1. Ch. 9 Patterns of Inheritance1

2. The science of genetics has ancient roots
The blending hypothesis, was suggested in the 19th century by scientists studying plants but later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.
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3. Experimental genetics began in an abbey garden
Heredity is the transmission of traits from one generation to the next.
Genetics is the scientific study of heredity.
Gregor Mendel began the field of genetics in the 1860s,deduced the principles of genetics by breeding garden peas, and relied upon a background of mathematics, physics, and chemistry.
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4. Experimental genetics began in an Abbey Garden
In 1866, Mendel correctly argued that parents pass on to their offspring discrete “heritable factors” and stressed that the heritable factors (genes), retain their individuality generation after generation.
A heritable feature that varies among individuals, is called a character (flower color)
Each variant for a character, is a trait (purple or white flowers)
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5. Character Traits Dominant Recessive Flower color Purple White
Figure 9.2D_1
Character
Traits
Dominant
Recessive
Flower color
Purple
White
Flower position
Axial
Terminal
Figure 9.2D_1 The seven pea characters studied by Mendel (part 1)
Seed color
Yellow
Green
Seed shape
Round
Wrinkled 5

6. Experimental genetics began in an abbey garden
True-breeding varieties result when self-fertilization produces offspring all identical to the parent
The offspring of two different varieties are hybrids
The cross-fertilization is a genetic cross
True-breeding parental plants are the P generation.
Hybrid offspring of the P generation are the F1 generation
A cross of F1 plants produces the F2 generation
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7. Mendel’s law of segregation describes the inheritance of a single character
A monohybrid cross is a cross between two individuals that differ in just one character
Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers.
100% of the F1 generation had purple flowers
When F1 plants were crossed to produce the F2 generation, 75% had purple flowers and 25% had white flowers
*Change percentages to ratios by dividing percentages by the smallest percentage*
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8. 25% of plants have white flowers 75% of plants have purple flowers
The Experiment
P generation (true-breeding parents) 
Purple flowers
White flowers
F1 generation
100% have purple flowers
Fertilization among F1 plants (F1  F1)
Figure 9.3A_s3 Crosses tracking one character (flower color) (step 3)
F2 generation
25% of plants have white flowers
75% of plants have purple flowers 8

9. Mendel’s law of segregation describes the inheritance of a single character
The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis
Mendel needed to explain why white color seemed to disappear in the F1 generation and reappear in one quarter of the F2 offspring
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10. Mendel’s Hypotheses
Mendel developed four hypotheses from these crosses
Alleles are alternative versions of a gene
An organism inherits two alleles (on homologs), one from each parent. The alleles can be the same (homozygous) or different (heterozygous)
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11. Homologous chromosomes
Gene loci
Dominant allele P a B
Homologous chromosomes P a b
Recessive allele
Genotype: PP aa Bb
Figure 9.4 Three gene loci on homologous chromosomes
Homozygous for the dominant allele
Homozygous for the recessive allele
Heterozygous, with one dominant and one recessive allele 11

12. Mendel’s Hypotheses
If the two alleles differ, the one that determines the organism’s appearance is the dominant allele. The recessive allele has no noticeable effect on the organism’s appearance
Phenotype is an organism’s traits
Genotype is an organism’s genetic makeup
4. A sperm or egg carries only one allele for each inherited character because allele pairs separate from each other during gamete formation (meiosis). This is called the law of segregation
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14. The law of independent assortment is revealed by tracking two characters at once
A dihybrid cross is a mating of parental varieties that differ in two characters
Mendel performed the following dihybrid cross with the following results:
P generation: round yellow seeds  wrinkled green seeds
F1 generation: 100% with round yellow seeds
F2 generation:
9/16 had round yellow seeds
3/16 had wrinkled yellow seeds
3/16 had round green seeds
1/16 had wrinkled green seeds 14

15. The law of independent assortment is revealed by tracking two characters at once
Mendel needed to explain why the F2 offspring had new non-parental combinations of traits and a 9:3:3:1 phenotypic ratio
Mendel suggested that the inheritance of one character has no effect on the inheritance of another, and that the dihybrid cross is the equivalent to two monohybrid crosses, and called this the law of independent assortment
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17. Geneticists can use the testcross to determine unknown genotypes
A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual
A testcross can show whether the unknown genotype includes a recessive allele
Mendel used testcrosses to verify true-breeding genotypes
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18. What is the genotype of the black dog?
Testcross 
Genotypes
B_? bb
Two possibilities for the black dog: BB or Bb
Figure 9.6 Using a testcross to determine genotype
Gametes B B b b Bb b Bb bb
Offspring
All black
1 black : 1 chocolate 18

19. Mendel’s laws reflect the rules of probability
Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected the segregation of allele pairs during gamete formation and the re-forming of pairs at fertilization.
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20. Mendel’s laws reflect the rules of probability
The probability of a specific event is the number of ways that event can occur out of the total possible outcomes
The probability of two independent events (independent assortment) uses the rule of multiplication.
The probability to independent events is the product of the separate probabilities
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21. Traits Inherited by a Single-Gene
Dominant Traits
Recessive Traits
Trait
Dominant
Recessive
Bent little finger
Have it
Don’t have it
Chin cleft
Dimples
Hand clasping
Left on top
Right on top
Tongue rolling
Can roll tongue
Can’t roll tongue
Freckles
No freckles
Figure 9.8A Examples of single-gene inherited traits in humans
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe 21

22. Genetic traits in humans can be tracked through family pedigrees
The inheritance of human traits follows Mendel’s laws
A pedigree shows the inheritance of a trait in a family through multiple generations, demonstrates dominant or recessive inheritance, and can also be used to deduce genotypes of family members
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23. Human Pedigree First generation (grandparents) Ff Ff ff Ff
Second generation (parents, aunts, and uncles)
FF or Ff ff ff Ff Ff ff
Third generation (two sisters) ff
FF or Ff
Figure 9.8B A pedigree showing the inheritance of attached versus free earlobes in a hypothetical family
Female
Male
Attached
Free 23

24. Many inherited disorders in humans are controlled by a single gene
Dominant inheritance: one dominant allele is needed to show the dominant phenotype. Dominant lethal alleles are usually eliminated from the population.
Recessive inheritance: two recessive alleles are needed to show the recessive phenotype (heterozygous parents are carriers of the recessive allele).
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25. Dd No Albinism (carrier) Dd No Albinism (carrier)
No Albinism Dd
No Albinism Dd
Parents  D
Sperm d
Dd No Albinism (carrier)
DD No Albinism D
Figure 9.9A Offspring produced by parents who are both carriers for a recessive disorder, a type of deafness
Eggs
Offspring
Dd No Albinism (carrier)
dd Albinism d 25

26. Many inherited disorders in humans are controlled by a single gene
Recessive Human Disorders
Cystic fibrosis (CF) is the most common fatal genetic disease in the U.S. (carried by about 1 in 31 Americans)
Dominant Human Disorders
Achondroplasia, (results in dwarfism)
Huntington’s disease (degenerative disorder of the nervous system)
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27. VARIATIONS ON MENDEL’S LAWS
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28. Incomplete dominance results in intermediate phenotypes
The F1 generation in Mendel’s pea crosses always looked like one of the parental varieties or showed complete dominance
Incomplete dominance
Effects the heterozygotes (hybrids); expression of one intermediate trait
Hypercholesterolemia, pink snapdragon flower color
The phenotype of the F1 hybrids is different from either parental variety and falls between the phenotypes of the parental varieties
Codominance
Effects the heterozygotes (hybrids); expression of both alleles
AB blood type 28

29. F2 generation Sperm R r R RR rR Eggs Rr rr r 1 1 2 2 1 2 1 2
Figure 9.11A_3 Incomplete dominance in snapdragon flower color (part 3) r 29

30. Many genes have more than two alleles in the population
Human ABO blood group phenotypes involve three alleles for a single gene (IA, IB and i) resulting in four human blood groups, A, B, AB, and O
Blood Group (Phenotype)
Carbohydrates Present on Red Blood Cells
Genotypes
IAIA or IAi
Carbohydrate A A
IBIB or IBi
Carbohydrate B B
Carbohydrate A and Carbohydrate B AB
IAIB ii O
Neither
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31. A single gene may affect many phenotypic characters
Pleiotropy occurs when one gene influences many characteristics
Sickle-cell disease is a human example of pleiotropy because it
affects the type of hemoglobin produced and the shape of red blood cells and
causes anemia, organ damage, other traits
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32. An individual homozygous for the sickle-cell allele
Produces sickle-cell (abnormal) hemoglobin
The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped
Sickled cell
The multiple effects of sickled cells
Figure 9.13B Sickle-cell disease, an example of pleiotropy
Damage to organs
Other effects
Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis)
Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections 32

33. A single character may be influenced by many genes
Polygenic inheritance is when a single phenotypic character results from the additive effects of two or more genes.
Ex. Human skin color
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34. aabbcc (very light) AABBCC (very dark)
P generation 
aabbcc (very light)
AABBCC (very dark)
Figure 9.14_1 A model for polygenic inheritance of skin color (part 1)
F1 generation 
AaBbCc
AaBbCc 34

35. Sperm F2 generation Eggs 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8
Figure 9.14_2 A model for polygenic inheritance of skin color (part 2) 8 1 8 1 64 1 64 6 64 15 64 20 64 15 64 6 64 1 35

36. Fraction of population
Figure 9.14_3 64 20 64 15
Fraction of population 64 6
Figure 9.14_3 A model for polygenic inheritance of skin color (part 3) 64 1
Skin color 36

37. THE CHROMOSOMAL BASIS OF INHERITANCE
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38. Chromosome behavior accounts for Mendel’s laws
The chromosome theory of inheritance states that genes occupy specific loci on chromosomes and chromosomes undergo segregation and independent assortment during meiosis. 38

39. Chromosome behavior accounts for Mendel’s laws
Mendel’s laws correlate with chromosome separation in meiosis
The law of segregation depends on separation of homologous chromosomes in anaphase I
The law of independent assortment depends on alternative orientations of chromosomes in metaphase I
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40. SEX CHROMOSOMES AND SEX-LINKED GENES
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41. Chromosomes determine sex in many species
Many animals have a pair of sex chromosomes, designated X and Y, that determine their sex
Male mammals have XY sex chromosomes, females have XX sex chromosomes X Y
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42. Chromosomes determine sex in many species
In some animals, environmental temperature determines the sex
For some species of reptiles, the temperature at which the eggs are incubated during a specific period of development determines whether the embryo will develop into a male or female.
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43. Sex-linked genes exhibit a unique pattern of inheritance
Sex-linked genes are located on either of the sex chromosomes
The X chromosome carries many genes unrelated to sex
The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait
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44. Female Male XRXR XrY Sperm Xr Y Eggs XR XRXr XRY R  red-eye allele
Figure 9.21B A homozygous, red-eyed female crossed with a white-eyed male
Eggs XR
XRXr
XRY
R  red-eye allele
r  white-eye allele 44

45. Female Male XRXr XRY Sperm xR Y XR XRXR XRY Eggs Xr XrXR XrY
Figure 9.21C A heterozygous female crossed with a red-eyed male
Eggs Xr
XrXR
XrY
R  red-eye allele
r  white-eye allele 45

46. Female Male XRXr XrY Sperm Xr Y XR XRXr XRY Eggs Xr XrXr XrY
Figure 9.21D A heterozygous female crossed with a white-eyed male
Eggs Xr
XrXr
XrY
R  red-eye allele
r  white-eye allele 46

47. Human sex-linked disorders affect mostly males
Human sex-linked disorders affect mostly males
If a sex linked trait is due to a recessive allele, a female will express the phenotype if she is homozygous recessive
Any male receiving the recessive allele from his mother will express the trait
For this reason, more males than females have sex-linked recessive disorders
Sex-linked recessive disorders: Hemophilia, red-green color blindness, and Duchenne muscular dystrophy
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