SECTION 11 .2

Chapter 11: Introduction to Genetics

Probability

Probability is the likelihood that something will happen

Example: Flipping a coinExample: Choosing the right question on a multiple choice test

Probability can be used to predict traits

Dominant vs. Recessive

Alleles are represented by lettersDominant alleles are represented by a capital

letter R

Recessive traits are represented by a lower case letter

r

Dominant alleles are expressed over recessive alleles If you have to alleles together the dominant one will

always be expressed or “visable”Example: Height Example: Brown hair

Key Words

Homozygous: organisms that have 2 identical alleles for a particular gene Homozygous dominant (2 dominant alleles)

RR Homozygous recessive (2 recessive alleles)

rr

Heterzgous: organisms that have 2 different alleles Have a dominant allele (written first) and a recessive

allele Rr

Practice

Label the following as either homozygous or heterozygous EE Tt Zz YY Yy pp bb NN

Key Words

Phenotype: physical expression of alleles, the traits, “what you see”

Genotype: genetic make-up, you cannot see it, this is the letter representation of the phenotype

YY, Ff, ff

Phenotype: Brown Genotype

Bb

Example

Practice

In people, brown eyes (B) are dominant to blue (b), from the following phenotypes identify possible genotypes:

BB or Bb bbBB or Bb bb

What are your traits?

Curley (H) hair is dominant to straight (h)Widow’s peak (W) is dominant to no widow’s

peak (w)Free ear lobes (F) are dominant to attached

ear lobes (f)

Hair Widow’s Peak

Ear lobes

Phenotype Straight Yes Free

Possible Genotypes

hh WW or Ww FF or Ff

Punnet Squares

Punnet Squares: use probability to help predict genotypes and phenotype combinations (ratios) in genetic crosses

Monohybrid Cross: examine the inheritance of only one specific trait.

Completeing Punnet Squares

1. Start with the Parents: Find the genotype of the parents

2. Figure out the Gametes: Determine the alleles that each parent could pass on to their offspring

3. Set-up the Table: Draw the Punnet Square and place the alleles and parents on the top and the left side of the square

4. Complete the Punnet Square: Fill in the table with offspring’s possible genotypes

5. Interpret the results

Step 1: Start with Parents

Long fins (L) in fish are dominant to short (l). You are breeding a homozygous dominant long fin male to a homozygous recessive short fin female. What are the possible genotypes of the F1 offspring?

What are the genotypes of the parents Male: LL Female: ll

Step 2: Figure out the gametes

What alleles can the male and female possibly give to their offspring?

Male Genotype: LL Can give an L allele or an L allele

Female Genotype: ll Can give an l allele or an l allele

LL

L L

ll

l l

Step 3: Set up the Punnet Square

L L

l

l

Step 4: Fill in Punnet Square

L L

l

l

Step 4: Completed Punnet Square

L L

l

lLl

LlLl

Ll

Step 5: Interpret Results

What were the possible genotypes from the cross?

Ll (heterozygous dominant)

What are the possible phenotypes from the cross?

All offspring will have long fins

Step 5: Interpret Results

What is the probability that the genotype will be Ll?

4/4 = 100%

What is the probability that the genotype will be ll?

0/4 = 0%

Step 5: Interpret the Results

What is the probability that the offspring will have long fins?

4/4 = 100%

What is the probability that the offspring will have short fins

0/4=0%

Testcross

A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype

If you have an organism with the dominant phenotype and you need to determine the possible genotype you can perform a test cross.

If you cross your Dominant organism with a recessive and produce recessive offspring you know the unknown organism must be heterozygous.

If no recessive offspring are produced can you be sure of the genotype of the unknown organism? NO

Testcross Example

Cattle are either horned or polled (no horns)Horns are dominant to polledPolled cattle are worth more so want

genotypes that carry the polled alleleRanchers will test cross cattle with horns to

determine if they are heterozygous or homozygous dominant

Questions??????