A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA
(or RNA, in some viruses)
A specific place along the length of a chromosome where a given gene is located
A haploid reproductive cell, such as an egg or sperm. Gametes unite during sexual reproduction to produce a diploid zygote.
The generation of offspring from a single parent that occurs *without the fusion of gametes* (by budding, division of a single cell, or division of the entire organism into two or more parts). In most cases, the *offspring are genetically identical to the parent*.
A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from both parents via the gametes
There are 46 chromosomes in human somatic cells. A chromosome is a cellular structure carrying
genetic material, found in the nucleus of eukaryotic cells. Each chromosome consists of one very
long DNA molecule and associated proteins.
How many chromosomes are in human cells? What is a chromosome?
Which type of reproduction will result in genetically identical offspring?
A somatic cell is any cell in a multicellular organism except a sperm or egg or their precursors. Examples may vary but could include bone cells, skin cells, blood cells, etc.
What is a somatic cell? Give examples of two human somatic cell types.
Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called haploid cells, and each has a haploid number of chromosomes (n). For humans, the haploid number is 23.
How does a somatic cell compare to a gamete in terms of chromosome number?
A chromosome responsible for determining the sex of an
individual; humans have 1
A chromosome that is not directly involved in determining
sex; not a sex chromosome; humans have 22
When images of the chromosomes are arranged in pairs, starting with the longest chromosomes, the resulting ordered display is called a karyotype. Karyotypes are prepared from isolated somatic cells, which are treated with a drug to stimulate mitosis and then grown in culture for several days. Cells arrested in metaphase, when chromosomes are most highly condensed, are stained and then viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is displayed on a computer monitor, and the images of the chromosomes are arranged into pairs according to their appearance.
What is a karyotype? How is it prepared?
Size of the chromosome
Position of the centromere
Pattern of the stained bands
What are three things that can be determined from a karyotype?
A pair of chromosomes of the same length, centromere position, and staining pattern
liver cell: diploid
egg cell: haploid
skin cell: diploid
somatic cell: diploid
sex cell: haploid
Haploid or diploid:
The only cells of the human body not produced by mitosis are the gametes, which develop from specialized cells called germ cells in the gonads—ovaries in females and testes in males.
Where are the gametes of an animal produced? Be specific as to male and female gametes.
By what process are gametes produced?
What is another term for a fertilized egg?
What is the chromosome number of the fertilized
egg? (Answer this in general terms, haploid, n, or diploid, 2n)
Meiosis is a modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell, producing gametes, and introducing genetic variability.
What is the purpose of meiosis?
You will see that plants have a life cycle that involves spores,
which form as a result of meiosis, so these spores are haploid. Notice also that both haploid and diploid cells can divide by mitosis. However, meiosis always begins with cells that are _____, and as a result of meiosis, daughter cells are formed that are always _____. These cells can be gametes (in animals) or spores (in plants).
A life cycle in which there is both a multicellular diploid form; characteristic of plants and some algae
Your study of plants this year will include knowing that they exhibit alternation of generations.
What does this mean?
Sporophyte and gametophyte
What are the two generations? (that exhibit alternation of generations)
sporophyte = diploid
gametophyte = haploid
Which, between sporophyte and gametophyte, is haploid, and which is diploid?
Alleles are any of the alternative versions of a gene that may produce distinguishable phenotypic effects. A possible example is the allele for freckles.
What are alleles? Give an example.
The pairing and physical connection of duplicated homologous chromosomes during prophase I of meiosis
The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis
The X-shaped, microscopically visible region where crossing over has occurred earlier in prophase I between homologous nonsister chromatids. Chiasmata become visible after synapsis ends, with the two homologs remaining associated due to sister chromatid cohesion.
During mitosis, individual chromosomes line up at the metaphase plate. During meiosis, pairs of homologous chromosomes line up at the metaphase plate.
How is the arrangement of chromosomes in metaphase I different from metaphase of mitosis?
In meiosis I, homologous chromosomes separate.
There are two divisions in meiosis. What will separate in the first division in meiosis I?
Now study the chromosomes in anaphase I and telophase I carefully. How many chromosomes are in each cell at the end of the first meiotic division?
Are the resultant daughter cells haploid, or diploid in the first meiotic division?
Sister chromatids separate during meiosis II.
During meiosis I, homologous chromosomes separate. What separates during meiosis II?
They are reduced by half
What happens to chromosome number in meiosis?
During which division is the chromosome number reduced?
To reduce the number of sets of chromosomes from two to one in gametes
What is the purpose of meiosis?
How many times does the cell divide in meiosis?
How many times do the chromosomes duplicate?
How many daughter cells are formed?
What is the chromosome number?
A pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci
What are homologs (homologous chromosomes)?
The pairing and physical connection of duplicated homologous chromosomes during prophase I of meiosis
What occurs in synapsis?
The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis
What is crossing over?
Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction
Mitosis role in the animal body
Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes
Meiosis role in the animal body
Number of DNA replications in Mitosis
1; occurs during interphase before meiosis I begins
Number of DNA replications in Meiosis
Number of divisions in Mitosis
Number of divisions in Meiosis
Number of daughter cells in Mitosis
Number of daughter cells in Meiosos
Chromosome number of daughter cells in Mitosis
Chromosome number of daughter cells in Meiosis
Synapsis and crossing over are unique to meiosis. During what specific phase do these occur?
Crossing over, a genetic rearrangement between nonsister chromatids involving the exchange of corresponding segments of DNA molecules, begins during pairing and synaptonemal complex formation, and is completed while homologs are in synapsis. A chiasma exists at the point where a crossover has occurred.
Explain the physical events of crossing over. You may wish to make a sketch of the event. Include these terms: synaptonemal complex, chiasmata, homologs, sister chromatids.
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 every other pair. This is called independent assortment. Each daughter cell represents one outcome of all possible combinations of maternal and paternal chromosomes.
independent assortment of chromosomes
Crossing over begins very early in prophase I as homologous chromosomes pair loosely along their lengths. Each gene on one homolog is aligned precisely with the corresponding gene on the other homolog. In a single crossover event, the DNA of two nonsister chromatids—one maternal and one paternal chromatid of a homologous pair—is broken by specific proteins at precisely corresponding points, and the two segments beyond the crossover point are each joined to the other chromatid. Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vice versa. In this way, crossing over produces chromosomes with new combinations of maternal and paternal alleles.
The random nature of fertilization adds to the genetic variation arising from meiosis. In humans, each male and female gamete represents one of about 8.4 million possible chromosome combinations due to independent assortment. The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about 70 trillion diploid combinations
END OF CHAPTER 13: TEST YOUR UNDERSTANDING ON SELF QUIZ
The explanation of heredity most widely in favor during the 1800s was the “blending” hypothesis, the idea that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paints blend to make green. This hypothesis predicts that over many generations, a freely mating population will give rise to a uniform population of individuals. However, our everyday observations and the results of breeding experiments with animals and plants, contradict that prediction. The blending hypothesis also fails to explain other phenomena of inheritance, such as traits reappearing after skipping a generation. An alternative to the blending model is a “particulate” hypothesis of inheritance: the gene idea. According to this model, *parents pass on discrete heritable units—genes—that retain their separate identities in offspring.*
In the 1800s the most widely favored explanation of genetics was “blending.” Explain the concept of blending, and then describe how Mendel’s “particulate” (gene) hypothesis was different.
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.
One of the keys to success for Mendel was his selection of pea plants. Explain how using pea plants allowed Mendel to control mating; that is, how did this approach let Mendel be positive about the exact characteristics of each parent?
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.
What is the difference between a character and a trait? Explain using an example.
First filial generation
Second filial generation
Explain how Mendel’s simple cross of purple and white flowers did the following:
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.
a. refuted blending:
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.
b. determined dominant and recessive characteristics:
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.
c. demonstrated the merit of experiments that covered multiple generations:
Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited from each parent.
In sexually reproducing organisms, why are there exactly two chromosomes in each homologous pair?
Alternative versions of genes account for variations in inherited characters.
Mendel’s Four Concepts: First Concept
For each character, an organism inherits two copies of a gene, one from each parent.
Mendel’s Four Concepts: Second 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 Four Concepts: Third Concept
The two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.*
Mendel’s Four Concepts: Fourth Concept (law of segregation)
A cross between two organisms that are heterozygous for the character being followed (or the self-pollination of a heterozygous plant)
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)
Each pair of alleles segregates independently of each other pair of alleles during gamete formation
Explain Mendel’s law of independent assortment
An event that is certain to occur has a probability of _, while an event that is certain not to occur has a
probability of _.
An event whose outcome is unaffected by what has happened on previous trials, such as in a sequence of coin tosses
In probability, what is an independent event?
The multiplication rule states that to determine this probability, we multiply the probability of one event by the probability of the other event. For example, by the multiplication rule, the probability that both coins will land heads up is ½ × ½ = ¼.
State the multiplication rule and give an original example.
The addition rule states that the probability that any two or more mutually exclusive events will occur is calculated by adding their individual probabilities.
State the addition rule and give an original example.
The probability is 1/16.
What is the probability that a couple will have a girl, a boy, a girl, and a boy in this specific order?
Incomplete dominance is the situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.
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.
Explain how incomplete dominance is different from complete dominance, and give an example of incomplete dominance.
In codominance, the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways, such as in the human MN blood group, determined by the codominant alleles for two specific molecules located on the surface of red blood
cells, the M and N molecules.
In incomplete dominance, the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele; neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of the two parental varieties.
Compare and contrast codominance with incomplete dominance.
Natural selection determines how common an allele is in the gene pool. For example, having six fingers (polydactyly) is dominant to five fingers, but the presence of six fingers is not common in the human gene pool.
Dominant alleles are not necessarily more common than recessive alleles in the gene pool. Explain why this is true.
Most genes exist in more than two allelic forms, for example, ABO blood groups.
Explain what is meant when a gene is said to have multiple alleles. Blood groups are an excellent human example of this.
Before working any problems, complete this ABO blood type chart.
Blood groups are so important medically that you should be able to solve genetics problems based on blood types. The first step in accomplishing that is to understand the genotypes of each blood type.
Half of the children would be expected to have type A blood and half type B blood.
If a man with type AB blood marries a woman with type O, what blood types would you expect in their children? What fraction would you expect of each type?
Pleiotropy is the ability of a single gene to have multiple effects. In humans, pleiotropic alleles are responsible for multiple symptoms associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.
hat is pleiotropy? Explain why this is important in diseases like cystic fibrosis and sickle-cell disease.
Epistatis is a type of gene interaction in which the phenotypic expression of one gene alters that of another independently inherited gene.
This dihybrid cross results in four yellow Labrador retrievers rather than three because the dominant allele, symbolized by E, results in the deposition of either black or brown pigment. If the Lab is homozygous recessive for the second locus (ee), then the coat is yellow, regardless of the genotype at the black/brown locus. The E/e gene is epistatic to the B/b gene.
Explain why the dihybrid cross detailed in Figure 14.12 in your text has four yellow Labrador retrievers instead of the three that would have been predicted by Mendel’s work.
For many characters, such as human skin color and height, an either-or classification is impossible because the characters vary in the population in gradients along a continuum.
Why is height a good example of polygenic inheritance?
Quantitative variation usually indicates ____
The outcome of a genotype lies within its norm of reaction, a phenotype range that depends on the environment in which the genotype of expressed. For some characters, such as the ABO blood group system, the norm of reaction has no breadth whatsoever. Other characteristics, such as a person’s blood count of red and white cells, varies quite a bit, depending on such factors as the altitude, the customary level of physical activity, and the presence of infectious agents. Genetics refers to such characters as multifactorial, meaning that many factors, both genetic and environmental, collectively influence phenotype.
Using the terms norm of reaction and multifactorial, explain the potential influence of the environment on phenotypic expression.
See page 276 in your text for the labeled figure.
Pedigree analysis is often used to determine the mode of inheritance (dominant or recessive, for example). Be sure to read the “Tips for pedigree analysis” in Figure 14.15 in your text; then complete the unlabeled pedigree by indicating the genotypes for all involved.
What is the mode of inheritance for this pedigree?
The presence of a free earlobe could indicate either an FF or Ff genotype, as F is the dominant allele, resulting in free earlobes.
The female with the recessive trait can only have one genotype. The female with the dominant trait could be homozygous or heterozygous.
In the pedigree you completed above, explain why you know the genotype of one female in the third generation, but are unsure of the other.
Thousands of genetic disorders are known to be inherited as simple recessive traits. These disorders range in severity from relatively mild, such as albinism (lack of pigmentation, which results in susceptibility to skin cancers and vision problems) to life-threatening, such as cystic fibrosis.
Describe what you think is medically important to know about the behavior of recessive alleles
a. cystic fibrosis: A human genetic disorder caused by a recessive allele for a chloride channel protein;
characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated.
b. sickle-cell disease: A recessively inherited human blood disorder in which a single nucleotide change in the β-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals.
c. achondroplasia: A form of dwarfism that occurs in one of every 25,000 people. Heterozygous individuals have the dwarf phenotype. Like the presence of extra fingers or toes, achondroplasia is a trait for which the recessive allele is much more prevalent than the corresponding dominant allele.
d. Huntington’s disease: A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.
You are expected to have a general knowledge of the pattern of inheritance and the common symptoms of a number of genetic disorders. Provide this information for the disorders listed below.
1. A sample of amniotic fluid can be taken starting at the fourteenth to sixteenth week of pregnancy.
2. Biochemical and genetic tests can be performed immediately on the amniotic fluid or later on the cultured cell.
3. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping.
1. A sample of chorionic villus tissue can be taken as early as the eighth to tenth week of pregnancy.
2. Karyotyping and biochemical and genetic tests can be performed on the fetal cells immediately, providing results within a day or so.
Amniocentesis and chorionic villus sampling are the two most widely used methods for testing a fetus for genetic disorders. Use the unlabeled diagram below to explain the three main steps in amniocentesis and the two main steps of CVS.
Strength of amniocentesis: In addition to fetal cells, amniotic fluid is also collected. Amniotic fluid can be used to detect additional enzymatic or developmental problems not detectable from the karyotype.
Weakness of amniocentesis: Cells must be cultured for several weeks before karyotyping, and the test cannot be performed until the fourteenth to sixteenth week.
Strength of CVS: These cells proliferate rapidly enough to allow karyotyping to be carried out immediately, and CVS can be performed as early as the eighth to tenth week.
Weakness of CVS: No amniotic fluid is collected with this technique.
What are the strengths and weaknesses of each fetal test?
The symptoms of phenylketonuria include an inability to metabolize the amino acid phenylalanine, causing severe mental intellectual disability. Some genetic disorders, including phenylketonuria, can be detected at birth by simple biochemical tests that are now routinely performed in most hospitals in the United States.
What are the symptoms of phenylketonuria (PKU)? How is newborn screening used to identify children with this disorder?
END OF CHAPTER 14: TEST YOUR UNDERSTANDING ON SELF QUIZ
The chromosome theory of inheritance is a basic principle in biology stating that genes are located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.
What is the chromosome theory of inheritance?
See page 287 in your text for the labeled figure.
The law of segregation states that the two alleles for each gene separate during gamete formation.
Explain the law of segregation. Use two different colored pencils to illustrate the segregation of alleles. You may want to consult Figure 15.2 in your text, and model your sketches on this.
See page 287 in your text for the labeled figure.
The law of independent assortment states that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes.
Explain the law of independent assortment. To demonstrate that you understand this concept, consider a cell with two pairs of chromosomes. Sketch the two different ways these chromosomes might be arranged during metaphase I.
1. Single mating will produce hundreds of offspring.
2. A new generation can be bred every two weeks.
3. The fruit fly has only four pairs of chromosomes, which are easily distinguishable with a light microscope.
Thomas Hunt Morgan selected Drosophila melanogaster as his experimental organism. List at least three reasons the fruit fly is an excellent subject for genetic studies.
The notation for wild type and mutant traits follows some accepted conventions. Notate the following genotypes for a female fruit fly:
a. a fly homozygous for red eyes
b. a fly heterozygous for red eyes
c. a fly homozygous for white eyes
See page 289 of your text for the labeled figure.
Parental generation: A red-eyed female was crossed with a white-eyed male.
F1 generation: All the offspring of the P generation had red eyes
F2 generation: The offspring showed a ratio of three red-eyed flies to one white- eyed fly. However, there were no white-eyed females.
When Thomas Hunt Morgan mated a white-eyed male fly with a red-eyed female, he came to the
startling conclusion that the trait for eye color was located on the chromosome that determines sex.
Show this cross. Begin with the parental generation, and go through the F2.
The white-eye trait showed up only in males.
What unusual result suggested that the eye-color trait is located on the X chromosome?
See page 290 of your text for the labeled figure.
There are several variations in the way sex is determined in different species. Complete the following figure to explain four different methods of sex determination.
SRY refers to the Sex determining Region of Y, a gene found on the Y chromosome that is required for the development of testes.
What is the SRY gene? Where is it found, and what does it do?
A gene located on either sex chromosome. Most sex-linked genes are on the X chromosome and show distinctive patterns of inheritance; there are very few genes on the Y chromosome.
What is the definition of a sex-linked gene?
Typically, the sex-linked traits are actually X-linked genes.
In humans, how has that term been historically modified?
1. Duchenne muscular dystrophy: A human genetic disease caused by a sex-linked recessive allele;
characterized by progressive weakening and a loss of muscle tissue
2. Hemophilia: A human genetic disease caused by a sex-linked recessive allele, resulting in the absence
of one or more blood-clotting proteins; characterized by excessive bleeding following injury
3. Color Blindness: A mild disorder almost always inherited as an X-linked allele
Name and describe three human sex-linked disorders.
(See page 291 of your text for the labeled figure.) If a carrier mates with a male who has normal color vision, there is a 50% chance that each daughter will be a carrier like her mother and a 50% chance that each son will have the disorder.
Try the following problem (Figure 15.7b in your text). A female who carries an allele for color blindness, but who is not color-blind, mates with a male who has normal color vision. What is the probability that they will have a son who is color-blind? A Punnett square to use for this problem is shown in the following figure.
A Barr body is a dense object lying along the inside of the nuclear envelope in cells of female mammals, representing a highly condensed, inactivated X chromosome. Female mammals, including humans, inherit two X chromosomes—twice the number inherited by males; females show a Barr body in their cells so that the cells of females and males have the same effective dose (one copy) of most X-linked genes.
What is a Barr body? Why do human females show a Barr body in their cells?
The selection of which X chromosome will form the Barr body occurs randomly and independently in each embryonic cell present at the time of X inactivation.
X inactivation maintains the proper gene dosage. How is the X chromosome inactivated?
The tortoiseshell gene is on the X chromosome, and the tortoiseshell phenotype requires the presence of two different alleles, one for orange fur and one for black fur. Normally, only females can have both alleles, because only they have two X chromosomes.
Why can you say that all calico cats are females?
Linked genes are genes located close enough together on a chromosome that they tend to be inherited together. These genes do not sort independently, but rather are transmitted as a unit. It is important to note that as Morgan’s experiments illustrated, some mechanism (later discovered to be “crossing over”) occasionally breaks the linkage between specific alleles of genes on the same chromosome.
What are linked genes? Do linked genes sort independently?
Geneticists call the offspring that show these new combinations recombinant types, or recombinants for short.
If two genes are linked on the same chromosome, we call this combination the parental combination. These genes will be transmitted as a unit and will not sort independently. However, during meiosis, crossing over occurs between homologous chromosomes, and the linked genes can become “unlinked.” In general, the farther two genes are from each other along the chromosome, the more often they will come “unlinked.” Genetic recombination is the process during which linked genes become unlinked. What do geneticists call the offspring that show these new combinations?
Review meiosis. When does crossing over occur?
A linkage map is a genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes.
Alfred H. Sturtevant, a student of Thomas Hunt Morgan, used assumptions from observations of crossovers to map genes. What is a linkage map
A map unit is a unit of measurement of the distance between genes. One map unit is equivalent to a 1% recombination frequency.
What is a map unit?
See page 295 in your text for the labeled figure. Since 391 of the total 2,300 offspring do not show the parental phenotypes, they are recombinants. Since the frequency of recombination is therefore 17%, the two genes are 17 map units apart.
Use the figure below, which is from Figure 15.10. It shows the results of a cross between a fruit fly that is heterozygous for a gray body with normal wings, and a fruit fly that has a black body with vestigial wings. Because these genes are linked, the results are not what might have been predicted. Show the phenotypes and number of each type of offspring. Indicate which offspring are the recombinants and which are the parental type. Finally, calculate the map distance between the two genes. Show all your work here.
Nondisjunction is an error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other.
What occurs in nondisjunction?
A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number. Trisomy 21 (Down syndrome) is an aneuploidy.
Refers to a diploid cell that has only one copy of a particular chromosome instead of the normal two. Turner syndrome is a human monosomy; the female has only one X chromosome.
Refers to a diploid cell that has three copies of a particular chromosome instead of the normal two. Trisomy 21 is trisomic for chromosome 21.
A chromosomal alteration in which the organism possesses more than two complete chromosome sets. It is the result of an accident of cell division.
Down syndrome is usually the result of an extra chromosome 21, so that each cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often called trisomy 21.
Four characteristics of Down syndrome:
1. Characteristic facial features
2. Short stature
3. Correctable heart defects
4. Developmental delays
What causes Down syndrome? What are four characteristics of Down syndrome?
Klinefelter syndrome: male sex organs, but abnormally small/ sterile testes; some breast enlargement and other female characteristics
human aneuploidies: XXY Male
Trisomy X: no unusual physical features other than being slightly taller than average; at risk for learning disabilities; fertile
human aneuploidies: XXX Female
Turner syndrome: phenotypically female, but sterile due to lack of maturation in sex organs; secondary sex characteristics developed with estrogen replacement; normal intelligence
human aneuploidies: XO Female
Normal sexual development; taller than average stature
human aneuploidies: XYY Male
A deficiency in a chromosome resulting from the loss of a fragment through breakage
An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated
An aberration in chromosome structure resulting from reattachment of a chromosomal fragment in a reverse orientation to the chromosome from which it originated
An aberration in chromosome structure resulting from attachment of a chromosomal fragment to a nonhomologous chromosome
Genomic imprinting occurs during gamete formation and results in the silencing of a particular allele of certain genes. Because these genes are imprinted differently in sperm and eggs, the zygote expresses only one allele of an imprinted gene that is inherited from either the female or the male parent. The imprints are then transmitted to all body cells during development.
A number of genes will cause a variation in phenotype, depending on whether the gene came from the father or the mother. This variation occurs because of genomic imprinting. Explain genomic imprinting.
Although you inherited one chromosome of each pair from your mother and your father, you have inherited a group of genes from your mother only. What genes are these?
Chloroplasts or mitochondria
You should have identified mitochondrial DNA as the correct response to question 29 above. What other organelle has its own genes? These are extranuclear genes.
END OF CHAPTER 13: TEST YOUR UNDERSTANDING ON SELF QUIZ
remember long FRQ, short FRQ and Punnett squares