Lab # 5
Genetic Inheritance
Section A: Patterns of Inheritance
Background Information
In the 19th century, a monk named Gregor Mendel conducted a series of experiments to determine how ‘traits’ or features of pea plants were passed on from generation to generation. He carefully studied seven traits of successive generations of pea plants to discover how many plants had traits such as smooth seeds or shriveled seeds. Eventually he came to the conclusion that these traits were passed from generation to generation through an inheritance of some kind of ‘factor’ (today known as genes). He also discovered that individuals possess more than one factor, even though only one is seen in any individual. For instance, two parents who both had smooth seeds might produce offspring with shriveled seeds. For this to happen, the parents must have been hiding a ‘factor’ (gene) for shriveled seeds. Today we know that there are dominant and recessive genes. A typical example is the gene for eye color in humans. The two most common eye colors are brown and blue. We know that brown eyes are dominant, thus, if an individual has even one gene for brown, they will have brown eyes. Blue eyes are recessive. In order to have blue eyes, both genes must be for blue.
In this lab you will solve an example of a Punnett square, a tool that researchers can use to determine the likelihood of any particular trait showing up in a population. To do this, imagine that you are a farmer who loves sunflowers. One year you buy a packet of seeds and plant them. When they bloom, you discover that some of the seeds produce white flowers while the rest produce yellow. The white flowers are very beautiful and you want to produce more of them. Understanding the genetic basis of those flowers will help you to increase your crop of white flowers the next year.
Procedure Part 1:
Genetic Traits are usually listed as letters. Capital letters represent dominant genes, lowercase letters represent recessive genes. Let's write a genetic code for two flowers: one white and one yellow. We will use an uppercase Y to represent yellow, and a lowercase y to represent white (you can use a w, but geneticists use the same letter to avoid confusion later on. Thus, the top of your page should look like this:
Parent A Parent B
Parent generation: YY yy
Mendel realized that only one of the two traits of each parent was handed down to the next generation during sexual reproduction. Thus, Parent A will hand one Y (yellow gene) down to the next generation and parent B will hand one y (white gene) down to the next generation. Thus, the second generation (called F1 generation) will look like this:
F1 generation: Yy Yy
In the next generation things start to get complicated, as there are a number of possibilities. To figure out the likelihood of any possibility, we can make a chart called a Punnett square.
Y y
Y
y
Y = yellow, y = white
Each parent from the F1 generation is represented on an axis (Parent A along the top and Parent B down the side). Each one will contribute one gene to the next generation. Because there are two genes from each parent, there are four possibilities for the next generation.
Fill in the Punnett square to show what gene combinations are possible and then answer the following questions:
Part 1 Questions:
1. How many of the new flowers will have two yellow genes? What percentage of the total is that?
2. How many of the new flowers will have two white genes? What percentage of the total is that?
3. How many of the new flowers will be yellow?
4. How many will be white?
5. Why will so many more of the new flowers be yellow than white?
Procedure Part 2:
Now, suppose that you, as a flower farmer, have discovered that there is also a recessive gene for ruffled petals although the majority of petals are strait. The white flowers with ruffled petals are in high demand and you want to figure out how many you will produce. You can make a more complicated Punnett square to figure out how many will be ruffled. Again, we can imagine the parent generation having one parent that has all dominant genes, and one that has all recessive:
Y=yellow y=white S=strait s=ruffled
Parent: YYSS yyss
Gametes: YS ys
F1 generation: YySs X (crosses with) YySs
A Punnett square representing a cross between F1 individuals will need to be 4 x4, and will show the 16 possible genotypic combinations in the F2 generation.
SY Sy sY sy
Part 2 Questions:
1. How many different types of gene combinations did you find? List all the possible combinations.
2. How many of each combination did you find?
3. How many flowers have each of the following appearances?
a. Yellow with strait petals
b. Yellow with ruffled petals
c. White with strait petals
d. White with ruffled petals
4. If the farmer lets nature take its course, what percentage of the total crop would you expect to be the highly prized white with ruffled petals?
Section B: Human Genetics
Below is a list of genetic traits that are determined by a single gene. You will determine your phenotype and possible genotype for the indicated traits. If the trait is a recessive trait, use two lower case letters to indicate that the inheritance is of two like alleles (homozygous recessive).
If the trait is the result of a dominant gene, the genotype may be homozygous dominant (indicated by two identical upper case letters) or it may be heterozygous (indicated by one upper case and one lower case letter). If you have a dominant trait and can analyze family members to be certain of the genotype, the known genotype should be posted with an explanation on how the genotypic determination was made. If the genotype cannot be determined with 100% certainty, both genotype possibilities should be listed. Record your results on the data sheet.
Inherited Traits
1. Widow's peak - The W for widow's peak is dominant over a straight hairline (w allele). A widow's peak is a distinct downward point in the frontal hairline.
2. Albinism - The A allele is dominant and leads to melanin production, thus pigmented skin. Individuals with albinism have the aa genotype.
3. Attached earlobes - The E allele for a free earlobe is dominant over the e allele for attached earlobes.
4. Eye color - The P allele produces pigment in the iris of the eye producing either brown, green, hazel or black eye color. Individuals that are homozygous recessive (pp) do not produce any pigment and as a result have either blue or gray eyes.
** Note that eye color is also influenced by more than one gene locus. This eye color gene refers to the pigment of the iris.
5. Mid-digit hair - Hair on the middle segment on the fingers (between two joint knuckles) is a dominant trait (M). Absence of hair in this location is due to a recessive gene in the homozygous condition (p).
6. Interlocking Fingers. Fold the hands together without thinking about it. If the left thumb is crossed over the right, the designation is C. If the right thumb is crossed over the left, the person is homozygous recessive for this trait (cc). Don't think about it too much when performing this action.
7. Bent finger - Lay the fingers flat on top of a desk and relax. If the little finger bends towards the index finger, the genotype is dominant for this trait (B).
8. Hitchhikers thumb - Bend the thumb backward. If the thumb bends back at the last joint at an angle greater than 60 degrees, then this indicates the recessive trait (hh).
9. Dimpled chin - Squeeze the chin (you need to do this one with a partner or a mirror). If there is a dimple, this indicates the dominant (D) allele.
10. Relative finger length. A sex-influenced trait (not sex-linked) relates to the relative lengths of the index and ring fingers. The allele for a short index finger, S', is dominant in males. The allele for long index finger is S. In females both copies of the S' allele must be present in order for the index fingers to be shorter than the ring fingers. (It is considered recessive in females.) In rare cases, each hand is different.
11. Tongue Rolling. The ability to roll one's tongue (the sides of the tongue curl up towards the center producing a U-shaped tongue) is due to a dominant allele, T. The homozygous recessive condition, tt, results in the inability to roll the tongue.
12. Freckles. The tendency to develop freckles (melanin spots) is a dominant trait (F) over the absence of freckles (ff).
13. Aromatic urine - The urine of some people has a very distinctive odor after eating asparagus (use A and a for alleles). This appears to be a recessive trait. Eat some asparagus. If the urine is "aromatic" afterwards, it will be obvious and indicates the recessive trait. Most of you will be able to answer this already without having to repeat this experiment.
14. Color vision. The ability to distinguish between red and green colors is due, in part, to proteins called opsins. The two genes for red opsin and green opsin are on the X chromosome. Often a person lacks both normally functioning opsin genes and has red/green color blindness. The inheritance pattern is X-linked recessive. The normal allele is XC. The abnormal recessive allele is Xc. In order for a woman to be color blind, both X chromosomes would lack the normal allele. For a man to be color blind, the X chromosome would lack the normal allele. The Y chromosome does not contain this gene locus.
15. Sex determination. In humans, the sex of an individual is determined by a pair of chromosomes designated X and Y. Males have one X and one Y chromosome. Females have two X chromosomes.
16. Long Palmar Muscle (Incomplete Dominance). Clench the left hand tightly and feel the wrist with the right hand; do the same for the right hand. Three tendons in a wrist indicate a long palmar muscle. If there are long palmar muscles in both wrists, the genotype is P1P1. If both wrists do not contain long palmar muscles, the genotype is P2P2. If there is a long palmar muscle in only one arm, the genotype is heterozygous, P1P2 for this trait. This trait would be an example of incomplete dominance.
17. Hair texture. Another example of incomplete dominance is hair texture. The texture of untreated hair is determined by protein content. If the hair is naturally curly, the genotype is C1C1 If it is wavy, the genotype is heterozygous C1C2; straight hair indicates a homozygous recessive, C2C2, genotype.
18. Hair Whorl direction. Hair at the back of the head should be observed. The dominant condition is when the hair is going in a clockwise direction. The recessive condition is when it is going in a counter clockwise direction. Use W for the dominant and w for the recessive conditions.
ONE NOTE: Remember the difference between phenotype and genotype. Phenotypes are descriptive words, not abbreviations like XY or Ss. For example, for sex: the phenotype would be 'male' or 'female' and the genotype would be 'XY' or 'XX', respectively. Please do not write genotypes in your phenotype column! This is a common, but easily avoidable, error.
ONLY SUBMIT THE PORTION BELOW THIS LINE
Lab 5
Patterns of Inheritance
&
Human Genetics
Name _____________________________
_______________________
Section A: Patterns of Inheritance
Part 1 Questions:
1. How many of the new flowers will have two yellow genes? What percentage of the total is that?
2. How many of the new flowers will have two white genes? What percentage of the total is that?
3. How many of the new flowers will be yellow?
4. How many will be white?
5. Why will so many more of the new flowers be yellow than white?
Part 2 Questions:
1. How many different types of gene combinations did you find? List all the possible combinations.
2. How many of each combination did you find?
3. How many flowers have each of the following appearances?
a. Yellow with strait petals
b. Yellow with ruffled petals
c. White with strait petals
d. White with ruffled petals
4. If the farmer lets nature take its course, what percentage of the total crop would you expect to be the highly prized white with ruffled petals?
Section B: Human Genetics
Inherited Traits Phenotype Genotype
1. Widow's Peak
2. Albinism
3. Attached earlobes
4. Eye color
5. Mid-digit hair
6. Interlocking Fingers
7. Bent Finger
8. Hitchhikers thumb
9. Dimpled chin
10. Relative finger length
11. Tongue Rolling
12. Freckles
13. Aromatic Urine
14. Color Vision
15. Sex Determination
16. Long Palmar Muscle
17. Hair Texture
18. Hair Whorl
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