1. 9A- Inheritance and variation
Lesson 3: Basic Genetics

2. Learning Objectives
Describe the work of Mendel in discovering the basics of genetics
dominant and recessive genes

3. Selective Breeding
Historically people have selectively bred plants and animals for specific traits (variations)
Cows- milk and meat
Chickens-eggs and meat
Horses/camels- speed and stamina
Corn/grain/vegetables- flavour and hardiness
Dogs- working and show
Flowers- beauty

4. Inheritance How are traits passed from parents to offspring?
Old theory- offspring a mixture of parents but this didn’t explain why some traits disappeared and then reappeared

5. Gregor Mendel (1866) selectively bred pea plants over years
he always found the same ratios when breeding pure strains of discontinuous characteristics
Flower colour (purple or white)
Stem length (long or short)
Seed shape (round or wrinkled)
Seed colour (yellow or green)
Pod colour (yellow or green)

6. Dominant and Recessive
He found that when he crossed a yellow pea plant with a green pea plant all the new plants were yellow pea plants.
BUT when he crossed these first generation yellow peas the next generation had the green pea return in a 3:1 ratio
What was going on?

7. Principles of inheritance
Traits are determined by “units” of inheritance (genes)
An individual inherits one unit (allele) from each parent for each trait
The trait may not show up in an individual but can be passed on to the next generation (carrier)
Dominant alleles will mask a Recessive allele
Genes pairs are assigned letters
Capital (W ) for dominant lower case (w) for recessive

8. Three possible combinations
WW- homozygous dominant
Ww- heterozygous
ww- homozygous recessive
WW and Ww will show the dominant trait
ww will show the recessive trait

9. Basic genetics
the pair of genes (alleles) of each parent separate and only one allele passes from each parent on to the offspring.
new combinations of genes from the parents occur randomly- creates variation

10. Punnett Square- predicting crosses

11. Punnett Square

12. Make three of your own

13. Task: Booklet Reference: textbook p.9
Worksheets: Mendel, 1.23, 9Ab-7,9Ab-9
Reference: textbook p.9