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GENETICS
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Genetics:
• The study of how traits encoded in our DNA are passed on.
• Used to predict the possible outcomes of a genetic cross–Depends on probability
–We might expect 2/4 offspring to look a certain way, and end up with 0/4 looking that way!
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History of Genetics as a science:
Gregor Mendel is the “Father of Genetics” - Austrian monk who studied pea plants to figure out how parents pass traits to their offspring
- Started with parents that were True breeding - means that when they self fertilize, their
offspring all look like them.
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Parents: (both true breeding)
white x purple
Expect???
What he got:
So… he crossed two of them….
Expect???
What he got:
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These crosses showed that there were “factors” being passed from parent to offspring even if it wasn’t being “used”
Now we call these factors GENES
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Genes – control a heritable feature;
Example: Hair color, seed shape, height;
Allele – controls the variation of a feature.Example: brown, blonde, black hair
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Characteristic/Gene?
Trait/Allele?
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CHARACTERS AND VARIANTS OF EACH
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TRAITS
?
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TRAITS
?
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RARE DOMINANT PHENOTYPE
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A chromosome = folded up string of many genes
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What are alleles?
Variations of a gene that occupy the same locus on homologous
chromosomes
Locus = position on a chromosome.
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P
pT
t
LONG
SHORT
GENE = FLOWER COLORGENE = STEM LENGTH
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Terms• Diploid (2n)• HAPLOID (n)• Egg• Sperm• Parent• Meiosis• Testes
• Gamete
• Zygote
• Progeny
• Offspring
• Fertilization
• Ovary
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Mendel’s laws of genetics
1. Law of segregation: only one allele for each gene is passed from a parent to the offspring.
Why? Has to do with separation of homologous chromosomes during meiosis.
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Segregation of Alleles
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Tongue Rolling
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2. Law of independent assortment:
Alleles for different genes are passed to offspring independently of each other.
The result is that new combinations of genes present in neither parent is possible.
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3. Law of complete dominance – some alleles over power others. So even if both alleles are present, we only “see” the dominant one.
- the “hidden” allele is called recessive
This only applies to SOME genes, not all.
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Remember Mendel’s pea plants?
- Purple was crossed with white and we got ALL purple. So which allele is dominant?
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Genotype: the alleles that an organism has.
- alleles are abbreviated using the first letter of the dominant trait. (with some exceptions that we will get to)
- a capital letter represents the dominant
ex: P for purple flower allele
- a lower case represents the recessive.
ex: p for white flower allele
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All diploid organisms have two alleles for each trait:
- you can have two of the same alleles
Ex: PP or pp
- such an individual is described as Pure or Homozygous.
OR
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All diploid organisms have two alleles for each trait:
- you can have two different alleles
Ex: Pp
- such an individual is described as hybrid or heterozygous
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Phenotype: physical appearanceExamples: brown hair, widows peak
- the trait that “wins” in the case of complete dominance;
- depends on the combination of alleles
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P generation: “parents;” First generation in the cross
F generations: results of the cross;
- F1 – 1st generation; offspring of P generation
- F2 – 2nd generation; offspring of F1 generation
Terminology for Genetic Crosses
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Monohybrid cross: cross that focuses on the alleles of a single trait;
How do we show the possibilities?
- punnett square
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PUNNETT SQUARE
Allele in Egg 1
Zygote formed if sperm 1
fertilizes egg 1
Allele in Egg 2
Allele in sperm 1
Allele in sperm 2
Zygote formed if sperm 2
fertilizes egg 1
Zygote formed if sperm 1
fertilizes egg 2
Zygote formed if sperm 2
fertilizes egg 2
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In pea plants tallness is dominant to short or dwarf. Cross a pure tall male to a pure dwarf female pea plant. Show both ratios phenotype & genotype for the offspring. Now cross two of the F1.
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• Take it step by step until you “get it”• Step 1: what are the parent’s genotypes?
–Mom?–Dad?
tt
TT
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• Step 2: Set up Punnett Square
t t
T
T
Tt Tt
TtTt
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• Step 3: ANSWER THE QUESTION
t t
T
T
Tt Tt
TtTt
Offspring genotypes:
Offspring phenotypes:
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• Step 4: Part II
T t
T
t
TT Tt
t tTt
Offspring genotypes:
Offspring phenotypes:
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Inheritance Patterns:
Every gene demonstrates a distinct phenotype when both alleles are combined (the heterozygote)
Complete dominance is one - when both alleles are present, only the dominant trait is seen.
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Inheritance Patterns:
Incomplete dominance - when both alleles are present, the two traits blend together and create an intermediate trait
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INCOMPLETE DOMINANCE
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Inheritance Patterns:
Co-dominance - when both alleles are present, both traits are visible
Different notation: Use first letter of the feature with a superscript for the trait. Example: CW or CB for white coat or black coat;
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Inheritance Patterns:
Co-dominance - when both alleles are present, both traits are visible
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Inheritance Patterns:
Each gene has a specific inheritance pattern. - you will either be told or be given a hint; look at the heterozygote!
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Women have two X’s but men only have one.
How do we deal with the genes on the X chromosome?
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Sex-linked traitAlleles for the trait are located on the X chromosome in humans.
- works the same in women as all the other traits.
BUT –
- men only inherit one such allele.
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Sex-linked traitFor females: have to figure out phenotype based on inheritance pattern.
For Males: phenotype is that of whatever allele they inherit.
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Example: color blindness
Seeing color (XC) is dominant to being color blind (Xc)
Identify the sex and trait of the following:
XCY XCXc XCXC
XcXc XcY
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Example: Color Blindness
Set up a punnett square crossing a heterozygous normal female with a normal male:
- what is mom’s genotype?
- what is dad’s genotype?
- what gametes can each give?
- what are the offspring’s geno’s?
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XC
Cross Number 1:
Xc
XC
Y
XCXC XCXc
Xc YXC Y
What % chance of having color blind daughter?
Son?
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SEX-LINKED TRAITS
COLOR BLINDNESS
AFFLICTS 8% MALES AND 0.04% FEMALES.
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If we are dominant, how can we figure out our genotype?
What are the possibilities?
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Test cross: a cross that determines genotype of dominant parent
- Cross unknown dominant parent (possibilities BB or Bb)
with a recessive parent
then analyze the offspring.
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B ?
b
b
Bb ?b
?bBb
If some of the offspring have the recessive trait, then the unknown parent has to be heterozygous
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B ?
b
b
Bb ?b
?bBb
If all offspring are dominant, unknown parent HAS to be
homozygous
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Multiple alleles: Some genes have more than two variations that exist, although we still only inherit 2
Example: Human blood types
Three alleles:
IA
IB
i
Genotype Phenotype
IAIA A
IAi A
IBIB B
IBi B
IAIB AB
ii 0
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Polygenic –
Multiple genes each with 2 alleles
Creates additive/
quantitative effect
SKIN PIGMENTATION
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Dihybrid cross:
A cross that focuses on possibilities of inheriting two traits
- two genes, 4 alleles
Black fur is dominant to brown fur
Short fur is dominant to long fur
What is the genotype of a guinea pig that is heterozygous for both black and short fur?
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Dihybrid cross:
Parent phenotypes: BbSs x BbSs
Figure out the possible gametes:
Then set up punnett square
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Dihybrid cross:
BS Bs bS bs
BS
Bs
bS
bs
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Linked Genes: genes that are on the same chromosome.
Does the law of independent assortment apply?
Can they be separated? Will they always separate?
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What does this mean? • It means that you can pass on an allele that
you got from your mom and an allele you got from your dad ON THE SAME CHROMOSOME
• However, it is more likely that two alleles that start on the same chromosome will get passed on together.