Mendelian Inheritance 1Outline Blending Inheritance Monohybrid Cross  Law of Segregation Modern Genetics  Genotype vs. Phenotype  Punnett Square Dihybrid Cross  Law of Independent Assortment Human Genetic Disorders

Mendelian Inheritance G90 2

3 Gregor Mendel

Mendelian Inheritance 4 Gregor Mendel Austrian monk  Studied science and mathematics at University of Vienna  Conducted breeding experiments with the garden pea Pisum sativum  Carefully gathered and documented mathematical data from his experiments Formulated fundamental laws of heredity in early 1860s  Had no knowledge of cells or chromosomes  Did not have a microscope

5 Fruit and Flower of the Garden Pea

6 Garden Pea Traits Studied by Mendel

Mendelian Inheritance 7 Blending Inheritance Theories of inheritance in Mendel’s time:  Based on blending  Parents of contrasting appearance produce offspring of intermediate appearance Mendel’s findings were in contrast with this  He formulated the particulate theory of inheritance  Inheritance involves reshuffling of genes from generation to generation

Mendelian Inheritance 8 One-Trait Inheritance Mendel performed cross-breeding experiments  Used “true-breeding” (homozygous) plants  Chose varieties that differed in only one trait (monohybrid cross)  Performed reciprocal crosses ­Parental generation = P ­First filial generation offspring = F 1 ­Second filial generation offspring = F 2  Formulated the Law of Segregation

9 Mendel’s Monohybrid Crosses: An Example

Mendelian Inheritance 10 Law of Segregation Each individual has a pair of factors (alleles) for each trait The factors (alleles) segregate (separate) during gamete (sperm & egg) formation Each gamete contains only one factor (allele) from each pair Fertilization gives the offspring two factors for each trait

Mendelian Inheritance 11 Modern Genetics View Each trait in a pea plant is controlled by two alleles (alternate forms of a gene) Dominant allele (capital letter) masks the expression of the recessive allele (lower- case) Alleles occur on a homologous pair of chromosomes at a particular gene locus  Homozygous = identical alleles  Heterozygous = different alleles

12 Homologous Chromosomes

Mendelian Inheritance 13 Genotype Versus Phenotype Genotype  Refers to the two alleles an individual has for a specific trait  If identical, genotype is homozygous  If different, genotype is heterozygous Phenotype  Refers to the physical appearance of the individual

Mendelian Inheritance 14 Punnett Square Table listing all possible genotypes resulting from a cross  All possible sperm genotypes are lined up on one side  All possible egg genotypes are lined up on the other side  Every possible zygote genotypes are placed within the squares

15 Punnett Square Showing Earlobe Inheritance Patterns

Mendelian Inheritance 16 Monohybrid Testcross Individuals with recessive phenotype always have the homozygous recessive genotype However, Individuals with dominant phenotype have indeterminate genotype  May be homozygous dominant, or  Heterozygous Test cross determines genotype of individual having dominant phenotype

17 One-Trait Test Cross Unknown is Homozygous Dominant

Mendelian Inheritance 18 Two-Trait Inheritance Dihybrid cross uses true-breeding plants differing in two traits  Observed phenotypes among F 2 plants  Formulated Law of Independent Assortment ­The pair of factors for one trait segregate independently of the factors for other traits ­All possible combinations of factors can occur in the gametes

19 Two-Trait (Dihybrid) Cross

Meiosis & Sexual Reproduction FTEU 20

Meiosis & Sexual Reproduction 21Outline Reduction in Chromosome Number  Meiosis Overview  Homologous Pairs Genetic Variation  Crossing-Over  Independent Assortment  Fertilization Phases of Meiosis  Meiosis I  Meiosis II Meiosis Compared to Mitosis Human Life Cycle

Meiosis & Sexual Reproduction 22 Meiosis: Halves the Chromosome Number Special type of cell division Used only for sexual reproduction Halves the chromosome number prior to fertilization  Parents diploid  Meiosis produces haploid gametes  Gametes fuse in fertilization to form diploid zygote  Becomes the next diploid generation

Meiosis & Sexual Reproduction 23 Homologous Pairs of Chromosomes In diploid body cells chromosomes occur in pairs Humans have 23 different types of chromosomes Diploid cells have two of each type Chromosomes of the same type are said to be homologous  They have the same length  Their centromeres are positioned in the same place  One came from the father (the paternal homolog) the other from the mother (the maternal homolog)  When stained, they show similar banding patterns  Because they have genes controlling the same traits at the same positions

24 Homologous Chromosomes

Meiosis & Sexual Reproduction 25 Homologous Pairs of Chromosomes Homologous chromosomes have genes controlling the same trait at the same position  Each gene occurs in duplicate  A maternal copy from the mother  A paternal copy from the father Many genes exist in several variant forms in a large population Homologous copies of a gene may encode identical or differing genetic information The variants that exist for a gene are called alleles An individual may have:  Identical alleles for a specific gene on both homologs (homozygous for the trait), or  A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)

26 Overview of Meiosis

Meiosis & Sexual Reproduction 27 Phases of Meiosis I: Prophase I & Metaphase I Meiosis I (reductional division):  Prophase I ­Each chromosome internally duplicated (consists of two identical sister chromatids) ­Homologous chromosomes pair up – synapsis ­Physically align themselves against each other end to end ­End view would show four chromatids – Tetrad  Metaphase I ­Homologous pairs arranged onto the metaphase plate

Meiosis & Sexual Reproduction 28 Phases of Meiosis I: Anaphase I & Telophase I Meiosis I (cont.):  Anaphase I ­Synapsis breaks up ­Homologous chromosomes separate from one another ­Homologues move towards opposite poles ­Each is still an internally duplicate chromosome with two chromatids  Telophase I ­Daughter cells have one internally duplicate chromosome from each homologous pair ­One (internally duplicate) chromosome of each type (1n, haploid)

Meiosis & Sexual Reproduction 29 Phases of Meiosis I: Cytokinesis I & Interkinesis Meiosis I (cont.):  Cytokinesis I ­Two daughter cells ­Both with one internally duplicate chromosome of each type ­Haploid ­Meiosis I is reductional (halves chromosome number) Interkinesis  Similar to mitotic interphase  Usually shorter  No replication of DNA

Meiosis & Sexual Reproduction 30 Genetic Variation: Crossing Over Meiosis brings about genetic variation in two key ways:  Crossing-over between homologous chromosomes, and  Independent assortment of homologous chromosomes 1. Crossing Over:  Exchange of genetic material between nonsister chromatids during meiosis I  At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues ­Holds homologues together ­Aligns DNA of nonsister chromatids ­Allows crossing-over to occur  Then homologues separate and are distributed to different daughter cells

31 Crossing Over

Meiosis & Sexual Reproduction 32 Genetic Variation: Independent Assortment 2. Independent assortment:  When homologues align at the metaphase plate: ­They separate in a random manner ­The maternal or paternal homologue may be oriented toward either pole of mother cell  Causes random mixing of blocks of alleles into gametes

33 Independent Assortment

34 Recombination

Meiosis & Sexual Reproduction 35 Genetic Variation: Fertilization When gametes fuse at fertilization:  Chromosomes donated by the parents are combined  In humans, (2^ 23 ) 2 = 70,368,744,000,000 chromosomally different zygotes are possible If crossing-over occurs only once  (4^23) 2, or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible

Meiosis & Sexual Reproduction 36 Genetic Variation: Significance Asexual reproduction produces genetically identical clones Sexual reproduction cause novel genetic recombinations Asexual reproduction is advantageous when environment is stable However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous

Meiosis & Sexual Reproduction 37 Phases of Meiosis II: Similar to Mitosis Metaphase II  Overview ­Unremarkable ­Virtually indistinguishable from mitosis of two haploid cells  Prophase II – Chromosomes condense  Metaphase II – chromosomes align at metaphase plate  Anaphase II ­Centromere dissolves ­Sister chromatids separate and become daughter chromosomes  Telophase II and cytokinesis II ­Four haploid cells ­All genetically unique

38 Meiosis I & II in Plant Cells

Meiosis & Sexual Reproduction 39 Meiosis versus Mitosis Meiosis RRRRequires two nuclear divisions CCCChromosomes synapse and cross over CCCCentromeres survive Anaphase I HHHHalves chromosome number PPPProduces four daughter nuclei PPPProduces daughter cells genetically different from parent and each other UUUUsed only for sexual reproduction Mitosis RRRRequires one nuclear division CCCChromosomes do not synapse nor cross over CCCCentromeres dissolve in mitotic anaphase PPPPreserves chromosome number PPPProduces two daughter nuclei PPPProduces daughter cells genetically identical to parent and to each other UUUUsed for asexual reproduction and growth

40 Meiosis Compared to Mitosis

41 Meiosis I Compared to Mitosis

42 Meiosis II Compared to Mitosis

43 PLC02FBC20B99B6B4D

Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Complete dominance

46 Complete dominance

Mendelian Inheritance 47 Incomplete Dominance Heterozygote has phenotype intermediate between that of either homozygote  Homozygous red has red phenotype  Homozygous white has white phenotype  Heterozygote has pink (intermediate) phenotype Phenotype reveals genotype without test cross

Fig Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW

Fig Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2

Fig Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 F 2 Generation Sperm Eggs CRCR CRCR CWCW CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 1/21/2 1/21/2 1/21/2 1/21/2

Mendelian Inheritance 51 Multiple Alleles and Codominance Some traits controlled by multiple alleles The gene exists in several allelic forms (but each individual only has two) ABO blood types The alleles:  I A = A antigen on red cells  I B = B antigen on red cells  I = Neither A nor B antigens

Fig IAIA IBIB i A B none (a) The three alleles for the ABO blood groups and their associated carbohydrates Allele Carbohydrate Genotype Red blood cell appearance Phenotype (blood group) I A I A or I A i A B I B I B or I B i IAIBIAIB AB iiO (b) Blood group genotypes and phenotypes

53 Inheritance of Blood Type

Mendelian Inheritance 54 Polygenic Inheritance Occurs when a trait is governed by two or more genes having different alleles Each dominant allele has a quantitative effect on the phenotype These effects are additive Result in continuous variation of phenotypes

55 Height in Human Beings

56 Frequency Distributions in Polygenic Inheritance

Fig. 14-UN2 Degree of dominance Complete dominance of one allele Incomplete dominance of either allele Codominance Description Heterozygous phenotype same as that of homo- zygous dominant Heterozygous phenotype intermediate between the two homozygous phenotypes Heterozygotes: Both phenotypes expressed Multiple alleles In the whole population, some genes have more than two alleles CRCRCRCR CRCWCRCW CWCWCWCW IAIBIAIB I A, I B, i ABO blood group alleles PP Pp Example

Mendelian Inheritance 58 Red-Green Colorblindness Chart

Mendelian Inheritance kM 59 Human Genetic Disorders