1. Gregor Mendel and an Introduction to Punnett Squares

2. What is Heredity?
The passing on of characteristics from parents to offspring is called HEREDITY
The branch of biology that studies this passing on is called GENETICS.
Characteristics that are inherited are called TRAITS and are carried on GENES, made of DNA.

3. Mendel’s Pea Plants
A. Gregor MENDEL predicted how traits are passed using PEA
PLANTS
Mendel transferred pollen from one plant to female gamete of another with different trait = CROSS FERTILIZATION
Mendel was the 1st to conduct CONTROLLED genetics experiments = Father of Genetics
Only 1 trait at a time
Mathematical analysis
Plants came from fields of similar plants
Placed bag around plant

4. Mendel’s Pea Plants
B. First generation and second generations of Mendel’s plants
1st generation (F1) = Tall X Short = ALL TALL (short trait disappeared)
2nd generation (F2) crossed two of the offspring (self-pollinate)
75% of offspring were as tall as tallest parent
25% were as short as short plants from original plants
Mendel repeated experiment with SEVEN pairs of traits

5. What did Mendel Conclude?
A. Organisms have 2 factors for each trait = RULE OF UNIT FACTORS
Factors are called GENES and are located on CHROMOSOMES
Genes (segments of DNA) exist is alternative forms called ALLELES.
Alleles, genes and chromosomes are ALL made of DNA!
Mendel’s plants all had two alleles for height = Could be 2 tall, 2 short, or 1 tall and 1 short

6. What did Mendel Conclude?
B. Alleles are located on homologous chromosomes - one inherited from the FEMALE parent and one from the MALE parent
Only one trait is observed = DOMINANT
The disappeared trait (in F1 generation) = RECESSIVE
One allele (tall) dominated the other allele (short) = RULE OF DOMINANCE
The Law of Segregation
The 2 alleles for each trait SEPARATE when gametes are formed. (during meiosis/anaphase I)
Parent passes on at random only 1 allele per trait to each offspring
This conclusion = The Law of Segregation

7. Phenotypes and Genotypes
2 organisms can look alike but have different underlying gene combinations
The way an organism looks = PHENOTYPE
A tall plant’s phenotype is tall, regardless of genes
The gene combination of an organism = GENOTYPE
A tall plant’s genotype can be either TT or Tt
You cannot always know the genotype by looking at phenotype
Genotypes are designated as Homozygous or Heterozygous
If both alleles are the same = HOMOZYGOUS (also purebred)
TT = HOMOZYGOUS DOMINANT
tt = HOMOZYGOUS RECESSIVE
If they are different = HETEROZYGOUS (Tt) (also called hybrid)

8. Mendel’s Dihybrid Cross
Mendel then combined pea plants that differed in more than one trait (dihybrid cross = 2 traits)
First Generation
Crossed dominant round, yellow pea plants (RRYY) with homozygous recessive (rryy) wrinkled and green
The first generation was all ROUND AND YELLOW.
He then let F1 plants self-pollinate to make 2nd/F2 generation
Results: 9 yellow round: 3 round green: 3 wrinked yellow: 1 wrinkled green

9. Mendel’s Dihybrid Cross
Mendel concluded another law: LAW OF INDEPENDENT ASSORTMENT
Genes for different traits are inherited INDEPENDENTLY of each other
Alleles of same trait separate and alleles of other trait separate
Can recombine in FOUR different ways
This principle is only true if genes are located far apart on chromosomes or on different chromosomes (why??)
His work was supported by the model of DNA by Watson and Crick/Franklin and Wilkins and the MAPPING OF GENES on chromosomes.

10. Punnett Squares
Reginald PUNNETT (English) devised a short-hand way to find expected proportions of possible genotypes in the offspring cross.
This method is called the PUNNETT SQUARES
Takes into account fertilization happens at random (Mendel’s law of segregation)
If you know genotypes of parents, use Punnett Squares to predict GENOTYPES and PHENOTYPES of offspring.
Punnett Squares can also be used in TEST crosses to determine genotypes. (Cross unknown dominant with a known homozygous recessive)