Mendelian Genetics 1

Transcription

1 Mendelian Genetics

2 Genetics Mendel and the Gene Idea Genetics: The study of heredity. 2

3 Heredity Two possible explanations for heredity: The Blending Hypothesis Genetic material from the parents mix. Similar to the idea of making green from blue and yellow. The Particulate Hypothesis Parents pass on discrete units (genes), which then affect the offspring. 3

4 Mendelian Genetics Gregor Johann Mendel ( ) Augustinian monk, Czech Republic Foundation of modern genetics Studied segregation of traits in the garden pea (Pisum sativum) beginning in 854 Published his theory of inheritance in 865. Experiments in Plant Hybridization Mendel was rediscovered in 902 Ideas of inheritance in Mendel s time were vague. One general idea was that traits from parents came together and blended in offspring. Thus, inherited information was predicted to change in the offspring, an idea that Mendel showed was wrong. Characters, or what we now call alleles, were inherited unchanged. This observation and the pattern of inheritance of these characters gave us the first definition of a gene 4

5 Crossing Pea Plants Removed stamens from purple flower 2 Transferred spermbearing pollen from stamens of white flower to eggbearing carpel of purple flower Parental generation (P) 3 Pollinated carpel matured into pod Stamens Carpel (male) (female) 4 Planted seeds from pod First generation offspring (F ) 5 Examined offspring: all purple flowers 5

6 Mendel s Studied Discrete Traits 6

7 Genetic Vocabulary Character: a heritable feature, such as flower color Trait: a variant of a character, such as purple or white flowers Each trait made of two copies of a gene One from the mother and the other from the father Alternative forms of traits are called alleles 7

8 Genetic Vocabulary Phenotype observable characteristic of an organism Genotype pair of alleles present in and individual Homozygous two alleles of trait are the same (YY or yy) Heterozygous two alleles of trait are different (Yy) Capitalized traits = dominant phenotypes Lowercase traits= recessive phenotypes 8

9 Genetic Vocabulary Generations: P = parental generation F = st filial generation, progeny of the P generation F2 = 2nd filial generation, progeny of the F generation (F3 and so on) Crosses: Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait. Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits. 9

10 Phenotype vs Genotype Phenotype Genotype Purple PP (homozygous) 3 Purple (heterozygous) 2 (heterozygous) Purple pp White (homozygous) Figure 4.6 Ratio 3: Ratio :2: 0

11 Dominant & recessive Phenotype alleles (Fig. 0.7): vs Genotype

12 Heredity Concepts. Alternative versions of genes account for variations in inherited characters, which are now called alleles Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers 2. For each character an organism inherits two alleles, one from each parent, A genetic locus is actually represented twice 3. If the two alleles at a locus differ, the dominant allele determines the organism s appearance 4. The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes 2

13 Monohybrid Cross Classical Punett's Square is a way to determine ways traits can segregate Parental P 0 cross P P p p F cross P p P PP p pp Determine the genotype and phenotype 3

14 Mendel s Monohybrid Cross White (pp) Purple () Purple (PP) Gametes p p P p Purple () Gametes P P Gametes Gametes PP P p pp F generation All purple F 2 generation ¾ purple, ¼ white 4

15 Test Cross To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype: Alternative Plant with dominant phenotype is homozygous Recessive phenotype pp Gametes p p Gametes P P Dominant Phenotype PP? Dominant Phenotype Recessive phenotype pp? Gametes p p Alternative 2 Plant with dominant phenotype is heterozygous Gametes P p 5

16 Test Cross To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype: Alternative Plant with dominant phenotype is homozygous Recessive phenotype pp Gametes p p Gametes P P Dominant Phenotype PP? Dominant Phenotype Recessive phenotype pp? Gametes p p Alternative 2 Plant with dominant phenotype is heterozygous Gametes P p pp pp If all offspring are purple; unknown plant is homozygous. If half of offspring are white; unknown plant is heterozygous. 6

17 Dihybrid Cross Dihybrid cross - parental generation differs in two traits example-- cross round/yellow peas with wrinkled/green ones Round/yellow is dominant RY Ry ry ry RY Ry ry ry What are the expected phenotype ratios in the F 2 generation? round, yellow = round, green = wrinkled, yellow = wrinkled, green = 7

18 A Dihybrid Cross How are two characters transmitted from parents to offspring? As a package? Independently? A dihybrid cross Illustrates the inheritance of two characters Produces four phenotypes in the F2 generation EXPERIMENT Two true-breeding pea plants one with yellow-round seeds and the other with green-wrinkled seeds were crossed, producing dihybrid F plants. Self-pollination of the F dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. CONCLUSION The results support the hypothesis ofindependent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. P Generation F Generation YYRR Gametes YR yr Hypothesis of dependent assortment 2 YR 2 yr Eggs YR F 2 Generation 2 YYRR YyRr (predicted offspring) 2 yr YyRr yyrr Sperm Phenotypic ratio 3: YyRr yyrr 4 YR Yr yr yr Hypothesis of independent assortment Sperm 4 YR Yr 4 yr 4 4 yr Eggs YYRR YYRr YyRR YyRr YYrr YYrr YyRr YyRR YyRr yyrr YyRr Yyrr yyrr Yyrr yyrr yyrr Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3: 8

19 Mendel s conclusions Genes are distinct entities that remain unchanged during crosses Each plant has two alleles of a gene Alleles segregated into gametes in equal proportions, each gamete got only one allele During gamete fusion, the number of alleles was restored to two 9

20 Summary of Mendel s Principles Mendel s Principle of Uniformity in F: F offspring of a monohybrid cross of true-breeding strains resemble only one of the parents. Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds (alleles). Mendel s Law of Segregation: Recessive characters masked in the F progeny of two true-breeding strains, reappear in a specific proportion of the F2 progeny. Two members of a gene pair segregate (separate) from each other during the formation of gametes. Mendel s Law of Independent Assortment: Alleles for different traits assort independently of one another. Genes on different chromosomes behave independently in gamete production. 20

21 Exceptions To Mendel s Original Principles Incomplete dominance Codominance Multiple alleles Polygenic traits Epistasis Pleiotropy Environmental effects on gene expression Linkage Sex linkage 2

22 Incomplete dominance Neither allele is dominant and heterozygous individuals have an intermediate phenotype For example, in Japanese Four o clock, plants with one red allele and one white allele have pink flowers: P Generation Red C R C R White C W C W Gametes C R C W F Generation Pink C R C W Gametes 2 2 C R C R F 2 Generation Eggs 2 2 C R C R C R 2 C R C R C R C W Sperm 2 C w C R C W C W C W 22

23 Codominance Neither allele is dominant and both alleles are expressed in heterozygous individuals Example ABO blood types 23

24 Polygenic Traits Most traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. These are called polygenic traits. Polygenic traits often show continuous variation, rather then a few discrete forms: 24

25 Epistasis Type of polygenic inheritance where the alleles at one gene locus can hide or prevent the expression of alleles at a second gene locus. Labrador retrievers one gene locus affects coat color by controlling how densely the pigment eumelanin is deposited in the fur. A dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat However, a second gene locus controls whether any eumelanin at all is deposited in the fur. Dogs that are homozygous recessive at this locus (ee) will have yellow fur no matter which alleles are at the first locus: 25

26 Epistasis ee No dark pigment in fur E_ Dark pigment in fur eebb eeb_ E_bb E_B_ Yellow fur Yellow fur Brown fur Black fur Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 26

27 Pleiotropy This is when a single gene locus affects more than one trait. For example, in Labrador retrievers the gene locus that controls how dark the pigment in the hair will be also affects the color of the nose, lips, and eye rims. 27

28 Environmental Effects on Gene Expression The phenotype of an organism depends not only on which genes it has (genotype), but also on the environment under which it develops. Although scientists agree that phenotype depends on a complex interaction between genotype and environment, there is a lot of debate and controversy about the relative importance of these 2 factors, particularly for complex human traits. 28