Exploring Mendelian Genetics

11.3

Biology

Mr. Hines

What happens when you consider more than one gene when you

cross 2 parents?

Independent assortment

Mendel performed more experiments with the pea plants to see what would happen if 2 genes were followed.

This is known as a Two-Factor Cross or a Dihybrid cross.

The Two Factor Cross: F1Mendel isolated the following pea plants

1. Plants that produced only round yellow peas (genotype RRYY – homozygous dominant)

2. Plants that produced wrinkled green peas (genotype rryy – homozygous recessive)

Results – all of the F1 offspring produced round yellow peas.

Show dihybrid segregation

RRYY

rryy

Question

If mom has dominant round and yellow seeds and dad has recessive wrinkled green seeds, can the offspring have all 4 possibilities?

Round yellow

Round green

Wrinkled yellow

Wrinkled green

Draw dihybrid punnett square

100% of the offspring were Round and Yellow.

Where did the wrinkled green trait go?

This did not show whether genes will assort independently

It did however set up heterozygous offspring (RrYy)

All offspring are heterozygous dominant for Round yellow seeds.

The Two factor cross: F2

Mendel knew that all plants were heterozygous dominant for Round, yellow peas (RrYy)

How would the genes segregate when the F1 generation was crossed?

Remember that the F1 generation was formed by the fusion of gametes carrying the dominant RY alleles and the other carrying the recessive ry alleles.

Does this mean that the dominant alleles always stay together?

Or would they segregate independently?

556 peas were produced in the F2 generation

Mendal’s data:556 Total Peas in F2 generation

• Phenotype Number of Peas

Yellow and Round (dominant traits)

315

Wrinkled and Green (recessive traits)

32

Other combinations - Yellow and wrinkled/Green and Round

209

Perform this cross

RrYy x RrYy

Section 11-3

Figure 11-10 Independent Assortment in Peas

Notice the other combinations

These phenotypes –

Yellow and wrinkled

Green and round

WERE NOT SEEN IN EITHER PARENT

What does this mean?

This meant that the alleles for seed shape segregated independently of those for seed color.

This is known as Independent Assortment.

Independent assortment – independent segregation of genes during the formation of gametes.

Independent assortment explains why traits such as nose shape and eye color do not always go together.

The shape of your dad’s nose and his eye color will segregate separately.

This is why you can have dad’s nose and mom’s eyes

Practice with dihybrid cross examples

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Ability?

Can you understand the language of genetics?

Read the top of page 272 “A SUMMARY OF MENDEL’S PRINCIPLES.

Can you read this and understand it?

• The inheritance of biological characteristics is determined by individual units known as genes. Genes are passed from parents to their offspring.

• In cases in which 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.

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

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

Beyond Dominant and Recessive Alleles

Mendel’s contribution to genetics was great, but there is more to the story – most of which is not covered in this course.

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

Incomplete dominance

There is a plant named “four o’clock” (genus Mirabilis)

Mirabilis can have flowers of 3 colors – Red, white, and pink.

If you cross Red flowers with white flowers, you get pink flowers.

Section 11-3

Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

Mirabilis Red x White

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Section 11-3

Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

Consider a heterozygous cross of Pink vs pink

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Codominance

Some chickens demonstrate codominance.

This next example is not true of all chickens.

If you cross a black chicken and a white chicken, the offspring will be speckled with black and white feathers.

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Codominance

Other examples of codominance

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Multiple alleles

Many genes have more than 2 alleles.

These are said to have multiple alleles.

This does not mean that one individual will have more than 2 alleles in the genotype, it simply means that there are many alleles in the population

Rabbit hair color is a good example of multiple alleles.

There is one gene for hair color and there are four different alleles.

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Practice for advanced multiple allele monohybrid cross

Genotype Phenotype

cchch

chch

ccCch

cchcch

Perform this monohybrid cross

cchch x Cch

Draw Punnett square

Predict the Probability for each phenotype.

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Polygenic Traits

Some traits are created as a result of many genes.

These are called polygenic traits.

It is believed that human skin color is controlled by 4 genes.

Genetics and the environment

Genes don’t control everything.

A plant can a have a “tall” gene, but if it does not receive enough sunlight, it will not be tall.

A human can have an “obese” gene, but if he does not have enough nourishment, he will not be obese.

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