1. Genes & Heredity
Ms. Lefebvre
SBI3U

3. Genes & Heredity ~20 000 genes determine human characteristics
Some human traits are determined by a single gene
Trait Dominant Recessive
Earlobe Unattached Attached
Tongue Rolling Can roll Can’t roll
Hairline Widow’s Peak Straight
Finger hair Present Absent
Folded hands Left thumb over right Rt over Left
Chin dimple Dimple in middle No dimple

4. …genes and heredity… the humble beginnings

5. Gregor Mendel Moravian monk who studied mathematics & botany
1853 Century work
Experimented with Pisum sativum
“Father of Modern Genetics”

6. Mendel’s Garden Augustinian Monastery St. Thomas, Brno, Czech RepublicF2 F1
Dihybrid Cross

7. Why Pea Plants? They were commerically available in Europe
2. Easy to grow & matured quickly
3. Sexual Organs enclosed in the flower
4. Different varieties with different traits

8. Cross-Pollination of Pea Plants

9. Characteristics of Garden Peas Studied by Mendel
Dominant Traits
Recessive Traits

11. Some Basic Genetics Terminology
Alleles
Loci
Genotype
Phenotype
Dominant
Recessive

12. Gene
A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism

13. Alleles
Any one of two or more alternative forms of a gene that may occur alternatively at a given site on a chromosome. An individual cannot possess more than two alleles for a given trait.

14. Genotype Phenotype Alleles that the organism has The genetic makeup
Observable traits
Gene interaction & environment

15. Homozygous & Heterozygous
These alleles determine the expression of the genetic trait in the offspring
Alleles of this type are overruled by dominant alleles
Homozygous = AA or Aa
Heterozygous = Aa

16. Generations in Genetic Crosses
Parental Generation (P)
First Filial Generation (F1)
Second Filial Generation (F2)

17. True-breeding Hybrids
Establishes pure lines
A group of identical individuals that always produce offspring of the same phenotype when intercrossed
Hybrids
• A individual offspring produced
from any cross
involving parents of
differing genotypes.
•Offspring unlike
parents.

18. Determine the Phenotypic and Genotypic Ratios for the F2 generation.
Punnett Squares
A chart used by geneticists to show the possible combinations of alleles in offspring
Phenotypic Ratios
F1: 100% Green, Round
Genotypic Ratios
F1: 100% Yy
Determine the Phenotypic and Genotypic Ratios for the F2 generation.

19. Single Trait Analysis Monohybrid CrossTT tt
A cross which looks at the inheritance of a single character or trait.
Ex. Flower colour, plant height, seed shape…
Ex. TT x tt
Ex. Tt x Tt Tt
True-Breeding
Upon self-pollination, all offspring are of same variety.

20. Crossing the F1 Generation
When the F1 generation is allowed to self-pollinate, common ratios of the two varieties occur in the F2 generation.

21. Mendelian Ratios for Monohybrid Crosses
F2 Ratios
Genotypic Ratio 1:2:1
Phenotypic Ratio 3:1

22. F = dominant- freckles present f = recessive – freckles absent
Mono-hybrid Crosses
F = dominant- freckles present
f = recessive – freckles absent
Perform the monohybrid cross for the given parents genotypes.
Determine the genotypes of the offspring.
3. Determine the genotypic and phenotypic ratios for the F1 generation.
4. What type(s) of children would result if a homozygous dominant male mated with a heterozygous female?

23. White = WW or Ww and Black = ww 50% are black and 50% are white
The Test Cross
White = WW or Ww and Black = ww
ww x W?
Crossing an individual of UNKNOWN genotype, which shows a dominant trait, with an individual that has a recessive trait.
If any offspring show the recessive trait, then the individual is heterozygous; but if all offspring show the dominant trait, then the individual is homozygous. W w
50% are black and 50% are white w Ww ww w Ww ww W W w Ww Ww
100% are white w Ww Ww

24. Mendel’s Laws Mendel’s First Law – The Law of Segregation
During gamete formation (meiosis) two alleles of a gene pair segregate or separate from each other. The gametes are not a blend of the two traits.

25. Mendel’s Laws Mendel’s Second Law – The Law of Independent Assortment
Segregation for different pairs of alleles occurs independently. A plant that is heterozygous for two pairs of alleles can form four types of gametes.

26. Multiple Trait Analysis Dihybrid Cross Ex. Seed colour and seed shape
A cross which looks at the inheritance patterns for two characters or traits.
Ex. Seed colour and seed shape
Ex. RRYY x rryy
Ex. RrYy x RrYy P F1 F2

27. Crossing the F1 Generation
When the F1 generation is allowed to self-pollinate, common ratios of the two traits occur in the F2 generation.

28. Mendelian Ratios for Dihybrid Crosses
F2 Ratios
Phenotypic Ratio 9:3:3:1

29. B = black fur b = brown fur
Guinea Pig Genetics
B = black fur b = brown fur
S = short fur s = long fur
P: BBSS x bbss
F1: BbSs
F2: ?

31. Mice Genetics
In mice, grey coat colour, G, is dominant to white coat, g, and having a long tail, T, is dominant to short, t.
What fraction of F1 mice would you predict to have grey coats and short tails when a male ggTt parent is crossed with a female GGtt parent?
Answer: % Grey coat, short tails
50% Ggtt and 50% GgTt 50% Grey coat, long tails