Chapter 14: Menelian Genetics

Objectives1. Understand the two laws Mendel came up with

2. Understand the laws of probability associated with Mendalian Genetics

3. You will know that inheritance is more complicated then what Mendel predicted

4. Many human traits follow Mendelian Genetics patterns

Why Pea Plants?

1. Peas have different characters or traits like color that were distinct

2. Short generations

3. Large number of offspring

4. Could control pollination

5. Picked true breeding plants (All self-pollinated offspring have same traits)

6. He could cross pollinate or hybridize two true breeding parental generations

7. He could gather good data on the First and second generations

Mendel’s Conclusions

Conclusion 1.

Traits occur in different versions called alleles

Conclusion 2. An organism inherits one allele from mom and one from dad.

Conclusion 3.

Alleles can be either dominant or recessive with the recessive only being expressed in a homozygous

Conclusion 4.

“Law of Segregation”

Two alleles segregate separately during meiosis into different haploid sex cells.

“Law of Independent Assortment”

-Pairs of alleles or genes segregate independently from other allele pairs

-Law doesn’t apply to genes that are located near each other on the same chromosome

Probability

Monohybrid Cross = one character

B b

b Bb bb

b Bb bb

Hair Color

Eye Color

Genotypic

Ratios

AA = ¼

Aa = 2/4

aa = ¼

Genotypic

Ratios

BB = 0

Bb = 2/4

bb = 2/4

1. What is the probability of getting an offspring that is homozygous recessive for both traits?

2. What is the probability of getting an offspring that expresses both dominant phenotypes?

Multiplication Rule: States multiplication is used to determine the probability of two or more independent events will occur at the same time

Addition Rule: States addition is used to determine the probability of one of two or more events occuring.

Complex Genetics Problems use both multiplication and addition

1. Three characters (Flower color, Seed color, and Pod shape) are considered in a cross between two pea plants (PpYyIi X ppYyii) What fraction of offspring would be predicted to be homozygous recessive for at least two of the three charactersA a

A AA Aa

a Aa aaTrihybrid Cross

Use multiplying ratios or the punnet square

Pp = ___/4

pp = ___/4

YY = ___/4

Yy = ___/4

yy = ___/4

Ii = ___/4

Ii = ___/4

Possible Genotypes

1.

2.

3.

4.

5.

Complex Inheritance Patterns

1. Degrees of Dominance (Read p. 272 “Relationship between…”)

a. Complete (Normal Dominant vs Recessive)

b. Incomplete (Heterozygous and neither allele is completely dominant

c. Codominance (Both alleles are expressed equally)

2. Multiple alleles

3. Pleiotropy – Gene codes for more then one phenotype

4. Two or more genes code for same phenotype

Multiple genes coding for one trait

Epistasis

- One gene can alter the expression of other genes

Polygenic Inheritance -Quantitative characters or amount of alleles determines color