Mendelian Genetics

Chapter 14

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Mendel’s Big Ideas

The Law of Segregation The 2 alleles of a gene separate (segregate) during gamete formation, so that a sperm or egg only carries 1 allele of each pair Explains 3:1 ratio found in hybrid crosses

The Law of Independent Assortment Each pair of alleles segregates into gametes independently of other pairs Explains 9:3:3:1 ratio found in hybrid crosses

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Vocabulary

Character – heritable feature Trait – variant of a character (heritable feature) True Breed – Whatever traits the parent has are expressed in ALL subsequent populations

For example, self-pollinating a purple flowered plant produces a generation of only purple flowered plants. Typically, need to do this for at least two (2) generations to ensure that the parent is a true-breed

Hybrid – Mating (crossing) of two (2) true-breeding varieties of true-breeds

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Vocabulary (Page 2)

P Generation – Parent generation Two (2) true-breeding parents being crossed

F1 – First filial (child or son) generation Hybrids

F2 – Second filial generation Each member of F1 self-pollinates Hybrids again 3:1 ratio and 9:3:3:1 ratio when 2 characters are considered

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More Vocab

Dominant trait One that will mask the recessive trait if found together

Recessive trait Trait that will be masked if found with the dominant trait

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What Mendel found (stretched the truth about)

Only looked at “all-or-nothing” traits Sometimes called binary traits – “yes” vs. “no”

Mendel took true breeding for 1 trait, and pollinated it with a true breed for another trait What is the difference between trait & character? Purple flowered + White flowered

F1 = All colored purple (all expressed only one trait)

F2 = 3:1 ratio of one trait to the other

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___ GenerationWhat type of plants?____________

___ GenerationWhat type of plants?____________

___ GenerationWhat type of plants?____________

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Mendel’s Model

1. Alternate versions of genes account for variations in inherited characteristics The alternative versions are called alleles One plant had the allele for purple flower color while the other had the allele for white flower color

2. For each character, an organism inherits two (2) alleles, one from each parent The two alleles may be the same or different

If the 2 alleles are the same = Homozygous Different = Heterozygous

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Mendel’s Model (Page 2)

3. If the alleles differ (heterozygous), then the dominant allele determines the organism’s appearance Heterozygous individuals display dominant trait Homozygous Dominant display _________ trait Homozygous Recessive display _________ trait

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Mondel’s Model (Page 3)

4. Law of Segregation 2 alleles for a heritable character segregate (separate)

during gamete formation and end up in different gametes

We already know that homologous chromosomes assort independently during meiosis into gametes But Mendel did not know about chromosomes

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Questions

What is the difference between a gene and a character?

What is the difference between a gene and an allele?

What is the difference between a character and a trait?

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Important Vocab.

Phenotype - appearance Characters

Traits are different types of the character IF character is eye color, trait is brown, blue, etc

Genotype - genetic makeup Genes

Alleles are different types of genes Gene for eye color, alleles = brown (dominant), blue

(recessive), etc.

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Punnett Squares

One parent at top, other on the left

Here we are crossing homozygous dominant (HD) in the form of [AA] with Heterozygote (Ht) as [Aa]

A A

A AA AA

a Aa Aa

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Cross all 6 combinations

Cross Genotypic Ratio

Phenotypic Ratio

HomoD x HomoD

HomoR x HomoR

Heter x Heter

HomoD x HomoR

HomoD x Heter

HomoR x Heter

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Pattern Recognition

What happens when you have 1 parent who is homozygous Dominant (HD)?

What must the parental genotypes be to get 100% recessive phenotype offspring?

What must the parental genotypes be to get 1:1 phenotypic ratio of offspring?

What about 3:1 phenotypic ratio of offspring?

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TestCross

A dominant phenotype crossed with a recessive phenotype We know the genotype of recessive phenotype (hr) We do NOT know the genotype of the dominant

phenotype (could be HD or Ht)

So we turn the lights down, put on some Maxwell, and let nature take its course... If all the offspring are dominant phenotype, then? If the offspring are 1:1, then?

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Monohybrid vs. Dihybrid

Monohybrid Cross – Take pure breeds for 1 character and cross (AA x aa) We got these

Dihybrid Cross – Take pure breeds for 2 characters and cross (AABB x aabb) 2 Characters like seed color + seed shape

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Repeat but use Probability

Compute P(green & wrinkled) from the table Now do so from Punnett Squares for each character

Compute P(Yellow & Round) from table Now do so from Punnett Squares for each character

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Law of Independent Assortment

States that each pair of alleles segregates independently of other pairs of alleles during gamete formation This rule really only pertains to genes (allele pairs) on different chromosomes

If on the same chromosome = linked genes Cannot do Mendelian genetics with linked genes, so we save this for next chapter (CH 15)

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Probability Laws

Multiplication rule – probability of a compound event is equal to the product of the individual probabilities of the independent events P(flip coin twice / Both times heads) = * = ½ ½ ¼

Addition rule – If an event that can occur in two or more independent, mutually exclusive ways, its probability is the sum of the individual probabilities P(roll dice / Get “1” or “2”) = [1/6 + 1/6] = 2/6 = 1/3

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Probability Basics

Probabilities range from 0 to 1

0 = Not gonna happen 1 = gonna happen

P(event) = Probability of event occurring

Sum of probabilities of all possible outcomes = 1 P(heads) + P(Tails) = 1

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Multiplication Rule (AND Rule)

If I flip a coin twice, what is the P(heads on first flip AND second)? P(heads) = ½ P(heads on 1st flip) = ½ P(heads on 1st flip) = ½ P(Heads on flip 1 AND 2) = ½ * ½ = ¼

Only caveat: Are the flips independent of each other? If NOT, cannot use the multiplication rule

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Addition Rule (OR rule)

When there are 2 or more ways independent & mutually exclusive ways to get some result, you can add up the individual probabilities for the compound result

What is the probability of rolling a “4” or higher on a die?

= P(4) or P(5) or P(6)= 1/6 + 1/6 + 1/6= 3/6 = 1/2

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Bringing Laws Together

What is the probability that if you flip a coin twice, it will only land heads once? P(flip coin 2x / Heads once & Tails once) = P(Heads 1st AND Tails 2nd) OR P(Tails 1st AND Heads 2nd) = [ * ] + [ * ] = + = ½ ½ ½ ½ ¼ ¼ ½

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Problems

In a dihybrid cross, what is the possibility of heterozygous in one trait and homozygous recessive in the other, given both heterozygous parents?

In a trihybrid cross, what is the probability of getting exactly 2 recessive phenotypes, if one parent is heterozygous for all 3 genes and the other is homozygous recessive for 2 genes, and hetero for the 3rd?