WHAT IS GENETICS?
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Transcript of WHAT IS GENETICS?
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WHAT IS GENETICS?
GENETICS is the study of how traits are passed from parent to offspring in the form of Genes.
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HISTORY!Gregor Mendel
Born 1822
Austrian Monk
Examined reproduction of pea plants
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Plants reproductive organs
are called FLOWERS
A flower has both male and female parts.
The pea plants Mendel was working with were typically TRUE-BREEDING, meaning they self-pollinated
EX, TALL pea plants would always be pollinated by tall pea plants and produce tall offspring!
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WHAT WE KNOW (MENDEL DIDN’T)
Genes – control a heritable feature (characteristic);
Example: Hair color, seed shape, height;
Allele – controls the variation of a feature (trait).
Example: brown, blonde, black hair
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REVIEW TIME: What are homologous chromosomes???
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Homologous chromosomes may……-Both have the same alleles HOMOZYGOUS (aka: pure or true-breeding)-Both have different alleles HETEROZYGOUS (aka: hybrid)
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Mendel’s IdeaCross two pea plants with different contrasting traits!
Ex: First cross : Crossed true breeding purple with
true-breeding white plants.
Called offspring F1 GenerationResults were that offspring were_100%
PURPLE_Had the white allele disappeared????
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Mendel’s Law of 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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Second cross two of the purple F1 Offspring
Called offspring the F2 Generation
Results- 75 % purple- 25 % were white
White trait had reappeared!
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“The Traits (genes) Mendel looked at
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Mendel’s Law of Segregation
during meiosis, the pair of alleles in a parent will separate.
Only ONE allele for EACH TRAIT will pass from each parent to the offspring
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Ex. sugar beet preference.dominant allele (A) prefers sugar beetsrecessive allele (a) does not. Heterozygote produces gametes
50% chanceGet AGet a
Question: If a heterozygous sugar beet eater marries a non-sugar beet eater, what possible offspring could they have?
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Mendel’s 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.
How many allele combinations could the following genotype produce?RRYYRRYyRrYy
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Genetic TermsDiploid (2n)- Two sets of chromosomes.
Somatic Cells
Haploid (n)- One set of homologous Chromosomes(gametes)
Egg- Female haploid gamete
Sperm- male haploid gamete
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Parent – Seriously, you should know this
Meiosis – Cell division that produces haploid gametes
Testes – Site of male meiosis
Gamete – Haploid sex cell (sperm, egg, pollen)
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Zygote- Single cell (result of sperm and egg)
Progeny - Offspring
Offspring – see above
Fertilization – gametes fuse into zygote
Ovary- site of female meiosis - eggs
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Genotype: the alleles that an organism has.
- alleles are abbreviated using the first letter of the dominant trait.
- capital letter represents the dominant - ex: P for purple flower allele
- lower case represents the recessive. - ex: p for white flower allele
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All diploid organisms have two alleles for each trait:
Can be two of the same alleles Ex: PP or pp called Pure or Homozygous.
ORCan be two different alleles
Ex: Pp described as Hybrid or Heterozygous
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Phenotype: physical appearance
Examples: brown hair, widows peak, purple flowers
the trait that “wins” in the case of complete
dominance;
depends on the combination of alleles
GENOTYPE
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MENDEL’S CROSSES
P Generation: “parents;”
F1 Generation offspring of P generation
F2 Generation offspring of F1 generation
Punnet Squares
How we show allele combinations in crosses
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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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Monohybrid CrossTall vs. Short Example
Tall allele T Short allele t
P Cross TT x tt
F1 GenerationGenotypes
Phenotypes
T T
t
t
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F2 Generation
F1 Generations 100% Tt
Tt x Tt
F2 Generation
Genotypes-
Ratio =
Phenotypes-
Ratio =
t
tT
T
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Sample ProblemsHomozygous Tall x Heterozygous Tall
Heterozygous Tall x Homozygous Short
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ProbabilityProbability is only the LIKELIHOOD of an event
happening.
It does not mean it is what HAS to happenEx. Coin Toss. Two tosses, always one heads and
one tails?
What happens when we look at very large samples?Ex. Male/female ratio of a family vs. the world!
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INHERITENCE PATTERNSEvery gene demonstrates a distinct phenotype when
both alleles are combined (the heterozygote)
Complete dominance is when both alleles are present, only the dominant trait is seen.
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 CR for white petals or red petals;
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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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ProbabilitiesQuestion 1: What is the probability of having a
female offspring?
Question 2: After having 4 sons in a row, what is the probability the next kid will be male?
Question 3: What is the probability of having three daughters in a row?
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Sex-Linked TraitsRefers to traits coded by genes found on the X
chromosomeFemales will have 2 copies of these genesMales will have 1 copy of these genes
Significance???
If males get a bad (recessive) allele for a sex-linked trait, THEY WILL EXPRESS THE RECESSIVE TRAIT!
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Example – Color BlindnessSeeing 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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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 TRAITSCOLOR BLINDNESS
AFFLICTS 8% MALES AND 0.04% FEMALES.
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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.
Ex. B- Black Hair b- white hair
You are given a black-haired guinea pig and need to determine whether homozygous dominant or heterozygous.
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Multiple AllelesGenes may have more than two alleles.
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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 PhenotypeIAIA A
IAi AIBIB B
IBi BIAIB ABii 0
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Polygenic –
Multiple genes code for a trait each with 2 allelesExamples in humans:
Skin ColorEye ColorHeight
Why so many possibilities???
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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Di-hybrid Cross Generalization
Laws of probability indicate a 9:3:3:1 phenotypic ratio of F2 offspring resulting in the following:
9/16 of the offspring are dominant for both traits3/16 of the offspring are dominant for one trait
and recessive for the other trait3/16 of the offspring are dominant and recessive
opposite of the previous proportions; and 1/16 of the offspring are recessive for both traits.
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Linkage and Gene MapsWhen genes are one separate chromosomes,
they independently assort.
If on the same chromosome, they will rarely separate and be inherited together (gene linkage)
Actually it is the chromosomes that assort independently, not the genes. Mendel was just lucky with the genes he was looking at!
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Crossing over in meiosis often separates linked genes.
The distance between the two genes on the same chromosome are from each other affects the frequency of separation from each other during crossing-over.Further Apart Closer together
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The frequency of crossing over between genes is actually an indicator of how far apart different genes are located from each other on the same chromosome.
Use the frequency rates to make gene maps that show relative locations of genes with respect to each other.
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