Mendelian Genetics

VocabularyHeredityGeneticsTraitGametePollinationFertilization Law of segregation Law of independent assortment

Dominant

Recessive

Hybrid

Phenotype

Genotype

Homozygous

Heterozygous

Allele

Genetics

• Scientific study of heredity

• Heredity-passing of traits from parents to offspring

• Trait -characteristic that is inherited

• Many had realized family resemblance but didn’t know how

Gregor Mendel

•Austrian monk

•1822-1884

•Came up with the laws of heredity

•Importance of work not realized until 1900

Mendel’s Work

• First to succeed in predicting how traits would be transferred from one generation to the next

• Used garden peas (reproduced sexually via gametes)

• Pollination -transfer of male pollen grains to the pistil of flower (female)

Pollination

• Results in fertilization (fusion of male and female gametes)

• Pea plants normally went through self-pollination (gametes from self)

• Mendel needed to cross pollinate

• Taking pollen from one plant and using it to fertilize another plant

Monohybrid Crosses

• Crosses where the parents only differed by one trait (mono)

• Hybrid -offspring formed by parents having different forms of a trait

• First trait he looked at was height

First Generation

• Six-foot-tall pea plant that came from a population of pea plants, all of which were over six feet tall

• Short pea plant that was less than two feet tall and which came from a population of pea plants that were all short

• Cross-pollinated the two plants

Result

• All of the offspring grew to be as tall as the taller parent

• It was as if the shorter parent had never existed

• Ratio of 4:0

Second Generation

• Allowed the tall plants in this first generation to self-pollinate

• Counted more than 1000 plants in this second generation

• ¾ of the plants were tall• ¼ of the plants were short• Ratio of approximately three tall

plants to one short plant (3:1)

Generations

• Original parents, the true-breeding tall and short plants, are known as the P1 generation (P- parent)

• Cross of P1 are known as the F1 generation (F- filial or son/daughter)

• Cross two F1 plants with each other equals F2 generation

Further Experimentation

• Did similar monohybrid crosses with a total of seven pairs of traits

• Studying one pair of traits at a time

• In every case, he found that one trait of a pair seemed to disappear in the F1 generation, only to reappear unchanged in one-fourth of the F2 plants

Genes

• Located on chromosomes, units of heredity

• The rule of unit factors

• Each organism has two factors that control each of its traits

• Genes exists in alternative forms

Alleles

• Alternative forms of a gene for each variation of a trait of an organism

• Could have 2 alleles for tallness

• 2 alleles for shortness • 1 allele for tallness and 1 for

shortness • Receive 1 allele from each

parent

The Rule of Dominance

• One gene in a pair masks another

• Dominant -observed trait of an organism that masks the recessive form of a trait

• Recessive -trait of an organism that can be masked by the dominant form of a trait

How to Write the Alleles

• Each gene is expressed as a letter

• Dominant allele is expressed as a uppercase letter

• Recessive allele is expressed as a lower case letter

• Previous example: T for tall, t for short

Law of Segregation

• 1. There are alternative forms for genes.

• 2. For each characteristic or trait organisms inherit two alternative forms of that gene, one from each parent. These alternative forms of a gene are called alleles.

Law of Segregation

• 3. When gametes (sex cells) are produced, allele pairs separate or segregate leaving them with a single allele for each trait.

• 4. When the two alleles of a pair are different, one is dominant and the other is recessive.

Phenotypes and Genotypes

• What was the difference between the tall plants in P1 and the tall plants in F1?

• Two organisms, therefore, can look alike but have different underlying gene combinations

Phenotype

• Outward appearance of an organism, regardless of its genes

• How the organism looks

• A tall TT plant and a tall Tt plant look what?

• Their phenotype would be tall

Genotype

• Combination of genes in an organism

• Does the tall TT plant and the tall Tt plant have the same genotype?

• Can’t always tell an organism’s genotype by looks

Homozygous vs. Heterozygous

• Homozygous- when there are two identical alleles for a trait

• TT and tt

• Heterozygous- when there are two different alleles for a trait

• Tt

Law of Independent Assortment

• Genes for different traits are inherited independently of each other

• For example, seed shape and seed color

Punnett Squares

• Shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross

How to Work Punnett Square

• Step #1: Determine the genotypes of the parent organisms

• Step #2: Write down your "cross" (mating).  Write the genotypes of the parents in the form of letters (ex: Tt x tt)

Step #3: Draw a p-square

T is the allele for being tall and t is the allele for being short, cross one plant that is homozygous recessive (tt) with another that is heterozygous tall (Tt).

Step #4: "Split" the letters of the genotype for each parent & put them "outside" the p-square

t t

T

t

Step #5: Determine the possible genotypes of the offspring by filling in the p-square