1. Patterns of Inheritance
Chapter 8
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2. Mendel and the Garden Pea
Advantages of Garden Pea for research:
Many hybrids previously produced.
Large number of true-breeding varieties.
Small and easy to grow.
Short generation time.
Sexual Organs enclosed in flower.
Self-fertilization
Cross fertilization
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3. Mendel and the Garden Pea
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.
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4. Mendel and the Garden Pea
F1 Generation (First Filial)
Offspring of white flower and purple flower cross had flower color resembling one parent (no blending inheritance).
All flowers exhibited purple flowers (dominant trait) and none exhibited white flowers (recessive trait).
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5. F2 Generation (Second Filial)
Mendel’s Results
F2 Generation (Second Filial)
Cross between seeds of F1 generation produced some plants exhibiting white flowers (recessive form reappeared).
Dominant : Recessive ratio among F2 plants was always close to 3:1.
Mendelian Ratio
Discovered ¼ of recessives were always true breeding.
Disguised 1:2:1 ratio
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6. Mendel’s Model of Heredity
Parents transmit discrete physiological trait information (factors) to offspring.
Each individual receives two factors that may code for same, or alternative, character traits.
Not all copies of a factor are identical.
Alleles
Homozygous - Same alleles.
Heterozygous - Different alleles.
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7. 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.
Phenotype - Physical appearance.
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8. Interpretation of Mendel’s Results
Notational Convention
P - Dominant allele (purple)
PP - Homozygous Dominant
p - Recessive allele (white)
Pp - Heterozygous
pp - Homozygous Recessive
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9. Interpretation of Mendel’s Results
F1 Generation
PP x pp (parental generation) yielded all Pp offspring.
F2 Generation
Pp x Pp yielded: (1:2:1) ratio
1 PP
2 Pp
1 pp
Punnett Squares
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10. Mendelian Inheritance
Mendel’s First Law of Heredity
(Law of Segregation)
Alternative alleles of a character segregate from each other in heterozygous individuals and remain distinct.
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11. TestCross
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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12. Mendelian Inheritance
Mendel’s Second Law of Heredity
(Law of Independent Assortment)
Genes that are located on different chromosomes assort independently of one another.
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13. Mendelian Inheritance
Phenotype Considerations
Continuous Variation
The greater the number of genes influencing a character, the more continuous the expected distribution of character variation will be.
Example: eye color, hair color, height
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14. Phenotypic Considerations
Incomplete Dominance
Heterozygotes are intermediate in color.
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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15. Not all defects are recessive. Huntington’s Disease
Gene Disorders
Gene Disorder refers to the harmful effect a detrimental allele produces when it occurs at a significant frequency in a population.
Most disorders are rare because affected individuals often die at a relatively young age, or cannot reproduce.
Not all defects are recessive.
Huntington’s Disease
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16. Multiple Alleles: ABO Blood Group
Codominance - No single allele is dominant, and each allele has its own effect.
ABO Blood Groups
Human gene that encodes enzyme that adds sugar molecules to lipids on the surface of red blood cells.
IB adds galactose
IA adds galactosamine
i adds no sugar
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17. Mutations are accidental changes in genes.
Pedigrees
Mutations are accidental changes in genes.
Rare, random, and usually result in recessive alleles.
Pedigrees used to study heredity.
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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18. 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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19. Curable Defects 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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20. Chromosomes and Inheritance
In early 20th century, it was not obvious chromosomes were vehicles of heredity information.
Chromosomal Theory of Inheritance first formulated in 1902.
Problems quickly arose in trying to track independent assortment.
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21. Chromosomes and Inheritance
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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22. Chromosomes and Inheritance
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23. 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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24. 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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25. 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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26. 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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27. Nondisjunction in Sex Chromosomes
XXX or XXY yields Klinefelter syndrome
XO yields Turner Syndrome
Y Chromosome
XYY – Antisocial ?
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28. 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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29. 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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30. Can be used to determine sex of the individual
Karyotype Analysis
Karyotype-photograph of a person’s chromosomes during metaphase of mitosis
Can be used to determine sex of the individual
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31. Can be used to find major chromosome errors
Karyotype Analysis
Can be used to find major chromosome errors
Trisomy 21
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