APW

• If you flip a coin, what are the chances that it will land on heads? Tails?

• Suppose you flip the coin once, get heads, and then flip it again. What are the chances that you will get heads again? Explain.

APW 3-20-08

• Fill in the following Punnett Square.

• Define: Purebred -• Define: Hybrid-• Define: Testcross-

D d

d

d

APW 3-20-08• Fill in the following Punnett Square.

• Define: Purebred-having the same alleles• Define: Hybrid-having two different alleles• Define: Testcross-when you cross an organism with a

homozygous recessive genotype with a dominant phenotype to figure out the dominant trait organisms genotype.

D d

d Dd dd

d Dd dd

APW 3-24-08

• Fill in the following Punnett Squares.

d d

d

d

D D

D

D

Chapter 3 Section 2

Traits and Inheritance

Objectives

1. Explain how genes and alleles are related to genotype and phenotype

2. Use the information in a Punnett square

3. Explain how probability can be used to predict possible genotypes in offspring.

4. Describe three exceptions to Mendel’s observations

Genes• Genes are one set of

instructions for an inherited trait.

• Each parent gives one set of genes to offspring.

• Therefore, offspring has two sets of a gene for every characteristic.

• Genes are located on chromosomes.

Bigger

Smaller

Alleles

• Alleles are the different forms of a gene.

• Characteristics are to traits as genes are to alleles.

Phenotype vs. Genotype

• A phenotype is any observed quality of an organism, such as its morphology, development, or behavior.

• A genotype is the inherited instructions an organism carries, which may or may not be expressed.

Phenotype

Genotype

Phenotypes of Pea Plants• Name 2 possible phenotypes for flower color. ______________ _______________

• Name 2 possible phenotypes for seed shape.______________ _______________

• Name 2 possible phenotypes for seed color.______________ _______________

Genotypes of Pea Plants• Name 3 possible genotypes for flower color if the alleles

are F and f. ___________ ___________ ___________

• Name 3 possible genotypes for seed shape if the alleles are S and s.

___________ ___________ ___________

• Name 3 possible genotypes for seed color if the alleles are C and c.

___________ ___________ ___________

Genotype: Homozygous vs. Heterozygous

• A Homozygous plant is one with two dominant or two recessive alleles.

• A Heterozygous plant is a plant with two different alleles

HH or hh

Hh

Punnett Squares

• Punnett Squares are used to organize all of the possible combinations of offspring from particular parents.

• What is the ratio of dominant phenotypes to recessive phenotypes?

_______________

Using a punnett square to model Mendel’s first experiments

Punnett Squares• Punnett Squares are used to

organize all of the possible combinations of offspring from particular parents.

• What is the ratio of dominant phenotypes to recessive phenotypes?

_______________

Using a Punnett square to model Mendel’s second experiments

What are the chances?

• When the alleles a parent has are different, the chances of receiving them are fifty-fifty, like a coin toss.

• Probability is the mathematical chance that something will happen. In this case, it is the chance that a trait will be inherited.

• Probability is usually written as a fraction or a percentage.

Probability Practice

• If you roll a pair of dice, what is the probability that you will roll 2 threes?______ x ______ = ______

• If you flip a coin twice, what is the probability that you will get 2 heads?

______ x ______ = ______• If you roll a single die, what is the probability you will

roll an even number?______ + ______ + ______ = ______

Probability Practice

• If both parents have Pp genes, what is the probability that the offspring will have PP genes?

______ x ______ = ______• If both parents have Pp genes, what is the probability

that the offspring will have Pp genes?______ x ______ + ______ x ______ = ______• If both parents have Pp genes, what is the probability

that the offspring will have pp genes?______ x ______ = ______

Exceptions to Mendel’s Principles

• Incomplete Dominance• Codominance• One gene, many traits• Many genes, one trait• Environmental Effects

Incomplete Dominance

• When an individual is heterozygous for a trait, instead of one trait being completely dominant over another, the traits mix.

• Thus, you can tell the genotype by looking at the phenotype for incomplete dominance. R r

R RR Rr

r Rr rr

Codominance

• In codominance, neither phenotype is recessive. Both traits are fully expressed.

• However, both codominant traits are dominant over the recessive trait.

• Blood type is an exampleIA i

IB IAB IBi

i IAi ii

One Gene, Many Traits

• What do you notice about the tiger?

• Sometimes one gene influences more than one trait.

• In tigers, the gene that influences eye color also influences hair color.

Many genes, One trait

• Skin color and eye color are influenced by many genes.

• That is why there are intermediary colors.

Environment• Environments can also affect

phenotype (hair cutting)

Objectives

1. Explain how genes and alleles are related to genotype and phenotype

2. Use the information in a Punnett square

3. Explain how probability can be used to predict possible genotypes in offspring.

4. Describe three exceptions to Mendel’s observations