Dihybrid Crosses

Learning Check:

What is a monohybrid cross?

A: Cross of two individuals that differ by one trait

Learning Check:

What is the law of segregation?A: Traits are determined by pairs of alleles that segregate during meiosis so that each gamete receives one allele

Learning Check:

What is a genotype?A: genotype the combination of alleles for any given trait, or the organism’s entire genetic make-up

Learning Check:

In what ratio do physical traits appear in the F2 generation when the P generation of plants are true-breeding plants with opposite forms of a trait?A: 3:1

Learning Check:

How does the F1 generation from a cross between true-breeding plants with opposite forms of a trait differ from the F2 generation? A: F1 100% of the offspring have the same genotype, therefore 100% same phenotype (Heterozygous dominant) whereas F2 has a mixed ratio of genotypes producing a variety of phenotypes (Genotype- 1:2:1 Phenotype- 3:1)

Learning Check:

List all the possible different gametes that could be produced from the following parents: a. Tt b. GG c. ff

A: a. T or tb. G onlyc. f only

Dihybrid Crossu When more than one trait is involved we can make the

Punnett Square larger to predict the probability of the different type of offspring.

u All of the steps of a monohybrid cross still apply, the only difference is that you now have to consider the different combinations of alleles that could be present in the gametes produced by either parent.

u A cross involving TWO different traits is called a dihybrid cross

u Mendel’s work with peas and subsequent punnettsquares illustrated the Law of Dominance or Complete dominance, where in a heterozygous genotype, the dominant allele completely masks the effects of the recessive allele. There are many examples in nature where this is NOT the case.

Learning Check:

What is the difference between a dihybrid cross and a monohybrid cross?A: A monohybrid cross is a cross of two individuals that differ in ONE trait due to ONE different gene whereas a dihybrid cross examines the cross beween two individuals that differ in two traits due to two different genes

Learning Check:

List all the possible diff erent gametes that could be produced from parents with the following genotypes. a. TtGg b. AABbcc

A: a. TG, Tg, tG, tgb. ABc and Abc ONLY

Learning Check:

Complex Patterns of Inheritance

Incomplete Dominance:

u Incomplete dominance describes a condition in which neither of the two alleles for the same gene can completely conceal the presence of the other.

u As a result, a heterozygote individuals look like a blend of the dominant and recessive traits

Codominance:

u Codominance is a situation in which neither of a pair of alleles is dominant over the other and so both alleles are fully expressed. Heterozygous individuals have both alleles expressed in the phenotype

u A roan animal is an excellent, visible example of codominance. A roan animal is a heterozygote in which both the base colourand white are fully expressed. If you look closely at the individual hairs on a roan animal, such as the cow below, you will see a mixture of red hairs and white hairs. One allele is expressed in the white hairs, and the other allele is expressed in the red hairs.

Multiple Allelesu Many traits in humans and other species are the result of

the interaction of more than two alleles for one gene. u A gene with more than two alleles is said to have

multiple alleles.u As you know, any individual has only two alleles for

each gene—one allele on each homologous chromosome.

u However, many different alleles for a gene can exist within the population as a whole.

Multiple Alleles:An example would be blood types: A single gene determines a person’s ABO blood type. This gene determines what type of an antigen protein, if any, is attached to the cell membrane of red blood cells• Alleles A and B are codominant• Allele O is recessive to both A and B

u https://www.youtube.com/watch?v=9O5JQqlngFY

Polygenic Traits

u Traits that are controlled by more than one gene are called polygenic traits. These traits exhibit continuous variation, where the phenotypes vary gradually from one extreme to another.

u Some examples of traits that show continuous variation include height and skin colour in humans, ear length in corn, and kernel colour in wheat. Continuous traits cannot be placed into discrete categories because they vary over a continuum. For example, height in humans varies over a wide range of values. People cannot be categorized as only short or tall.

Polygenic Inheritance:

u An example of polygenic inheritance is melanin, the pigment that determines skin and eye colour. Melanin production is controlled by several different genes.

0 1 2 3 4 5 6

Number of Dominant AllelesNumber of Dominant Alleles

Freq

uenc

yFr

eque

ncy

aabbcc AABBCC

AaBbCcaaBbCCAAbbCcAabbCCAABbccaaBBCcAaBBcc

AaBbccAabbCcaaBbCcAAbbccaaBBccaabbCC

aaBBCCAAbbCCAABBccAaBbCCAaBBCcAABbCc

AabbccaaBbccaabbCc

AaBBCCAABbCCAABBCc

Skin Colour

Polygenic InheritanceMendel carefully selected plants that had very diff erent heights so there would be no question about phenotypes. However, there are traits that exhibit continuous variation. Th ese are traits for which the phenotypes vary gradually from one extreme to another.

Some examples of traits that show continuous variation include height and skin colour in humans, ear length in corn, and kernel colour in wheat. Continuous traits cannot be placed into discrete categories because they vary over a continuum. For example, height in humans varies over a wide range of values. People cannot be categorized as only short or tall.

Traits that exhibit continuous variation are usually controlled by more than one gene. For some traits this can involve several genes. Traits that are controlled by many genes are called polygenic traits. A group of genes that all contribute to the same trait is called a polygene. Each dominant allele contributes to the trait. Recessive alleles do not contribute to the trait. For skin colour, the more dominant alleles a person has, the darker their skin. Th e graph in Figure 6.9 shows that there are more intermediate phenotypes than extreme phenotypes.

continuous variation a range of variation in one trait resulting from the activity of many genes

polygenic trait a trait that is controlled by more than one gene

Figure 6.9 This graph shows possible shades of skin colour from three of the sets of alleles that determine this trait.

Predict the effect of more gene pairs on the possible phenotypes.

A polygenic trait is one that is controlled by more than one gene and shows continuous variation. In this activity, you will choose one human trait that you hypothesize is controlled by more than one gene and shows continuous variation. You will then collect data from your classmates to test your hypothesis.

Materials• ruler or measuring tape (if necessary)• graph paper

Procedure1. In your group, choose one human trait that you think

is polygenic. Make sure your choice is one for which data can be easily and respectfully collected from your classmates.

2. Construct a data table to organize your data. Keep in mind that you will be measuring a particular trait and recording the number of times that measurement of the trait occurs.

3. Collect your data from your classmates.

4. Create a line graph of your data. Your graph should refl ect the actual measurements you took and the frequency of the values that you measured.

Questions1. Do your data support your hypothesis that the trait

you selected is polygenic? Explain.

2. How could this activity be improved to provide a clearer picture of the inheritance pattern of the trait you selected?

Activity 6.1 Identifying a Polygenic Trait

248 MHR • Unit 2 Genetic Processes

S61_BIO11.indd 248S61_BIO11.indd 248 14/05/10 1:44 PM14/05/10 1:44 PM

For skin colour, the more dominant alleles a person has, the darker their skin. The graph shows that there are more intermediate phenotypes than extreme phenotypes.

u Length of corn ears is also controlled by two genes. The longest ears of corn have a genotype of AABB. The shortest ears of corn have a genotype aabb. All other genotypes produced intermediate lengths

Lesson 3 Word List

u Law of Segregationu Law of Dominanceu Law of Independent Assortmentu Alleleu Homozygousu Heterozygousu Dominantu Recessive

Lesson 4 Word List

u Dihybridu Incomplete Dominanceu Codominanceu Multiple Allelesu Polygenic Traits