Mendelian Genetics CH 6 Section 6.3 – 6.5. Slide 2 of 26.

Mendelian Genetics

CH 6Section 6.3 – 6.5

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??

Sometimes, certain traits seem to disappear for a generation but then return in the next generation.

How does this happen?

Is this a “law” or “theory”? Law since we are describing something, not

trying to explain it.

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Vocabulary

Character – heritable feature Hair Color Height of plant Eye color Pea color Pea shape

Trait – variant of a character (heritable feature) Brown hair color Tall plant height Blue eye color Yellow pea color

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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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Breeding

True Breed (purebred)– 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 (purebred)

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 Dominant to Recessive ratio

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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 purebreds for 1 trait, and pollinated it with a purebred 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 Dominant to Recessive trait

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___ GenerationWhat type of plants?____________



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Genes

Gene Portion of DNA that provides the instructions for making

a particular protein Each gene has a particular location on a chromosome

called its locus Like a house’s address

Allele One of the different forms of the gene Mendel looked at genes that had ONLY 2 alleles

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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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Allele Dominance

Just as there are dominant and recessive traits, there are dominant and recessive alleles

Dominant allele If present, always expressed Represented by a capitalized letter: A

Recessive allele Only expressed when BOTH alleles are recessive is represented by a lowercase letter: a

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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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Monohybrid vs. Dihybrid

Monohybrid Cross – Take pure breeds for 1 character and cross (AA x aa) You should already understand 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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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

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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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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?

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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 separates independently (of other

allele pairs) into gametes Genes assort independently of one another

Mendelian Genetics CH 6 Section 6.3 – 6.5. Slide 2 of 26.

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