Omics-C12 Flashcards
1) What was the most significant conclusion that Gregor Mendel drew from his experiments with pea plants?
A) There is considerable genetic variation in garden peas.
B) Traits are inherited in discrete units and are not the result of “blending.”
C) Recessive genes occur more frequently in the F1 generation than do dominant ones.
D) Genes are composed of DNA.
B) By crossing true-breeding plants with different traits, Mendel observed that traits do not blend but are inherited in discrete units (now called genes).
Laws of Probability: Mendel’s results were explained using the concepts of dominant and recessive alleles. He developed the law of segregation and law of independent assortment. What do each of these say?
Laws of Inheritance:
Law of Segregation: Alleles separate during gamete formation, ensuring offspring receive one allele from each parent.
Law of Independent Assortment: Different traits are inherited independently of each other (It states that alleles for different traits assort independently of each other during gamete formation, leading to genetic variation.)
What is the difference between a dominant and a recessive allele?
A dominant allele is expressed even if only one copy is present, while a recessive allele is only expressed if both copies are present.
What is a monohybrid cross?
A cross between two organisms that are heterozygous for one trait. The F2 generation typically exhibits a 3:1 phenotypic ratio.
The F2 generation revealed the presence of recessive traits that had been masked in the F1 generation, supporting the idea of discrete inheritance.
What is the significance of the F2 generation in Mendel’s experiments?
What is a dihybrid cross?
A cross between organisms that are heterozygous for two traits. It typically results in a 9:3:3:1 phenotypic ratio in the F2 generation.
A dihybrid cross is a genetic cross between two individuals that are heterozygous for two traits. It is used to determine how different genes are inherited together. The classic example of a dihybrid cross involves Mendel’s pea plants, specifically looking at the traits of seed color (yellow vs. green) and seed shape (round vs. wrinkled).
Example: Dihybrid Cross in Pea Plants
Traits Involved:
- Seed color:
- Yellow (Y) is dominant.
- Green (y) is recessive.
- Seed shape:
- Round (R) is dominant.
- Wrinkled (r) is recessive.
In a dihybrid cross, Mendel crossed two pea plants that were heterozygous for both traits:
- Parent Genotype: YyRr × YyRr
Step-by-Step Process:
-
Parental Genotypes:
- Each parent is heterozygous for both seed color and seed shape. Their genotype is YyRr, which means they have:
- One dominant allele for yellow (Y) and one recessive allele for green (y).
- One dominant allele for round (R) and one recessive allele for wrinkled (r).
- Each parent is heterozygous for both seed color and seed shape. Their genotype is YyRr, which means they have:
-
Gamete Formation:
- According to Mendel’s law of independent assortment, alleles for different traits segregate independently of each other during gamete formation. Each parent can produce four types of gametes based on their alleles for seed color and seed shape:
- YR, Yr, yR, and yr.
- According to Mendel’s law of independent assortment, alleles for different traits segregate independently of each other during gamete formation. Each parent can produce four types of gametes based on their alleles for seed color and seed shape:
-
Punnett Square:
To predict the outcome of the cross, we can use a Punnett square to show all possible combinations of the gametes from each parent. The Punnett square for this dihybrid cross has 16 boxes because each parent produces four types of gametes.
| YR | Yr | yR | yr |
|———–|———|———|———|———|
| YR | YYRR | YYRr | YyRR | YyRr |
| Yr | YYRr | YYrr | YyRr | Yyrr |
| yR | YyRR | YyRr | yyRR | yyRr |
| yr | YyRr | Yyrr | yyRr | yyrr | -
Phenotypic Ratios:
From the Punnett square, you can see the possible combinations of alleles and predict the phenotypes (observable traits):- 9: Yellow, Round (YYRR, YYRr, YyRR, YyRr)
- 3: Yellow, Wrinkled (YYrr, Yyrr)
- 3: Green, Round (yyRR, yyRr)
- 1: Green, Wrinkled (yyrr)
Summary of Results:
- 9 plants will have yellow seeds and round shapes (dominant for both traits).
- 3 plants will have yellow seeds and wrinkled shapes (dominant for color, recessive for shape).
- 3 plants will have green seeds and round shapes (recessive for color, dominant for shape).
- 1 plant will have green seeds and wrinkled shapes (recessive for both traits).
Key Points:
- Independent Assortment: The alleles for color and shape assort independently, meaning the inheritance of seed color does not affect the inheritance of seed shape.
- 9:3:3:1 Ratio: This ratio is typical for a dihybrid cross between two heterozygous individuals when both traits follow simple dominant-recessive inheritance patterns.
This example illustrates how two traits are inherited together and how the combinations of alleles lead to predictable phenotypic ratios in the offspring.
Why did Mendel choose pea plants for his experiments?
Pea plants
* reproduce quickly
* have easily observable traits
* can self-fertilize
which made them ideal for controlled genetic studies.
3) A sexually reproducing animal has two unlinked genes, one for head shape (H) and one for tail length (T). Its genotype is HhTt. Which of the following genotypes is possible in a gamete from this organism?
A) Hh
B) HhTt
C) T
D) HT
D
The genotype HhTt indicates that the organism is heterozygous for two unlinked genes: one for head shape (Hh) and one for tail length (Tt).
During gamete formation (via meiosis), the alleles for these genes will segregate independently according to Mendel’s law of independent assortment. This means that each gamete will receive one allele from each gene.
A gamete will only contain one allele for each gene, either H or h for head shape and either T or t for tail length. Therefore, a possible combination of alleles in a gamete from this organism could be HT, Ht, hT, or ht.
Why other answers are incorrect:
A) Hh: This is not a valid gamete because gametes only receive one allele per gene, not both.
B) HhTt: This is the genotype of the organism, not a gamete. Gametes only carry one allele for each gene.
C) T: A single gene’s allele by itself cannot be a gamete. Gametes must contain alleles from all genes being inherited, so T alone is incomplete.
Thus, the correct genotype for a possible gamete is HT.
What is a genotype?
A genotype is the genetic makeup of an organism, specifically the combination of alleles (variants of a gene) that an individual possesses for a particular gene or set of genes. The genotype determines the organism’s potential traits, though not all of these traits may be physically expressed (those that are expressed are referred to as the phenotype).
Mendel’s observation of the segregation of alleles in gamete formation has its basis in which of the following phases of cell division?
A) prophase I of meiosis
B) anaphase II of meiosis
C) metaphase II of meiosis
D) anaphase I of meiosis
The correct answer is: D) anaphase I of meiosis
Explanation:
Mendel’s observation of the segregation of alleles is based on the behavior of chromosomes during meiosis, the process by which gametes (sperm and eggs) are formed.
During anaphase I of meiosis, homologous chromosomes (each containing two sister chromatids) are pulled apart and move toward opposite poles of the cell. This is the point at which the two alleles for a gene, which are located on homologous chromosomes, are segregated into different gametes.
A) Prophase I of meiosis: This is when homologous chromosomes pair up and crossing over occurs, but segregation of alleles hasn’t occurred yet.
B) Anaphase II of meiosis: In this phase, the sister chromatids are separated, but the key segregation of homologous chromosomes (and therefore alleles) happens earlier, in anaphase I.
C) Metaphase II of meiosis: Chromosomes line up at the metaphase plate, but the actual separation of alleles doesn’t happen in this phase.
Thus, anaphase I is the phase where homologous chromosomes, and the alleles they carry, are segregated into different gametes, which reflects Mendel’s law of segregation
How many unique gametes could be produced through independent assortment by an individual with the genotype AaBbCCDdEE?
A) 4
B) 8
C) 16
D) 64
The correct answer is: B) 8
Explanation:
The number of unique gametes an individual can produce due to independent assortment is determined by the number of heterozygous gene pairs in the genotype. Each heterozygous gene pair can produce two possible alleles in a gamete.
For the given genotype AaBbCCDdEE, let’s break it down:
A and a (heterozygous) → 2 possible alleles.
B and b (heterozygous) → 2 possible alleles.
C and C (homozygous) → 1 possible allele (only C).
D and d (heterozygous) → 2 possible alleles.
E and E (homozygous) → 1 possible allele (only E).
Now, calculate the number of unique gametes:
The total number of unique gametes is determined by multiplying the number of allele possibilities for each gene:
2 (for A) × 2 (for B) × 1 (for C) × 2 (for D) × 1 (for E) = 8 unique gametes.
Thus, the number of unique gametes that could be produced through independent assortment is 8.
Which of the following scenarios describes an example of epistasis?
A) Recessive genotypes for each of two genes (aabb) results in an albino corn snake.
B) In rabbits and many other mammals, one genotype (ee) prevents any fur color from developing.
C) In Drosophila (fruit flies), white eyes can be due to an X-linked gene or to a combination of other genes.
D) In cacti, there are several genes for the type of spines.
The correct answer is: B) In rabbits and many other mammals, one genotype (ee) prevents any fur color from developing.
Explanation:
Epistasis is a genetic phenomenon where the expression of one gene is affected or masked by another gene. In other words, one gene can interfere with or completely prevent the expression of another gene, even if the second gene would typically show its trait.
B) In rabbits and many other mammals, one genotype (ee) prevents any fur color from developing: This is a classic example of epistasis. The ee genotype (often called the epistatic gene) masks the expression of any other gene responsible for fur color. Even if a rabbit has alleles for brown or black fur, the presence of ee will result in the rabbit having no fur color (albino or white).
Why Other Options Are Incorrect:
A) Recessive genotypes for each of two genes (aabb) result in an albino corn snake: This describes a case of two recessive genes leading to a phenotype but does not involve one gene masking another; thus, it is not epistasis.
C) In Drosophila, white eyes can be due to an X-linked gene or to a combination of other genes: This example involves different genetic pathways leading to the same trait (white eyes) but doesn’t describe one gene masking the effect of another gene.
D) In cacti, there are several genes for the type of spines: This describes polygenic inheritance, where multiple genes contribute to the expression of a trait, but it’s not an example of epistasis.
Therefore, B describes epistasis because the ee genotype prevents the expression of fur color, even if the organism carries alleles for other colors.
In epistasis, a gene that is unrelated to the specific trait (such as fur color) can indeed prevent the expression of other genes that directly control that trait. The key aspect of epistasis is that one gene can mask or modify the expression of another gene, even if it does not directly determine the specific trait itself.
