Mendel & Inheritance Sherry Flint-Garcia USDA ARS MU Division of Plant Sciences

Genetic variation In the beginning, geneticists studied differences they could see (i.e. morphological differences). Individual variants are referred to as phenotypes. –tall vs. short corn plants –red vs. green grapes Source: USDA Source: MGDB

Trait A broad term encompassing a distribution of phenotypic variation. Two types of traits: Qualitative or Discrete Trait: Kernel Color Phenotype: Purple vs. Yellow Governed by one or few genes Quantitative or Continuous Trait: Corn Rootworm Resistance Phenotype: 1 (no damage) to 6 (severe damage) Usually governed by many genes Source: USDA Source: MGDB

Advantages of Plants Can make controlled crosses. –Including making inbreds (in many plant species) without the severe effects that are typical in animals. One plant can produce tens to hundreds of progeny, and we can store the progeny for long periods of time. Less costly and time consuming to maintain than animals. –Generation time is often much shorter. Source: MGDB

Pre-Mendel Blending Theory –Traits present in the parents are mixed (or blended) in offspring. Inheritance of Acquired Characteristics –Traits present in parents are modified, through use, and passed on to the offspring in their modified form They didn’t know about genes or DNA!!!!

Gregor Mendel Performed several experiments between 1856 and 1863 that are the basis for what we know about heredity today. Published his work in His papers were largely ignored for more than 30 years until other researchers appreciated the significance of his work. Described the behavior of particulate (discrete) bodies = genes.

Garden Pea Pisum sativum Many morphological differences. Diploid with short generation time. Ability to make controlled pollinations. Each phenotype Mendel studied was controlled by a single gene.

Terms Wild-type is the phenotype that would normally be expected. Mutant is the phenotype that deviates from the norm: it is unexpected but heritable. –Notice that this definition does not imply that all mutants are bad. In fact, many beneficial mutations have been selected by plant breeders. Phenotype is the observable or measurable characteristic of an individual. Genotype is the genetic constitution of an individual.

Terms continued… An allele is a particular form of a gene. –The “tall” gene has two alleles: T (tall) & t (short) A homozygous individual (homozygote) has two identical alleles, e.g. TT or tt. A heterozygous individual (heterozygote) has two different alleles, e.g. Tt. The dominant phenotype is expressed in either a homozygous or heterozygous genotype. –TT and Tt condition tall plants. Tall is dominant over short. The recessive phenotype is only expressed in a homozygous genotype. –tt conditions short plants. Short is recessive to tall.

Mendel’s Experiment Parental (Inbred) Lines Round Wrinkled × ALL Round F 1 progeny 3 Round : 1 Wrinkled Self-pollinate Round 5474 Wrinkled 1850 F 2 progeny Since the F 1 progeny are all round, we know that the round phenotype is dominant and the wrinkled phenotype is recessive.

Mendel’s Results Parental CrossF 1 PhenotypeF 2 data Round x wrinkled seedRound5474 : 1850 Yellow x green seedYellow6022 : 2001 Purple x white flowerPurple705 : 224 Inflated x constricted podInflated882 : 299 Green x yellow podGreen428 : 152 Axial x terminal flowerAxial651 : 207 Long x short stemLong787 : 277 All approximately 3 : 1 The 3 : 1 ratio is the key to interpreting Mendel’s data.

Important Observations F 1 progeny are heterozygous, but express only one phenotype, the dominant one. In the F 2 generation, plants with both phenotypes are observed: i.e. some plants have recovered the recessive phenotype. In the F 2 generation, there are approximately three times as many of one phenotype as the other.

Mendel’s Conclusions Law of Unit Character –Characteristics of an individual are controlled by hereditary factors (genes) that maintain their characteristics when passed down. These factors occur in pairs. Law of Dominance –Some inherited factors are dominant and can mask the other, recessive factor. Aa

Mendel’s First Law: Law of Segregation Each gamete contains only one factor from each pair of factors (from the parent). The pair of factors from a parent separate (or segregate) during reproduction, such that an offspring receives one factor from each parent. Each gamete is equally likely to contain either factor of the pair. Fertilization restores the paired condition in the offspring.

First Law  Meiosis Cellular division resulting in four gametes, each possessing half the number of chromosomes of the original cell. A B a b A A B B a a b b A A b b a a B B A b a B a B A b “Pair of Factors” “Factor” “Segregation of Factors”

Round vs. Wrinkled The wild-type allele (W) is round, and the mutant allele (w) is wrinkled. W is dominant to w. The enzyme SBEI (starch-branching enzyme) makes branched starch (amylopectin). Round peas (W-) have a functional SBEI, resulting in amylopectin. Wrinkled peas (ww) have a non- functional SBEI and, therefore, do not make amylopectin. Amylopectin causes the seed to shrink uniformly. As a result, round peas shrink uniformly and remain round. Wrinkled peas shrink non-uniformly and become ‘wrinkled.’

Punnett Square Grid used to determine the results of simple genetics crosses. WwWwWwWww WwWwWwWww WW Male Female WW ww × WwWw Round Wrinkled Round FemaleMale All Round (Ww)

Punnett Square WwWwWwWww WwWwWwWww WW Male Female WwWwWwWww WwWwWwWww WW Male Female WW ww × WwWw Round Wrinkled Round FemaleMale WW ww WwWw Round Wrinkled Round round : 1 wrinkled (phenotypic ratio) Self-pollinate 1 WW : 2 Ww : 1 ww (genotypic ratio)

Chi-Squared (  2 ) Analysis Tests if your observations are statistically different from your expectation. For example, does the Mendel data (round vs wrinkled) fit the 3:1 hypothesis?  2 =  [(observed-expected) 2 /expected] Observed Expected Expectedobs - exp (obs – exp) 2 Data RatioData Round Wrinkled Total73244  2 = 722 Look up in  2 tables…The difference between observed and expected is NOT significant. Therefore, the observed data fit the 3:1 hypothesis.

Testcross / Backcross Used to determine whether a dominant individual is homozygous or heterozygous. –Cross the dominant individual to a homozygous recessive individual. UNKNOWN could be WW or Ww ww × ALL round Ww Wrinkled Round WwWw Two possible outcomes: ww Wrinkled Unknown was WW Unknown was Ww 1 : 1

Mendel’s Second Law: Law of Independent Assortment Each pair of factors (genes) segregates (assorts) independently of the other pairs in a di-hybrid (tri-hybrid, etc.) cross. Different pairs of alleles are passed to offspring independently so that new combinations are possible. Example: yellow seed color in pea plants can appear in combination with either the round or wrinkled phenotype.

Independent Assortment During Meiosis-1 A B a b A A B B a a b b A A b b a a B B A a b B A A b b a a B B A a B b OR

W = round; w = wrinkled; G = yellow; g = green Di-hybrid (2-Gene) Cross Round Yellow Wrinkled Green All F 1 are Round, Yellow Round Yellow 315 Round Green 108 Wrinkled Yellow 101 Wrinkled Green 32 × Self-Pollinate W is dominant to w; G is dominant to g.

Take It One Gene at a Time Round Yellow 315 Round Green 108 Wrinkled Yellow 101 Wrinkled Green 32 Round = 423 Wrinkled = 133 Yellow = 416 Green = 140 Each gene has a 3 : 1 ratio.

Put Two Genes Together WwGg WG ¼ Wg ¼ wG ¼ wg ¼ F1F1 Gametes & Frequencies If a gamete contains W, the probability that it contains G is equal to the probability that it contains g (i.e. W/w and G/g are independent of each other).

The BIG Punnett Square WGWgwGwg WGWWGGWWGgWwGGWwGg WgWWGgWWggWwGgWwgg wGWwGGWwGgwwGGwwGg wgWwGgWwggwwGgwwgg

Trait-Based Punnett Square Yellow ¾ Green ¼ Round ¾ Round, Yellow ¾ x ¾ = 9/16 Round, Green ¾ x ¼ = 3/16 Wrinkled ¼ Wrinkled, Yellow ¼ x ¾ = 3/16 Wrinkled, Green ¼ x ¼ = 1/

Violations of Mendel’s First Law Non-Dominant Gene Action –Partial or incomplete dominance Flower color in snapdragon –Co-dominance ABO blood groups Sex-linked inheritance –Human red-green colorblindness Polygenic Inheritance –Human skin color "Ishihara Color Vision Test," developed by Dr. Shinobu Ishihara Copyright © 2006 Pearson Prentice Hall Inc ABCABcAbCAbcaBCaBcabCabc ABC ABc AbC Abc aBC aBc abC Abc

Violations of Mendel’s First Law Organellar Inheritance (Maternal Effect) –Mitochondria/chloroplast inherited from mother Epigenetic –Imprinting, paramutation, gene silencing Penetrance, e.g. hemachromatosis Expressivity, e.g. timing of onset or severity Epistasis –Interaction between different genes and alleles one gene masks the effect of another gene two gene pairs complement each other

Violations of Mendel’s Second Law Linkage –Genes are physically linked to one another on chromosomes –Genes that are close together will NOT assort independently Deviation from the 9:3:3:1 ratio –Recombination (during meiosis) may break these linkages Recombination is a function of distance ACAC ACAC B B acac acac b b LMNOP lmnop

Summary of Mendel Inherited traits are controlled by factors (genes) present in the gametes. In a diploid, the pair of factors segregate during gamete formation, and progeny inherit one factor of each pair of factors (one allele) from the mother and one from the father. Each pair of factors assorts independently, such that new combinations are possible in progeny. With some exceptions (e.g. due to epigenetics and linkage), Mendel established the foundation of modern genetics.