Unit Three Notes
Heredity
 The transmission of traits from one generation to the next.
Greg Mendel –Austrian Monk---1800’s
 Observed some traits disappeared in one generation, only to reappear in the next.
Hypothesis
 Some traits are stronger than others.
Experimental Design
 Needed something which could be easily manipulated.
 Something with a variety of visible characteristics.
The Garden Pea
 Quick generation time
 Does not require much space
 Peas undergo self-fertilization
o Pollinate themselves
 Could also pry open petal to make specific crosses
between plants.
 Many visible traits:
Flower color
Seed color
Seed shape
Plant height
Parental Generation
o For each trait studied he wanted a true breeding plant to begin the experiment.
o True BreedingGeneration after generation breeds true to a single visible trait.
Experiment
o Cross two different parental generation plants (each One displays One version of the
trait)
o Parental white flower x purple flower cross them and plant the seeds to get the first
generation
o F1—First Filial (Latin for son/daughter) all purple 100%.
Recorded The Numbers & Traits Of All Generations
o Allowed F1 to self-fertilize.
o Counted the F2 for number of each trait.
o 75% purple, 25% white.
Dominant—Trait which was solely visible in F1 generation.
Recessive—Trait which disappeared in F1 reappeared in F2.
Allowed F2 to self-fertilize.
o F2 white only produced white.
o Recessive individuals always bred true.
o F2 purple
o Progeny a mix of Purple individuals & white individuals.
Mendel’s Theory
o Individuals possess two copies of ea. trait factor (called genes).
o The two trait factors may be identical to each other or not.
Homozygous
o The two trait factors in an individual are identical for a specific trait.
Heterozygous
o The two trait factors are different for a specific trait.
Mendelian Notation
o Each trait designated by a single letter.
o Dominant version upper case.
o Recessive version lower case.
Alleles - alternate forms of particular trait
Trait
Flower color
Plant Height
Seed Shape
Alleles
White
Purple
Tall
Short
Round
Wrinkled
Notation
p
P
T
t
R
r
Genotype: Sum of all alleles of an individual.
o Homozygous / Heterozygous
RR or rr
Rr
Phenotype: Physical Representation of genotype, What we see
o PP Purple
o Pp Purple
o Dominance Relationship is important in the heterozygote
Punnett Squares
o Allows prediction of progeny genotypes based upon parental genotypes.
o Determine all gamete possibilities of both parents.
o Write one parental set of gametes across the Horizontal axis, write the other parental set
of gametes down the vertical axis of the Punnett Square.
One Factor Cross
RR x rr
Determine possible gametes for each parent
Write one parent along the side the other across the top.
Off Spring Genotypes In The Squares--F1 In Boxes.
Genotypically - Heterozygous
Phenotypically - Round
Cross Rr x Rr
Set up Punnett Square
Determine Gametes
Test Cross
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Used to determine an unknown individual’s genotype.
Cross unknown with a known homozygous.
*Homozygous Recessive
 Allows you to deduce the genotype by offspring produced.
 If any offspring are recessive then the unknown must have been
Heterozygous.
You have a purple pea plant. Can you tell by looking whether it is PP or Pp?
Set up a test cross to determine the genotype.
What genotype will you cross it with?
Determine the gametes for each plant.
Set up the Punnett Square.
Two Factor Crosses
 Predicting the offspring characteristics for two different traits at the same time
 Illustrates Independent Assortment
 16 boxes in Punnett square rather than 4
 ***Always write the alleles for a single trait together
o ie Rr Tt not RT rt
For the parental cross of a Round Tall plant with wrinkled short
RR TT x rr tt
All progeny will be
Rr Tt
(Round & Tall)
Now what will happen when this F1 generation self fertilizes?
Rr Tt x Rr Tt
Make sure you have every possible combination of gametes for the individual
Rr Tt
Set up the Punnett Square the same way. One set of gametes across the top and the other down
the side.
Ratio Phenotypes
Genotypes
Epistasis
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The interaction of multiple gene products to produce a phenotype.
Most traits are controlled by multiple genes.
Labrador Coat Color
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Two genes determines coat color.
Pigment (Black, brown) and Deposition (E yes, e no)
Combination of these produce the Black, Brown (chocolate), Yellow phenotypes
BB EE x bb ee
All puppies will be what color?
Now cross 2 Heterozygotes
BbEe X BbEe
List Gametes
In class discussion problems:
How can a brown lab & a yellow lab produce only brown & yellow puppies?
How can a black lab & a yellow lab only have black or brown puppies.
Other Types Of Dominance
Codominance
 Neither allele overpowers the other.
 Each allele is fully expressed.
 A, B, O, BLOOD GROUPS
 A, B defines sugar groups found on Red Blood Cell (R.B.C.)
 An enzyme (protein) adds these to the R.B.C.
 Protein A adds the A sugar group
 Protein B adds the B sugar group
 Individuals who DO NOT synthesize EITHER of these proteins are type O
Alleles – Gene is designated by I
 IA=Put on sugar A.
 IB=Put on sugar B.
 i = O=No sugar.
 Immune system identifies with these sugars as self.
The +/- of blood types is from an additional gene. These two genes are treated as independent ie
a two factor cross (not epistasis).
Although there are 3 alleles for blood type, each individual will only have TWO!!!!
IA IB
Gametes:
X
IA i
Question:
One parent is type A and the other is type B. What are their genotypes if they have
children with A, B, AB, O blood types?
Gametes:
Incomplete Dominance


Heterozygous has intermediate phenotype.
Red flower X White flower
RR
X
rr
Rr=Pink flower.
 Not the same as Co-dominance.
Genotype
RR
Rr
rr
Normal Dominance
Co-Dominance
Incomplete
Dominance
Environmental Factors Influence Gene Expression
 Gene products are proteins.
 Proteins are affected by temp, PH, salt concentration, etc.
 Some gene products appear differently in different environments.
Artic Fox
 Protein production (Pigment) temperature regulated.
 WarmProduce pigmentBrown.
 ColdNo pigmentWhite.
Siamese Cat
 Cooler areas produce pigment.
 Ears/nose & extremities.
 If cat gets obese the layer of fat will insulate the skin from the body heat, entire cat will
now be pigmented.
Genetic Issues
Recessive does not denote good … Dominant does not denote good
 Perfect VisionRecessive
 Huntington’s diseaseDominant
How many alleles are required for Huntington’s disease? How many alleles are required for a
recessive disorder?
Carrier – When referring to a genetic recessive disease the heterozygote is called a carrier. A
carrier individual has one copy of the recessive allele but is not afflicted by the disease
Other Genetic Anomalies
Non Disjunction
 The failures of chromosomes to separate during Anaphase.
 Results in one gamete with too many chromosomes & one gamete with too few.
Down’s Syndrome: Trisomy 21
 Individual has three copies of chromosome 21.
 More common in older mothers.
 Males produce sperm throughout their physically mature lifetime
 Females are born with all their eggs.
 Eggs are stalled at the end of meiosis I.
 Upon physical maturity, 1-2 eggs per month complete meiosis, two are ovulatedOlder
mothers….older eggs. Any damage accumulates over the life-time and can result in non
disjunction in meiosis 2.
Nondisjunction In Sex Chromosomes
 Can occur in both male and females.
XX
Female Gametes
XX

X
XY
Male Gametes
XY
XX
YY
Offspring
XXX Sterile female
XXY Sterile male
XYY Fertile male
X
Sterile female
Y
Non viable
Normal mental
Some female characteristics, possible mental.
(super male)
(short, webbed neck, Turner Syndrome, low mental).
Sex Linked Traits
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These traits are encoded on the X chromosome
The gender of the individual is linked to the expression of these traits
Sex Chromosomes X, Y
XX=Female
XY=Male.
Male sperm carry either X or Y determines gender of offspring.
Female eggs only carry an X for sex chromosome.
Since female have two X chromosomes, they follow standard dominance patterns for gene
carried on X chromosome.
Males only have one X chromosome. Any genes on their one chromosome are automatically
expressed.
Color blindness often said to pass from mother to son. Why?
How many X chromosomes do females
have?
Where do females get each of their X
chromosomes?
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How many X chromosomes do males have?
Where do males get of their X chromosome
from?
Sex linked is x linked
Males their X chromosome is automatically expressed
Females follow normal dominance patterns for traits on the X chromosome
Female XXc
Male XcY
1. What % of daughters will be color blind?
2. What % of sons will be color blind?
3. What % of daughters will be carriers.
**Males cannot be carriers for sex link traits – have the trait or not
Autosomal Disorders … Carried on non-sex chromosome.
 Humans have 23 pairs of chromosomes
o 22 pairs of AUTOSOMES
o 1 pair of Sex Chromosomes
 Autosomal disorders have the same inheritance pattern for both males and females
Genetic Diabetes
 Autosomal recessive disorder
 Must be homozygous recessive to be afflicted by disease
 Either parent can be carrier
 *** Pedigree Assignment
 DD Normal
 Dd Carrier
 dd Affilicted
Female Dd
x
Male Dd
1What % of offspring will have diabetes?
2 What % of offspring will be carriers?
3 What % of offspring will be genotypically homozygous dominant?
How do our genes make us who we are??
Genes are the construction plans for protein
 DNA transcribed into RNA (Single stranded)
 Ribosomes can only read mRNA
 Must transcribe DNA into a form the ribosome can read
 Called mRNA—Messenger RNA
RNA Polymerase
 Enzyme which reads DNA & makes the mRNA copy.
 mRNA copy made by complimentary base pairing to the DNA
 Binds to promoter at the beginning of each gene
 Read DNA one base at a time
Transcription
1. RNA Polymerase binds to promoter
2. Reads one base at a time synthesizing single stranded mRNA from the DNA template
3. mRNA transported to the cytoplasm through the nuclear pores
The Genetic Code
 DNA 4 bases Adenine, Thymine, Guanine, Cytosine
 RNA 4 bases Adenine, Uracil, Guanine, Cytosine
 20 Amino Acids (A.A,).
 How to specify each individual A.A.
 Uses a distinct, unique set of 3 bases.
 Called a codon.
 Each A.A. coded for by at least one codon.
 Special codons.
 Start/F metheonine AUG
 Stop UAA, AAG, UGA
Translation
 Protein synthesis
 Ribosomes read the mRNA.
 Assemble A.A. in order by reading one codon at a time.
How do the A.A. get to the ribosomes?
tRNA—Transfer RNA
 Molecules which bring A.A. to ribosomes.
 Has Anti codon at one end to temporarily base pair to mRNA.
 Has corresponding A.A. at other end
Initiation
 Small ribosome subunit binds to mRNA at start codon AUG.
 Large subunit then binds to complex.
Elongation
 Ribosome moves down mRNA one codon at a timeadding one amino acid at a time.
 tRNA comes in and binds by complimentary base pairing.
 Peptide bond is formed between the new amino acid and the peptide chain
 Continues down mRNA until stop codon reached.
 Transfer RNA only carries specific A.A.
Termination
 Stop codon; signal the two ribosome subunits to break apart
 Completed protein released
Anatomy Of A Gene
Promoter~~~~~~~~~CodingSequence~~~~~~~~~~~~Termination.
Promoter
 Sequence which signals DNA polymerase and RNA polymerase to start
 Will include start codon for ribosome
Coding Sequence
 Sequence of bases specifying the amino acids in the proper order for the protein
Termination
 Sequence which signals the end of the gene
Regulation Of Gene Expression
 Differentiation requires that some of our genes be turned on or off in specific cells.
 Remember, each and every cell has Full set of DNA.
 Cells express different genes according to their functions.
Repressors
 Prevents transcription.
 Bind over Promoter.
 Blocks RNA ploymerase from binding.
Activators
 Increase rate of transcription.
 Bind upstream of Promoter.
 Assist RNA polymerase in binding to DNA.