Chapter 3: Mendelian Genetics Flashcards
What factors were crucial to the success of Gregor Mendel’s experiments?
He kept detailed quantitative records.
He observed only one or very few traits in any given experiment.
He chose traits that were not greatly influenced by the environment.
Mendel often crossed different individuals in his experiments.
Which of the following would be true of a plant heterozygous for a single gene controlling flower color? Assume complete dominance.
If crossed with another heterozygous plant, the majority of progeny will have the dominant flower color.
Does the 3:1 phenotypic ratio observed among progeny of an F1 X F1 cross requires random union of gametes?
TRUE
The 1:2:1 genotypic ratio represents relative probabilities of gamete combinations based on the assumption that gamete union is random.
What would be the best way to determine which of 2 alleles of a gene is dominant to the other?
Observe the relevant phenotype in the progeny that result from a cross between individuals from two different pure-breeding lines.
All progeny will be heterozygous for the trait in question and will display the phenotype that corresponds with the dominant allele.
In an individual that is heterozygous for a particular trait, is the recessive allele expressed?
NO
Only the dominant allele is expressed in an individual that is heterozygous for a particular trait.
Round (R) seed shape is dominant to wrinkled (r) seed shape in pea plants. If an RR plant is crossed with an rr plant, what is the frequency of phenotypes in the F2 generation?
3 round seeds, 1 wrinkled seed
The F1 generation would have the genotype Rr, so crossing two heterozygotes would result in 3 plants with round seeds and 1 plant with wrinkled seeds.
If the first three F2 offspring grown from the cross described above are round, what is the probability that the next F2 offspring will be wrinkled?
25%
The proportion of rr is ¼, so the next offspring has a 25% chance of being wrinkled.
Which two genotypes in an individual would be expressed as the same phenotype in a diploid organism?
homozygous dominant and heterozygous
Both of these genotypes would express the dominant phenotype. The homozygous dominant individual would have both copies of the dominant allele, and the heterozygous individual would have one copy of the dominant allele and one copy of the recessive allele, but only the dominant phenotype would be observed.
Mendel’s unit factors in pairs are most accurately known to be _______.
two alleles on paternal and maternal homologs
Mendel observed patterns of inheritance in offspring that stemmed from traits found in the parents. Mendel explained this phenomenon as the passage of “unit factors” that are transmitted from the parents to the offspring.
Homolog
A gene related to a second gene by descent from a common ancestral DNA sequence. The term, homolog, may apply to the relationship between genes separated by the event of speciation or to the relationship betwen genes separated by the event of genetic duplication.
One member of a chromosome pair.
Mendel’s Law of Segregation is supported by a 1:1 testcross ratio.
TRUE
To test Mendel’s Law of Segregation, the experimenter needs a minimum of two contrasting forms of a gene.
True
A 1:1 phenotypic ratio is expected from a monohybrid testcross with complete dominance.
TRUE
Does the law of independent assortment states that one gene in a pair is always dominant to the other?
NO
The law of independent assortment states that during gamete formation, segregating pairs of unit factors assort independently of each other; the law says nothing about dominance.
If a yellow pea plant with round seeds that has the genotype GgWw is crossed to itself, what proportion of the offspring will be green with round seeds?
3⁄16
This is the probability of offspring that are either green with round seeds or yellow with wrinkled seeds.
The following phenotypic ratios are determined for a trihybrid cross in which the gametes assort independently:
A 1 : A 2 = ¼ : ¾
B 1 : B 2 = ¼ : ¾
C 1 : C 2 = ¾ : ¼
What is the probability that the F2 offspring will have the phenotype A 1 B 1 C 1?
3⁄64
This is the probability that the offspring will be
A 1 B 1 C 1,
A 1 B 2 C 2,
or A 2 B 1 C 2.
Which of the following phenomena is a consequence of Independent Assortment?
The phenotypic ratio among phenotypes produced from an F1 X F1 dihybrid cross is 9:3:3:1.
The 9:3:3:1 ratio requires that the four gamete classes produced occur with equal frequency. This requires independent assortment.
The 9:3:3:1 ratio exhibited in the F2 generation of a dihybrid cross is a genotypic ratio.
FALSE
A pea plant is heterozygous for two genes; one controlling height, one controlling color. The genotype is written PpTt. Based on the Law of Independent Assortment, approximately what proportion of the pollen produced by this plant should have the genotype PT?
1/4
There are 4 different possible genotypes. Independent Assortment assumes that all are equally likely. Therefore, each of the 4 should occur with roughly equal frequency (1/4).
How many different kinds of gametes can be produced by an individual with the genotype AABbCCddEeFf ?
8 gametes
1 x 2 x 1 x 1 x 2 x2
Which of the following statements is true regarding a trihybrid cross between two true-breeding homozygous individuals with contrasting phenotypes?
The least frequent F2 phenotypic class is recessive for all three traits.
1/64 of the F2 phenotypes is triply recessive.
Which of the following statements is an example of independent assortment?
A tall pea plant is no more or less likely to have round seeds than a dwarf pea plant.
Traits that are not on the same chromosome are inherited independently of each other.
One character in peas that Mendel studied was yellow versus green seeds.
A cross between a homozygous yellow line (GG) and a homozygous green line (gg) will result in F1 plants that are heterozygous (Gg) for this trait and produce yellow seeds.
Cross between homozygous yellow and green pea plants.
When an F1 plant undergoes meiosis, what gamete types will it produce, and in what proportions?
Mendel’s Law of Segregation states that allele pairs segregate equally into gametes during meiosis. This means that a gamete will have only one allele of any given gene, and that the probability of a gamete having one allele or the other is equal (and therefore ½, or 50%, for either allele).
This Punnett square shows the results of a Gg x Gg cross to form F2 progeny.
Use your understanding of Mendel’s Law of Segregation and the rules of probability to complete the Punnett square for this cross.
Identify the gametes. Identify the male and female gamete types and the gamete frequencies.
Then identify the F2 progeny. Identify the progeny genotypes and the progeny frequencies.
A Punnett square is a convenient method for representing Mendel’s Law of Segregation in a visual form. Using a Punnett square allows one to easily see gamete types and frequencies, as well as the genotypes and frequencies of progeny formed by random gamete fusion.
The genotype frequencies inside a Punnett square are calculated using the product rule: The probability of two independent events occurring simultaneously is the product of their individual probabilities.
In this example, the genotype frequencies inside the square (¼) are the product of the gamete frequencies that led to their formation (½ x ½ = ¼).