Genetics and Mendel

Gregor Mendel Austrian Monk Studied peas 1860 (Darwin’s voyage ended 1836) Developed laws of inheritance of traits Did not know about genes or DNA

Mendel’s Experiments

Mendel’s Peas After many test crosses of plants Mendel decided some traits were dominant (expressed) and some traits were recessive (not expressed) but still resided in the organism

Law of Dominance Only the dominant trait will appear in the offspring The other trait does not appear and is recessive When crossing two purebred individuals:

Mendel’s Laws of Inheritance Each individual has two factors for each traitEach individual has two factors for each trait The factors segregate (separate) during the formation of gametesThe factors segregate (separate) during the formation of gametes Each gamete contains only one factor from each pair of factorsEach gamete contains only one factor from each pair of factors Fertilization gives each new individual two factors for each traitFertilization gives each new individual two factors for each trait Factors sort independently of each otherFactors sort independently of each other All possible combinations of factors can occur in the gametesAll possible combinations of factors can occur in the gametes

Modern Genetics Each person has two alleles for each trait, one on each homologous chromosome Meiosis separates the alleles when gametes are formed Each gamete receives one of the alleles Fertilization gives each new individual two alleles for each gene of a trait During metaphase I the tetrads arrange randomly at the equatorial plane All possible combinations are in the gametes

Alleles Alternate forms of the same gene Dominant alleles – – –expressed – –capital letter used Recessive alleles – – –suppressed by the dominant allele – –not expressed – –Lower case letter used Gene locus - – –specific location on homologous chromosomes

Homozygous vs. Heterozygous Homozygous – – –Two identical alleles for the trait Heterozygous – – –Two different alleles for the trait Homozygous dominant for shape Heterozygous for pod color

Genotype vs. Phenotype Genotype –Genotype – –Alleles received –Genetic code of the individual PhenotypePhenotype –Physical appearance –Gene expression Genotype is homozygous dominant for shape Phenotype is Round Genotype is heterozygous for color Phenotype is Green pod color

Genotype vs. Phenotype Genotypes? Phenotypes? Homozygous recessive Heterozygous Wrinkled Green pod color

HumansHumans Alleles for finger length: –Short is dominant –Long is recessive What letter will indicate the dominant allele?What letter will indicate the dominant allele? S (capital) What letter will indicate the recessive allele?What letter will indicate the recessive allele? s (lower case) Genotype vs. Phenotype

What are the possible genotypes and phenotypes? Genotypes Phenotypes Genotypes Phenotypes Genotype vs. Phenotype SS Ss ss Short Fingers Long Fingers

Mendel discovered that tall pea plants (T) were dominant over short pea plants (t).Mendel discovered that tall pea plants (T) were dominant over short pea plants (t). If the phenotype of a pea plant is short, what is its genotype?If the phenotype of a pea plant is short, what is its genotype? Explain:Explain: The short gene is recessive. To be expressed, both recessive genes must be in the genotype Genotype vs. Phenotype tt

Predicting Results Punnett Square: predicts the probability of the offspring having a specific genotype and phenotype STEPS: 1) Assign a symbol for each allele 2) Determine genotype of each parent 3) Determine all possible gametes each parent can produce (set up Punnett square) 4) Determine all allele combinations of offspring 5) Determine phenotype of each offspring from genotype

Given: Purple flowers are dominant over white flowers Two heterozygous purple flowers are crossed Predicting Results 1)Assign a symbol for each allele Purple: P White: p 2)Determine genotype of each parent Heterozygous: Pp

Given: Purple flowers are dominant over white flowers Two heterozygous purple flowers are crossed Predicting Results 3) Determine all possible gametes each parent can produce 4) Determine all allele combinations of offspring Father’s gametes Mother’ s gametes P P p p p p ppP PPP First write the letters at the top of each column in each square below Then write the letters at the beginning of each row in each square across Separate and write one parent’s alleles across the top of each column Place the other parent’s alleles down the side in front of each row

Predicting Results Given: Purple flowers (P) are dominant over white flowers (p) Two heterozygous purple flowers are crossed P P p p p p ppP PPP 5) Determine phenotype of each offspring from genotype PP = 1 Purple Pp = 2 Purple pp = 1 White What percentage of offspring are purple? What percentage of offspring are white? ¾ = 75% ¼ = 25%

Genotypes vs. Phenotypes Which color is dominant? Which color is recessive? Which plants are homozygous? Which plants are heterozygous?

Generations The original parents: – –Called the P generation – –Produce P gametes Offspring of P generation: –Called the F 1 generation –Produce F 1 gametes

Two Trait Inheritance Yellow (YY) Round (RR) (YYRR) Green (yy) Wrinkled (rr) (yyrr) P gametes YRyr YyRr F 1 Generation Only combinations possible since they are homozygous for both traits

Two Trait Inheritance Yellow, Round (YyRr) F 1 gametes: YR Yr yR yr YR Yr yR yr Y Y Y Y Y Y Y Y RR RR RR RR r r r r r r r r y y y y y y y y Y R Y R Y r Y r y R y R y r y r

F 2 Generation YR Yr yR yr YYRRYYRR YYRrYYRr YyRRYyRR YYrrYYrrYyRrYyrr YyRrYyRrYyRRYYRr YyRrYyRr yyRRyyRR YyrrYyrr YyRr yyrryyrryyRryyRr yyRr How many: Yellow Smooth? Yellow wrinkled? Green Smooth? Green wrinkled? 93 31

What to Know: Know and apply terminology for geneticsKnow and apply terminology for genetics Describe genotypes and phenotypesDescribe genotypes and phenotypes Identify and explain dominant and recessive traitsIdentify and explain dominant and recessive traits Identify gametes of parents for one and two traitsIdentify gametes of parents for one and two traits Set up and complete Punnett squares for both one and two traitsSet up and complete Punnett squares for both one and two traits