AY Honors/Heredity/Answer Key

1. What is meant by the term "heredity"?

Heredity refers to the transfer of biological characteristics from a parent organism to offspring, and is practically a synonym for genetics, as genes are now recognized as the carriers of biological information.

2. Draw a picture of an animal cell and label the following parts: Cell membrane, cytoplasm, nucleus, nuclear membrane, ribosomes.

3A. What is a chromosome and where are they located?

A chromosome is a very long, continuous piece of DNA which contains many genes. Chromosomes are located within the nucleus of a cell.

3B. What is a gene and where is it found?

Genes are the units of heredity in living organisms. They are encoded in the organism's genetic material (usually DNA or RNA), and control the physical development and behavior of the organism. They are located on the chromosomes.

4A. What is meant by the term "allele"?

An allele is any one of a number of viable DNA codings of the same gene occupying a given position on a chromosome. Humans have paired chromosomes in their somatic cells, and these contain two copies of each gene. In some cases the two copies of the gene are identical — that is, have the same allele. In other cases, the two copies are different. Humans inherit two copies of every gene, one copy comes from the mother, and the other comes from the father.

4B. How does a dominant allele differ from a recessive allele?

Characteristics associated with a certain allele can sometimes be dominant or recessive, but often they are neither. A dominant trait will be expressed when at least one allele of its associated type is present, whereas a recessive trait will be expressed only when both alleles are of its associated type.

4C. Determine which allele you have of the following genetic traits:

• 

  Widow's Peak

• 

  Free earlobe(left) and attached earlobe (right)

• Dimples.jpg

  Dimpled cheeks

Widow's Peak

A widow's peak is a descending V-shaped point in the middle of the hairline (above the forehead). The trait is inherited genetically and dominant. The term comes from English folklore, where it was believed that this hair formation was a sign of a woman who would outlive her husband.

Free earlobe

The free earlobe is a dominant trait, and its counterpart is the attached earlobe. Geneticists are unsure if it is the result of a single gene or if multiple genes are involved.

Dimples

Dimples are small indentations in the cheeks most evident when a person possessing this trait smiles.

Curved thumb

Bent pinky

Digit hair

Rolling tongue

Second toe longer

4D. Using a punnet square, predict the ratio of offspring produced from these monohybrid crosses: TT (tall) crossed with tt (short), Tt (tall) crossed with Tt (tall), Tt (tall) crossed with tt (short).

A punnet square is a genetic diagram used to determine the probability of an offspring expressing a particular genotype. An allele can be dominant or recessive. If a dominant allele (represented as a capital letter) is present, the trait will be expressed. The recessive trait will be expressed only of both alleles are recessive (represented as a lower case letter).

In our example, tallness is the dominant allele and it is represented by the capital 'T'. Shortness is the recessive allele, and it is represented as a lowercase 't' (not by an S). First we construct a table showing the alleles of the parents. The mother (with alleles TT) is generally shown across the top, and the father (with alleles tt) is shown down the left column:

	T	T
t
t

Next we copy the alleles from the mother's row and the father's column, placing the dominant allele ahead of the recessive one (so we would always show Tt rather than tT). In this case, all four outcomes are identical: Tt.

	T	T
t	Tt	Tt
t	Tt	Tt

Next we show a Tt mother and a Tt father. In this case, the pair has produced one TT, two Tt's, and one tt.

	T	t
T	TT	Tt
t	Tt	tt

Finally, we show the cross between a Tt and a tt. In this case, we get two Tt's, and one tt.

	T	t
t	Tt	tt
t	Tt	tt

So what does all this mean? A TT individual has two dominant alleles for tallness. All of that individual's offspring will be tall, but all will not necessarily be TT. A Tt individual will also be tall, but can have short offspring if crossed with either another Tt (75% tall, 25% short), or if crossed with a tt (50%-50%). Only individuals with tt alleles will be short, but if crossed with a TT or a Tt, can still have tall offspring (though all offspring will be carriers for shortness).

5A. What does the process of mitosis accomplish?

Mitosis is the scientific term for cell division. Before mitosis begins, the cell will have already made two copies of its genetic material. Mitosis is the separation of these two copies into two new cells.

5B. Draw a sequence of cells that shows the process of mitosis including: prophase, metaphase, anaphase, and telophase.

Prophase

Prophase: The two round objects above the nucleus are the centrosomes. Note the condensed chromatin.

Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin. When prophase begins, chromatin condenses together into a highly ordered structure called a chromosome. Since the genetic material has already been duplicated earlier, the chromosomes have two sister chromatids, bound together at the centromere by a protein. Just outside the nucleus are two centrosomes. The two centrosomes sprout microtubules (which may be thought of as cellular ropes or poles). By repulsive interaction of these microtubules with each other, the centrosomes push themselves to opposite ends of the cell.

Metaphase

Metaphase: The chromosomes have aligned at the metaphase plate.

The nuclear envelope dissolves, the microtubules enter the nucleus, and attach to points on the chromatids. As microtubules find and attach to these points, the centromeres of the chromosomes gather on an imaginary line called the metaphase plate that is equidistant from the two centrosome poles. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug of war between equally strong people.

Anaphase

Anaphase: Microtubules shorten.

During anaphase, two events occur in order:

1. The proteins that bind sister chromatids together are split, allowing them to separate. These sister chromatids turned sister chromosomes are pulled apart because the microtubules attached to the chromosomes become shorter, pulling them toward the centrosomes to which they are attached.
2. The unattached microtubules elongate, pushing the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell.

At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations.

Telophase

Telophase: The pinching is known as the cleavage furrow. Note the decondensing chromosomes.

Telophase is a reversal of the prophase events. It "cleans up" the aftereffects of mitosis. At telophase, the unattached microtubules continue to lengthen, elongating the cell even more. Corresponding sister chromosomes attach at opposite ends of the cell. A new nuclear envelope forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin.

5C. Briefly explain how the DNA in the chromosomes is copied during this process.

The DNA replicates itself and then splits apart.

6A. What does the process of meiosis accomplish and how does it differ from mitosis?

Meiosis forms new body cells, which for humans each contain 46 chromosomes to be used anywhere in the body, but for reproduction. Mitosis forms sex or somatic cells, each containing 23 chromosomes because when two meet (one from a female and one from a male) they combine to have a full 46 chromosomes.

8. What is mutation? Using diagrams or models created in question 7, illustrate the effect of a mutation on the genetic trait.

Mutation is any change in an organism's genetic material (DNA) caused by a mutagen, which is any material that drives this change.

Mutation can cause changes both big and small in a gene, either by point mutations and insertion/deletion of nucleotides on the small end of the scale or by mutations that have larger effects such as the duplication of a certain gene, the translocation of a certain chromosome, or the inversion of a chromosomal segment.

Many mutiations result in improperly built RNA which translates into improperly formed proteins and even malformed body structures and uncontrolled cell growth (cancer).

9. Know at least five genetic disorders and tell a story about a famous person or someone that you know who has had one of these disorders.

• Color-blindness
  • Bill Clinton, former President of the United States, and Bob Dole, Former Majority Leader of the U.S. Senate. These two ran against each other in the 1996 Presidential election, and during their debates, the colors normally used had to be changed to accomodate them both.
• Cystic fibrosis
• Down syndrome
  • Stephane Ginnsz, actor (Duo (film)) First actor with Down syndrome in the lead part of a motion picture.
  • Chris Burke, actor (Life Goes On) and autobiographer
  • Andrea Friedman, actor (Life Goes On), guest appearances on many other shows
  • Pascal Duquenne, actor (Le Huitième Jour aka The Eighth Day, Toto le héros aka Toto the Hero)
  • Anne de Gaulle (1928-1948), daughter of Charles de Gaulle
• Hemophilia
• Huntington's Disease
  • Woody Guthrie
• Sickle Cell Anemia
• Spina bifida
  • Olympian and eight-time Boston Marathon winner Jean Driscoll[1]
  • 1980s rock star, John Mellencamp
  • Welsh Paralympian, Tanni Grey-Thompson
  • U.S. country music singer, Hank Williams
• Tay-Sachs disease

References

• K12science.org - Genetic traits
• CVD (Color Vision Deficiency) Books