Chapter 12 Chromosomal Patterns of Inheritance

Chapter 12 Chromosomal Patterns of Inheritance

The chromosomal theory of inheritance and its history, sex chromosomes and X linkage, and gene linkage are detailed in this chapter. The construction and interpretation of a chromosome map is outlined. The abnormal situations of chromosome number change (e.g., monosomy, trisomy, etc.) are described, as are the situations of chromosome structural change (e.g., deletion, translocation, etc.). Many human diseases involving chromosomal abnormalities are described.

Chapter Outline

12.1 Chromosomal Inheritance

1.Genes are located on chromosomes; this is called the chromosome theory of inheritance.

2.Chromosomes can be categorized as two types:

a.Autosomes are non-sex chromosomes that are the same number and kind between sexes.

b.Sex chromosomes determine if the individual is male or female.

3.Sex chromosomes in the human female are XX; those of the male are XY.

4.Males produce X-containing and Y-containing gametes; therefore males determine the sex of offspring.

5.Besides genes that determine sex, sex chromosomes carry many genes for traits unrelated to sex.

6.An X-linked gene is any gene located on X chromosome; used to describe genes on X chromosome that are missing on the Y chromosome.

A.X-Linked Alleles

1.Work with fruit flies (Drosophila) by Thomas Hunt Morgan (early 1900s) confirmed genes were on chromosomes.

a.Fruit flies are easily and inexpensively raised in common laboratory glassware.

b.Females only mate once and lay hundreds of eggs.

c.The fruit fly generation time is short, allowing rapid experiments.

2.Fruit flies have an XY sex chromosome system similar to the human system; experiments can be correlated to the human situation.

a.Newly discovered mutant male fruit flies had white eyes.

b.Cross of the hybrids from the white-eyed male crossed with a dominant red-eyed female yielded the expected 3:1 red-to-white ratio; however, all of the white-eyed flies were males.

c.An allele for eye color on the X but not on the Y chromosome supports the results of this cross.

d.Behavior of this allele corresponds to the behavior of the chromosome; this confirmed the chromosomal theory of inheritance.

3.X-Linked Problems

a.X-linked alleles are designated as superscripts to the X chromosome.

b.Heterozygous females are carriers; they do not show the trait but can transmit it.

c.Males are never carriers but express the one allele on the X chromosome; the allele could be dominant or recessive.

d.One form of color-blindness is X-linked recessive.

B.Human X-Linked Disorders

1.More males have X-linked traits because recessive alleles on the X chromosome in males are expressed in males.

2.Color Blindness

a.Color blindness can be an X-linked recessive disorder involving mutations of genes coding for green or red sensitive cone cells, resulting in the inability to perceive green or red, respectively; the pigment for blue-sensitive protein is autosomal.

b.About 8% of Caucasian men have red-green color blindness.

3.Muscular Dystrophy

a.Duchenne muscular dystrophy is the most common form and is characterized by wasting away of muscles, eventually leading to death; it affects one out of every 3,600 male births.

b.This X-linked recessive disease involves a mutant gene that fails to produce the protein dystrophin.

c.Signs and symptoms (e.g., waddling gait, toe walking, frequent falls, difficulty in rising) soon appear.

d.Muscles weaken until the individual is confined to a wheelchair; death usually occurs by age 20.

e.Affected males are rarely fathers; the gene passes from carrier mother to carrier daughter.

f.Lack of dystrophin protein causes calcium ions to leak into muscle cells; this promotes action of an enzyme that dissolves muscle fibers.

g.As the body attempts to repair tissue, fibrous tissue forms and cuts off blood supply to the affected muscles.

h.A test now detects carriers of Duchenne muscular dystrophy; treatments are being attempted.

3.Hemophilia

a.About one in 10,000 males is a hemophiliac with impaired ability of blood to clot.

b.The two common types: Hemophilia A, due to the absence of clotting factor IX; Hemophilia B, due to the absence of clotting factor VIII.

c.Hemophiliacs bleed externally after an injury and also suffer internal bleeding around joints.

d.Hemorrhages stop with transfusions of blood (or plasma) or concentrates of clotting protein.

e.Factor VIII is now available as a genetically-engineered product.

f.Of Queen Victoria’s 26 offspring, five grandsons had hemophilia and four granddaughters were carriers.

4.Fragile X Syndrome (See Health and Focus box)

a.In this case, the X chromosome is nearly broken; most often found in males.

b.This affects one in 1,500 males and one in 2,500 females.

c.As children, they are often hyperactive or autistic with delayed or repetitive speech.

d.As adults, males usually have larger testes, unusually protruding ears, and other symptoms.

e.About one-fifth of males with fragile X do not show symptoms.

f.Fragile X passes from a symptomless male carrier to grandson.

g.It has been traced to excessive repeats of base triplet CGG (cytosine-guanine-guanine); up to 230 copies compared to normal 6–to–50 copies.

12.2 Gene Linkage

1.Fruit flies have four pairs of chromosomes to hold thousands of genes; therefore each chromosome must hold many genes.

2.All alleles on one chromosome form a linkage group that are inherited together except when crossing over occurs.

3.Crossing-over causes recombinant gametes and at fertilization, recombinant phenotypes.

1. Linked alleles do not obey Mendel’s laws because they tend to go into the gametes together.
2. A linkage map (or chromosome map)tells the relative distances between gene loci on a chromosome.

A.Constructing a Chromosome Map

1.The percentage of recombinant phenotypes, which is statistically related to the frequency of crossing over between specific loci, is used to measure the distance between genes.

2.Crosses involving linked genes do not give the same results as unlinked genes.

3.A heterozygote forms only two types of gametes and produces offspring with only two phenotypes.

B.Linkage Data

1.Linked genes indicate the distance between genes on the chromosomes.

2.If 1% of crossing-over equals one map unit, then 6% recombinants reveal 6 map units between genes.

3.If crosses are performed for three alleles on a chromosome, only one map order explains map units.

4.Humans have few offspring and a long generation time, and it is impossible to designate matings; therefore biochemical methods are used to map human chromosomes.

12.3 Changes in Chromosome Numbers

1.Chromosomal mutations are changes in chromosome number or structure.

2.Mutations, along with crossing-over, recombination of chromosomes during meiosis, and gamete fusion during fertilization, increase the amount of variation among offspring.

3.The correct number of chromosomes in a species is called euploidy; changes in chromosome number include polyploidy and aneuploidy.

A.Polyploidy

1.A polyploid is a eukaryote with three or more complete sets of chromosomes.

2.Polyploid organisms are named according to the number of sets of chromosomes they have: triploids (3n), tetraploids (4n), etc.

3.Polyploidy is not often seen in animals.

4.Polyploidy is a major evolutionary mechanism in plants; it is probably involved in 47% of flowering plants including some important crops (wheat, corn, fruits, etc.).

5.Polyploidy generally arises following hybridization (reproduction between two different species); a hybrid may have an odd number of chromosomes and thus be sterile, but if the chromosomes in the hybrid double in number, the now-even number of chromosomes can undergo synapsis during meiosis; successful polyploidy thus results in a new species.

1. Aneuploidy

1. Aneuploidy is the condition in which an organism gains or loses one or more chromosomes.
2. Monosomy (2n – 1) occurs when an individual has only one of a particular type of chromosome.
3. Trisomy (2n + 1)occurs when an individual has three of a particular type of chromosome.
4. Nondisjunction is the failure of chromosomes to separate at meiosis—both members of the homologous pair go into the same gamete.
5. Monosomy and trisomy occur in plants and animals; in autosomes of animals, it is generally lethal.
6. Trisomy 21 is the most common autosomal trisomy.

a.Trisomy 21 (also called Down syndrome) occurs when three copies of chromosome 21 are present.

b.Usually two copies of chromosome 21 are contributed by the egg; in 23% of the cases, the sperm had the extra chromosome 21.

c.Over 90% of individuals with Down syndrome have three copies of chromosome 21.

d.Chances of a woman having a Down syndrome child increase with age, starting at age 40.

e.Chorionic villi sampling testing or amniocentesis and karyotyping (see the Science Focus box for a detailed description of this procedure) detects a Down syndrome child; however, risks for young women exceed likelihood of detection.

f.A Down syndrome child has many characteristic signs and symptoms, including a tendency for leukemia, cataracts, faster aging, mental retardation, and an increased chance of developing Alzheimer disease later in life.

g.The Gart gene, located on the bottom third of chromosome 21, leads to a high level of purines and is associated with the signs and symptoms of Down syndrome; future research may lead to suppression of this gene.

C.Changes in Sex Chromosome Number

1.Nondisjunction during oogenesis can result in too few or too many X chromosomes; nondisjunction during spermatogenesis can result in missing or too many Y chromosomes.

2.Turnersyndrome females have only one sex chromosome, an X; thus, they are XO, with O signifying the absence of a second sex chromosome.

a.Turner females are short, have a broad chest and folds of skin on back of neck.

b.Ovaries of Turner females never become functional; therefore, females do not undergo puberty.

c.They usually have normal intelligence and can lead fairly normal lives with hormone supplements.

3.Klinefelter syndrome maleshave one Y chromosome and two or more X chromosomes (e.g., XXY).

a.Affected individuals are sterile males; the testes and prostate are underdeveloped.

b.Individuals have large hands and feet, long arms and legs, and lack facial hair.

c.Presence of the Y chromosome drives male formation but more than two X chromosomes may result in mental retardation.

d.A Barr body, usually only seen in the nuclei of a female’s cells, is seen in this syndrome due to the two X chromosomes.

4.Poly-X females (or superfemale) have three or more X chromosomes and therefore extra Barr bodies in the nucleus.

a.There is no increased femininity; most lack any physical abnormalities.

b.XXX individuals are not mentally retarded but may have delayed motor and language development; XXXX females are usually tall and severely mentally retarded.

c.Some experience menstrual irregularities but many menstruate regularly and are fertile.

5.Jacobs syndrome (XYY) are males with two Y chromosomes instead of one.

a.This results from nondisjunction during spermatogenesis.

b.Males are usually taller than average, suffer from persistent acne, and tend to have speech and reading problems.

c.Earlier claims that XYY individuals were likely to be aggressive were not correct.

12.4 Changes in Chromosome Structure

1.Environmental factors including radiation, chemicals, and viruses, can cause chromosomes to break; if the broken ends do not rejoin in the same pattern, this causes a change in chromosomal structure.

2.Deletion: a type of mutation in which an end of a chromosome breaks off or when two simultaneous breaks lead to the loss of a segment.

3.Translocation: a chromosomal segment is removed from one chromosome and inserted into another nonhomologous chromosome; in Down syndrome, 5% of cases are due to a translocation between chromosome 21 and 14, a situation that runs in the family of the father or mother.

4.Duplication: the presence of a chromosomal segment more than once on the same chromosome.

1. A broken segment from one chromosome can simply attach to its homologue or unequal crossing-over may occur.
2. A duplication may also involve an inversion where a segment that has become separated from the chromosome is reinserted at the same place but in reverse; the position and sequence of genes are altered.

A.Human Syndromes

1.Deletion Syndromes

a.Williams syndrome occurs when chromosome 7 loses an end piece: children look like pixies, have poor academic skills but good verbal and musical skills; lack of elastin causes cardiovascular problems and skin aging.

b.Cri du chat syndrome (“cry of the cat”) is a deletion in which an individual has a small head, is mentally retarded, has facial abnormalities, and an abnormal glottis and larynx resulting in a cry resembling that of a cat.

2.Translocation Syndromes

a.If a translocation results in the normal amount of genetic material, the person will remain healthy; if a person inherits only one of the translocated chromosomes, that person may have only one allele or three alleles rather than the normal two.

b.In Alagille syndrome, chromosomes 2 and 20 exchange segments, causing a small deletion on chromosome 20 that may produce some abnormalities.

Lecture Enrichment Ideas

Experience Base: Laboratory experience with slide preparations is critical if students are to meaningfully understand autosomes and sex chromosomes. Chromosome mapping is a difficult concept and will best be understood if actual experiments with Drosophila can be done—three-point crosses work the best. Sex linkage is complex enough to also require visuals. Early karyotypes were not easily spread and identified; for a time, researchers thought human cells had 48 chromosomes; accordingly, students should understand that not all current knowledge was immediately evident.

1. Discuss why only a few trisomies (of chromosomes 21, 13, and 18) are seen in live human births, and why these reduce the life span, often drastically. Why aren’t trisomies of the larger chromosomes (numbers 1 to 7) able to produce a live individual?

2. Describe the methods of prenatal diagnosis and the means of recognizing carriers so that couples at risk for having a homozygous recessive child will know to test for a particular disorder.

3. Highlight the fact that in history, the woman is usually blamed for lack of male offspring. Yet, gametes from which sex actually determine the gender of the child? History notes that some powerful individuals (e.g., King Henry VIII) and cultures erroneously believed that the female was at fault if there was no male child born. Now that we know that it is the male’s sperm that determines sex of a child, why do we not reverse the perspective and “blame” the father? This topic allows you to lead discussion to reveal how this knowledge should give us freedom from unjust actions. It is also possible to bring in the history of biology, including the belief that either the ova or the sperm contained fully preformed individuals and only received a “growth factor” from the other parent.

1. Consider the possible effects of an X-linked disease on a carrier female; given the random inactivation of one X in each cell, half of her cells would be expected to have the abnormal X active. Lead students through examples of color blindness or Duchenne muscular dystrophy.
2. If available, present to the students results obtained for three phenotypic traits (e.g., eye color, wing shape, body type) in an F2 of Drosophila. Have them construct a chromosome map of the three loci governing those traits. Obviously, this will have much more impact if the students can actually conduct the experiment in the lab themselves.

Critical Thinking

Question 1.The Y chromosome is shorter than the Xchromosome, accounting for about four percent less mass to the sperm cell. However, when millions are deposited and must travel a relatively long distance with equal propulsion, this difference in mass should result in what difference in male and female zygotes produced?

Answer:Because the Y chromosomes are hypothetically lighter weight, Y-carrying sperm should pull ahead in this race to the egg, resulting in more males being produced. In actuality, slightly more males than females are born. However, far more males are conceived and the attrition rate for them is greater as miscarriages; therefore, this hypothesis is supported.

Question 2.Lysenko claimed that he could “vernalize” wheat or make it cold-hardy by storing seeds or growing it in refrigerated conditions: the inheritance of acquired characteristics. He and his coworkers did get survival of some wheat plants farther north than common wheat usually grows. However, Western critics claimed he did not use pure strains of wheat, and mixed seed would allow these results under standard genetics without any inheritance of acquired characteristics. Explain how mixed seed or impure strains would mimic “vernalization.”

Answer:Because Lysenko did not breed his wheat in pure strains, it was likely to contain a wide range of genes for cold-tolerance, perhaps as recessive genes or as a polygenic trait with a certain combination of genes contributing to cold-hardiness. His “vernalization” served not to make all plants more cold-tolerant but merely selected for those recombinations that were cold-hardy.

Technology from McGraw-Hill

Please consult your Course Integration Guide for technology correlations for this chapter.

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