Chapter 8 and 9

Which processes lead to most genetic variation in sexually reproducing organisms?

-Crossing Over -Random Fertilization -Independent orientation of chromosome in meiosis

Crossing over occurs during _____.

Prophase I Crossing over that results in genetic recombination occurs during this phase.

In corn, blue kernels are produced by a dominant allele of a coloration gene, and white kernels are produced in individuals homozygous for a recessive allele of the same coloration gene. Another gene has two alleles for shape, with smooth kernels being dominant to wrinkled. A plant heterozygous for both genes is testcrossed (crossed to a homozygous recessive white, wrinkled strain). The testcross offspring consist of the following types: 1,447 blue smooth; 1,436 white wrinkled; 150 blue wrinkled; 145 white smooth. Explain the inheritance of the coloration and shape traits.

The coloration and shape genes are linked on the same chromosome. The fact that there isn't the 1:1:1:1 ratio of phenotypes expected in this cross if the alleles assorted independently and that there is a pair of large phenotypic classes (1,447 blue smooth; 1,436 white wrinkled) and a pair of small phenotypic classes (150 blue wrinkled; 145 white smooth) indicates that the genes are linked.

Imagine you're counseling a couple who have undergone carrier screening for Tay-Sachs disease. The man is a carrier, and the woman does not carry the Tay-Sachs allele. How should you advise them?

They should be informed that if they have a child, the child will not have Tay-Sachs disease but will have a 50% chance of being a carrier of the Tay-Sachs allele. Because the woman will transmit a normal allele to the child, the child will not be affected; however, there is a 50% chance that the child will inherit a Tay-Sachs allele from the father and thus be a carrier.

In theory, when a nondisjunction for chromosome 18 occurs during meiosis I, four gametes can be produced. If these gametes are fertilized with unaffected gametes from the second parent, what observations would you make concerning the resulting embryos?

Two of the embryos will be trisomic for chromosome 18, and two will contain a single copy of chromosome 18. A nondisjunction that occurs in the first meiotic division will result in one daughter cell having two copies of chromosome 18. This will be trisomic upon fertilization. The other daughter cell will have no copies of chromosome 18 and will contain a single copy of chromosome 18 upon fertilization, a condition known as monosomy.

Meiosis I produces______ cells, each of which is ______.

Two........haploid. At the end of meiosis I there are two haploid cells.

A woman and her male partner have normal color vision. However, her father and her first son are colorblind. What is her genotype? Use C as the gene for colorblindness.

X^C X^c Because this is an X-linked recessive condition, she must possess at least one allele for color blindness to have an affected son. Because she has normal color vision, she must also have one normal allele.

most human genes come in alternate versions called _____.

alleles

If an organism has two non-identical versions of a gene, the one that is expressed in the organism is called the _____ allele.

dominant

The _______ is the genetic makeup of an organism.

genotype

Hypercholesterolemia is a disorder in which blood cholesterol levels are elevated. The H allele is incompletely dominant to the h allele, with hh homozygotes having extremely high levels of blood cholesterol. A husband and wife are both Hh heterozygotes. What is the chance that their first child will have normal levels of blood cholesterol?

1/4 This is the probability of giving birth to an HH homozygote, the only genotype conferring normal cholesterol levels.

Assume that in cattle a spotted coat is dominant to an even coat, short horns are dominant to long horns, and the traits for coat type and horn length assort independently. In a cross between cattle that are each heterozygous for both traits, what proportion of their offspring are expected to have long horns?

1/4. Because these traits assort independently, this dihybrid cross can be considered as two separate monohybrid crosses, with the consequence that one-fourth of the offspring of cattle heterozygous for the horn length trait will express the recessive long-horn trait.

In humans, the haploid number of chromosomes is 23. Independent assortment has the possibility of producing __________ different gametes.

2^23 For each gamete, there are two possibilities for each chromosome (the paternal or the maternal chromosome), and there are 23 different chromosomes per gamete. Crossing over can increase this number even further.

Synapsis occurs during ____.

Prophase I Synapsis, the pairing of homologous chromosomes, occurs during prophase I.

During ____ a spindle forms in a haploid cell.

Prophase II. It is essentially the same as mitotic prophase except that the cells are haploid.

The exchange of parts between nonhomologous chromosomes is called _____.

Reciprocal translocation. It is the exchange of genetic material between nonhomologous chromosomes.

One version of a gene may encode __________, whereas a different version of the same gene may encode __________.

Red eyes; white eyes Different versions of a gene could encode different eye colors. Alternately, they could encode different coat colors.

An example of a cell that is 2n is a __________.

Somatic Cell It is used to refer to the body. These body cells are diploid and have two homologous copies of each chromosome.

Which of the following is a key difference between meiosis and mitosis?

Synapsis occurs. The pairing of homologous chromosomes during prophase I of meiosis is called synapsis.

At the end of _____ and cytokinesis, haploid cells contain chromosomes that each consist of two sister chromatids.

Telophase I At the end of telophase I and cytokinesis, there are two haploid cells with chromosomes that consist of two sister chromatids each.

At the end of _____ and cytokinesis there are four haploid cells.

Telophase II. At the end of telophase II and cytokinesis there are four haploid cells.

Homologous chromosomes migrate to opposite poles during _____.

Anaphase I. During anaphase I sister chromatids remain attached at their centromeres, and homologous chromosomes move to opposite poles.

During ______ sister chromatids separate.

Anaphase II. It is essentially the same as mitotic anaphase except that the cell is haploid.

A _____ is an inherited feature that varies from individual to individual.

Character

Imagine that long fins in zebrafish is a dominant trait. A breeder wants to set up a breeding program beginning with homozygous dominant long-finned fish. If she obtains a handful of the long-finned fish, how can she tell which, if any, of these are homozygous for the trait?

Cross the long-finned fish with short-finned fish; if the offspring are all long-finned, the long-finned parent is homozygous. This cross is a testcross (the short-finned fish must be homozygous recessive), and having all offspring show the dominant trait proves that the long-finned parent is homozygous.

In Morgan's testcross of a gray-bodied, long-winged heterozygous female Drosophila with a homozygous recessive black-bodied, vestigial-winged male, the following offspring were obtained: 965 gray body, long wing; 944 black body, vestigial wing; 206 gray body, vestigial wing; 185 black body, long wing. Focusing only on the recombinant classes (gray body, vestigial wing and black body, long wing), the numbers of offspring of each type are similar (206 and 185). What accounts for the similar number of offspring of each recombinant phenotype?

Crossing over between chromosomes is reciprocal, so whenever a recombinant chromosome of one type is produced, there's a recombinant of the opposite type that is also produced. Every time a recombinant chromosome of one type is produced, there is a complementary type of recombinant chromosome that is also produced.

An insect that has the genotype EeGGcc will have the same phenotype as an insect with the genotype __________.

EEGgcc Individuals with one or two dominant alleles of a gene are indistinguishable, so EE and Ee have the same phenotype, as do GG and Gg.

What is the typical result when a diploid cell undergoes meiosis?

Four haploid cells. The two cell divisions of meiosis, I and II, produce four haploid daughter cells, which are not genetically identical to the diploid parent cell.

Meiosis II typically produces _______ cells, each of which is ___________.

Four......haploid. At the end of meiosis II there are typically 4 haploid cells

At the conclusion of meiosis I, the daughter cells are _____.

Haploid and the sister chromatids are joined. The daughter cells of meiosis I are haploid, and the sister chromatids are still joined. During meiosis II, the sister chromatids separate.

In breeding pure-breeding large and small strains of mice, you cross individuals of each strain and note that their offspring are intermediate in size. Two models (explanations) to account for this result are (1) that body size in these strains is due to one gene with alleles that show incomplete dominance and (2) that body size is a polygenic trait. How could you distinguish between these models?

Intercross the F1 and see if the F2 contains three size classes (consistent with the incomplete dominance model) or if there is a range of sizes (consistent with the polygenic model). This is exactly what is predicted by each model of inheritance.

During _____ chromosomes align single file along the equator of a haploid cell.

Metaphase II. It is essentially the same as mitotic metaphase except that the cell is haploid.

After fertilization, the resulting zygote begins to divide by_______.

Mitosis It ensures that all somatic cells receive copies of the parental chromosomes.

Trisomy for most autosomes is fatal, yet trisomy or even tetrasomy (four copies) of the X chromosome is not. What is the explanation for this difference?

Only one copy of the X chromosome is functional within any given cell, regardless of the total number of X chromosomes. This is the reason that alterations in the number of X chromosomes are much better tolerated than autosomal trisomy.

A single allele that controls more than one character is said to be __________.

Pleiotropic The ability of a gene to affect an organism in many ways (pleio = more) is called pleiotropy.

John and Jane are planning a family, but each has a brother who has sickle-cell disease, so they are concerned that their children may develop sickle-cell disease. Neither John nor Jane nor their respective parents have the disease. They consult a genetic counselor who tells them that __________.

it's possible that they cannot produce a child with sickle-cell disease, but blood tests on them both will be required to make sure. The blood tests will determine if either or both carry the sickle-cell allele.

A genetic cross involving parents that differ in a single character is called a ____

monohybrid cross

The physical traits of an organism are called its _____.

phenotype

If an organism that has non-identical versions of a gene, the one that is not expressed in the organism is called the ____ allele.

recessive

A _____ is one particular variation of a character.

trait

Although in humans there are 22 pairs of autosomal chromosomes, only three different chromosomal trisomies are commonly seen in newborns. Of the remaining 19 autosomes, many trisomies have not been seen in newborns. Why not?

Trisomy for the other autosomal chromosomes is often lethal, and the affected embryos are miscarried. Unfortunately, these trisomies do occur and are commonly observed in spontaneously aborted embryos and fetuses.