Assignment: Introduction to Genetics & Blood Phenotype

Lab – Introduction to Genetics

Practice Problems

Background Information and Terminology

Why is there so much variation in some traits, yet so much similarity in other traits, even with closely related individuals?

The variety occurs because there are many different alleles for a specific trait in a population, and sexual reproduction mixes up the possibilities of various combinations of alleles. Every child receives half of their genetic information from thier mother and half from their father. That is to say, a parent contributes one of the two alleles present at every location (i.e., locus) on a gene to his or her offspring. However, as a result of randomness of meiosis, not all offspring receive the same combination of alleles (other than identical twins), thus the process of meiosis mixes up the possibilities of allele combinations, increasing genetic variation.

Letters are commonly used to denote alleles, so we can track how they are passed on from parent to offspring. Various forms of inheritance of these alleles from parents have been documented: complete dominance, incomplete dominance, and codominance. If two of the same alleles for a trait are received by the offspring, we call this a homozygous genotype (e.g., AA or aa); if two different alleles for a trait are received by the offspring, we call this a heterozygous genotype (e.g., Aa).

Notice the upper and lowercase letters that were used in the previous paragraph. An uppercase letter is used to denote a version of a trait that is dominant; a lowercase letter is used to denote a version of a trait that is recessive. It is the combination of a recessive and a dominant allele that makes a heterozygous genotype (e.g., Aa). In a case of complete dominance, we would see the version of the trait represented by the A in a heterozygous Aa genotype (A masks a), whereas in a case of incomplete dominance, we would see a new version of a trait, an intermediate phenotype, represented by a mix of the A and a alleles in a heterozygous genotype. In a case of codominance, we could see a phenotype that is represented by two dominant alleles (e.g., AB blood genotype = Type AB blood phenotype), of which neither allele masks the other allele.

Some human traits are determined by one gene (i.e., two alleles; e.g., insulin production), but others are determined by more than one gene (i.e., four, six, or eight alleles; e.g., skin tone). This possibility of polygenic traits contributes to a wide range of phenotypes possible among individuals in a population. These types of traits are likely controlled by genes at more than one locus; this can sometimes result in epistasis.

Answers will be posted following the due date.

Questions

Complete the Punnett square and answer the questions below.

What is the genotypic ratio of the potential offspring?

What is the phenotypic ratio of the potential offspring?

What is the probability that a child will have attached earlobes if the father is heterozygous and the mother is homozygous recessive (complete the Punnett square)?

Unattached earlobes = dominant

Attached earlobes = recessive

Flat feet are inherited through a recessive allele (f). Two people who have normal arches produced a child with flat feet.

What are the genotypes of all the family members?

Father’s genotype:

Mother’s genotype:

Child’s genotype:

What is the probability of another child with flat feet (complete the Punnett square)?

Marfan syndrome (M) is a dominant trait. Chris Patton, a University of Maryland basketball player in the late 1970s, died while playing basketball. An autopsy showed that Patton’s aorta had burst because of the lifelong weakening effects of Marfan syndrome. Assume Chris’s mother had Marfran syndrome and his father was normal. If Chris had married a normal woman, and had a child before his death, what is the probability that this child would inherit Marfran disease (complete the Punnett square below).

What are the genotypes of all the family members?

Chris’s father’s genotype:

Chris’s mother’s genotype:

Chris’s wife’s genotype:

Chris’s genotype:

Probability of a Marfran syndrome child:

Dihybrid Cross. Refer to your textbook Figure 7.6 (page 123) for information on setting up a Punnett square for a dihybrid cross.

In horses, the coat color black (B) is dominant over chestnut (b). The trotting gait is dominant (T) over the pacing gait (t). If a homozygous black pacer is mated to a homozygous chestnut, heterozygous trotter, what will be the genotic and phenotypic ratios of the F1 generation? (complete the Punnett square below).

Cross:

Genotypic ratio:

Phenotypic ratio:

See write-up near the end of this documentfor a discussion on incomplete dominance, codominance, and sex -inked traits.

6. Incomplete Dominance(see page 126 of your textbook for more information on incomplete dominance)

Zinnia flower color is determined by two alleles that have an incomplete dominance pattern of inheritance.

Zinnias with a blue color have the genotype C^BC^B

Zinnias with a yellow color have the genotype C^YC^Y, and

Zinnias with a green color have the genotype C^BC^Y

Cross a blue flower with a yellow flower, that is to say, cross two true-breeding parents. Complete the Punnett square below.

Cross:

Write the genotype(s) possible in the F₁ offspring:

Write the phenotype(s) possible in the F₁ offspring:

Now cross two F₁ offspring. Complete the Punnett square below.

Write the genotypic ratio in the F₂ offspring:

Write the phenotypic ratio in the F₂ offspring:

7. Codominance(see page 126 of your textbook for more information on codominance)

Blood types in humans is a case of a codominant pattern of inheritance. There are three possible alleles, although each person attains one allele from each parent. Thus, two alleles determine the blood type for each person. The possible alleles are A, B, and O, depending upon the presence of an antigen (an agglutinogen protein) on the surface of the red blood cells. These proteins assure that your body only has your blood type allele(s) in it, and your body attacks against foreign allele-types. (Application: hemolytic disease)

Individuals with the A antigen, have the Type A blood phenotype; individuals with the B antigen, have the Type B blood phenotype; individuals with both the A and B antigens, have the Type AB blood phenotype; and individuals with neither the A nor B antigen, have the Type O blood phenotype. Both the A and B alleles are dominant, thus if you inherit one from each parent, you will have both of them expressed in the genotype (AB) and in the phenotype (Type AB blood), as they are codominant alleles. The O allele is recessive, thus if it is inherited with an A or a B allele, it will be masked by the A or B.

Mr. and Mrs. Smith had a baby girl they named Samantha. Their next-door neighbor, Mr. Jones, filed suit to gain custody of Samantha, claiming the baby was his. Blood tests were conducted with the following results:

Samantha type O

Mr. Smith type AB

Mrs. Smith type B

Mr. Jones type A

You are the judge. How would you rule? Explain your answer.

How would you rule if the blood types had come out as follows?

Samantha type AB

Mr. Smith type AB

Mrs. Smith type B

Mr. Jones type A

8. Sex-Linked Traits(see pages 139 – 145 of your textbook for more information on sex-linked genes)

Some genes occur on only sex chromosomes, not the autosomes (i.e., the other chromosomes). Assignment: Introduction to Genetics & Blood Phenotype

Write the sex chromosomes you have in your cells:

Name the female gamete cell type:

Name the male gamete cell type:

Which sex chromosome(s) is in an ovum?

Which sex chromosome(s) is in a sperm?

Which parent’s gamete determines the sex of the offspring in humans?

Color-blindness is a sex-linked trait; it occurs only on the X chromosome, not on the Y chromosome. Normal color vision (X^N) is dominant over color blindness (Xⁿ).

Note: When setting up a Punnett square for sex-linked crosses make sure you indicate which sex chromosome (i.e., X or Y) the allele is located on. For example, cross between carrier female (X^A X^a) and normal male (X^A Y)

X^A

X^a

X^A

X^A X^A

X^A X^a

X^A Y

X^a Y

If a colorblind mom (XⁿXⁿ) and a dad with normal color vision (X^NY) have a child together,

What is the genotypic ratio of the first generation offspring?

What is the phenotypic ratio of the first generation offspring?

Now consider that one of the sons from the family above marries a woman with normal color vision whose father was colorblind.

What is the % chance that their sons (i.e., the second generation offspring) will be colorblind?

What is the % chance that their daughters will be colorblind?

Both the husband and wife have normal vision. Their daughter is color blind. What can you conclude about the girl’s father?

If you remove a cell from the inside of your cheek or your elbow, how many sex chromosomes would you find inside each of these cells?

Assuming that both parents plants in the diagram below are homozygous, why would all of the F1 generation have yellow phenotypes?

***IMPORTANT. Complete all questions and tables then save this document using the format lastname_firstinital.doc. Go to D2L Brightspace and upload your saved document by the due date to the assignment folder/dropbox labeled “Introduction to Genetics.”

DISCUSSION OF INCOMPLETE DOMINANCE, CODOMINANCE, AND SEX-LINKED TRAITS

The following video describes incomplete dominance, codominance, and sex linked traits: ()

On page 126 of the textbook incomplete dominance is defined as an inherited trait in which neither allele is able to exert its full extent, so a heterozygote displays an intermediate phenotype. The textbook example given is dogs homozygous for allele I (II) are large and dogs homozygous for allele B (BB) are small and dogs heterozygous (IB) are medium (intermediate of large and small).

The INCOMPLETE DOMINANCEexample for this worksheet is for the trait flower color (C) for zinnias. Assignment: Introduction to Genetics & Blood Phenotype

Zinnia flower color is determined by two alleles that have an incomplete dominance pattern of inheritance. THE TRAIT IS REPRESENTED BY THE SYMBOL C, THEREFORE TO REPRESENT THE TWO FLOWER COLORS (AS NEITHER ALLELE IS DOMINANT, SO WE CANNOT USE UPPERCASE AND LOWERCASE SYMBOLOGY, E.G., Bb). WE MUST USE TWO DISTINCT SYMBOLS FOR EACH COLOR. IN THIS CASE, B REPRESENTS BLUE AND Y REPRESENTS YELLOW. THUS,

Zinnias with a blue color have the genotype C^BC^B

Zinnias with a yellow color have the genotype C^YC^Y, and

Zinnias with a green color have the genotype C^BC^Y

(THE PHENOTYPE IS GREEN BECAUSE NEITHER ALLELE (C^BOR C^Y) IS DOMINANT, THUS BOTH ARE PARTIALLY EXPRESSED RESULTING IN THE GREEN FLOWER COLOR PHENOTYPE)

Cross a blue flower with a yellow flower, that is to say, cross two true-breeding parents.

C^BC^Bx C^YC^Y

THE PUNNETT SQUARE METHODOLOGY IS THE SAME, PLACING THE POSSIBLE ALLELES OF ONE PARENT WITHIN THE TOP TWO BOXES AND THE OTHER PARENTS ALLELS WITHIN THE LEFTHAND SIDE TWO BOXES. THE RESULT FOR THIS QUESTION IS ALL HETEROZYGOUS OFFSPRING (C^BC^Y)

For CODOMINANCE both alleles are expressed equally. For blood type alleles A and B are codominant while the allele for O is recessive to both A and B. Thus, a person with genotype AB (heterozygous) has blood type (i.e., phenotype) AB. However, a person with genotype AO has blood type A, as allele A is dominant to allele O. Assignment: Introduction to Genetics & Blood Phenotype

SEX-LINKED TRAITS are traits that are located on the sex chromosomes. The X chromosome is larger than the Y and therefore contains more genetic material, and hence alleles.The example given is for colorblindness where normal color vision (X^N) is dominant over color blindness (Xⁿ). Assignment: Introduction to Genetics & Blood Phenotype

The trait for colorblindness (and hence normal color vision) is located on the X chromosome and not on the Y chromosome. Therefore, the genotype for normal color vision for a male is: X^NY, in which XY represents the sex chromosomes and the superscript N represents the allele for normal color vision.

The genotype for colorblind for a male is: XⁿY, in which XY represents the sex chromosomes and the superscript n represents the allele for colorblindness.

The genotype for normal color vision for a female is: X^NX^N or X^NXⁿ in which XX represents the sex chromosomes and the superscript N represents the allele for normal color vision and the superscript n represents the allele for colorblindness.

The only combination for a female to be colorblind is the genotype XⁿXⁿ (homozygous recessive).

Assignment: Introduction to Genetics & Blood Phenotype