Chapter 14: Mendel and the Gene Idea Flashcards
Why does meiosis result in four haploid gametes?
Meiosis consists of two nuclear divisions. In the first nuclear division, Meiosis I reduces the
number of chromosome sets: from two (diploid) to one (haploid), resulting in two haploid
cells. In Meiosis II, the sister chromatids separate, producing four haploid sex cells.
What is the difference between a character and a trait? Explain using an example.
A heritable feature that varies among individuals, such as flower color, is called a character.
Each variant for a character, such as purple or white color for flowers, is called a trait.
For example, the varying color of the flowers on pea plants is a character, and the specific variations, white and purple, are traits.
How did Mendel control the crosses so he was always certain of their parentage?
The reproductive organs of a pea plant are in its flowers, and each pea flower has both pollen-producing organs (stamens) and an egg-bearing organ (carpel). In nature, pea plants
usually self-fertilize: Pollen grains from the stamens land on the carpel of the same flower,
and sperm released from the pollen grains fertilize eggs present in the carpel. To achieve
cross-pollination (fertilization between different plants), Mendel removed the immature
stamens of a plant before they produced pollen and then dusted pollen from another plant
onto the altered flowers. Each resulting zygote then developed into a plant embryo encased
in a seed (pea). Mendel could thus always be sure of the parentage of new seeds.
Define the following terms:
P generation
F1 generation
F2 generation
Parent generation
First filial generation
Second filial generation
Explain how Mendel’s simple cross of purple and white flowers refuted the “blending” hypothesis.
The reappearance of white-flowered plants in the F2
generation was evidence that the heritable factor causing white flowers had not been
diluted or destroyed by coexisting with the purple-flower factor in the F1 hybrids.
Explain how Mendel’s simple cross of purple and white flowers determined dominant and recessive characteristics.
Mendel reasoned that the heritable
factor for white flowers did not disappear in the F1 plants, but was somehow hidden, or masked when the purple-flower factor was present. In Mendel’s terminology, purple flower color is a dominant trait, and white flower color is a recessive trait.
Explain how Mendel’s simple cross of purple and white flowers demonstrated the merit of experiments that covered multiple generations.
Had Mendel stopped his experiments with the F1 generation, the basic patterns of inheritance would
have escaped him. Mendel’s quantitative analysis of the F2 plants from thousands of genetic crosses like these allowed him to deduce two fundamental principles of heredity: the law of segregation and the law of independent assortment.
Label the allele for both purple and white flower color,
a homologous pair, and the locus of the flower color gene.
Why is a particular trait recessive? Note the difference between the DNA nucleotide sequences for the two different color alleles. Explain, at the molecular level, why purple or white flowers result.
The paternally inherited chromosome (blue) has an allele for purple flowers that begins
with a nucleotide sequence of CTA. The rest of the DNA sequence is like that of the maternal chromosome. The nucleotide sequence of this allele codes for the production of an
enzyme that helps synthesize purple pigment.
The maternally inherited chromosome (red) has an allele for white flowers that begins with
a nucleotide sequence of ATA. The nucleotide sequence of this allele results in the absence
of the enzyme for synthesizing pigment.
In sexually reproducing organisms, why are there exactly two chromosomes in each
homologous pair?
Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited
from each parent.
Mendel’s model consists of four concepts. Describe the first concept.
Alternative versions of genes account for variations in inherited characters.
Mendel’s model consists of four concepts. Describe the second concept.
For each character, an organism inherits two copies of a gene, one from each parent.
Mendel’s model consists of four concepts. Describe the third concept.
If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
Mendel’s model consists of four concepts. Describe the 4th concept, Law of Segregation
The two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.
Alleles separate in Anaphase I
homozygous
An organism that has a pair of identical alleles for a gene. PP or pp could be examples.
Punnett square for a cross between
true-breeding plants with yellow and true-breeding plants with green pods. Yellow is recessive.
a. What is the F2 phenotypic ratio?
b. What is the F2 genotypic ratio?
c. Which generation is heterozygous?
d. Which generation has both heterozygous and homozygous offspring?
a. 3 green:1 yellow
b. 1 GG: 2Gg: 1gg
c. F1
d. F2
In pea plants, T indicates the allele for tall plants, and t is the allele for dwarf plants. If
you have a tall plant, demonstrate with a testcross how it could be determined if the plant is homozygous tall or heterozygous tall.
A testcross always involves crossing the unknown phenotype with an individual that is
homozygous recessive for the trait in question. In this case, a homozygous tall plant
crossed with a homozygous recessive dwarf will yield all tall offspring. A heterozygous
tall plant crossed with a dwarf will yield an offspring ratio of one tall plant to one dwarf
plant. The presence of dwarf plants indicates that the previously unknown tall plant is heterozygous.
Explain the event in meiosis when independent assortment occurs.
The Law of Independent Assortment of chromosomes occurs at metaphase I.
The homologous pairs, each consisting of one maternal and one paternal chromosome, are situated at the metaphase plate. Each pair may orient with either its maternal or paternal homolog closer to a given pole- its orientation is as random as the flip of a coin. Thus, there is a
50% chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a 50% chance that it will get the paternal chromosome.
Because each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of each other pair.
