000,000)
2. Codominance: Both alleles in a heterozygote express themselves fully
Ex: Human Blood Type
Antigen (carbohydrate) on RCB
Genotype
Serum Agglutinin
A = A type
IAIA or IAi
Anti-B
B = B type
IBIB or IBi
Anti-A
AB = Both types
O = None
IAIB
ii
None (Universal
Recipient)
Both Anti-A & Anti-B
(Universal Donor)
3. Polygenic Traits: A trait controlled by 2 or more genes
Ex: Eye color, Skin color, many others
Multiple Alleles: 3 or more alleles in a population
Pleiotropy: Occurs when a single gene affects more than one trait
Environment: May play a role in the expression of certain traits
Ex: Himalayan rabbit fur (temperature affects color)
Western White butterfly: darker in spring, which help attracts
heat and allows it to fly (can only fly in
certain temperatures)
What other traits do you think are affected by environment?
Can you think of any traits in humans that are affected by
environment?