Mendelian Genetics Introduction to Genotype and

Phenotype

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Genetic TerminologyGenetic TerminologyTrait Trait - any characteristic - any characteristic

that can be passed from that can be passed from parent to offspring parent to offspring

HeredityHeredity - passing of traits - passing of traits from parent to offspring from parent to offspring

GeneticsGenetics - study of - study of heredity heredity

copyright cmassengale

Genotype is… our genetic make-upwe cannot see our genes!

We get one copy of our genes from our mother and one from our father. The genes on these chromosomes might code for different forms of the same trait. Different forms of genes are called alleles.

Image retrieved from http://www.emunix.emich.edu/~rwinning/genetics/inherit2.htm

Genotype is determined…

by how the parent’s alleles combine during reproduction.

Here, the mother has only the “a” form of the gene, while the father has only the “A” form of the same gene. In this scenario, the offspring can have only one genotype: Aa

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Determining Genotypes

Here, each parent has both the “A” and the “a” allele for this gene.

The offspring can have any of the following genotypes: AA, Aa or aa

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So what?The genotype of an organism is the code for the physical features of the organism (phenotype) Phenotype is the physical appearance of the organism

For example, the allele, “A” might code for a normal skin pigment, while the “a” allele might code for albinism.

Image retrieved from http://www.bbc.co.uk/schools/gcsebitesize/biology/variationandinheritance/0dnaandgenesrev5.shtml

What is the relationship between genotype and phenotype?

Image retrieved from http://bio1151.nicerweb.com/doc/class/bio1151/Locked/media/ch14/14_06PhenotypeVsGenotype_L.jpg

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More TerminologyMore Terminology

GenotypeGenotype - - gene gene combination for a traitcombination for a trait (e.g. RR, Rr, rr) (e.g. RR, Rr, rr)

PhenotypePhenotype - - the physical the physical feature resulting from a feature resulting from a genotypegenotype (e.g. red, white) (e.g. red, white)

How is phenotype determined?Generally, certain alleles of a gene completely

override other alleles of the same gene (they are completely dominant).

The allele for a tall plant is dominant in this example. If the tall parent has only tall alleles, all of its offspring will receive a tall allele from this plant. Since the tall allele is dominant to the short allele, all of the offspring will be tall.

Image retrieved from http://web.pdx.edu/~cruzan/Kid

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Punnett SquarePunnett Square

Used to help Used to help solve solve genetics genetics problemsproblems

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Designer Designer “Genes”“Genes”AllelesAlleles - - two forms of a two forms of a gene gene

(dominant & recessive)(dominant & recessive) DominantDominant - - stronger of two genes stronger of two genes

expressed in the hybrid; expressed in the hybrid; represented byrepresented by aa capital letter (R)capital letter (R)

RecessiveRecessive - - gene that shows up gene that shows up less often in a cross; represented less often in a cross; represented by aby a lowercase letter (r)lowercase letter (r)

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Genotype & Phenotype in Genotype & Phenotype in FlowersFlowers

Genotype of alleles:Genotype of alleles:RR = red flower= red flowerrr = yellow flower= yellow flower

All genes occur in pairs, so All genes occur in pairs, so 22 allelesalleles affect a characteristic affect a characteristic

Possible combinations are:Possible combinations are:

GenotypesGenotypes RRRR RRrr rrrr

PhenotypesPhenotypesRED RED RED RED YELLOWYELLOW

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GenotypesGenotypesHomozygousHomozygous genotype - gene genotype - gene

combination involving 2 combination involving 2 dominant or 2 recessive genes dominant or 2 recessive genes (e.g. RR or rr);(e.g. RR or rr); also called also called pure pure 

HeterozygousHeterozygous genotype - gene genotype - gene combination of one dominant & combination of one dominant & one recessive allele    (one recessive allele    (e.g. Rr);e.g. Rr); also calledalso called hybridhybrid

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Trait: Seed ShapeTrait: Seed Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: Cross: RoundRound seedsseeds xx Wrinkled Wrinkled seedsseeds

RRRR xx rr rr

PP11 Monohybrid Cross Monohybrid Cross

R

R

rr

Rr

RrRr

Rr

Genotype:Genotype: RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: All alikeAll alike

PhenotypicPhenotypicRatio:Ratio: All alike All alike

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PP11 Monohybrid Cross Monohybrid Cross ReviewReview

Homozygous dominant x Homozygous dominant x Homozygous recessiveHomozygous recessive

OffspringOffspring allall HeterozygousHeterozygous (hybrids)(hybrids)

Offspring calledOffspring called FF11 generation generation

Genotypic & Phenotypic ratio Genotypic & Phenotypic ratio isis ALL ALIKEALL ALIKE

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Trait: Seed ShapeTrait: Seed Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: Cross: RoundRound seeds seeds xx Round Round seedsseeds

RrRr xx Rr Rr

FF11 Monohybrid Cross Monohybrid Cross

R

r

rR

RR

rrRr

Rr

Genotype:Genotype: RR, Rr, RR, Rr, rrrr

PhenotypePhenotype: Round Round && wrinkled wrinkled

G.Ratio:G.Ratio: 1:2:11:2:1

P.Ratio:P.Ratio: 3:1 3:1copyright cmassengale

Given alleles T(tall) and t(short)

…-What are the possible genotypes of a tall plant?

-What would be the phenotype of Tt?-What would be the phenotype of tt?

-What would be the phenotype of TT?

-What are the possible genotypes of a short plant?

In a famous experiment, a man named Gregor Mendel mated pea plants.

When he mated a pure bred smooth seed with a pure bred wrinkled seed, he found that the wrinkled seed phenotype only occurred about 25% of the time.

This happened because the allele for smooth seeds (S) is dominant to the allele for wrinkled seeds (s).

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Mendel and the relationship between genotype and phenotype

Click on the link that appears below to see an animation about Gregor Mendel and the relationship between genotype and phenotype

**Once you get to the sight, click on animation at the

bottom of the page to see the animation!!**

http://www.dnaftb.org/dnaftb/2/concept/index.html

Go to the following link and see how changing the genotype of a fictional dragon will change its phenotype.

http://biologica.concord.org/webtest1/web_labs_genophenotype.htm

Then go to the following link and watch the video. This may help you remember what you have learned.

http://youtube.com/watch?v=EvR_Sdm1orU