1. Patterns of Inheritance
Chapter 12
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2. Early Ideas of Heredity Mendel Gene Disorders Multiple Alleles
Outline
Early Ideas of Heredity
Mendel
Gene Disorders
Multiple Alleles
Pedigrees
Gene Disorders Due to Protein Alteration
Chromosome and Inheritance
Genetic Recombination
Human Chromosomes
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3. Early Ideas of Heredity
Classical Assumption
Heredity occurs within boundaries of a particular species.
Traits are transmitted directly from parents to offspring.
Paradox –If no variation enters a species from the outside, then all members of same species should eventually have the same appearance.
Hybrids ?
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4. (Early Ideas of Heredity)
Early geneticists demonstrated some forms of an inherited character can:
Disappear and then reappear in future generations.
Segregate among offspring.
Be more likely to be represented than alternative forms.
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5. Mendel’s use of Mathematics to study the Garden Pea
Advantages of Garden Pea :
Production of hybrids
allowed segregation of traits.
Large number of true-breeding varieties .
Small and easy to grow.
Short generation time.
Sexual Organs enclosed in flower.
Self-fertilization
Cross fertilization

6. (Mendel’s Work)
Mendel’s Experimental Design
Allowed pea plants to self-fertilize for several generations.
Assured constant traits.
Performed crosses between varieties exhibiting alternative character forms.
Permitted hybrid offspring to self-fertilize for several generations.

7. F1 Generation (First Filial)
Mendel’s Results:
F1 Generation (First Filial)
Offspring of white flower and purple flower cross had flower color resembling one parent (no blending inheritance).
Result: All plants exhibited purple flower color (dominant trait) and none exhibited white flower color (recessive trait).
Genotype: 4PP Genotypic Ratio 4:0
Phenotype: 4 purple Phenotypic Ratio 4:0
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8. (Mendel’s Results)
F2 Generation (Second Filial)
Cross between seeds of F1 generation produced some plants exhibiting white flower color (recessive form reappeared).
Dominant : Recessive ratio among F2 plants was always close to 3:1 (phenotypic ratio).
Mendelian Ratio
Discovered ¼ of recessives were always true breeding.
Resulted in 1:2:1 ratio (genotypic ratio) that is disguised

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10. Mendel’s Model of Heredity
Parents transmit discrete physiological trait information (factors) to offspring.
Factors = genes=Alleles
Each individual receives two factors that may code for same, or alternative, character traits.
Not all copies of a factor are identical.
Homozygous - Same alleles.
Heterozygous - Different alleles.
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11. (Mendel’s Model of Heredity)
The two alleles do not influence each other in any way.
Presence of a particular allele does not ensure its encoded trait will be expressed in an individual carrying that allele.
Genotype - Totality of alleles (PP or Pp).
Phenotype - Physical appearance (purple/white).
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12. Mendelian Inheritance
Mendel’s First Law of Heredity:
Law of Segregation
Two alleles of a gene segregate during gamete formation (meiosis) and are rejoined at random, one from each parent, during fertilization
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13. Interpretation of Mendel’s Results
Notational Convention (Punnet Square- Monohybrid Cross)
P = purple
(Dominant allele)
p = white
(Recessive allele)
Gentoypes:
PP - Homozygous Dominant
Pp – Heterozygous
pp - Homozygous Recessive
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14. Interpretation of Mendel’s Results
P Generation (parental generation)
PP x pp yielded all Pp offspring (F1)
- Genotypic ratio: 4:0 Phenotypic ratio: 4:0
F1 Generation = 1st filial generation
Pp X Pp yielded PP, Pp, and pp offspring (F2)
Genotypic ratio: 1:2:1 Phenotypic ratio: 3:1
F2 Generation = 2nd filial generation
Pp x Pp yielded: (1:2:1) ratio
1 PP
2 Pp
1 pp
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15. Pedigrees Used to study heredity
Allow deduction of trait Inheritance patterns in families
Mutations are accidental changes in genes.
Rare, random, and usually result in recessive alleles.
Hemophilia - Inherited condition where blood is slow to clot or does not clot at all.
Only expressed when individual has no copies of the normal allele.
Royal Hempohilia - Sex-linked
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16. Pedigrees

17. Dihybrid Crosses Involves 2 traits Genotypes:
Phenotypes:
Flower color: P= purple; p= white
Height: T= tall; t= short
Heterozygous purple tall x Heterozygous purple tall
PpTt x PpTt
Possible allelic combinations: PT, Pt, pT, pt
♂ ♀ PT Pt pT pt
Genotypic ratio:
Phenotypic ratio:
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18. Mendelian Inheritance
Mendel’s Second Law of Heredity
Law of Independent Assortment
In a diahybrid cross, the alleles of each gene assort independently of one another; the segregation of different allele pairs is independent.
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19. Rules of Probability 1. Rule of Multiplication 2. Rule of Addition
Cross: round yellow seeds
What is the probability of round yellow (R_Y_)?
What is the probability of round green (R_yy)?
What is the probability of wrinkled yellow (rrY_)?
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20. Rules of Probability 1. Rule of Multiplication 2. Rule of Addition
A trihybrid cross in which one parent has purple flowers and yellow round seeds and another parent has purple flowers and green wrinkled seeds.
What is the probability of the offspring that would exhibit the recessive phenotypes for at least two of the three traits?
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21. TestCross
Is conducted when an individual with an unknown genotype is crossed with a homozygous recessive gentoype
If you cross a plant with an unknown genotype (PP or Pp) with a homozygous recessive individual, the two possible genotypes will yield different results.
pp x PP = 100% (Pp)
pp x Pp = 50% (pp) : 50% (Pp)
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22. Mendelian Inheritance was not always easy:
Some characteristics are govern by
Polygenes (many genes that act
sequentially or jointly on a characteristic)
Vary in the population along a continuum
Called Polygenic inheritance or continous Variation
Continuous Variation
The greater the number of genes influencing a character, the more continuous the expected distribution of character variation will be.
Examples: hair and skin color; height; weight
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23. Pleiotrophic Effects
Individual alleles often have more than one effect on the phenotype; ability of a gene to have multiple phenotypic effects
Examples:
cystic fibrosis- symptoms are pleiotropic effects of single defect of the gene that encodes a chloride ion transmembrane channel
Sickle cell anemia – defect in the hemoglobin molecule causes anemia, heart failure, susceptibility to pneumonia, kidney failure, enlargement of the spleen
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24. Phenotypic Considerations
Incomplete Dominance
Heterozygotes are intermediate in color.
Codominace
Most genes in a population posses several different alleles, and often no single allele is dominant both are sometimes expressed in heterozygotes
Environmental Effects
Degree of allele expression may depend on the environment.
Epistasis
One gene interferes with the expression of another gene.
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25. Raven - Johnson - Biology: 6th Ed
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26. Raven - Johnson - Biology: 6th Ed
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27. Multiple Alleles: ABO Blood Group
Condominant - No single allele is dominant, and each allele has its own effect.
ABO Blood Groups
Human gene that encodes enzyme that adds sugar molecules (cell markers) to lipids on the surface of red blood cells.
IB adds galactose
IA adds galactosamine
i adds no sugar
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28. Human Genetics ≈ Mendelian Principles
Gene Disorder refers to the harmful effect a detrimental allele produces when it occurs at a significant frequency in a population.
Homozygous recessive Gene Disorders
Most gene disorders are rare because affected individuals often die at a relatively young age, or cannot reproduce.
Tay-Sachs disease
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29. Human Genetics ≈ Mendelian Principles
Not all gene disorders are recessive.
Huntington’s Disease – caused by a dominant allele
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30. Gene Disorders Due to Protein Alteration
Sickle-Cell Anemia is a recessive inherited disorder in which afflicted individuals have defective hemoglobin, and thus are unable to properly transport oxygen to tissues.
Homozygotes have Sickle-Cell.
Heterozygotes usually appear normal, but are resistant to malaria.
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31. Curable Defects in Future
Cystic Fibrosis
Body cells of affected individuals secrete thick mucus that clogs airways of lung.
Defect in cf gene.
Researchers currently working on methods of transmitting a working copy of cf gene via viruses.
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32. Chromosomes and Inheritance
In early 20th century, it was not obvious chromosomes were vehicles of heredity information.
Chromosomal Theory of Inheritance first formulated in (Walter Sutton)
Evidence:
1. Diploid/haploid cells
2. chromosomes segregate during meiosis
Problems : Why does the # of characters that assort independently in a given kind of organism often exceed the # of chromosome pairs the organism posses?
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33. Chromosomes and Inheritance
Thomas Hunt Morgan (1901)
A trait determined by a gene on the sex chromosome is said to be sex-linked.
In Drosophila, sex is determined by the number of copies of the x chromosome.
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34. Genetic Recombination
Crossing Over
Studies in 1931 conclusively found evidence of crossing over.
If two different genes are located relatively far apart on a chromosome, crossing over is more likely to occur than if they are close together.
Frequency of crossings can be used to construct a genetic map.
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35. Human somatic cells normally have 23 pairs of chromosomes.
Human Chromosomes
Human somatic cells normally have 23 pairs of chromosomes.
Divided into seven groups characterized by size and shape.
22 pairs of autosomes
1 pair of sex chromosomes
XX = Female
XY = Male
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36. Human Chromosomes
One x chromosome in females in inactivated early in embryonic development.
Visible as a darkly staining Barr body attached to the nuclear membrane.
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37. Abnormalities Due to Chromosome Number
Failure of chromosomes to separate correctly during meiosis I or II is called primary nondisjunction.
Down Syndrome caused by trisomy 21.
1 in 1700 for mothers < 20.
1 in 1400 for mothers >20<30.
1 in 750 for mothers >30<35.
1 in 16 for mothers >45.
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38. Nondisjunction in Sex Chromosomes
XXX or XXY yields Klinefelter syndrome
XO yields Turner Syndrome
Y Chromosome
XYY – Antisocial ?
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39. High-Risk Pregnancies Couples with recessive alleles.
Genetic Counseling
Genetic counseling identifies parents at risk of producing children with genetic defects and assesses the state of early embryos.
High-Risk Pregnancies
Couples with recessive alleles.
Mothers older than 35.
Amniocentesis
Chorionic villi sampling
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40. Aneuploidy or gross alterations. Proper enzyme functioning.
Genetic Counseling
Counselors can look for three things in cell cultures in search of genetic disorders:
Aneuploidy or gross alterations.
Proper enzyme functioning.
Association with known genetic markers.
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41. Early Ideas of Heredity Mendel Gene Disorders Multiple Alleles
Review
Early Ideas of Heredity
Mendel
Gene Disorders
Multiple Alleles
Pedigrees
Gene Disorders Due to Protein Alteration
Chromosome and Inheritance
Genetic Recombination
Human Chromosomes
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

42. Copyright © McGraw-Hill Companies Permission required for reproduction or display
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