3. Chromosome: a long chain of genes
Genes and Chromosomes
Write these down p. 74
Chromosomes are found in the nucleus of a cell.
Chromosome: a long chain of genes

4. Chapter 3 Section 1
What is Heredity?

5. Austrian monk born in 1822
Son of a farmer
Ordained as a priest
Lived in a monastery where over many years, he performed hundreds of experiments on pea plants relating to heredity
Known as the FATHER OF GENETICS

6. Heredity p. 74
Passing of physical characteristics from parent to offspring
Mendel’s study of pea plants would forever change biology

7. Allele: Specific characteristics, such as stem height or seed color
Remember…
Different forms of a gene
Allele:
Write this…

8. Remember??
For example…

9. Mendel’s Experiments p. 75
Genetics: The scientific study of heredity
Mendel wondered why different pea plants had different characteristics: some tall, some short, some with green seeds, some with yellow seeds…

10. FERTILIZATION: when egg and sperm join together
MENDEL’S EXPERIMENTS
Mendel “crossed” pea plants with different traits and recorded results
“Crossed” means he took pollen from one plant and put it on another plant to fertilize it
FERTILIZATION: when egg and sperm join together

11. Just listen…no need to highlight

12. Before we learn about Mendel’s experiments, let’s review the basics of sexual reproduction in flowering plants…
Flowers have both male and female reproductive structures.
The female part of the flower: pistil
The pistil produces egg cells.
The male part of the flower: stamen
The stamen produces pollen which contains sperm cells.
p. 75

13. The birds and the bees of plants…
When pollen lands on the pistil of a flower, sperm travels down to the egg cell, and the result is fertilization
Fertilization produces a tiny embryo, which is enclosed inside a seed

14. Pea plants are usually self pollinating
Since the reproductive structures are inside the flower, the sperm of the flower will usually fertilize the egg of the same flower.
Resulting embryos will have the same characteristics as their parents
Even though sexual reproduction has occurred – there is only 1 parent – 2 sex cells though…

15. MENDEL’S EXPERIMENTS Crossing Pea Plants
Mendel devised a way to cross-pollinate pea plants.

16. Mendel started with PUREBRED ORGANISMS
PUREBRED: The offspring of many generations that have the same form of a trait
Example: Purebred tall pea plants always come from tall parent plants.

17. Before we move on, you must be familiar with the following terms:
P generation: Parent generation
F1 generation: First generation of
offspring
F2 generation: second generation
of offspring
Hybrids: offspring of parents with
different traits
Which thumb do you put on top?

18. F1 Offspring p. 76 The offspring from the parent cross
Filial comes from filia and filius – the Latin words for “daughter” and “son”

19. F1 Offspring
The offspring from the parent cross – (1st generation offspring)
Mendel crossed purebred tall plants with purebred short and all F1 offspring were tall.

20. F2 Offspring
The offspring from the F1 generation (2nd generation offspring)
Mendel let the hybrid tall plants from the first generation self pollinate – the result was a mix of tall and short plants. ¾ were tall and ¼ were short.

21. Evidently the F1 “tall” offspring must have been carrying the short trait, but it had been hidden.
The short trait had been passed down to the offspring and it reappeared in the F2 generation.

22. Experiments with other traits
Mendel studied other traits such as seed shape and color, pea pod shape and color.
In all of Mendel’s crosses, he found that only one form of the trait appeared in the F1 generation. However in the F2 generation, the “lost” form of the trait always reappeared in about ¼ of the plants.
Example…P generation (tall x short)
F1 generation = all tall
F2 generation = ¾ tall, ¼ short

23. Results of a Cross – figure 2 pg. 76

24. Results of a Cross – figure 2 pg. 76

25. How Do Alleles Affect Inheritance?
Mendel reached several conclusions
P. 77
Sets of genetic information must control the inheritance of traits
The factors that control each trait exist in pairs
The female parent contributes one factor, the male contributes the other
One factor in a pair can mask, or hide, the other factor

26. Two forms for each trait:

27. How Do Alleles Affect Inheritance?
Gene
Factors that control a trait
Do you know this Gene???

28. ALLELES Different forms of a gene
EXAMPLE: The gene that controls stem height in peas has one allele for tall stems and one for short stems.

29. ALLELES ARE INHERITED
Each pea plant inherits two alleles from its parents – one allele from MOM (the egg) and the other from DAD (the sperm)
Our little sweet pea!!

30. ALLELES ARE INHERITED
An organism’s traits are controlled by the alleles it inherits from its parents. Some alleles are dominant, while other alleles are recessive
A dominant allele can cover up or mask a recessive allele

31. Dominant vs. Recessive Alleles
Dominant alleles: trait that always shows up when the allele is present
Recessive Alleles: trait that is “hidden” whenever the dominant allele is present
(this trait will only show up if the dominant allele is not present)

32. Circle the picture of each dominant form of the trait in the P generation.

33. Predict: Under what conditions would the recessive form of one of these traits reappear?

34. Predict: Under what conditions would the recessive form of one of these traits reappear?
If an offspring has two recessive alleles.

35. Alleles in Mendel’s Crosses
Hybrid: An organism with two different alleles for a trait
(1 dominant and 1 recessive)
The F1 plants were all hybrids - one tall allele and one short allele
The dominant allele masked the recessive allele, so all the plants were tall in the F1

36. Symbols for Alleles Geneticists use letters to represent alleles
Dominant allele – represented by a capital letter
Recessive allele – represented by a lowercase version of the same letter
Example: T = tall stems
t = short stems
TT = tall plant
Tt = tall plant
Tt = short plant

37. Dominant and Recessive Alleles
Figure 4
Fill in the missing allele symbols and descriptions.
Use the word bank to complete the statements. (Terms will be used more than once).
Draw the two possible ways the F2 offspring could look. T t
Tall
dominant t
recessive
Hybrid
dominant
dominant
recessive
recessive

38. Apply it! P. 79
WW and ww
About ¾ will have long wings and ¼ will have short wings
All will have long wings; Ww

39. Significance of Mendel’s Contribution
Mendel’s discovery of genes and alleles eventually changed scientists’ minds about heredity
Before Mendel, most people thought traits were just a blend of the parent’s characteristics.
Mendel showed that offspring traits are determined by the individual, separate alleles inherited from each parent.

40. Significance of Mendel’s Contribution
Known as the Father of Genetics