Genetics Test: Chapters 13, 14, 15

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.

Sperm

Eggs

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.

Asexual reproduction

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.

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.

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.

Size of the chromosome
Position of the centromere
Pattern of the stained bands

A pair of chromosomes of the same length, centromere position, and staining pattern

liver cell: diploid
gamete: haploid
egg cell: haploid
zygote: diploid
skin cell: diploid
sperm: haploid
somatic cell: diploid
sex cell: haploid

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.

Meiosis

Zygote

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.

diploid; haploid

A life cycle in which there is both a multicellular diploid form; characteristic of plants and some algae

Sporophyte and gametophyte

sporophyte = diploid
gametophyte = haploid

Alleles are any of the alternative versions of a gene that may produce distinguishable phenotypic effects. A possible example is the allele for freckles.

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.

In meiosis I, homologous chromosomes separate.

3

haploid

Sister chromatids separate during meiosis II.

They are reduced by half

Meiosis I

To reduce the number of sets of chromosomes from two to one in gametes

2

None

4

n

A pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci

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

Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction

Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

1

1; occurs during interphase before meiosis I begins

1

2

2

4

2n

n

Prophase I

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.

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.

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

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.*

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.

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.

Parent generation

First filial generation

Second filial generation

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.

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.

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.

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

Alternative versions of genes account for variations in inherited characters.

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

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.

The two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.*

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

1; 0

An event whose outcome is unaffected by what has happened on previous trials, such as in a sequence of coin tosses

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.

The addition rule states that the probability that any two or more mutually exclusive events will occur is calculated by adding their individual probabilities.

The probability is 1/16.

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.

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.

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.

Most genes exist in more than two allelic forms, for example, ABO blood groups.

Before working any problems, complete this ABO blood type chart.

Half of the children would be expected to have type A blood and half type B blood.

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.

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.

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.

polygenic inheritance

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.

See page 276 in your text for the labeled figure.

Recessive

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.

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.

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.

Amniocentesis:
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.
CVS:
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.

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.

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.

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.

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.

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.

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.

a. Xw+Xw+
b. Xw+Xw
c. XwX

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.

The white-eye trait showed up only in males.

See page 290 of your text for the labeled figure.

SRY refers to the Sex determining Region of Y, a gene found on the Y chromosome that is required for the development of testes.

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.

Typically, the sex-linked traits are actually X-linked genes.

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

(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.

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.

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.

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.

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.

Geneticists call the offspring that show these new combinations recombinant types, or recombinants for short.

Prophase I

A linkage map is a genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes.

A map unit is a unit of measurement of the distance between genes. One map unit is equivalent to a 1% recombination frequency.

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.

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.

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

Klinefelter syndrome: male sex organs, but abnormally small/ sterile testes; some breast enlargement and other female characteristics

Trisomy X: no unusual physical features other than being slightly taller than average; at risk for learning disabilities; fertile

Turner syndrome: phenotypically female, but sterile due to lack of maturation in sex organs; secondary sex characteristics developed with estrogen replacement; normal intelligence

Normal sexual development; taller than average stature

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.

Mitochondrial DNA

Chloroplasts or mitochondria