Patterns of Inheritance Mendelian Genetics

Mendel’s Principles 1. Principle of Segregation 2. Principle of Independent Assortment Punnett Squares and Pedigrees

Genetics is the science of heredity.  The science of heredity dates back to ancient attempts at selective breeding.

Some definitions and rules  Allele : an alternative form of a single gene passed from generation to generation  Dominant : typically overshadows recessive allele, phenotype is seen  Capital letter represents dominance and phenotype  Example: Black is dominant over white so B for black  Recessive : hidden trait, unless both alleles are recessive  Lower case letter, same as dominant allele  Again black dominant over white so b for white BB Bb BB B b B = black fur B = white fur

Some definitions and rules  Homozygous : an organism with two of the same alleles for a particular trait  BB or bb  Heterozygous : an organism with two different alleles for a particular trait  Bb BB Bb BB B b B = black fur B = white fur

Mendel’s Principles Discovered Experimental genetics began in an abbey garden  Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants  Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation  Mendel studied seven pea characteristics

Stamen Carpel This illustration shows his technique for cross-fertilization 1 Removed stamens from purple flower White Stamens Carpel Purple PARENTS (P) OFF- SPRING (F 1 ) 2 Transferred pollen from stamens of white flower to carpel of purple flower 3 Pollinated carpel matured into pod 4 Planted seeds from pod

FLOWER COLOR FLOWER POSITION SEED COLOR SEED SHAPE POD SHAPE POD COLOR STEM LENGTH PurpleWhite AxialTerminal YellowGreen RoundWrinkled InflatedConstricted GreenYellow TallDwarf Seven pea characteristics

The parent generation is also known as the P generation.

Offspring of the P cross are the first filial (F 1 ) generation. (Filial: having or assuming the relation of a child or offspring)

Principle of Segregation  Mendel’s principle of segregation describes the inheritance of a single characteristic.  From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic.  One characteristic comes from each parent. P GENERATION (true-breeding parents) F 1 generation F 2 generation Purple flowersWhite flowers All plants have purple flowers Fertilization among F1 plants (F 1 x F 1 ) 3 / 4 of plants have purple flowers 1 / 4 of plants have white flowers

A sperm or egg carries only one allele of each pair.  The pairs of alleles separate when gametes form.  This process describes Mendel’s principle of segregation.  Alleles can be dominant or recessive.

P GENERATION (true-breeding parents) F 1 generation F 2 generation Purple flowersWhite flowers All plants have purple flowers Fertilization among F1 plants (F 1 x F 1 ) 3 / 4 of plants have purple flowers 1 / 4 of plants have white flowers GENETIC MAKEUP (ALLELES) Parent PLANTS F 1 PLANTS (hybrids) F 2 PLANTS PPpp All P All p All Pp 1/2 P1/2 P 1/2 p1/2 p Eggs P p P PP p Sperm Pp pp Gametes: sperm or egg carry only ONE allele Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp F 1 generation Gametes: sperm or egg carry only ONE allele

P GENERATION (true-breeding parents) F 1 generation F 2 generation Purple flowersWhite flowers All plants have purple flowers Fertilization among F1 plants (F 1 x F 1 ) 3 / 4 of plants have purple flowers 1 / 4 of plants have white flowers F 1 PLANTS (hybrids) P Generation Cross Punnett Square Genotypic Ratio? Phenotypic Ratio? F 1 Generation Cross Punnett Square Genotypic Ratio? Phenotypic Ratio?

Homologous chromosomes  Homologous chromosomes bear the two alleles for each characteristic.  Alternative forms of a gene (alleles) reside at the same locus (location) on homologous chromosomes. GENE LOCI PaB DOMINANT allele RECESSIVE allele Pab GENOTYPE: PPaaBb HOMOZYGOUS for the dominant allele HOMOZYGOUS for the recessive allele HETEROZYGOUS

Principle of Independent Assortment  The principle of independent assortment is revealed by tracking two characteristics at once.  Example: pea color and seed shape  By looking at two characteristics, Mendel found that the alleles of a pair segregate independently of other allele pairs during gamete formation  This is known as the principle of independent assortment

Geneticists use the testcross to determine unknown genotypes.  The offspring of a testcross often reveal the genotype of an individual when it is unknown. TESTCROSS: B_ GENOTYPES bb BBBb or Two possibilities for the black dog: GAMETES OFFSPRING All black1 black : 1 chocolate B b B b b Bb bb If all of the offspring are black, then dad was homozygous.

Mendel’s principles reflect the rules of probability.  Inheritance follows the rules of probability  The rule of multiplication and the rule of addition can be used to determine the probability of certain events occurring  Rule of Multiplication (F 1 s)  Rule of Addition (F 2 s) F 1 GENOTYPES Bb female F 2 GENOTYPES Formation of eggs Bb male Formation of sperm 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4 BB BB B B b b b b bb

Punnett Squares  Use Punnett Squares to visualize the probabilities of offspring genotypes.  Monohybrid cross : a cross that involves hybrids for a single trait.  Dihybrid cross : the simultaneous inheritance of two or more traits in the same organism.

Monohybrid Cross F1 and F2 Generations for Pea Color

Dihybrid Punnett Square

Let’s Practice!  Punnett Squares Packet  Read Example One. Discuss:  Law of Segregation  Conventions  Genotypic Percents  Phenotypic Percents  Monohyrid Practice Problems (1-5)  ADVANCED: Turn to Example 3 and try the Dihybrid Crosses.