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Patterns of Inheritance

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1 Patterns of Inheritance
Chapter 13

2 Outline Early Ideas of Heredity Mendel Gene Disorders Multiple Alleles Pedigrees Gene Disorders Due to Protein Alteration Chromosome and Inheritance Genetic Recombination Human Chromosomes

3 Early Ideas of Heredity
Classical assumptions First, heredity occurs within species. species maintained without significant change since time of creation Second, traits are transmitted directly and independently. paradox - all members of same species should eventually have the same appearance hybrids differ in appearance

4 Early Ideas of Heredity
Early geneticists demonstrated some forms of an inherited character can: disappear in one generation and reappear, unchanged, in future generations. segregate among offspring of a cross. be more likely to be represented than alternative forms.

5 Mendel and the Garden Pea
Advantages of garden pea : many hybrids previously produced expect 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 and the Garden Pea
Mendel’s experimental design allowed pea plants to self-fertilize for several generations assured pure-breeding traits performed crosses between varieties exhibiting alternative character forms permitted hybrid offspring to self-fertilize for several generations

7 Mendel’s Experiments

8 What Mendel Found F1 Generation (first filial) Offspring of white flower and purple flower cross had flower color resembling one parent (no intermediate color). All flowers exhibited purple flowers (dominant trait) and none exhibited white flowers (recessive trait).

9 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

10 Second Filial Generation

11 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

12 Mendel’s Model of Heredity
Alleles do not influence each other in any way. Presence of a particular allele does not ensure its encoded trait will be expressed. genotype - totality of an individual’s alleles phenotype - physical appearance

13 Interpretation of Mendel’s Results
Notational convention P - dominant allele (purple) p - recessive allele (white) PP - homozygous dominant Pp - heterozygous pp - homozygous recessive

14 Interpretation of Mendel’s Results
F1 generation PP x pp (parental generation) yielded all Pp offspring F2 generation Pp x Pp yielded: (1:3:1) ratio 1 PP 3 Pp 1 pp Punnett squares

15 Mendel’s Cross

16 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.

17 Testcross Cross of a plant with an unknown genotype (PP or Pp) with a homozygous recessive individual, will yield one of two possible results: pp x PP = 100% (Pp) pp x Pp = 50% (pp) : 50% (Pp)

18 Testcross

19 Mendelian Inheritance
Mendel’s Second Law of Heredity (Law of Independent Assortment) Genes that are located on different chromosomes assort independently of one another.

20 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. pleiotropic effects Individual alleles often have more than one effect on the phenotype.

21 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 coat color in Labrador retrievers

22 Epistatic Interactions

23 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 disease

24 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

25 ABO Blood Groups

26 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 hemophilia - sex-linked

27 Royal Hemophilia Pedigree

28 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.

29 Sickle Cell and Malaria

30 Curing Defects with Gene Therapy
Cystic fibrosis body cells of affected individuals secrete thick mucus that clogs airways of lung defect in cf gene Researchers are currently working on transmitting a working copy of cf gene using viruses. Early attempts using adenovirus vectors produced mixed results.

31 Chromosomes and Mendelian 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

32 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. Mendelian traits assort independently because chromosomes assort independently.

33 Sex Linkage in Drosophila

34 Genetic Recombination
Crossing over Genes located relatively far apart on a chromosome are more likely to cross over than genes located closer together. Frequency of crossings can be used to construct a genetic map. measures distance between genes in terms of recombination frequency

35 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

36 Human Chromosomes One x chromosome in females is inactivated early in embryonic development. Visible as a darkly staining Barr body attached to the nuclear membrane.

37 Alterations in 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.

38 Nondisjunction in Sex Chromosomes
XXX or XXY yields Klinefelter syndrome XO yields Turner syndrome Y Chromosome XYY - Jacob syndrome

39 Nondisjunction

40 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

41 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

42 Summary 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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