11-1 The Work of Mendel• What does every living thing inherit from

their parents?• Genetics – the study of heredity

Look around at your classmates and make a list of some of the traits that are inherited.

What did Mendel already know…• Each flower produces pollen (sperm) and

egg cells

• Cross fertilization (sexual) – male and female cells join

• Self-pollination (asexual) – pollen fertilizes eggs from same plant

• Mendel’s pea plants were true-breeding – A tall plant with green seeds would produce a

tall plant with green seeds

Genes, Alleles and Dominance

• Trait – specific characteristic– Mendel studied 7 different pea plant traits

• What are some examples? Plant Height, Seed Shape, Pod Color

Genes, Alleles and Dominance

• Genes – the chemical factors that determine traits (the segment of DNA) ex. pea plant: height

• Alleles – different forms of a gene from each parent ex. pea plants: tall and short (T or t)

Principle of Dominance

• Principle of dominance: some alleles are dominant and others are recessive

• Dominant allele – the trait is always shown– Capital letter (T = tall)

• Recessive allele – the trait that will only show if there is no dominant allele– Lower case letter (t = short)

• Heterozygous – organisms that have 2 different alleles for the same trait– Hybrid for that trait– Ex: Tt

• Homozygous – organisms that have 2 identical alleles for a particular trait

• True-breeding for a particular trait– Ex: TT or tt

Genes, Alleles and Dominance• Phenotype – physical characteristics

– Ex: tall, short, yellow, green

• Genotype – genetic make-up– Ex: TT, Tt, tt

• Tall plants have the same phenotype (tall), but not the same genotype (TT or Tt)

• Why are TT and Tt genotypes for tallness, but tt is not???

11-2 Probability and Punnett Squares

•Mendel realized…the principles of probability could be used to explain the results of genetic crosses.

Genetics and Probability• Probability – the likelihood a particular event

will occur.– Ex: probability of flipping a coin to heads = ½ or

50%– Probability of head 3 times in a row = ½ x ½ x ½ =

1/8 – The greater the number or trials, the closer to the

expected ratio– Past outcomes do not affect future outcomes

• Alleles segregate randomly (like a coin)

Principle of Segregation• Segregation =

separation

• The alleles for tall vs. short separate during the formation of gametes – sex cells

• Each gamete carries one allele for each gene

The Two-factor Cross: F1• 4 alleles (2 genes) at the same time• Provides the hybrid plants (F2 generation)• Crossed a homozygous RRYY (round yellow peas)

with a homozygous rryy (wrinkled, green peas)

11.3 Beyond Dominant and Recessive Alleles• Genes can act in various ways

1. Dominant vs. Recessive – one allele completely masks another allele.

2. Incomplete Dominance1. Definition : one allele is not completely

dominant over another

Ex: flowers – white x red flowers = pink flowers

11.3 Beyond Dominant and Recessive

• Codominance– Definition: both alleles contribute to the

phenotype of the organism

Ex: chicken feather – black and white alleles = black and white feathers

Colors don’t blend like incomplete dominance

11.3 Multiple Alleles• Definition: more than two alleles

• (more than 2 alleles exist in a population not an individual) Blood TypesEx: rabbit’s coat color

Ex: human’s blood type

11.3 Polygenic Traits• Definition: traits that are controlled by 2 or

more genesEx: fruit fly red eyes - 3 genes involved in making

pigment– Diff. combo of genes produce different eye colors

Ex: Human skin color – more than 4 different genes

Human height – more than 50 genes

11.3 Genetics and the Environment• Genes provide a plan for development, but

how the plan unfolds also depends on the environment:

• Ex. Butterflies have different wing colors depending on when they hatch

• Hydrangea flowers are different colors depending on soil pH.

Mendel Questions

Using tall and short pea plants and the letters T = dominant and t = recessive describe Mendel’s F1 generation and F2 generation.

Provide the genotype and phenotype for each of four offspring from each generation. You do not need to use a punnett square.

Meiosis

Ttrr x ttRr

ttRr x TtRr

Questions1. Genetics is the study of ________.

2. Alternate characteristics of an organism such as height, hair color, eye color, etc. are called _______.

3. Why did pea plants make such a good subject for Mendel to study?

4. What does “true breeding” mean?

Considering the allele related to short and tall pea plants.

1. Plants of the P generation (tall) will produce only_____________ if not crossed with plants having other alleles.

2. Describe the phenotype (what we can see) of Mendel’s F1 Generation.

3. What was the genotype (use letters “T” and “t”) for Mendel’s F1 generation?

4. How did the F2 generation suggest that genes were simply segregated, not lost?