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Patterns of Inheritance Chapter 9 BIOL 1010. GENETICS: the scientific study of heredity Genome: complete set of an organism’s gene Gene: unit of heredity.

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Presentation on theme: "Patterns of Inheritance Chapter 9 BIOL 1010. GENETICS: the scientific study of heredity Genome: complete set of an organism’s gene Gene: unit of heredity."— Presentation transcript:

1 Patterns of Inheritance Chapter 9 BIOL 1010

2 GENETICS: the scientific study of heredity Genome: complete set of an organism’s gene Gene: unit of heredity which codes for a protein

3 Gregor Mendel ( ): Gained posthumous fame as the founder of genetics Was the first person to analyze patterns of inheritance: “heritable factors” Deduced the fundamental principles of genetics Gregor Mendel: “Father of Modern Genetics”

4 Rule of Multiplication: probability of a compound event is the product of the separate probabilities of the independent events P(A and B) = P(A)*P(B) Rule of Addition: if events are mutually exclusive, then the probability of either/or is the sum of the separate probabilities of the independent events P(A or B) = P(A) + P(B) Rules of Probability The probability of event A OR event B occurring is the chance of event A occurring added to the chance of event B occuring The probability of event A AND event B occurring is the chance of event A occurring multiplied by the chance of event B occuring

5 Heredity: transmission of traits from one generation to the next Phenotype (Character): heritable feature (i.e. flower color) based on genotype (genetic makeup) Trait: variant of a character Wild-type: variant found most often in nature True-breeding: purebred, offspring are identical to the parent Hybrids: offspring of two different true-breeding parents Genetic Nomenclature

6 Inherited Traits in Humans: Controlled by a Single Gene

7 P generation: two different pure-breeding parental plants F1 hybrids: the first generation plants obtained from crossing two selected pure breeding plants. F1 hybrids do not produce seed that is the same as the parent plants F2 hybrids: second generation plants (result of self or cross fertilization of F1 hybrids Genetic Crosses Monohybrid Cross

8 A monohybrid cross is a cross between purebred parents that differ in only one characteristic F1 generation: all show the trait of one parent (i.e. purple flowers) F2 generation: show the two traits of the parents in a 3:1 ratio (i.e. purple to white flowers) Monohybrid Cross

9 1.There are alternate versions of genes 2.For each inherited character, an organism inherits two alleles, one from each parent Homozygous: two identical alleles Heterozygous: two different alleles 3.If the two alleles differ (the individual is heterozygous) Dominant allele: determines the phenotype (character) Recessive allele: no noticeable effect on the phenotype (character) 4.Law of segregation: A sperm or egg carries only one allele for each character because the allele pair segregates during meiosis Mendel’s Law of Segregation

10 Show the results of random fertilization Each axis shows possible alleles from each parent Punnett Squares

11 Mating of an individual of dominant phenotype but unknown genotype to a homozygous recessive individual Test Crosses: Determining Unknown Genotypes

12 Modern genetics Homologous chromosomes contain the same genes at the same loci but may contain different alleles (alternative forms of a gene) Gene locus (plural-loci): specific location of a gene on the chromosome Modern Genetics

13 A dihybrid cross is a cross between purebred parents that differ in two characteristics F1 generation: all show the dominant trait of the two characters F2 generation: show four combinations of traits in a 9:3:3:1 ratio Dihybrid Cross

14 Each pair of alleles assorts independently of the other pairs of alleles during gamete formation (the inheritance of one character has no effect on the inheritance of another) Mendel’s Law of Independent Assortment

15 Mendel’s Laws and Meiosis Chromosome Theory of Inheritance: genes are located at specific loci on chromosomes and that the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns

16 Male Female Affected Male Affected Female Mating between related individuals Family Pedigrees A family pedigree showing inheritance of free versus attached earlobes

17 Some Autosomal Disorders in Humans Recessive Disorders Albinism (1/22,000) Cystic fibrosis (1/1,800) Phenylketonuria (1/10,000) Sickle cell disease (1/500) Tay Sachs disease (1/3,500) Dominant Disorders Achondroplasia (1/25,000) Alzheimer’s disease (early onset) Huntington’s disease (1/25,000) Hypercholesterolemia (1/500)

18 Inbreeding: mating between close blood relatives Increases the likelihood of homozygosity of a recessive allele in children of inbred parents Most genetic disorders are not evenly distributed across all ethnic groups Result of prolonged isolation of certain populations Inbreeding Increases the Likelihood that a Recessive Trait will be Inherited Dd DD Dd DD Dd dd=deaf

19 Dominant Disorders One allele is all that is required to cause the phenotype –i.e. Achondroplasia: dwarfism (homozygosity=lethal) Dominant lethal alleles are rare in populations –i.e. Huntington’s disease: causes degeneration of the nervous system in middle age

20 About 200,000 new breast cancer cases are diagnosed each year Approximately 10% of these are heritable; they run in families There are options if the results of genetic testing show a positive result for a BRCA mutation Genetic Testing Carrier screening Prenatal diagnostic testing: –i.e. amniocentesis Newborn screening Genealogical DNA testing Predicting adult-onset disorders Estimating risks of disease development Confirmational diagnosis Forensic/identity testing

21 Variations on Mendel’s Laws Incomplete dominance: F1 has an appearance in between the two parental phenotypes Codominance: both alleles are fully expressed in heterozygous individuals Pleiotropy: single gene influences more than one character Polygenic Inhertitance: additive effects of two or more genes on a single phenotype Environmental Factors: non-genetic, non-hereditary factors that contribute to phenotype

22 An Example of Incomplete Dominance Hypercholesterolemia

23 An Example of Codominance ABO Blood Groups

24 An Example of Pleiotropy Sickle-Cell Disease

25 An Example of Polygenic Inheritance Skin Color

26 Linked Genes Thomas Hunt Morgan: In 1916, published a paper on genes in Drosophila melanogaster (fruit fly) –Found the recombinant frequency to be 17%, not 50%

27 Linked Genes Genetic recombination (crossing over): usually ensures that genes on the same chromosome still assort independently Linked genes: genes so close together on a chromosome that they do not assort independently but tend to travel together Linkage map: diagrams describing relative gene locations using recombination frequencies –The shorter the distance between two genes, the less likely a crossover event will happen between them

28 Sex Chromosomes and Sex-Linked Genes Sex chromosomes: X and Y –XX  Female –XY  Male Sex-linked gene: any gene located on a sex chromosome X-linked genes: the X chromosome contains many more genes (~2000 genes) than the Y chromosome (~78 genes) Sex-linked disorders: disorders associated with a defective gene found on a sex (usually X) chromosome

29 –Is characterized by a malfunction of light- sensitive cells in the eyes. –Males are more likely to be color blind than females Figure 9.30 Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings Sex-Linked Disorders in Humans Red-green color blindness: recessive X-linked trait

30 Figure 9.32 Sex-Linked Disorders in Humans Hemophilia: “The Royal Disease” Sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises


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