Gregor Mendel. Monohybrid Cross. Patterns of Inheritance. Chapter 12 Founder of genetics. Augustinian monk ( )

Transcription

1 Gregor Mendel Patterns of Inheritance Chapter 12 Founder of genetics Augustinian monk ( ) First to use scientific method to study inheritance Removed stamens from purple flower Garden Pea Monohybrid Cross Transferred spermbearing pollen from stamens of white flower to eggbearing carpel of purple flower Parental generation (P) Carpel Stamens Pollinated carpel matured into pod Mendel used traits with only 2 variants (alleles) P Generation (true-breeding parents) White F1 Generation (hybrids) Planted seeds from pod First generation offspring (F1) Examined offspring: all purple All plants had purple

2 Monohybrid Cross Punnett Square P Generation Appearance: Genetic makeup: PP White pp P Generation (true-breeding parents) White Gametes F 1 Generation P p Appearance: Genetic makeup: Pp F 1 Generation (hybrids) All plants had purple Gametes: 1 2 P P 1 2 p F 1 sperm p F 2 Generation F 2 Generation P F 1 eggs p PP Pp Pp pp 3 : 1 First filial generation F 1 generation Offspring produced by crossing 2 truebreeding strains For every trait Mendel studied, all F 1 plants resembled only 1 parent Referred to this trait as dominant Alternative trait was recessive No plants with characteristics intermediate between the 2 parents were produced 7 F 2 generation Second filial generation Offspring resulting from the selffertilization of F 1 plants Although hidden in the F 1 generation, the recessive trait had reappeared among some F 2 individuals Counted proportions of traits Always found about 3:1 ratio 8

3 3:1 is really 1:2:1 Pea Characters Phenotype Genotype 3 PP (homozygous Pp (heterozygous 1 Mendel produced truebreeding pea plants with 7 different traits 2 Pp (heterozygous 1 White pp (homozygous 1 Ratio 3:1 Ratio 1:2:1 Pea Characters Mendel s First Hypothesis Genes for genetic characters occur in pairs One gene inherited from each parent Alleles are different versions of a gene Diploid: two copies of each gene Homozygous 2 of the same allele Heterozygous different alleles

4 Mendel s Second Hypothesis If two alleles of a gene are different, one allele is dominant over the other Dominant allele is expressed Recessive allele is masked Recessive alleles only expressed when two copies of the allele present Genotype total set of alleles an individual contains Phenotype physical appearance Mendel s Third Hypothesis Two alleles of a gene segregate (separate) and enter gametes singly Half the gametes carry one allele, half carry the other allele (haploid) Principle of Segregation Two gametes fuse to produce a zygote that contains two alleles (diploid) Punnett square Cross purple-flowered plant with white-flowered plant P is dominant allele purple p is recessive allele white True-breeding white-flowered plant is pp Homozygous recessive True-breeding purple-flowered plant is PP Homozygous dominant Pp is heterozygote purple-flowered plant 15 16

5 Human traits Some human traits are controlled by a single gene Some of these exhibit dominant and recessive inheritance Pedigree analysis is used to track inheritance patterns in families Dominant pedigree juvenile glaucoma Disease causes degeneration of optic nerve leading to blindness Dominant trait appears in every generation

6 Recessive pedigree albinism Condition in which the pigment melanin is not produced Pedigree for form of albinism due to a nonfunctional allele of the enzyme tyrosinase Males and females affected equally Most affected individuals have unaffected parents Dihybrid crosses Examination of 2 separate traits in a single cross Produced true-breeding lines for 2 traits RR YY x rryy The F 1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait Allow F 1 to self-fertilize to produce F 2 F 1 self-fertilizes RrYy x RrYy The F 2 generation shows all four possible phenotypes in a set ratio 9:3:3:1 R_Y_:R_yy:rrY_:rryy Round yellow:round green:wrinkled yellow:wrinkled green 23 24

7 25 26 Mendel s Fourth Hypothesis Principle of Independent Assortment In a dihybrid cross, the alleles of each gene assort independently The segregation of different allele pairs is independent Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs Probability Rule of addition Probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUS probability of obtaining pp (1/4)! +! = " 27 28

8 Testcross Rule of multiplication Probability of 2 independent events occurring simultaneously is the product of their individual probabilities When crossing Pp x Pp, the probability of obtaining pp offspring is Probability of obtaining p from father = " Probability of obtaining p from mother = " Probability of pp= " x " =! Cross used to determine the genotype of an individual with dominant phenotype Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent Extensions to Mendel Mendel s model of inheritance assumes that Each trait is controlled by a single gene Each gene has only 2 alleles There is a clear dominant-recessive relationship between the alleles Most genes do not meet these criteria 31 32

9 Polygenic inheritance Occurs when multiple genes are involved in controlling the phenotype of a trait The phenotype is an accumulation of contributions by multiple genes These traits show continuous variation and are referred to as quantitative traits For example human height Histogram shows normal distribution Pleiotropy Refers to an allele which has more than one effect on the phenotype Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions This can be seen in human diseases such as cystic fibrosis or sickle cell anemia Multiple symptoms can be traced back to one defective allele 35 Multiple alleles May be more than 2 alleles for a gene in a population ABO blood types in humans 3 alleles Each individual can only have 2 alleles Number of alleles possible for any gene is constrained, but usually more than two alleles exist for any gene in an outbreeding population 36

10 Incomplete dominance Heterozygote is intermediate in phenotype between the 2 homozygotes Red x white = pink Codominance Heterozygote shows some aspect of the phenotypes of both homozygotes Type AB blood Human ABO blood group The system demonstrates both Multiple alleles 3 alleles of the I gene (I A, I B, and i) Codominance I A and I B are dominant to i but codominant to each other 39 40

11 Environmental influence Epistasis Coat color in Himalayan rabbits and Siamese cats Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment Allele produces an enzyme that allows R.A. Emerson crossed 2 white varieties of pigment corn production F only at 1 was all purple temperatures F 2 was 9 purple:7 white not expected below 30 o C