Like humans, plants have genes, and different alleles exist for those genes. Gene: Petal Color Red allele Yellow allele Purple allele

This is a particular type of plant called a pea plant: Pea plant flowers Peas Seeds of the pea plant

Allele 1 Allele 1 Allele 1 Gene Gene Gene Allele 2 Allele 2 Allele 2 Each pea plant gene has two alleles. Allele 1 Allele 1 Allele 1 Allele 1 Gene Gene Gene Gene Flower Color Flower Position Seed Color Seed Shape Pod Shape Pod Color Plant Height Purple Purple Axial Yellow Round Inflated Green Tall White Terminal Green Wrinkled Constricted Yellow Short Allele 2 Allele 2 Allele 2 Allele 2

Like humans, plants pass their genes to the next generation by producing gametes.

As with us, plant gametes combine during fertilization As with us, plant gametes combine during fertilization. Offspring receive two copies of every gene. Purple White Purple Purple White Purple

The difference is that flowering plants produce both male and female gametes. (Polllen) Eggs

Pollen The male gametes of a plant. G P

Flowers can reproduce through self-pollination…

Or cross-pollination. C

Cross-pollination is usually facilitated by insects.

Only cross-pollination can produce hybrid offspring. Self-pollination Cross-pollination

Gregor Mendel, a 19th-century Austrian monk and scientist, used his knowledge of plant reproduction to create one of the first genetics experiments.

Gregor Mendel An early plant geneticist who discovered the laws of inheritance. G L

Mendel knew nothing about DNA or genes (his experiment was conducted about the same time as the civil war.)

We did have microscopes, though, so we knew about gametes and fertilization.

Mendel set out to answer three questions about inheritance.

1. Why do some allele blend, while others don’t? BROWN HAZEL

2. Why do some alleles appear to skip a generation?

3. Do alleles for different genes stay together? Gene: Fur color Gene: Fur pattern Alleles: Brown, tan Alleles: Solid, stripe Tan, stripe Brown, solid Brown, stripe

To answer these questions, Mendel produced hybrids. Flower Color Flower Position Seed Color Seed Shape Pod Shape Pod Color Plant Height Purple Purple Axial Yellow Round Inflated Green Tall White Terminal Green Wrinkled Constricted Yellow Short

To make hybrids, Mendel had to ensure that his plants did not accidentally self-pollinate. Self-pollination Cross-pollination

To prevent self-pollination, Mendel removed the male parts of one flower and dusted it with pollen from another flower. S M F P F Pollen transfer Flower 1 Flower 2

Mendel began his experiment by crossing one tall pea plant and one short pea plant. He called these plants the P generation (parent generation). T = Tall t = short X P generation

My parents were both tall! My parents were both short! Remember that living things have 2 copies of every gene. My parents were both tall! My parents were both short! TT tt T = Tall t = short X P generation

Mendel hypothesized that the tall and short alleles would blend in the hybrid offspring. ? T = Tall t = short = X P generation

F1 generation (hybrids) TT tt X P generation F1 generation (hybrids)

F1 generation (hybrids) All of the F1 hybrids were tall; Mendel’s hypothesis was not supported. T H S TT tt X P generation F1 generation (hybrids)

F1 generation (hybrids) Each of the F1 hybrids inherited one tall allele and one short allele. So why didn’t the alleles blend? TT Tt Tt Tt Tt tt X P generation F1 generation (hybrids)

Mendel concluded that some alleles are dominant, while others are recessive.

F1 generation (hybrids) Dominant alleles are always expressed, even if only 1 copy is inherited. D E TT Tt Tt Tt Tt tt X P generation F1 generation (hybrids)

F1 generation (hybrids) Recessive alleles are only expressed when 2 copies are inherited. R E TT Tt Tt Tt Tt tt X P generation F1 generation (hybrids)

Brown eyes (B) are dominant, while blue eyes (b) are recessive. BB Bb bb

The green eye allele (G) is somewhat recessive to brown, but dominant to blue. GG GB Gb

Dominant = more protein made Blue allele Brown allele From Dad From Mom

Dominant = more protein made Green allele Brown allele From Dad From Mom

PRINCIPLE OF DOMINANCE 1. Why do some allele blend, while others don’t? PRINCIPLE OF DOMINANCE BROWN HAZEL

Mendel wanted to know what would happen if he cross-pollinated the F1 plants. He crossed two of the F1 hybrids and created an F2 generation. Tt Tt X F1 F2 generation

¼ of the F2 plants were short. Tt Tt tt X F1 F2 generation

Why did shortness reappear in the F2 generation?

In order for 2 recessive alleles to pair up during fertilization, the allele for shortness (t) must somehow separate, or segregate, from the allele for tallness (T). T t T t

Segregation Bb Bb B b B b The separation of alleles when gametes form.

Why shortness reappeared in F2 Tt Tt F1 generation Segregation T t T t Gametes T t T t Fertilization F2 generation TT Tt Tt tt

TT Tt Tt tt F2 generation

Because of segregation, 2 recessive alleles were able to pair up during fertilization to produce a short plant. S R F S Tt Tt F1 generation Segregation T t T t Sex cells Fertilization F2 generation TT Tt Tt tt

PRINCIPLE OF SEGREGATION 2. Why do some alleles appear to skip a generation? PRINCIPLE OF SEGREGATION

We can use Mendel’s principles of dominance and segregation to explain many phenomena in genetics.

Genetics Problem #1 The allele for brown eyes (B) is dominant, while the allele for blue eyes (b) is recessive. What is the only possible allele combination for a blue-eyed person? bb What are two possible allele combinations for a brown-eyed person? BB Bb

Brown = B (dominant); Blue = b (recessive) Bb Bb Parents Segregation Gametes B b B b Fertilization Offspring BB Bb Bb bb

Brown = B (dominant); Blue = b (recessive) 34. Explain why the parents in this diagram have brown eyes, even though they both carry the allele for blue eyes. 35. Explain why bb is the only possible allele combination for a blue-eyed person. 36. Explain why two recessive alleles were able to pair up during fertilization to produce a blue-eyed baby. 37. The blue-eyed baby has a brown-eyed sister from the same parents. Explain how this is possible. Dominant One copy Recessive Two copies Segregation BB or Bb

Genetics Problem #2

Genetics Problem #2 The allele for dark skin (D) is dominant, while the allele for light skin (d) is recessive. What is the only possible allele combination for a light-skinned person? dd What are two possible allele combinations for a dark-skinned person? DD Dd

Dark = D (dominant); Light = d (recessive) Parents Dd dd Segregation Gametes D d d d Fertilization Offspring Dd Dd dd dd

Brown = B (dominant); Blue = b (recessive) 34. Explain why the father in this diagram has dark skin, even though he carries the allele for light skin. 35. Explain why dd is the only possible allele combination for the mother. 36. Explain why two recessive alleles were able to pair up during fertilization to produce a light-skinned baby. 37. Explain why one of the sisters has dark skin. Dominant One copy Recessive Two copies Segregation Dd