Chapter 12 Patterns of Inheritance

Chapter 12 Patterns of Inheritance
12.1 What is the physical basis of inheritance? 12.2 How did Gregor Mendel lay the foundation for modern genetics? 12.3 How are single traits inherited? 12.4 How are multiple traits on different chromosomes inherited? 12.5 How are genes located on the same chromosome inherited?

Chapter 12 Patterns of Inheritance, cont.
12.6 How is sex determined, and how are sex-linked genes inherited? 12.7 Do the Mendelian rules of inheritance apply to all traits? 12.8 How are human genetic disorders investigated? 12.9 How are human disorders caused by single genes inherited? 12.10 How do errors in chromosome number affect humans?

12.1 What is the physical basis of inheritance?
Inheritance is the process by which the characteristics of individuals are passed to their offspring Genes encode these characteristics

Genes A gene is a unit of heredity that encodes information for the form of a particular characteristic The location of a gene on a chromosome is called its locus

Alleles Homologous chromosomes carry the same kinds of genes for the same characteristics Genes for the same characteristic are found at the same loci on both homologous chromosomes

Alleles Genes for a characteristic found on homologous chromosomes may not be identical Alternate versions or forms of genes found at the same gene locus are called alleles

Alleles Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus

Alleles If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)

FIGURE 12-1 The relationships among genes, alleles, and chromosomes
Each homologous chromosome carries the same set of genes. Each gene is located at the same relative position, or locus, on its chromosome. Differences in nucleotide sequences at the same gene locus produce different alleles of the gene. Diploid organisms have two alleles of each gene.

12.2 How Did Gregor Mendel Lay the Foundation for Modern Genetics?
Gregor Mendel studied many subjects (including botany and math) in the 2 years at the Univ. of Vienna He settled down as a monk in the monastery of St. Thomas (in what is now Brno in the Czech Republic) He did groundbreaking research on peas, despite no knowledge of genes or chromosomes

Portrait painted about 1888
Figure: 12-1 Title: Gregor Mendel Caption: A portrait of Mendel, painted in about 1888, after he had completed his pioneering genetics experiments. Portrait painted about 1888

Doing It Right: The Secrets of Mendel’s Success 3 steps to doing an experiment right Choosing the right organism Designing and performing the experiment correctly Analyzing the data properly

Each flower has pollen grains (male gametes) which fertilize the eggs (female gametes) Peas are usually self-fertilized A plant which is homozygous for a trait is called true-breeding Peas can easily be cross-fertilized The traits Mendel looked at (like color) only had one gene that coded for that trait.

FIGURE 12-3 Flowers of the edible pea
In the intact pea flower (left), the lower petals form a container enclosing the reproductive structures—the stamens (male) and carpel (female). Pollen normally cannot enter the flower from outside, so peas usually self-fertilize. If the flower is opened (right), it can be cross-pollinated by hand.

Section 12.3 Outline 12.3 Inheritance of Single Traits
The Language of a Genetic Cross Mendel’s Flower Color Experiments Alleles of a Gene Are Dominant or Recessive How Meiosis Separates Genes: Segregation Understanding the Results of Mendel’s Flower Color Experiments “Genetic Bookkeeping” Practical Application: The Test Cross

The Language of a Genetic Cross
The parents used in a cross are part of the parental generation (known as P) The offspring of the P generation are members of the first filial generation (F1) Offspring of the F1 generation are members of the F2 generation, etc.

pollen Parental generation (P) pollen cross-fertilize true-breeding,
purple-flowered plant true-breeding, white-flowered plant First-generation offspring (F1) Figure: 12-UN1 Title: First generation offspring of the flowering pea all purple-flowered plants Copyright © 2005 Pearson Prentice Hall, Inc.

generation offspring (F1)
First- generation offspring (F1) self-fertilize Second- generation offspring (F2) Figure: 12-UN2 Title: Second generation offspring of the flowering pea 3/4 purple 1/4 white Copyright © 2005 Pearson Prentice Hall, Inc.

12.3 How are single traits inherited?
After that, Mendel allowed the F2 plants to self-fertilize a third generation (F3) The white plants always produced white plants – no matter how many generations The purple-flowered F2 plants were of 2 types 1/3 true-breeding purple 2/3 were hybrids and produced purple and white flowered peas F2 generation ratio was ¼ true breeding white : ½ hybrid : ¼ true breeding purple

Inheritance of dominant & recessive
Each trait is determined by a pair of discrete units now called genes The pairs separate during gamete formation (meiosis). This is called Mendel’s Law of Segregation Which allele goes in which gamete is random (otherwise the math doesn’t work) One type of allele can mask the other allele (dominant and recessive) but the dominant does not alter the recessive allele

homozygous parent gametes A A A A
Figure: 12-UN3 Title: Homozygous parent True-breeding organisms have two of the same allele for a given gene. This is also called homozygous.

heterozygous parent gametes A a A a
Figure: 12-UN4 Title: Heterozygous parent Hybrids organisms have two of the different alleles for a given gene. This is also called heterozygous. Capital letters for dominant and lower case for recessive

purple parent PP P P all P sperm and eggs white parent pp p p
Figure: 12-UN5 Title: Homozygous allele distribution pp p p all p sperm and eggs Copyright © 2005 Pearson Prentice Hall, Inc.

F1 offspring sperm eggs P p Pp or p P Pp Figure: 12-UN6 Title:
Heterozygous first generation offspring allele distribution p P Pp

1/4 1/4 1/4 1/4 gametes from F1 plants F2 offspring sperm eggs P PP P
Figure: 12-UN7 Title: Heterozygous second generation offspring allele distribution 1/4 p p pp

Dominant and Recessive Alleles
The particular combination of the two alleles carried by an individual is called the genotype (PP, Pp, or pp) The physical expression of the genotype is known as the phenotype (e.g. purple or white flowers)

Genetic Bookkeeping Punnett Square Method predicts offspring genotypes from combinations of parental gametes First assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive) 2. Determine the gametes and their fractional proportions (out of all the gametes) from both parents…

Genetic Bookkeeping Write the gametes from each parent, together with their fractional proportions, along each side of a 2 x 2 grid (Punnett square) Fill in the genotypes of each pair of combined gametes in the grid, including the product of the fractions of each gamete (e.g. ¼ P with ½ p = 1/8 Pp)…

Pp self-fertilize 1 2 1 2 P eggs p 1 2 P 1 4 1 4 sperm PP Pp 1 2 p 1 4
Figure: 12-4 part a Title: Determining the outcome of a single-trait cross part a Caption: (a) The Punnett square allows you to predict both genotypes and phenotypes of specific crosses; here we use it for a cross between plants that are heterozygous for a single trait, flower color. (1) Assign letters to the different alleles; use uppercase for dominant and lowercase for recessive. (2) Determine all the types of genetically different gametes that can be produced by the male and female parents. (3) Draw the Punnett square, with each row and column labeled with one of the possible genotypes of sperm and eggs, respectively. (We have included the fractions of these genotypes with each label.) (4) Fill in the genotype of the offspring in each box by combining the genotype of sperm in its row with the genotype of the egg in its column. (Multiply the fraction of sperm of each type in the row headers by the fraction of eggs of each type in the column headers.) (5) Count the number of offspring with each genotype. (Note that Pp is the same as pP.) (6) Convert the number of offspring of each genotype to a fraction of the total number of offspring. In this example, out of four fertilizations, only one is predicted to produce the pp genotype, so 1/4 of the total number of offspring produced by this cross is predicted to be white. To determine phenotypic fractions, add the fractions of genotypes that would produce a given phenotype. For example, purple flowers are produced by 1/4PP + 1/4Pp + 1/4pP, for a total of 1/4 of the offspring. 1 4 1 4 sperm PP Pp 1 2 p 1 4 1 4 pP pp

genotypic ratio phenotypic ratio
offspring genotypes genotypic ratio (1:2:1) phenotypic ratio (3:1) sperm eggs 1 2 1 4 P 1 2 P 1 4 PP PP 1 2 P 1 2 p 1 4 Pp 3 4 purple 2 4 Pp Figure: 12-4 part b Title: Determining the outcome of a single-trait cross part b Caption: (b) Probability theory can also be used to predict the outcome of a single-trait cross. Determine the fractions of eggs and sperm of each genotype, and multiply these fractions together to calculate the fraction of offspring of each genotype. When two genotypes produce the same phenotype (e.g., Pp and pP), add the fractions of each genotype to determine the phenotypic fraction. 1 2 P 1 2 P 1 4 pP 1 2 1 2 1 4 1 4 1 4 P p pp pp white

A test cross Take a dominant phenotype (with unknown second allele)
Cross it with a known recessive homozygous And you will get one of two results...

pollen PP or Pp sperm unknown pp all eggs p if PP if Pp p egg p egg
1 2 P P Figure: 12-UN8 Title: Test cross 1 2 all Pp Pp sperm 1 2 p 1 2 pp

12.4 How Are Multiple Traits on Different Chromosomes Inherited?
Mendel hypothesized that genes on different chromosomes are inherited independently He looked at many pea traits If these traits were linked in some way, then the ratios shouldn’t agree with hypothesis Remember that for the math to work out, you have to do lots of replicates – and Mendel also did that correctly.

Trait Dominant form Recessive form Seed shape smooth wrinkled Seed
color yellow green Pod shape inflated constricted Pod color green yellow Flower color purple white Figure: 12-5 Title: Traits of pea plants that Mendel studied Flower location at leaf junctions at tips of branches Plant size tall (1.8 to 2 meters) dwarf (0.2 to 0.4 meters)

Traits Are Inherited Independently
Punnett Square from SSYY x ssyy cross Gametes ¼sy ¼sy ¼sy ¼sy ¼SY SsYy SsYy SsYy SsYy F1: All SsYy Smooth yellow peas ¼SY SsYy SsYy SsYy SsYy ¼SY SsYy SsYy SsYy SsYy ¼SY SsYy SsYy SsYy SsYy

self-fertilize sperm SsYy eggs SY Sy sY sy SSYY SSYy SsYY SsYy SSyY
1 4 1 4 1 4 1 4 SY Sy sY sy 1 4 SY 1 16 1 16 1 16 1 16 SSYY SSYy SsYY SsYy 1 4 Sy 1 16 1 16 1 16 1 16 SSyY SsyY Ssyy Figure: 12-6 part a Title: Predicting genotypes and phenotypes for a cross between gametes that are heterozygous for two traits part a Caption: In pea seeds, yellow (Y) is dominant to green (y), and smooth (S) is dominant to wrinkled (s). (a) Punnett square analysis. In this cross, an individual heterozygous for both traits self-fertilizes. Note that the Punnett square predicts both the frequencies of combinations of traits (9/16 smooth yellow, 3/16 smooth green, 3/16 wrinkled yellow, 1/16 wrinkled green) and the frequencies of individual traits (3/4 yellow, 1/4 green, 3/4 smooth, and 1/4 wrinkled). SSyy sperm 1 4 sY 1 16 1 16 1 16 1 16 sSYY sSYy ssYY ssYy 1 4 sy 1 16 1 16 1 16 1 16 sSyY sSyy ssyY ssyy

seed shape seed color phenotypic ratio (9:3:3:1) smooth yellow
4 3 4 9 16 smooth yellow smooth yellow 3 4 1 4 3 16 smooth green smooth green 1 4 3 4 3 16 wrinkled yellow wrinkled yellow 1 4 1 4 1 16 wrinkled green wrinkled green Figure: 12-6 part b Title: Predicting genotypes and phenotypes for a cross between gametes that are heterozygous for two traits part b Caption: (b) Probability theory states that the probability of two independent events is the product (multiplication) of their individual probabilities. Seed shape is independent of seed color. Therefore, multiplying the independent probabilities of the genotypes or phenotypes for each trait produces the predicted frequencies for the combined genotypes or phenotypes of the offspring. These ratios are identical to those generated by the Punnett square. Exercise Use Punnett squares to determine if the genotype of a plant bearing yellow smooth seeds can be revealed by a test cross with a plant bearing green wrinkled seeds. And of course, his experiments came out just like this. This idea is called the Law of Independent Assortment Copyright © 2005 Pearson Prentice Hall, Inc.

The End of the Mendel Story
In 1865, Gregor Mendel presented his work and published the following year He was pretty much ignored In 1900, 3 biologists were about to publish on the same subject They did a literature search and found Mendel’s work, and gave him all the credit He had died in 1884

12.5 How are genes located on the same chromosome inherited?
Genes on the same chromosome tend to be inherited together. This is called linkage. One of the first pairs of linked genes was found on the sweet pea (a different species from Mendel’s garden pea)

Genes on the Same Chromosome
Not all genes independently assort Mendel’s Law of Independent Assortment only works for genes whose loci are on different chromosomes

chromosomes replicate
pairs of alleles on homologous chromosomes in diploid cells S s Y y chromosomes replicate S s Y y replicated homologues pair during metaphase of meiosis I, orienting like this or like this meiosis II meiosis I S s Y y Figure: 12-7 Title: Independent assortment of alleles Caption: Chromosome movements during meiosis produce independent assortment of alleles of two different genes. Each combination of alleles is equally likely to occur. Therefore, an F1 plant would produce gametes in the predicted proportions 1/4SY, 1/4sy, 1/4sY, and 1/4Sy. S s Y y independent assortment produces four equally likely allele combinations during meiosis SY sy Sy sY

flower color gene pollen shape gene purple allele, P long allele, L
Figure: 12-UN9 Title: Heterozygous pea plant chromosomes red allele, p round allele, l

12.5 How are genes located on the same chromosome inherited?
Despite being on the same gene, crossing over can occur Thereby creating new linkages. Exchanging DNA between homologous chromosomes is genetic recombination The farther apart genes are on a chromosome, the more likely crossing over will occur between them Mendel wasn’t just a good scientist, he was really lucky he didn’t find linked traits

flower color gene pollen shape gene sister chromatids homologous
chromosomes (duplicated) at meiosis I purple allele, P long allele, L sister chromatids Figure: 12-UN10 Title: Sweet pea at meiosis I red allele, p round allele, l Copyright © 2005 Pearson Prentice Hall, Inc.

L P p L recombined chromatids l P p l
Figure: 12-UN12 Title: Sweet pea at anaphase I Copyright © 2005 Pearson Prentice Hall, Inc.

P L p L P l Figure: 12-UN13 Title: Sweet pea at meiosis II p l

12.6 How Is Sex Determined, and How Are Sex-Linked Genes Inherited?
Females have two identical sex chromosomes XX Men have two different sex chromosomes XY That means the men control the sex of the offspring

FIGURE 11-9 The karyotype of a human male
Staining and photographing the entire set of duplicated chromosomes within a single cell produces a karyotype. Pictures of the individual chromosomes are cut out and arranged in descending order of size. The chromosome pairs (homologues) are similar in both size and staining pattern and have similar genetic material. Chromosomes 1 through 22 are the autosomes; the X and Y chromosomes are the sex chromosomes. Notice that the Y chromosome is much smaller than the X chromosome. If this were a female karyotype, it would have two X chromosomes.

Y chromosome X chromosome Figure: 12-8 Title:
Photomicrograph of human sex chromosomes Caption: Notice the small size of the Y chromosome, which carries relatively few genes. Y chromosome X chromosome

12.6 How Is Sex Determined, and How Are Sex-Linked Genes Inherited?
Sex linked genes are only found on the X or the Y gene But the Y gene is small, so most sex linked problems are found on the X Women have XX, so they can have dominant and recessive X alleles Men only have one X, and so whatever is on their X is expressed

female parent X1 X2 eggs X1 X2 X1 Xm X2 Xm male parent Xm
Figure: 12-9 Title: Sex determination in mammals Caption: Male offspring receive their Y chromosome from the father; female offspring receive the father's X chromosome (labeled Xm). Both male and female offspring receive an X chromosome (either X1 or X2) from the mother. Xm female offspring Xm Y sperm X1 Y X2 Y Y male offspring

female parent R r XR Xr XR Xr R r eggs XR Xr R R R r R
all the F2 females have red eyes male parent Figure: 12-10 Title: Sex-linked inheritance of eye color in fruit flies Caption: The gene for eye color is located on the X chromosome; the Y chromosome does not contain an eye color gene. Red (R) is dominant to white (r). When a white-eyed male is mated to a homozygous red-eyed female, all the offspring have red eyes: F1 females are heterozygous, receiving the r allele from their father and the R allele from their mother, while male offspring receive only the R allele from their mother. In the F2 generation, the single R allele of the F1 male parent is passed on to his daughters, so all the F2 daughters have red eyes. The F2 sons receive a Y chromosome from their father, and either the R or r allele on the X chromosome from their mother, so half the sons have white eyes and half have red eyes. Question What would the phenotypic ratios in the offspring of this cross be if the alleles for eye color showed incomplete dominance, as shown in Fig ? R XR XR XR Xr XR female offspring sperm half the F2 males have red eyes, half have white eyes Y R r XRY XR Y XR Y Xr Y male offspring

12.7 Do the Mendelian Rules of Inheritance Apply to All Traits?
Incomplete dominance: The phenotype of heterozygotes is intermediate between the phenotypes of the homozygotes In other words, neither allele is completely dominant or recessive This is more like sharing or blending

Figure: 12-11 Title: Incomplete dominance Caption:
RR R¢R¢ F1: RR¢ RR¢ F2: 1 2 R eggs 1 2 R¢ 1 2 Figure: 12-11 Title: Incomplete dominance Caption: The inheritance of flower color in snapdragons is an example of incomplete dominance. In such cases, we will use capital letters for both alleles, here R and R'. Hybrids RR' have pink flowers, whereas the homozygotes are red (RR) or white R'R'. Because heterozygotes can be distinguished from homozygous dominants, the distribution of phenotypes in the F2 generation (1/4 red: 1/4 pink: 1/4 white) is the same as the distribution of genotypes (1/4RR : 1/4RR' : 1/4R'R'). Question Is it possible for plant breeders to develop a true-breeding pink-flowered snapdragon? R 1 4 1 4 RR RR’ sperm 1 2 R¢ 1 4 RR¢ 1 4 R’R¢

Figure 12-24 Incomplete dominance
The inheritance of hair texture in humans is an example of incomplete dominance. In such cases, we use capital letters for both alleles, here C1 and C2. Homozygotes may have curly hair (C1C1) or straight hair (C2C2). Heterozygotes (C1C2) have wavy hair. The children of a man and a woman, both with wavy hair, may have curly, straight, or wavy hair, in the approximate ratio of 1/4 curly, 1/2 wavy, 1/4 straight.

12.6 Do the Mendelian Rules of Inheritance Apply to All Traits?
A single gene may have multiple alleles Eye color in Drosophilia has more than a 1,000 possible alleles There are hundreds of alleles for Marfan syndrome and cystic fibrosis Blood type has multiple alleles with dominant and recessive and codominance

Table: 12-T1 Title: Human Blood Group Characteristics

Figure: E12-1 Title: Cystic fibrosis Caption: A child is treated for cystic fibrosis. Gentle pounding on the chest and back while the child is held upside-down helps dislodge mucus from the lungs. A device on the child's wrist injects antibiotics into a vein. These treatments combat the numerous lung infections to which cystic fibrosis patients are vulnerable.

Polygenic Inheritance
Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci Human skin color is controlled by at least 3 genes, each with pairs of incompletely dominant alleles

R1R1¢R2R2¢ eggs R1R2 R1R2¢ R1¢R2 R1¢R2¢ R1R1R2R2 R1R1R2R2 ¢ R1R1¢R2R2
Figure: 12-12 Title: Polygenic inheritance of grain color in wheat Caption: At least two separate genes, each with two incompletely dominant alleles, determine the color of wheat grains. A cross between two wheat plants, both heterozygous for both genes, produces five colors of offspring. sperm R1R1R2R2 ¢ R1R1R2¢R2¢ R1R1¢R2R2¢ R1R1¢R2¢R2¢ R1¢R2 R1R1¢R2R2¢ R1R1¢R2R2 R1R1¢R2R2 ¢ R1¢R1¢R2R2 R1¢R1¢R2R2¢ R1¢R2¢ R1R1¢R1R2¢ R1R1¢R2¢R2¢ R1¢R1¢R2R2¢ R1¢R1¢R2¢R2¢

Figure 12-25 Polygenic inheritance of skin color in humans
(a) At least three separate genes, each with two incompletely dominant alleles, determine human skin color (the inheritance is actually much more complex than this). The backgrounds of each box indicate the depth of skin color expected from each genotype. (b) The combination of complex polygenic inheritance and environmental effects (especially exposure to sunlight) produces an almost infinite gradation of human skin colors.

Pleiotropy Some alleles of a characteristic may create multiple phenotypic effects (pleiotropy) Mendel’s rules specify only one phenotype possible for any allele Example: The SRY gene in male humans SRY gene stimulates development of gonads into testes, which in turn stimulate development of the prostate, seminal vesicles, penis, and scrotum

Environmental Influence
The environment can module how genes are expressed. Examples: Intelligence and height is based on genetics AND environment Himalayan rabbit Himalayan rabbits have the genotype for black fur all over the body Black pigment is only produced in colder areas of the body: the nose, ears, and paws The Himalayan rabbit has dark skin below 34º C (93º F)

Figure: 12-13 Title: Environmental influence on phenotype Caption: The expression of the gene for black fur in the Himalayan rabbit is a simple case of interaction between genotype and environment producing a particular phenotype. The gene for black fur is expressed in cool areas (nose, ears, and feet).

12.8 How Are Human Genetic Disorders Investigated?
Many experiments on humans are not allowed (not ethical) Human geneticists study medical, historical and family records Family pedigrees are records extending across several generations These help figure out which diseases are genetic, and how they are passed

Figure 12-27 Family pedigrees
(a) A pedigree for a dominant trait. Note that any offspring showing a dominant trait must have at least one parent with the trait. (b) A pedigree for a recessive trait. Any individual showing a recessive trait must be homozygous recessive. If that person's parents did not show the trait, then both of the parents must be heterozygotes (carriers). Note that the genotype cannot be determined for some offspring, who may be either carriers or homozygous dominants.

12.9 How Are Human Disorders Caused by Single Genes Inherited?
Some human genetic disorders are caused by recessive alleles Heterozygous individuals are called carriers and often don’t show any symptoms (although sometimes they show some) Related couples are more likely to express a recessive genetic disorder

Albinism is a single, recessive allele
Figure: 12-15 Title: Albinism Caption: Albinism is controlled by a single, recessive allele. Melanin is found throughout the animal kingdom, so albinos of many species have been observed. The female wallaby, having mated with a normally pigmented male, carries a normally colored offspring in her pouch. Human Rattlesnake Wallaby Albinism is a single, recessive allele

Sickle-cell anemia is caused by a defective allele for hemoglobin synthesis The heterozygous have some abnormal hemoglobin, but not enough to cause much problems The heterozygous also help protect against malaria

Figure: 12-16 Title: Sickle-cell anemia Caption: (a) Normal red blood cells are disc-shaped with indented centers. (b) Sickled red blood cells in a person with sickle-cell anemia occur when blood oxygen is low. In this shape they are fragile and tend to clump together, clogging capillaries.

Some human genetic disorders are caused by dominant alleles Wikipedia Huntington's Disease Some human genetic disorders are sex-linked Color blindness and hemophilia Queen Victoria and Hemophilia

Figure: part a Title: Color blindness, a sex-linked recessive trait part a Caption: (a) This figure, called an Ishihara chart after its inventor, distinguishes color-vision defects. People with red-deficient vision see a 6, and those with green-deficient vision see a 9. People with normal color vision see 96.

= heterozygous carrier female, normal color vision
maternal grandfather II aunts mother father III sister G. Audesirk T. Audesirk ? ? IV daughter Figure: part b Title: Color blindness, a sex-linked recessive trait part b Caption: (b) Pedigree of one of the authors (G. Audesirk), showing sex-linked inheritance of red-green color blindness. Both the author and his maternal grandfather are color deficient; his mother and her four sisters carry the trait but have normal color vision. This pattern of more-common phenotypic expression in males and transmission from affected male to carrier female to affected male are typical of sex-linked recessive traits. or = colorblind = heterozygous carrier female, normal color vision or = normal color vision (not carrier)

Copyright © 2004 Pearson Prentice Hall, Inc.
unaffected male hemophiliac male unaffected female carrier female Edward Duke of Kent Victoria Princess of Saxe-Coburg Albert Prince of Saxe- Coburg-Gotha Victoria Queen of England Figure: 12-18 Title: Hemophilia among the royal families of Europe Caption: A famous genetic pedigree involves the transmission of sex-linked hemophilia from Queen Victoria of England (seated center front, with cane, 1885) to her offspring and eventually to virtually every royal house in Europe. Because Victoria's ancestors were free of hemophilia, the hemophilia allele must have arisen as a mutation either in Victoria herself or in one of her parents (or as a result of marital infidelity). Extensive intermarriage among royalty spread Victoria's hemophilia allele throughout Europe. Her most famous hemophiliac descendant was great-grandson Alexis, Tsarevitch (crown prince) of Russia. The Tsarina Alexandra (Victoria's granddaughter) believed that only the monk Rasputin could control Alexis's bleeding. Rasputin may actually have used hypnosis to cause Alexis to cut off circulation to bleeding areas by muscular contraction. The influence that Rasputin had over the imperial family may have contributed to the downfall of the tsar during the Russian Revolution. In any event, hemophilia was not the cause of Alexis's death; he was killed with the rest of this family by the Bolsheviks (Communists) in 1918. Edward VII King of England Alexandra of Denmark Leopold Duke of Albany Helen Princess of Waldeck-Pyrmont Louis IV Grand Duke of Hesse-Darmstadt Alice Princess of Hesse several unaffected chidren Beatrice Henry Prince of Battenburg present British royal family (unaffected) Victoria Elizabeth Alexandra Tsarina Nicholas II of Russia Frederick Ernest Mary Victoria Irene Alexander Albert Alfonso XII Victoria Queen of Spain Leopold Maurice Mary carrier daughter and hemophiliac grandson ? ? ? ? ? ? ? ? Olga Tatiana Maria Anastasia Alexis Tsarevitch Alfonso Crown Prince Juan Beatrice died in infancy Marie Jaime Gonzalo Copyright © 2004 Pearson Prentice Hall, Inc.

12.9 How Do Errors in Chromosome Number Affect Humans?
Some genetic disorders are caused by abnormal numbers of sex chromosomes Nondisjunction means errors in meiosis which cause a different number of chromosomes Most embryos of this type would abort

Figure: 12-T2 Title: Effects of Nondisjunction of the Sex Chromosomes During Meiosis Klinefelter Syndrome Gives a Genetic Twist to Tales about George Washington, Napoleon, Lincoln

Figure: 12-19 Title: Trisomy 21, or Down syndrome Caption: (a) This karyotype of a Down syndrome child reveals three copies of chromosome 21. (b) These girls have the relaxed mouth and distinctively shaped eyes typical of Down syndrome.

Figure: 12-20 Title: Down syndrome frequency increases with maternal age Caption: The increase in frequency of Down syndrome after maternal age 35 is quite dramatic.

Rosy periwinkle – Leukemia drug
Figure: E12-2 part a Title: Medicinal plants part a Caption: (a) The rosy periwinkle provides several anticancer drugs. Rosy periwinkle – Leukemia drug

Calophyllum lanigerum – AIDS drug
Figure: E12-2 part b Title: Medicinal plants part b Caption: (b) Calophyllum lanigerum is the source of drugs that show great promise for treating AIDS. Calophyllum lanigerum – AIDS drug

Chapter 12 Patterns of Inheritance

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