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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-1 Classical Genetics — Lecture I Dr. Steven J. Pittler.

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Presentation on theme: "Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-1 Classical Genetics — Lecture I Dr. Steven J. Pittler."— Presentation transcript:

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2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-1 Classical Genetics — Lecture I Dr. Steven J. Pittler VH375B Office Cell Suggested Reading: Lewis 2nd Edition Chapter on Mendelian Inheritance

3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-2 Inheritance Parents and offspring often share observable traits. Grandparents and grandchildren may share traits not seen in parents. Why do traits disappear in one generation and reappear in another?

4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-3 Gregor Mendel: The father of modern genetics Combined plant breeding Statistics Careful recordkeeping Described hypothesis of transmission of traits now considered the laws of inheritance

5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-4 Mendel studied pea traits with two distinct forms

6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-5 True breeding plants Plants which consistently have offspring with same trait as parent are true breeding plants.

7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-6 Monohybrid cross What happens when true breeding plants with two distinct forms of a trait are crossed? Progeny show only one form of the trait. The observed trait is called dominant. The masked trait is called recessive.

8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-7 Test cross Is a plant showing the dominant trait true-breeding or not? Test by crossing with a plant showing the recessive trait. All tall offspring indicate parent is true-breeding Mixed offspring indicate parent is hybrid

9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-8 Crossing hybrids to each other Hybrid parents show the dominant trait (tall). Offspring: Mendel concluded that among the hybrid parents the short trait (recessive) was hidden but not absent Dominant trait (tall) and true breeding (1/4 total) Recessive trait (short) and always true breeding (1/4 total) Dominant trait (tall) and NOT true breeding (1/2 total)

10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-9 Mendel’s data 1. Crossed true-breeding plants differing at one of seven traits. 2. Crossed hybrid offspring to each other (all show dominant trait). 3. Counted offspring of hybrid crosses. TraitOffspring # with dominant trait # with recessive trait Ratio dominant : recessive Seed form7,3245,4741, : 1 Seed color8,0236,0222, : 1 Seed coat color : 1 Pod form1, : 1 Pod color : 1 Flower position : 1 Stem length1, : 1

11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display If you know the genotype of the parents, it is possible to determine the gametes and use a Punnett square to determine the phenotypic ratio among the offspring. When a monohybrid reproduces with a monohybrid, the results are 3 : 1. This ratio is used to state the chances of a particular phenotype. A 3 : 1 ratio means that there is a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype.

12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Punnett Squares tTt T The genes from one parent go here. The genes from the other parent go here.

13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Punnett Squares TT tTt t F 1 generation

14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gamete Formation (sperm and eggs) Because homologous pairs separate during meiosis, a gamete has only one allele from each pair of alleles. If the allelic pair is Tt, a gamete would contain either a T or a t, but not both. Tt represents the genotype of an individual. Gametes are represented by T or t.

15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display One-Trait Crosses and Probability Laws of probability alone can be used to determine results of a cross. The laws are: (1) the probability that two or more independent events will occur together is the product of their chances occurring separately, and (2) the chance that an event that can occur in two or more independent ways is the sum of the individual chances.

16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display In the cross of Tt x Tt, what is the chance of obtaining either a T or a t from a parent? Chance of T = ½, or chance of t = ½ The probability of these genotypes is: The chance of TT = ½ x ½ = ¼ The chance of Tt = ½ x ½ = ¼ The chance of tT = ½ x ½ = ¼ The chance of tt = ½ x ½ = ¼ The chance of tall plants (TT, Tt, tT) is ¼ + ¼ + ¼ = ¾ or 75%.

17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Law of segregation Why do traits “disappear” in one generation only to reappear in a subsequent generation? Each plant has two distinct separable units (alleles) for each trait inherited from each parent. (sister chromatids that together make a chromosome) Only one version is observed in an individual. Gametes contain ONE allele for each trait. The unit (allele) does not disappear. It may be present but hidden.

18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gene locus (locus = location)

19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Law of segregation

20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Alleles Mendel’s units (or “elementen”) are called alleles. versions of the same gene or DNA sequence. differ in DNA sequence at one or more sites.

21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Genotype indicates the combination of alleles present Homozygous alleles are the same Heterozygous alleles differ Phenotype indicates the trait observed. Terms distinguish the observed form and the underlying alleles present.

22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Genotype and phenotype Phenotype Tall plant Short plant Genotype Homozygous dominant “tall-associated” alleles Heterozygous Homozygous recessive “short-associated” alleles Abbreviation of genotype TT Tt tt

23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Modern terms for Mendel’s crosses Mendel’s true-breeding plants were homozygous for the alleles of a trait. Mendel’s hybrid plants were heterozygous for the alleles of a trait.

24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Wildtype most common version in the general population wildtype phenotype mutant phenotype wildtype allele mutant allele most common phenotype phenotype different from the wildtype most frequent allele associated with the common phenotype homozygous. allele associated with the mutant phenotype.

25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Law of segregation: the monohybrid cross Two heterozygous parents produce gametes with T or t allele equally frequently. Offspring genotypes 1/4 TT : 1/2 Tt : 1/4 tt Offspring phenotypes 3/4 tall : 1/4 short

26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mode of inheritance indicates the patterns with which the mutant phenotype is associated. Most common Autosomal recessive Autosomal dominant X-linked recessive X-linked dominant Y-linked (holandric) mitochondrial

27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Autosomal dominant inheritance Heterozygotes exhibit the affected phenotype. Males and females are equally affected and may transmit the trait. Affected phenotype does not skip generations.

28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Autosomal recessive inheritance Heterozygotes carry the recessive allele but exhibit the wildtype phenotype. Males and females are equally affected and may transmit the trait. May skip generations.

29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Comparison of autosomal dominant and autosomal recessive inheritance NoYes At least one parent of affected child must be affected? YesNoTrait skips generations? Yes Males and females transmit the trait? Yes Males and females affected? Autosomal recessive Autosomal dominant

30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Law of independent assortment Two genes on different chromosomes segregate their alleles independently. The inheritance of an allele of one gene does not influence which allele is inherited at a second gene.

31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Law of independent assortment

32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Independent assortment of two traits In a dihybrid cross, parents with two differing traits are crossed. Which allele is dominant? Heterozygous peas are round and yellow. Therefore round is dominant to wrinkled yellow is dominant to green

33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Two traits segregating independently

34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Pedigrees symbolic representations of family relationships and inheritance of a trait

35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Autosomal dominant inheritance of brachydactyly Heterozygotes exhibit the phenotype.

36 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Autosomal recessive inheritance of albinism Heterozygotes carry the recessive allele but exhibit the wildtype phenotype

37 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Genetic predictions What is the chance that Ellen’s child has a sickle cell anemia allele (a)? EllenMichael ? Ellen and Michael’s parents must be carriers. A a AaAa AAAa aa Ellen is not affected and cannot carry aa genotype chance child inherits sickle cell allele = 1/2 Overall chance child carries sickle cell allele from Ellen = 2/3 x 1/2 = 1/3 chance Ellen is a carrier = 2/3 Ellen’s brother Michael has sickle cell anemia, an autosomal recessive disease.


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