 DNA – double helix DNA coils to form a chromosome You receive 1 set of chromosome from mom & one from dad Chromosomes have areas that code for a gene 1 gene = 1 protein The Central Dogma!

Number of Genes on Chromosomes

Introduction to Genetics Chapter 9 Section 1 – Mendel’s Legacy Section 2 – Genetic Crosses

Mendel’s Legacy  Gregor Mendel was a priest in the mid 19 th Century.  Studied different pea plants  Father of GENETICS – the scientific study of heredity  What is heredity?-passing down of physical characteristics from parent to offspring

Mendel’s Experiments  Self pollinated  Developed a method of cross pollination  P generation, F 1 generation, F 2 generation TRAITS: plant height, flower position, pod color, pod appearance, seed texture, seed color, flower color

The Outcome of Mendel’s Work  In all of Mendel’s experiments  First generation ONLY produced one form of the trait (i.e. tall)  Second generation the “lost” form the trait reappeared in about ¼ of the plants

Mendel’s Findings TODAY  Factors that control the traits are called GENES.  Alleles – contrasting forms of a gene  i.e. Tall vs. Short; round vs. wrinkled  Inherited from parents  Dominant Allele – trait always shows up when the allele is present  Recessive Allele – “masked” or hidden by the dominant trait

 TERMS TO KNOW  Phenotype – the physical characteristic displayed by the organism (PHYSICAL = PHENOTYPE)  Genotype – the actual combination of alleles held by the organism (GENOTYPE = GENES) Genotype = Rr Phenotype = Red

Mendel concluded that the paired “factors” or alleles separate when making reproductive cells  This means that each gamete only gets one copy of each allele  When fertilized, the offspring would have 2 copies Law of Segregation  Alleles are segregated or separated during the formation of gametes.

Mendel also observed that dominant traits do not always appear together.  Mendel concluded that these factors are not connected  Law of Independent Assortment  “Factors” or alleles for different characteristics are distributed to gametes independently  Explain this in YOUR words

Representing Alleles  Use letters to represent alleles  Use the same letter for one trait (T for height)  Capital letter for dominant (T = Tall)  Lowercase letter for recessive letter (t = short) TT tTt t Purebred Tall Parent Purebred Short Parent Hybrid Offspring

Genetic Crosses  What is the chance that a coin will land on heads?  Probability – a number that describes how likely it is that an event will occur.

Interdependence of Events  If you toss a coin once and then toss again, the first toss does not affect the second  Each TIME you toss the coin, you have a 1 in 2 chance of getting heads.

Probability & Genetics  How is probability related to Genetics? (Think about Mendel’s results)  Each time Mendel repeated his experiment, he got similar results. WHY?  Mendel was the first to discover that you could PREDICT the outcome of a cross. Purebred Tall Plant X Purebred Short Plant ALL TALL PLANTS

Punnett Squares  Tool used by scientists to determine the outcome of a genetic cross  Shows all the possible combinations of offspring TT tTt t Purebred Tall Parent Purebred Short Parent Hybrid Offspring OUTCOME: TALL OFFSPRING

Monohybrid Crosses  Involves ONE PAIR of contrasting traits  Homozygous – purebred (has the same type of alleles)  Heterozygous – hybrid (has two types of alleles) TT t t Genetic cross between a tall plant and a short plant Genetic cross between 2 heterozygous tall plants Tt T t

 TERMS TO KNOW  Phenotype – the physical characteristic displayed by the organism (PHYSICAL = PHENOTYPE)  Genotype – the actual combination of alleles held by the organism (GENOTYPE = GENES) Genotype = Rr Phenotype = Red

Ratio’s  Phenotypic Ratio – the ratio of phenotypes that appear in the offspring  Genotypic Ratio – the ratio of genotypes that appear in the offspring Genotypic Ratio = 4:0 Phenotypic Ratio = 4:0

Monohybrid Punnett Square Review  Heterozygous Tall x Heterozygous Tall  Give phenotypic and genotypic ratios  TT x Tt  Give phenotypic and genotypic ratios

 Dihybrid Crosses  Involves TWO PAIRS of contrasting traits  Homozygous – purebred (has the same type of alleles)  Heterozygous – hybrid (has two types of alleles) Genetic cross between a Purebred tall plant, purple flower and a Purebred short plant, white flower TTPP x ttpp What are the possible gametes? (ALL THE POSSIBLE COMBINATION OF GAMETES)

Determining Gametes – all possible combinations PARENT 1 TtPp TP TptP tp PARENT 2 TtPp TP TptP tp

What are the possible gametes for these parents? Heterozygous Tall plant w/ purple flower (TtPp) x Heterozygous Tall plant w/ purple flower (TtPp) TPTptPtp TP Tp tP tp

Other Types of Dominance  Incomplete Dominance  Two or more alleles influence the phenotype  An intermediate phenotype results; a blending of traits Red Flower x White Flower (RR) (rr) Pink Flower (Rr)

Red Flower x White Flower Outcome = ALL PINK FLOWERS RR rRr r Incomplete dominance Punnett Squares Pink Flower x Pink Flower Outcome = Red, White & Pink Flowers Rr RRRRr r rr

Other Types of Dominance  Codominance  Two or more alleles are expressed in the phenotype  A combination phenotype results Red Cow x White Cow (RR) (WW) Roan Cow (RW)

Red Cow x White Cow Outcome = ALL ROAN COWS RR WRW W Codominance Punnett Squares Roan Cow x Roan Cow Outcome = Red, White & Roan Cows RW RRRRW W WW