1. Mendel & Heredity
Gregor Mendel
“Father of genetics”

2. Mendel Successes
1st to predict how traits are transferred from one generation to next
Successful b/c studied only one trait at a time to control variables & analyzed his data mathematically

3. Mendel’s experiments
Self-pollination  true-breeding parent, only 1 plant
Cross-pollination (making a cross)  offspring from two different parent plants

4. More on Mendel’s experiments
Mendel studied 7 traits on pea plants - Seed shape & color, pod color & shape, flower color, flower position, & plant height
Conducted monohybrid crosses looking at only 1 trait
Example – Flower color

6. What’s going on? The white flower trait had reappeared.
Mendel repeated for other traits and in every case he found that one trait seemed to disappear in the 1st generation only to reappear unchanged in ¼ of the 2nd generation.

7. Mendel’s hypothesis
Organisms have 2 factors that described each trait  alleles
Inherited one allele from father & other from mother

9. Explaining Mendel’s results
A purebred parent plant would have 2 identical alleles for purple flowers (PP) or 2 alleles for white flowers (pp)
F1 offspring have 1 allele for purple flowers & 1 allele for white flowers (Pp)
F2 offspring - pp, Some PP, Pp

10. Mendel’s Law of dominance
Some alleles are dominant (trait is expressed in the offspring) & other alleles are recessive (trait is only exhibited if dominant is absent)
Ex: Purple & white flowers

11. Law of segregation
Alleles for a trait will separate when the sex cells (gametes) are formed during meiosis.
Evidence – F2 offspring show recessive trait.

12. Law of Independent Assortment
Genes for different traits are inherited independently from each other
Mendel discovered this through his experiments w/dihybrid crosses (2 parents w/2 different traits)
Ex: tall, yellow pea plant with a short, green pea plant

13. Environment affects gene expression
Internal - Age, gender (hormonal differences)
Ex: male bird feathers colorful
External - Temperature, nutrition, light, radiation, chemicals, viruses
Ex: leaves at top of trees

14. Review Phenotype Genotype
What the organism looks like (physical appearance)
Ex: brown hair
Genotype
genetic makeup of an organism (we cannot see this)
Ex: Bb, BB

15. Two forms of Genotype Homozygous
both identical alleles are either dominant or recessive.
Ex: BB or bb
Heterozygous
different alleles, one dominant & one recessive
Ex: Bb

16. Punnett Squares

17. Punnett Squares (PSq) Developed in 1905 by Reginald Punnett.
Used to predict & compare possible genetic variations resulting from a cross
Probabilities, not exact results

18. Symbols used in PSq Original parents  P1 generation
Offspring of the parent plants  F1 generation
Offspring of the F1 generation  F2 generation

19. Example

20. Steps to solving PSq Identify the dominant & recessive alleles
Write the genotypes of the parents
Determine the possible gametes the parents can produce (how the alleles will separate)

21. Enter the possible gametes at top (#1 parent) & side (#2 parent) of the PSq
Complete the PSq  write the alleles from the gametes in the appropriate boxes
Determine the phenotypes of the offspring and percentages of each

22. PSq Practice Heterozygous yellow seeds x homozygous green seeds Key :
Y yellow seeds
y green seeds Y y

23. Answer Y y Yy yy

24. Now try this sex-linked trait
Heterozygous normal female x normal male
Key :
X N normal
X h hemophilia
Y  normal
X N
X h Y

25. Answer
X N
X h
X N X N
X N X h Y
X N Y
X h Y

26. Now try problems on: Working with Punnett squares worksheet
STOP click here

28. Dihybrid cross – like a puzzle
Can 2 parents with Heterozygous round, yellow seeds produce offspring with wrinkled, green seeds?
1st – determine possible gametes/ allele partners
First
Outside
Inside
Last
Set up PSq
boxes

30. Practice Dihybrid Cross
Heterozygous Purple Flower & Homozygous Tall X
Homozygous white flower & Heterozygous Tall
Key: F purple flower T tall
f white flower t short