Chapter 11-3: Exploring Chapter 11-3: Exploring Mendelian GeneticsMendelian Genetics

To determine if the To determine if the segregation of one pair of segregation of one pair of alleles affects the segregation alleles affects the segregation of another pair of alleles, of another pair of alleles, Mendel performed a Mendel performed a two-two-factor crossfactor cross (dihybrid cross). (dihybrid cross).

Independent AssortmentIndependent Assortment

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The Two-Factor Cross: FThe Two-Factor Cross: F1 1   

•Mendel crossed true-breeding plants that Mendel crossed true-breeding plants that produced round yellow peas (genotype produced round yellow peas (genotype RRYYRRYY) ) with true-breeding plants that produced with true-breeding plants that produced wrinkled green peas (genotype wrinkled green peas (genotype rryyrryy).).

•All of the FAll of the F11 offspring produced round yellow offspring produced round yellow peas (peas (RrYyRrYy).).

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The alleles for round (The alleles for round (RR) and yellow ) and yellow ((YY) are dominant over the alleles for ) are dominant over the alleles for wrinkled (wrinkled (rr) and green () and green (yy).).

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The Two-Factor Cross: FThe Two-Factor Cross: F22  

•Mendel crossed the heterozygous FMendel crossed the heterozygous F11 plants (plants (RrYy)RrYy) with each other to with each other to determine if the alleles would segregate determine if the alleles would segregate (separate) from each other in the F(separate) from each other in the F22 generation.generation.

RrYy RrYy ×× RrYyRrYy

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The Punnett square predicts a The Punnett square predicts a

9 : 3 : 3 :1 9 : 3 : 3 :1 phenotype ratiophenotype ratio in the F in the F22 generation.generation.

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dihybrid

Phenotype ratio from dihybrid cross is

or 9:3:3:1

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9 3 3 116 16 16 16: : :

-9 of 16 show both -9 of 16 show both dominant traitsdominant traits-3 of 16 show one dom., -3 of 16 show one dom., one recessiveone recessive-3 of 16 show one rec., -3 of 16 show one rec., one dom.one dom.-1 of 16 show both -1 of 16 show both recessiverecessive

Dihybrid phenotype ratio:Dihybrid phenotype ratio:

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Law of independent Law of independent assortmentassortment

•During meiosis, genes for During meiosis, genes for different traits are different traits are separated and distributed separated and distributed to gametes to gametes independentlyindependently of one another of one another (if the (if the genes are on different genes are on different chromosomes).chromosomes).

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The alleles for seed shape segregated The alleles for seed shape segregated independently of those for seed color. independently of those for seed color. This principle is known as This principle is known as independent assortmentindependent assortment..

Genes that segregate independently Genes that segregate independently do notdo not influence each other's influence each other's inheritance.inheritance.

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Trihybrid!

A Summary of Mendel's PrinciplesA Summary of Mendel's Principles• Genes are passed from parents to Genes are passed from parents to

their offspring. their offspring. • If two or more forms (alleles) of If two or more forms (alleles) of

the gene for a single trait exist, the gene for a single trait exist, some forms of the gene may be some forms of the gene may be dominantdominant and others may be and others may be recessiverecessive..

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• In most sexually reproducing organisms, In most sexually reproducing organisms, each adult has two copies of each gene. each adult has two copies of each gene. These genes are segregated from each These genes are segregated from each other when gametes are formed.other when gametes are formed.

• The alleles for different genes usually The alleles for different genes usually segregate independently of one segregate independently of one another. another.

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Incomplete DominanceIncomplete Dominance

•When both alleles When both alleles contribute to the contribute to the phenotype of a phenotype of a heterozygous individual to heterozygous individual to produce a trait not exactly produce a trait not exactly like either parent.like either parent.

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A cross A cross between red between red ((RRRR) and ) and white (white (WWWW) ) four o’clock four o’clock plants plants produces produces pink-colored pink-colored flowers (flowers (RWRW).).

WW

RR

1: 2: 1

Think of pink as anThink of pink as an““incomplete”incomplete”

red colorred color

CodominanceCodominance

•2 dominant alleles are 2 dominant alleles are expressed at the same expressed at the same time. They both contribute time. They both contribute to the phenotype.to the phenotype.

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Codominance: roan fur coat11-3

Codominance:Codominance:Human blood typesHuman blood types

3 Alleles: I3 Alleles: IAA, I, IBB, i, i

6 genotypes:6 genotypes:

IIAAIIAA, I, IAAIIBB, I, IAAi, Ii, IBBIIBB, I, IBBi, and ii i, and ii

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Having more Having more than 2 alleles than 2 alleles

is calledis calledMultiple Multiple allelesalleles

ABO Blood Group SystemABO Blood Group System

GenotypeGenotype Blood TypeBlood Type

IIAAIIAA or I or IAAii AA

IIBBIIBB or I or IBBii BB

IIAAIIBB ABAB

iiii OO

Multiple alleles:Multiple alleles:

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Polygenic TraitsPolygenic Traits    

•Traits controlled by two or more Traits controlled by two or more genes are said to be genes are said to be polygenic polygenic traits.traits.

•Skin color in humans is a Skin color in humans is a polygenic trait controlled by polygenic trait controlled by more than four different genes.more than four different genes.

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HumanHumanSkinSkinColorColor

Polygenic TraitsPolygenic Traits

One more thing:One more thing:X Inactivation in female X Inactivation in female mammalsmammals

http://arbl.cvmbs.colostate.edu/hbooks/genetics/medgen/chromo/mosaics.html

In a cross involving two pea plant traits, In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the observation of a 9 : 3 : 3 : 1 ratio in the FF22 generation is evidence for generation is evidence for

• the two traits being inherited the two traits being inherited together.together.

• an outcome that depends on the sex an outcome that depends on the sex of the parent plants.of the parent plants.

• the two traits being inherited the two traits being inherited independently of each other.independently of each other.

• multiple genes being responsible for multiple genes being responsible for each trait.each trait.

Traits controlled by two or more Traits controlled by two or more genes are called genes are called

• multiple-allele traits.multiple-allele traits.

• polygenic traits.polygenic traits.

• codominant traits.codominant traits.

• hybrid traits.hybrid traits.

In four o'clock flowers, the alleles for red In four o'clock flowers, the alleles for red flowers and white flowers show flowers and white flowers show incomplete dominance. Heterozygous incomplete dominance. Heterozygous four o'clock plants have four o'clock plants have

• pink flowers.pink flowers.

• white flowers.white flowers.

• half white flowers and half red flowers.half white flowers and half red flowers.

• red flowers.red flowers.

A white male horse and a tan female horse A white male horse and a tan female horse produce an offspring that has large areas of produce an offspring that has large areas of white coat and large areas of tan coat. This white coat and large areas of tan coat. This is an example ofis an example of

• incomplete dominance.incomplete dominance.

• multiple alleles.multiple alleles.

• codominance.codominance.

• a polygenic trait.a polygenic trait.

Mendel's principles apply to Mendel's principles apply to

• pea plants only.pea plants only.

• fruit flies only.fruit flies only.

• all organisms.all organisms.

• only plants and animals.only plants and animals.

Chimeras: combination of two Chimeras: combination of two very early embryos such that very early embryos such that their cells intermix and the their cells intermix and the resulting “conceptus” has cells resulting “conceptus” has cells from both original embryos from both original embryos

Sheep

Goat

http://arbl.cvmbs.colostate.edu/hbooks/genetics/medgen/chromo/mosaics.html

Chimeric mice and sheep-goat chimeras Chimeric mice and sheep-goat chimeras have been most useful in answering have been most useful in answering fundamental questions about fundamental questions about developmental biology and pathology. developmental biology and pathology. There is also some potential that this There is also some potential that this technique can be applied to problems technique can be applied to problems such as such as rescue of endangered speciesrescue of endangered species.. It It is possible, for example to construct a is possible, for example to construct a goat-sheep chimera such that a goat goat-sheep chimera such that a goat fetus is "encased" in a sheep placenta. fetus is "encased" in a sheep placenta. This enables a sheep to carry a goat to This enables a sheep to carry a goat to term, which will not occur if you simply term, which will not occur if you simply transfer goat embryos into sheep (the transfer goat embryos into sheep (the sheep will immunologically reject the sheep will immunologically reject the goat placenta and fetus). It may be goat placenta and fetus). It may be possible to extend this procedure to allow possible to extend this procedure to allow embryos from severely endangered embryos from severely endangered species to be carried by recipient mothers species to be carried by recipient mothers from another species.from another species.