Slide 1 of 31 Copyright Pearson Prentice Hall Biology.

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Copyright Pearson Prentice Hall

Biology

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11-3 Exploring Mendelian Genetics11–3 Exploring Mendelian Genetics

11–3 Exploring Mendelian Genetics

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Independent Assortment

To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two-factor cross – dihybrid cross (1)



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The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment (2)

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



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The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes.

Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.



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A Summary of Mendel's Principles


• Genes are passed from parents to their offspring.

• If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.


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

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



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Beyond Dominant and Recessive Alleles

Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.


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Incomplete Dominance

When one allele is not completely dominant over another it is called incomplete dominance (1)

In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes.

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A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).


WW

RR


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Codominance

In codominance (4)both alleles contribute to the phenotype.

In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.

Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately.


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Multiple Alleles

Genes that are controlled by more than two alleles are said to have multiple alleles (5)

An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.

A rabbit's coat color is determined by a single gene that has at least four different alleles.


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Different combinations of alleles result in the colors shown here.

Full color: CC, Ccch, Cch, or CcChinchilla: cchch, cchcch, or cchcHimalayan: chc, or chchAIbino: cc

KEY

C = full color; dominant to all other alleles

cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles

ch = Himalayan; color in certain parts of the body; dominant to c allele

c = albino; no color; recessive to all other alleles


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Polygenic Traits

Traits controlled by two or more genes are said to be polygenic traits (6)

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


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Applying Mendel's Principles

Applying Mendel's Principles

Thomas Hunt Morgan used fruit flies to advance the study of genetics.

Morgan and others tested Mendel’s principles and learned that they applied to other organisms as well as plants.

END OF SECTION

Slide 1 of 31 Copyright Pearson Prentice Hall Biology.

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