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Gregor Johann Mendel. Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants He found that the plants' offspring retained traits of the parents. Gregor Mendel. Called the “Father of Genetics
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Gregor Johann Mendel • Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants • He found that the plants' offspring retained traits of the parents
Gregor Mendel • Called the “Father of Genetics • Gregor Mendel (1860’s) discovered the fundamental principles of genetics by breedinggarden peas.
Pea Garden (Pisum sativum) • Easy to grow and can be grown in a small area • Produce lots of offspring • Produce pure plants when allowed to self-pollinate several generations (true breeding varieties) • Clearly defined characteristics or traits • Easy to be crossed between parents
Mendel cross-pollinated pea plants • Mendel probably chose to work with peas because they are available in many varieties. • The use of peas also gave Mendel strict control over which plants mated. • Fortunately, the pea traits are distinct and were clearly contrasting.
Mendel’s experimental design • Statistical analyses: • Worked with large numbers of plants • counted all offspring • made predictions and tested them • Excellent experimentalist • controlled growth conditions • focused on traits that were easy to score • chose to track only those characters that varied in an “either-or” manner
Typical breeding experiment P generation (parental generation) F1 generation (first filial generation, the word filial from the Latin word for "son") are the hybrid offspring. Allowing these F1 hybrids to self-pollinate produces: F2 generation (second filial generation).
Mendel Conclusion Factors are passed from one generation to the next.
Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds)
eye color locus B = brown eyes eye color locus b = blue eyes Paternal Maternal The Principle of Dominance
Dominant and Recessive alleles Dominant alleles – upper-case (B) a. homozygous dominant (BB – Brown eyes) Recessive alleles – lower case (b) a. homozygous recessive (bb – blue eyes) b. heterozygous dominant (Bb – Brown eyes)
Outward appearance Physical characteristics Examples: 1.Brown eyes 2.blue eyes Arrangement of genes that produces the phenotype Example: 1. TT, Tt 2. tt Phenotype vs. Genotype
Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.
Punnett Squares • Diagram used to predict genetic crosses • Tool for calculating genetic probabilities • A tool to predict the probability of certain traits in offspring that shows the different ways alleles can combine. • Diagram showing the probabilities of the possible outcomes of a genetic cross
How to use Punnett Squares • Choose a letter to represent the alleles in the cross. • Write the genotypes of the parents. • Determine the possible gametes (reproductive cells) that the parent can produce. • Enter the possible gamete at the top and side of the Punnett square. • Complete the Punnett square by writing the alleles from the gametes in the appropriate boxes. • Determine the phenotypes of the offspring.
Punnet Square Process • Determine alleles of each parent, these are given as TT, and tt respectively. • Take each possible allele of each parent, separate them, and place each allele either along the top, or along the side of the punnett square.
Punnett Square Process • Lastly, write the letter for each allele across each column or down each row. The resultant mix is the genotype for the offspring. In this case, each offspring has a Tt (heterozygous tall) genotype, and simply a "Tall" phenotype.
Punnett Square Process • Lets take this a step further and cross these F1 offspring (Tt) to see what genotypes and phenotypes we get. • Since each parent can contribute a T and a t to the offspring, the punnett square should look like this….
Punnett Square Process • Here we have some more interesting results: First we now have 3 genotypes (TT, Tt, & tt) in a 1:2:1 genotypic ratio. We now have 2 different phenotypes (Tall & short) in a 3:1 Phenotypic ratio. This is the common outcome from such crosses. Monohybrid cross (cross with only 1 trait)
Testcross Cross the dominant phenotype (unknown genotype) with the recessive phenotype (known genotype).
Dihybrid cross The cross with a pure-breeding (homozygous) two loci. F1 generation
Dihybrid cross • Take the offspring and cross them since they are donating alleles for 2 traits, each parent in the f1 generation can give 4 possible combination of alleles. TW, Tw, tW, or tw. F2 Generation
Dihybrid cross • Note that there is a 9:3:3:1 phenotypic ratio. 9/16 showing both dominant traits, 3/16 & 3/16 showing one of the recessive traits, and 1/16 showing both recessive traits. • Also note that this also indicates that these alleles are separating independently of each other. This is evidence of Mendel's Law of independent assortment
Mendel’s Principles • The inheritance of biological characteristics are determined by genes. • For two or more forms of a gene, dominance and recessive forms may exist. • Most sexually reproductive organisms have two sets of genes that separate during gamete formation. • Alleles segregate independently.
Law of Independent Assortment • Alleles for different traits are distributed to sex cells (& offspring) independently of one another. • Different genes on different chromosomes segregate into gametes independently of each other
Mother Father e E Replication e e E E e e E E OR n N N n n n N N e e E E N n N N n n Independent Assortment Alignment of Homologs at Metaphase I Telophase II
Hypothetical example of independent Assortment Eye color Hair color Gene for browneyes Gene for blueeyes Gene for black hair Gene for red hair
Independent Assortment OR Meiosis I & II Brown eyesBlack hair Blue eyesRed hair Blue eyesBlack hair Brown eyesRed hair
Three Conclusions of Mendel Experiment • Principle of Dominance and Recessiveness One allele in a pair may mask the effect of the other • Principle of Segregation The two alleles for a characteristic separate during the formation of eggs and sperm • Principle of Independent Assortment The alleles for different characteristics are distributed to reproductive cells independently.
Variations on Mendel’s Laws The relationship of genotype to phenotype is rarely simple • Mendel’s principles are valid for all sexually reproducing species • But genotype often does not dictate phenotype in the simple way his laws describe • There is an exceptional to Mendel Laws
Incomplete dominance Codominance Multiple alleles Polygenic traits Epistasis Pleiotropy Environmental effects on gene expression Linkage Sex linkage Exceptions To Mendel’s Original Principles
P Generation White CWCW Red CRCR Gametes CR CW Pink CRCW F1 Generation 1⁄2 1⁄2 Gametes CR CR 1⁄2 Sperm 1⁄2 CR CR Eggs F2 Generation 1⁄2 CR CR CR CR CW 1⁄2 Cw CW CW CR CW Incomplete dominance • The phenotype of the heterozygote is intermediate between those of the two homozygotes. • 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:
Incomplete Dominance Gametes CR CW CRCR CR CRCR CRCW Gametes CW F1 generation All CRCW CRCW CWCW F2 generation CWCW 1 : 2 : 1
Co-dominance Phenotype of both homozygotes are produced in heterozygotes individuals. Both alleles are expressed equally. Examples: Roan Cattle White-feathered birds are both homozygotes for both B and W alleles
Multiple Alleles • More than three alleles for a gene • Found among all individuals in a population • Diploid individuals only have two of the alleles • Phenotype depends on relationship between different pairs of alleles • Still follows Mendel’s principles
Multiple Alleles Small differences in DNA sequences result in multiple alleles
Human ABO Blood Group • Antigens • Glycoproteins on surface of red blood cells • IA allele produces A antigen (dominant) • IB allele produces B antigen (dominant) • i allele produces neither A nor B (recessive) • Blood types (phenotypes) • IAIA or IAi = type A blood • IBIB or IBi = type B blood • ii = type O blood • IAIB = type AB blood
Universal recipients Universal donors
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. • Allele of one locus inhibits or masks effects of allele at a different locus • Some expected phenotypes do not appear among offspring • 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:
Labrador Retrievers • Melanin pigment gene • B allele: black fur color (dominant) • b allele: brown fur color (recessive) • Pigment deposition gene • E allele: pigment deposition normal (dominant) • e allele: pigment deposition blocked (recessive) • Phenotypes • Black fur: BB EE, BB Ee, Bb EE, Bb Ee • Brown fur: bb EE, bb Ee • Yellow fur: BB ee, Bb ee, bb ee
Polygenic Inheritance • Most traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. These are called polygenic traits. • Several genes at different loci interact to control the same character • Produces continuous variation • Phenotypic distribution: Bell-shaped curve • Often modified by environmental effects
