Chapter 14: Mendel and the Gene Idea Flashcards

You may prefer our related Brainscape-certified flashcards:
1
Q

Why does meiosis result in four haploid gametes?

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the difference between a character and a trait? Explain using an example.

A

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 well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

How did Mendel control the crosses so he was always certain of their parentage?

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Define the following terms:
P generation
F1 generation
F2 generation

A

Parent generation

First filial generation

Second filial generation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Explain how Mendel’s simple cross of purple and white flowers refuted the “blending” hypothesis.

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Explain how Mendel’s simple cross of purple and white flowers determined dominant and recessive characteristics.

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Explain how Mendel’s simple cross of purple and white flowers demonstrated the merit of experiments that covered multiple generations.

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Label the allele for both purple and white flower color,
a homologous pair, and the locus of the flower color gene.

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

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.

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

In sexually reproducing organisms, why are there exactly two chromosomes in each
homologous pair?

A

Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited
from each parent.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Mendel’s model consists of four concepts. Describe the first concept.

A

Alternative versions of genes account for variations in inherited characters.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Mendel’s model consists of four concepts. Describe the second concept.

A

For each character, an organism inherits two copies of a gene, one from each parent.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Mendel’s model consists of four concepts. Describe the third concept.

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Mendel’s model consists of four concepts. Describe the 4th concept, Law of Segregation

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

homozygous

A

An organism that has a pair of identical alleles for a gene. PP or pp could be examples.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

heterozygous

A

An organism that has two different alleles for a gene, such as Pp.

17
Q

genotype

A

The genetic makeup of an organism such as PP or Pp or pp.

18
Q

phenotype

A

An organism’s appearance or observable traits such as purple flowers or white flowers.

19
Q

monohybrid cross

A

A cross between two organisms that are heterozygous for the character being followed (or the self-pollination of a heterozygous plant).

20
Q

parental cross

A

A genetic cross between two true-breeding parents. PP x pp.

21
Q

dihybrid cross

A

A cross between two organisms that are each heterozygous for both of the characters being followed (or the self-pollination of a plant that is heterozygous for both characters). YYRR x yyrr produces YyRr- a dihybrid cross

22
Q

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

a. 3 green:1 yellow
b. 1 GG: 2Gg: 1gg
c. F1
d. F2

23
Q

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

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.

24
Q

Explain how the gametes are derived for the F1 generation in the following cross.
YyRr × YyRr

A

An F1 plant will produce four classes of gametes in equal quantities:
YR,Yr, yR, and yr.

25
Q

What will the phenotypic ratio for the F2 generation be for the following cross: YyRr × YyRr

A

These combinations result in four phenotypic categories with a ratio of 9:3:3:1 (nine yellow-round to three green-round to three yellow-wrinkled to one green-wrinkled).

26
Q

What is Mendel’s law of independent assortment?

A

Two or more genes assort independently- that is, each pair of alleles segregates independently of any other pair of alleles during gamete formation. This law applies only to genes (allele pairs) located on different homologous chromosome pairs.

Line up randomly in Metaphase I

27
Q

Explain the event in meiosis when independent assortment occurs.

A

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.

28
Q

Alleles can show different degrees of dominance. Explain how incomplete dominance is different from complete dominance and give an example of incomplete dominance.

A

Incomplete dominance is the situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. If anything supported the “blending” theory, incomplete dominance would.

Complete dominance is the situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.
An example of incomplete dominance is the crossing of red snapdragons with white snapdragons to produce F1 hybrids with pink flowers.

29
Q

What pattern of inheritance is at play when genotypic ratio matches phenotypic ratio?

A

Incomplete Dominance
1:2:1

30
Q

What happens during meiosis to increase genetic variability, and why is it important?

A

Synapsis and crossing over produces recombinant chromosomes in prophase I, and independent assortment of chromosomes as homologous pairs of chomosomes line up in metaphase I and nonidentical sister chromatids line up metaphase II.
Genetic variability is important bc it allows a species to adapt to future environmental changes - survival!
Note: the random nature of fertilization also adds to genetic variablity.