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Mendelian Genetics.

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Chapter 14~ Mendel & The Gene Idea

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Presentation on theme: "Mendelian Genetics."— Presentation transcript:

1 Mendelian Genetics

2 Mendelian genetics Character (heritable feature, i.e., fur color)
Trait (variant for a character, i.e., brown) True-bred (all offspring of same variety) Hybridization (crossing of 2 different true-breds) P generation (parents) F1 generation (first filial generation)

3 Leading to the Law of Segregation
Alternative versions of genes (alleles) account for variations in inherited characteristics For each character, an organism inherits 2 alleles, one from each parent If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance The alleles for each character segregate (separate) during gamete production (meiosis). Mendel’s Law of Segregation

4 Genetic vocabulary……. Punnett square: predicts the results of a genetic cross between individuals of known genotype Homozygous: pair of identical alleles for a character Heterozygous: two different alleles for a gene Phenotype: an organism’s traits Genotype: an organism’s genetic makeup Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype)

5 Sample Crosses

6 The Law of Independent Assortment
Law of Segregation involves 1 character. What about 2 (or more) characters? Monohybrid cross vs. dihybrid cross The two pairs of alleles segregate independently of each other. Mendel’s Law of Independent Assortment


8 Non-single gene genetics, I
Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons Codominance: two alleles affect the phenotype in separate, distinguishable ways Ex: Tay-Sachs disease Multiple alleles: more than 2 possible alleles for a gene Ex: human blood types

9 Non-single gene genetics, II
Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height

10 Incomplete Dominance

11 Codominance

12 Human disorders The family pedigree
Recessive disorders: •Cystic fibrosis •Tay-Sachs •Sickle-cell Dominant disorders: •Huntington’s Testing: •amniocentesis •chorionic villus sampling (CVS)



15 What is Huntington's Disease?
Huntington's disease (HD) results from genetically programmed degeneration of brain cells, called neurons, in certain areas of the brain. This degeneration causes uncontrolled movements, loss of intellectual faculties, and emotional disturbance. HD is a familial disease, passed from parent to child through a mutation in the normal gene. Each child of an HD parent has a chance of inheriting the HD gene. If a child does not inherit the HD gene, he or she will not develop the disease and cannot pass it to subsequent generations. A person who inherits the HD gene will sooner or later develop the disease. Whether one child inherits the gene has no bearing on whether others will or will not inherit the gene. Some early symptoms of HD are mood swings, depression, irritability or trouble driving, learning new things, remembering a fact, or making a decision. As the disease progresses, concentration on intellectual tasks becomes increasingly difficult and the patient may have difficulty feeding himself or herself and swallowing. The rate of disease progression and the age of onset vary from person to person. A genetic test, coupled with a complete medical history and neurological and laboratory tests, help physician's diagnose HD. Presymptomic testing is available for individuals who are at risk for carrying the HD gene. In 1 to 3 percent of individuals with HD, no family history of HD can be found.

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