1. Variation & Inheritance
Chapter 11

2. Gregor Mendel Austrian monk Born in 1822
Studied math and science at the University of Vienna
Pea experiment
He used peas for 4 reasons:
Small Easy to grow
3. Produce many offspring 4. Self-fertilizing

3. Pea Plant Experiment
Used “true breeding plants” (produce offspring identical to the parent)
He cut away the pollen-bearing parts of two plants (one purple and one white) to prevent self- fertilization
He called these original 2 plants the Parent Generation (P)

4. Pea Plant Experiment
He then brushed pollen from the purple flower on the white flower and pollen from the white flower on the purple flower
All the offspring were purple flowers
He called this the F1 Generation

5. Pea Plant Experiment Mendel then let the F1 generation self-pollinate
He saw that 75% of the offspring (F2 generation) had purple flowers and 25% of the offspring had white flowers
Repeated the experiment with different traits and saw the same results

7. Genes & Alleles
Mendel concluded that an individual’s characteristics are determined by factors that are passed from one generation to the next
These “factors” are called genes
Different forms of the gene are called alleles

8. Dominant & Recessive Alleles
The other conclusion Mendel reached is called the Principle of Dominance
Some alleles are dominant and others are recessive
An organism with at least one dominant allele for a trait will show that form of the trait
The recessive allele is only shown if there is no dominant trait present
Which allele (purple or white) was dominant in Mendel’s experiment?

9. Alleles & Genotype The combination of alleles is called a genotype
Each organism has 2 alleles for each trait
One allele from mom, one allele from dad
There are 3 possible combinations of alleles:
Homozygous dominant (2 dominant alleles)
Homozygous recessive (2 recessive alleles)
Heterozygous (1 dominant and 1 recessive allele)
The combination of alleles is called a genotype

10. EXAMPLE: Think back to pea plants…
Gene:
Flower color
Alleles:
Purple flowers (P)
White flowers (p)
Genotypes:
PP Pp pp
Phenotypes:
Purple flowers (PP or Pp)
White flowers (pp)
Genotype & Phenotype
The physical expression of the alleles is called a phenotype

11. Punnett Squares
Helps predict the genotype and phenotype combinations in genetic crosses
Uses the genotypes of both parents to predict the probability of offspring having a certain trait

12. Punnett Square Tutorial
By drawing a Punnett square, you can determine the allele combinations that might result from a genetic cross. Follow the steps to make a correct Punnett Square.
EX. In cats, black fur is dominant over white fur. What are the genotypes and phenotypes of a cross between a HETEROZYGOUS black cat and a HOMOZYGOUS white cat?

13. EX. In cats, black fur is dominant over white fur
EX. In cats, black fur is dominant over white fur. What are the genotypes and phenotypes of a cross between a HETEROZYGOUS black cat and a HOMOZYGOUS white cat?
Black = B White = b
1. List out the dominant trait and the recessive trait (always pick the letter of the dominant trait)

14. EX. In cats, black fur is dominant over white fur
EX. In cats, black fur is dominant over white fur. What are the genotypes and phenotypes of a cross between a HETEROZYGOUS black cat and a HOMOZYGOUS white cat?
Parent 1-HETEROZYGOUS black Bb Parent 2-HOMOZYGOUS white bb
2. Use the problem to figure out the genotypes of Parent 1 and 2 (use homozygous and heterozygous for help with the parent genotypes)

15. EX. In cats, black fur is dominant over white fur
EX. In cats, black fur is dominant over white fur. What are the genotypes and phenotypes of a cross between a HETEROZYGOUS black cat and a HOMOZYGOUS white cat?
3. Put the genotype for Parent 1 across the top of the Punnett square, and the genotype for parent 2 down the side.
B b b

16. EX. In cats, black fur is dominant over white fur
EX. In cats, black fur is dominant over white fur. What are the genotypes and phenotypes of a cross between a HETEROZYGOUS black cat and a HOMOZYGOUS white cat?
4. Complete the Punnett square. Dominant alleles are always listed before recessive alleles.
B b b B b b b

17. EX. In cats, black fur is dominant over white fur
EX. In cats, black fur is dominant over white fur. What are the genotypes and phenotypes of a cross between a HETEROZYGOUS black cat and a HOMOZYGOUS white cat?
Homozygous Dominant 0 BB Homozygous Recessive 2 bb Heterozygous 2Bb
5. List out the offspring genotypes. How many are homozygous dominant? How many are homozygous recessive? How many are heterozygous?

18. EX. In cats, black fur is dominant over white fur
EX. In cats, black fur is dominant over white fur. What are the genotypes and phenotypes of a cross between a HETEROZYGOUS black cat and a HOMOZYGOUS white cat?
Dominant Trait 2 Black cats Recessive Trait 2 White cats
6. List out the offspring phenotypes. How many are going to SHOW the dominant trait? How many are going to SHOW the recessive trait?

19. Other Patterns of Inheritance
Sometimes traits don’t fall in the “one or the other” category (black or white, brown or blue, tall or short)
What happens if one allele is not completely dominant over the other?
What if one gene has several alleles?

20. Incomplete Dominance Some alleles are neither dominant nor recessive
In incomplete dominance, the heterozygous phenotype lies somewhere between the two homozygous phenotypes.

21. Codominance
In codominance, both alleles are expressed in the heterozygous phenotype
Alleles have equal dominance

22. Multiple Alleles
Many genes exist in several different forms and are therefore said to have multiple alleles
Example:
A rabbit’s coat color is determined by a single gene that has at least 4 different alleles
Human blood type

23. Polygenic Traits
Many traits are produced by the interaction of several genes
Polygenic traits often show a wide range of phenotypes
Variety of skin color in humans is partly because more than 4 genes control the trait

24. Karyotype

25. Dihybrid Cross
Until now, we have been crossing 1 gene between two parents
Sometimes, 2 genes need to be crossed
For this we use a dihybrid cross
Di = two

26. Dihybrid Cross
In a dihybrid, each parent is going to have 4 letters in their genotype
2 letters from mom, 2 letters from dad
A genotype in a dihybrid cross would look like this:
AaBb
Two different letters = Two different traits

27. Dihybrid Cross
In pea plants, purple flowers are dominant to white flowers. Green seeds are also dominant to yellow seeds. If a homozygous purple flowered plant with yellow seeds is crossed with a homozygous white flowered plant with green seeds, what are the genotypes and phenotypes of the offspring?

28. P = purple flowers p = white flowers G = green seeds g = yellow seeds
In pea plants, purple flowers are dominant to white flowers. Green seeds are also dominant to yellow seeds. If a homozygous purple flowered plant with yellow seeds is crossed with a homozygous white flowered plant with green seeds, what are the genotypes and phenotypes of the offspring?
P = purple flowers p = white flowers G = green seeds g = yellow seeds
1. Create a key for dominant and recessive traits.

29. 2. Use the problem to figure out the parent genotypes
In pea plants, purple flowers are dominant to white flowers. Green seeds are also dominant to yellow seeds. If a homozygous purple flowered plant with yellow seeds is crossed with a homozygous white flowered plant with green seeds, what are the genotypes and phenotypes of the offspring?
P1: “Homozygous purple flowered plant with yellow seeds” PPgg P2: “Homozygous white flowered plant with green seeds” ppGG
2. Use the problem to figure out the parent genotypes

30. PPgg Pg Pg Pg Pg ppGG pG pG pG pG
In pea plants, purple flowers are dominant to white flowers. Green seeds are also dominant to yellow seeds. If a homozygous purple flowered plant with yellow seeds is crossed with a homozygous white flowered plant with green seeds, what are the genotypes and phenotypes of the offspring?
PPgg Pg Pg Pg Pg ppGG pG pG pG pG
3. FOIL to find the possible combinations for each parent’s allele.

31. In pea plants, purple flowers are dominant to white flowers
In pea plants, purple flowers are dominant to white flowers. Green seeds are also dominant to yellow seeds. If a homozygous purple flowered plant with yellow seeds is crossed with a homozygous white flowered plant with green seeds, what are the genotypes and phenotypes of the offspring?
Pg Pg Pg Pg
4. Place one parent’s genotype possibilities across the top of the Punnett Square and the other parent’s possibilities down the side. pG pG pG pG

32. In pea plants, purple flowers are dominant to white flowers
In pea plants, purple flowers are dominant to white flowers. Green seeds are also dominant to yellow seeds. If a homozygous purple flowered plant with yellow seeds is crossed with a homozygous white flowered plant with green seeds, what are the genotypes and phenotypes of the offspring?
Pg Pg Pg Pg
5. Complete the Punnett Square EXACTLY the same way a regular Punnett square is completed. pG pG pG pG

33. In pea plants, purple flowers are dominant to white flowers
In pea plants, purple flowers are dominant to white flowers. Green seeds are also dominant to yellow seeds. If a homozygous purple flowered plant with yellow seeds is crossed with a homozygous white flowered plant with green seeds, what are the genotypes and phenotypes of the offspring?
Pg Pg Pg Pg
5. Complete the Punnett Square EXACTLY the same way a regular Punnett square is completed.
PpGg
PgGg pG pG pG pG