GENETICS AND MENDEL HISTORY OF DISCOVERERY OF

GENETICS AND MENDEL

HISTORY OF DISCOVERERY OF HEREDITY • 1851: Gregor Mendel, father of heredity – studied pea plants – prevented self pollination – used cross pollination – brought experimental and quantitative approach to genetics

Mendel’s Experiment

Genetic Terms Alleles: different forms of the same gene ex: tall / short green seed / yellow seed smooth / wrinkled curly hair/ straight hair Dominant: stronger trait which is expressed, written as a capital letter ex: T Recessive: weaker trait which is not expressed when paired with a dominant trait, written as a lower case letter ex: t Homozygous/Pure: appearance of 2 identical alleles of a gene on a chromosome ex: TT, tt Heterozygous/Hybrid: appearance of 2 different alleles of a gene on a chromosome ex: Tt

Genetic Terms • self pollination: fertilization of plant’s egg by pollen of same plant • cross pollination: fertilization of plant’s egg by pollen of another plant

Mendel: Experiment 1 1. he crossbred purebred plants of opposite traits (parental/P generation) 2. resulting offspring were first filial (F 1 generation) 3. he self pollinated F 1 generation 4. resulting offspring were second filial (F 2 generation) 5. he performed 100’s of crosses and documented results

Results: Experiment 1 P 100% homozygous dominant: SS F 1 100% heterozygous dominant: Ss F 2 3: 1 ratio dominant to recessive traits smooth : wrinked THIS WORK FORMED BASICS OF GENETICS AND HOLDS TRUE TODAY !

Mendel’s Three Laws of Heredity I. Law of Dominance and Recessiveness One factor (gene) in a pair may mask the other factor (gene) preventing it from having an effect. dominant: allele codes for a protein that works recessive: allele codes for a protein that doesn’t work **genes always occur in pairs** ex: TT, Tt : tall tt: short

Mendel’s Three Laws of Heredity II. Law of Segregation The two factors for a trait segregate (separate) during the formation of egg and sperm and each reproductive cell (gamete) receives only one factor for each trait ex: male would give one trait : T or t female would give one trait: T or t offspring could have these combinations: TT, Tt, tt

Mendel’s Three Laws of Heredity Ill. Law of Independent Assortment Factors (genes) for different traits are distributed to reproductive cells (gametes) independently of each other. - Mendel also crossed plants that differed in two characteristics ex: height, coat color - He found that traits from dominant factors did not appear together - Factors for each trait were not connected

Mendel was very innovative because he applied math (probability) to Biology. Probability Possibility that an event will occur Probability = # one kind of event # of all events

Genetic Crosses Punnett Square Chartused to predict probabilities of genetic crosses Phenotype: external appearance of an organism Genotype: actual genetic makeup of an organism

Monohybrid Crossing of one set of traits. Tt x Tt

Test Cross (Back Cross) • Offspring results are known • Solve for dominant genotypes of parental generation Method: cross dominant genotypes with homozygous genotype and determine which cross matches known offspring results.

Dihybrid Crossing of two sets of traits. Traits Y yellow R round y green r wrinkled P generation homozygous round, yellow X homozygous wrinkled, green RRYY x rryy

P Generation rryy ry ry ry RY Rr. Yy Rr. Yy RY Rr. Yy RY RRYY ry Rr. Yy F 1 generation: 100% Rr. Yy heterozygous round yellow (only genotype possible)

Cross F 1 generation: Rr. Yy RY r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy RRyy Rr. Yy Rryy r. Y Rr. Yy rr. YY rr. Yy ry Rr. Yy Ry Rr. Yy Rryy rr. Yy rryy F 2 generation: phenotype ratio 9 : 3 : 3 yellow green round wrinkled round : 1 green wrinkled When crossing two hybrids- phenotype ratio will always be 9: 3: 3: 1

Mendel Understood Probability Product Rule Chance of 2 or more independent events occurring together equals product of chances of each of the separate occurrences. (x and y occurring together) Ex. 1: probability of throwing 2 sixes 1/6 x 1/6 = 1/36 Ex. 2: probability of having 5 boys in a row ½ x ½ x ½ = 1/32 Ex. 3: If cross AABb. Cc x Aa. Bb. Cc, probability of offspring with Aa. Bbcc is: ½ x ¼ = 1/16

Product Rule Example

Mendel Understood Probability Sum Rule (Addition, “OR” Rule) Probability that 2+ mutually exclusive events will occur (events can’t occur together) (x or y occurring) - add together individual probabilities Ex. 1: chances of throwing a die that will land on 4 or 5? 1/6 + 1/6 = 1/3

Sum Rule- Example Predict the fraction of offspring from an Aa x Aa cross that will have the dominant phenotype (AA or Aa genotype). - In this cross, there are three events that can lead to a dominant phenotype: - Two A gametes meet (giving AA genotype), or - A gamete from Mom meets a gamete from Dad (giving Aa genotype), or - A gamete from Mom meets A gamete from Dad (giving Aa genotype) We can find that each individual event has a probability of ¼ Probability of offspring with dominant phenotype is: (1/4)+(1/4)= 3/4

Product Rule and Sum Rule Product Rule video Sum Rule

Theories of Heredity 1902: Walter Sutton, Columbia University - Observed that genes are located on chromosomes. - Realized chromosomes behaved exactly same as carriers of genetic information would do. 1903: Chromosome Theory of Heredity 1. Genes are located on chromosomes and each gene occupies a specific place (locus) on a chromosome. 2. Genes can exist in several forms. (alleles) 3. Each chromosome contains only one of the alleles for each of its genes. Sutton – believed that genes move in sets on a chromosome.

Theories of Heredity, cont. 1902/03: Thomas Hunt Morgan , Columbia Univ. - proved gene linkage, won Nobel prize in 1933 Gene linkage: attachment of certain genes to each other on a chromosome (by chemical bonds that keep them together) Linkage groups: group or packages of genes located on one chromosome which are usually inherited together (they do not undergo independent assortment) - groups can be independently assorted, but always go together

Exchange of chromatids pieces of a homologous pair during synapsis at a chiasma. . . is GREATER the FARTHER apart 2 genes are is proportional to relative distance between 2 linked genes

The further apart 2 genes on same chromosome, further the higher the probability of crossing over higher and the higher the recombination frequency higher

Morgan worked with Drosophila (new generation every 4 weeks) - demonstrated gene linkage: G grey body (dom) W normal wings (dom) g black body (rec) w small wings (rec) P GGWW x ggww F 1 100% Gg. Ww (grey normal wings) ---------------------------Gg. Ww x ggww (test cross)

Mendelian genetics would produce: 1 : 1 : 1 grey black norm. small Actual observation - 42% grey normal (84% chromosomes like parents) - 42% black small ------------------- - 8% black normal (16% new combinations) - 8% grey small Recombinants Individuals with new genetic combinations ***this indicated gene for body color and wing size were LINKED*****

Linked genes: located on same chromosome and tend to be inherited together during cell division

Calculating Recombination Frequency

Estimating Recombination and Distance Between Genes Two types of gametes are possible when following genes on the same chromosomes. • If crossing over does not occur, the products are parental gametes. • If crossing over occurs, the products are recombinant gametes • Recombinant gametes that are found in the lowest frequency ex: Rr. Tt x rrtt R red T tall r white t short Cross should yield a 1 : 1 : 1 ratio

Estimating Recombination and Distance Between Genes Rr. Tt x rrtt Red, tall x white, short 100 offspring were produced with the following results: 34 red, tall 19 red, short 16 white, tall 31 white, short Non- Mendelian ratio indicated that genes are linked and do not assort independently Problem: Determine the arrangement of alleles in the heterozygote parents and the map distance between the genes. To determine the linkage distance- divide the number of recombinant gametes into the total gametes Recombinants: red, short 19/100 = 19% 35% white, tall 16/100 = 16% Gene Map |_____35 mu_____| Color Height gene

Gene Linkage Map • genetic map that is based on % of cross-over events • 1 map unit = 1% recombination frequency • Express relative distances along chromosome • 50% recombination = genes on 2 different chromosomes

Gene Linkage Map Problem In pea plants, flower color and pollen shape are located on the same chromosome. A plant with purple flowers and long pollen (Aa. Bb) is crossed with one that is recessive for both traits (aabb). The results are as follows: a) What would be the expected offspring of this cross? 1 : 1 : 1 b) How far apart are the two alleles? purple long 47 red round 47 purple round 3 6% |_____ 6 mu_______| red round 3 color gene shape gene

Determining Expected Gametes and Frequency

Sex Determination 1905: Nettie Stevens – • studied mealworms - female cells had 20 large pairs of chromosomes - male cells had 19 lg. chrom, 1 small chromosome • she then studied drosophila - female cells had 4 pairs - male had 3 alike pairs, 1 mismatched (1 looked like a hook) ** autosomes – non sex chrom. matched ** sex chromosomes – mismatched pair - female chromosomes - X - male chromosome - Y - After meiosis what sex chromosomes will the gametes have? - What is probability of getting male or female in a cross?

Which sex determines sex of offspring? - homogametic sex (XX) 2 same sex chromosomes - heterogametic sex (XY) 2 different sex chromosomes

Human Development • • Y chromosome required for development of testes Embryo gonads indifferent at 2 months SRY gene: sex-determining region of Y Codes for protein that regulates other genes

Sex Linkage - 1909: discovered by Morgan (worked with drosophila) Experiment: - in a large batch of red eyed flies, they found 1 white eyed fly (actually a mutation) P F 1 **R red, dominant r white, recessive ----------------red eye female x white eye male 100% red eyed hybrids (Rr) F 2 Mated F 1 3 : 1 ¾ red ¼ white **this confirmed Mendels work **BIG DISCOVERY - ALL WHITE EYES WERE MALES ** MORGAN DISCOVERED A SEX LINKED TRAIT

How is the gene for white eyes related to sex? ? ?

Sex Linked Genes (X linked) - genes carried by either sex chromosome (generally carried on X chrom, missing on Y chromosome)

Lets revisit the experiment by Morgan R R r P X X x X Y red female white male F 1 R r R X X or X Y (all red eyes) red hetero hemizygous female Hemizygous: dominant gene present and expressed, recessive gene missing

R Cross F 1: F 2 X X r X x X R Y 3 : 1 red white phenotype X R X r R Y R R R r X X R X Y r 1 homo red female 1 hetero red female (carrier) genotypes 1 hemi red male 1 hemi white male X Y This is known as “criss cross inheritance” P F 1 F 2 male female male (express) (carrier) (1/2 sons express)

What would happen in F 3 if hybrid red female was crossed with expressing male? R r r X X x X Y r X Y R r R R F 3 X X Y r r X X X X Y 1 carrier female 1 normal male 1 male expresses 1 female expresses

• Why is this important? - sex linked traits not limited to drosophila - occur in all species including humans

REGULATION OF GENE EXPRESSION Gene expression (protein expression) Process by which a gene’s information is converted into the structures and functions of a cell. dominance: protein works recessiveness: protein does not work

Exceptions to Mendelian Inheritance Incomplete dominance Active allele does not entirely compensate for inactive allele. - heterozygous phenotype is between two homozygous phenotypes - only one dominant allele is active **3 rd phenotype is mixture**

Codominance Both alleles of a gene are expressed. - both alleles are active and expressed - very common in many organisms **3 rd phenotype has original phenotypes**

Incomplete dominance Co-dominance

Polygenic inheritance Two or more genes responsible for a single trait. Ex: skin color (4 -7 genes) eye color (at least 8 genes)

Genomic Imprinting: phenotypic effect of gene depends on whether from M or F parent Methylation: silence genes by adding methyl groups to DNA

Non-Nuclear DNA • Some genes located in organelles – Mitochondria, chloroplasts, plastids – Contain small circular DNA • Mitochondria = maternal inheritance (eggs) Variegated (striped or spotted) leaves result from mutations in pigment genes in plastids, which generally are inherited from the maternal parent