1. Segregation and Assortment BSC 315-001

1a. Mendel’s Law of Segregation

Trait: A particular characteristic of the organisms, eg., seed color, flower color, height, etc.

Phenotype: Appearance or form of physiological state with a trait. Genotype: description of the allele composition of an individual

o Every organisms has two* copies of every gene Exceptions will be discussed later

o If for gene “A” there are two alleles, A and a, then any individual has one of the three possible genotypes: AA, Aa, or aa.

o Homozygous genotype: two of the same allele. AA and aa.o Heterozygous genotype: two different alleles. Aa.

Genotype/Phenotype relationships Depends on the trait and the pair of alleles being considered Strict dominance/recessiveness

o Heterozygous genotype has the same phenotype as one of the homozygous genotypes

o Example: Aa has the same phenotype as AA The A allele is dominant; the a allele is recessive

o Example: Seed color (trait) in peas Two phenotypes:

Yellow: Genotypes YY and Yy Green: Genotype yy

Yellow is the dominant phenotype. The Y allele is dominant to the y allele.

Mendel Recognized the relationship between genotype and phenotype ~150 years ago Austrian monk; published studies on pea genetics in 1866 Started with pure-breeding strains of peas

o Pure-breeding: strain that produces the same phenotype generation after generation

Mendel’s observations:o Cross between two pure-breeding strains all of the progeny have one of

the two phenotypes.o When progeny are self-crossed both phenotypes are present in the second

generation progeny in a 3:1 ratio, ie., ¾ of one and ¼ of the other Self-cross: self-mating, or cross to a genetically identical individual.

o Only one of the phenotypes is observed in the first generationo Both phenotypes are observed in the second generation, in a 3:1 ratio

Mendel’s theoryo 1. Genotype/phenotype

Peas have 2 copies of each gene (diploid) Genes come in variants (alleles)

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1. Segregation and Assortment BSC 315-001

The Yy and YY genotypes have the same phenotype (complete dominance), but are different from the yy phenotype.

o 2. Segregation Gametes (egg/sperm) have only one copy of each gene (haploid) Fertilization occurs at random and restores the diploid gene number

Mendel’s crosseso Generations in a cross are named:

P (=parental) F1 (=first filial) F2 (=second filial) “x” indicates a mating

o Mendel’s P: Yellow seeds x green seeds (YY x yy) F1: all yellow seeds (all Yy) F2: ¾ yellow; ¼ green (YY and Yy and yy)

o Nomenclature for crosses: Hybrid: offspring of two true-breeding strains Monohybrid cross: one trait analyzed Dihybrid: two traits Trihybrid: three traits,etc.

Mendel’s Theory of Genetics (Law of Segregation) o Law of segregation: the two alleles of a gene segregate from each other

during gamete formation, and unite at random at fertilization, restoring the diploid chromosome number.

Mendel’s Test of his Theoryo Prediction: the yellow F2s contain both YY and Yy, in a 1:2 ratioo Test: determine the genotype of yellow F2s

Test cross: a cross to determine the genotype of an unknown individual

Procedure: cross unknown with homozygous recessive (tester strain)

1b. Independent Assortment

Law of Independent Assortment: Gametes carrying “Y” contain either “R” or “r” with equal probability. Ditto for gametes with allele “y.”

Dihybrid cross: two traits o P: Yellow round x green

wrinkled (YY RR x yy rr)o F1: yellow round x self (Yy

Rr x Yy Rr)o F2: 9 different genotypes, 4

different phenotypes (9:3:3:1 ratio)

9/16 Round; Yellow (R-Y-)

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1. Segregation and Assortment BSC 315-001

3/16 Round; Green (R-yy) 3/16 Wrinkled; Yellow (rrY-) 1/16 Wrinkled; Green (rryy)

Product rule: the fraction of cases in which two events both occur is the product of the fractions of each individual event.

o Example: In a self cross of F1 round yellow dihybrids, what fraction of F2s have round yellow phenotype?

Fraction of F2s with round yellow phenotypes=(fraction round)(fraction yellow)=(3/4)(3/4)=9/16

Fraction of F2s with genotype RRyy (fraction RR)(fraction yy)= fraction RR yy=(1/4)(1/4)=1/16

Fraction of F2s with genotype RrYy (fraction Rr)(fraction Yy)= fraction

RrYy=(1/2)(1/2)=1/4 Gamete types

o For “N” heterozygous genes, the number of genetically different gametes is 2N

o How many genetically different gametes are produced by an individual AaBbCcDd?

N=4, so 24=16o How many genetically different gametes are produced by

an individual AaBBCcDdee? N=3, so 23=8

o A trihybrid F1 individual (AaBbCc) is self-crossed. What fraction of the F2 individuals

(A-)(B-)(C-)=(3/4)(3/4)(3/4)=27/64 A-B-C-o What fraction will have the genotype AABbcc

(AA)(Bb)(cc)=(1/4)(1/2)(1/4)=1/32 Branch diagram method aaBBCcDdee x AaBbccDdee

o What fraction of the progeny have the same phenotype as the first parent? (1/2)(1/2)(1/2)(1/2)(1)

o What fraction have the same genotype as the first parent?

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