1. Mendelian Genetics

2. Do You Know These Words Heredity
Passing on of characteristics from parent to offspring
Genetics
Part of biology that studies heredity
Gene
Piece of DNA that codes for a polypeptide (specify certain trait)
Trait
Characteristic that is inherited
Homologous Chromosomes
Chromosome of the same type (pairs)
Locus
Specific location of a gene on a chromosome
Diploid
2 copies of each chromosome = 2n
Haploid
1 copy of each chromosome = 1n

3. Gregor Mendel Austrian monk A mathematician aware of probability
The “father” of genetics
Worked with pea plants

4. Why Peas?
Peas have many traits with 2 different forms that are easy to distinguish (ex: green or yellow seeds; tall or short plants)
Could easily control mating of the pea plants through cross-pollination
Pea plants are small, they grow quickly and easily, and they produce a lot of offspring.

5. Mendel’s Crosses
In one cross, Mendel crossed tall pea plants with short pea plants
All of the offspring were tall!
He then crossed two of the offspring tall plants
The next offspring were ¾ tall and ¼ short
Mendel said that 2 factors control each trait (these are genes found on chromosomes)
Each form of a gene is called an allele.
The two alleles are found on homologous chromosomes.

6. Mendel’s Crosses Parent x Parent = P1 generation
Offspring x Offspring = F1 generation
Offspring = F2 generation

7. Mendel’s Law of Dominance
Mendel called the observable trait (all tall plants) dominant and the trait that disappeared (short plants) recessive
The allele for tall plants is dominant
The allele for short plants is recessive
If T = tall and t = short
TT = tall plant, Tt = tall plant, tt = short plant

8. Mendel’s Theory of Segregation
Each parent passes on (at random) to offspring only one allele for each trait!
A plant that is Tt can pass either T or t in a gamete.
Ex: Humans have ____ chromosomes.
When eggs and sperm are made, only half of these chromosomes are passed…one of each kind from mom and one of each kind from dad
That way a fertilized egg (and the baby) will have the normal # of chromosomes

9. Mendel’s Theory of Segregation
Diploid cells have pairs of genes on pairs of homologous chromosomes.
The 2 genes of each pair are separated during meiosis and end up in different gametes.

10. More vocabulary! Phenotype = individual’s observable traits
Ex: (tall, short)
Genotype = genes an individual carries
Ex: TT, Tt
Homozygous = has 2 of the same alleles
Purebred, true-breeding
TT, tt
Heterozygous = has 2 different alleles
Hybrid Tt

11. Monohybrid Cross Cross a single trait
Cross 2 parents that are homozygous for different alleles of a single trait
Saw purple allele is dominant to (masks) white allele
Purple flower x White flower
All purple flower plants
705 purple
224 white
About a 3:1 ratio

12. Now lets investigate the heredity seen in Reebops!

13. Punnett Squares Can be used to show all possible offspring!
Ex.: R = round seed, r = wrinkled seed
Homozygous round x homozygous wrinkled
What F1 offspring can be produced? What are the genotypes and phenotypes?

14. F2 Generation
Now let’s cross two of the heterozygous offspring from the F1 generation.

15. Dihybrid Cross Cross two traits
Start with 2 parents that are homozygous for different alleles for 2 traits
Round Yellow Seed x Wrinkled Green Seed
R= round Y = yellow
r = wrinkled y = green

16. Punnett Square
Example: RRYY x rryy

17. Mendel’s Theory of Independent Assortment
Gene pairs for different traits are inherited independently of each other
Ex: Genes for round seed shape are inherited independently of genes for yellow seed color!
Note: We will see later that this does not always apply

18. Test Cross I have a guinea pig that is black.
I know that the black allele (B) is dominant to the brown allele (b).
What are the possible genotypes for my guinea pig?
I want to breed guinea pigs, and I want to have pure-bred guinea pigs!
How do I know if he is pure-bred?

19. Test Cross
My guinea pig may be BB or Bb. Both would show the black phenotype!
To see if a dominant phenotype is due to a homozygous dominant or a heterozygous phenotype, I can…
Cross it with one that is homozygous recessive! B B B b
b b