1. The Basic Laws of Genetics
Principles of Genetics

2. Genetic Information
Gene – basic unit of genetic information. Genes determine the inherited characters.
Genome – the collection of genetic information.
Chromosomes – storage units of genes.
DNA - is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life
Fig. 4: A chromosome is a long DNA molecule and contains many genes, each with it’s own coded information.
Image from
Fig. 5: The coded information in genes instructs the cell to make proteins that control specific traits or phenotypes such as the color of cells and tissues.

3. Chromosome Logical Structure
Locus – location of a gene/marker on the chromosome.
Allele – one variant form of a gene/marker at a particular locus.
Locus1
Possible Alleles: A1,A2
Locus2
Possible Alleles: B1,B2,B3

4. Human Genome Most human cells contain 46 chromosomes:
2 sex chromosomes (X,Y):
XY – in males.
XX – in females.
22 pairs of chromosomes named autosomes.
Image from

5. 1. Traits are passed or inherited from one generation to the next.

6. What part of your cells contain your genetic instructions?
Your genetic instructions are located in the nucleus of every cell, stored within chromosomes.

7. How will your traits be passed on to your future children?
Fathers pass genetic instructions to offspring through sperm cells.
Mothers pass genetic instructions to offspring through egg cells.

8. Chromosomes: How many? Where are they?
A human body cell contains 46 chromosomes, which are stored inside every cell’s nucleus.
Human sex cells (sperm or egg) contain 23 chromosomes each.

9. Chromosome # Egg (haploid # = 23) Sperm(haploid # = 23)
ZYGOTE (diploid # = 46)
Ex. Humans
=46

10. 2. Traits of an organism are controlled by genes.
A gene is a section of a chromosome, that codes for a specific trait.
Chromosomes are made of tightly wound strands of DNA and proteins.
gene

12. 3. Organisms inherit genes in pairs, one from each parent.
Human sex cells (sperm or egg) contain chromosomes each.
When the egg is fertilized, the new baby will have chromosomes in each of its cells, which is the correct number of chromosomes for a human.

13. Sexual Reproduction (2 parents) creates unique offspring.
In plants, male chromosomes are passed through pollen grains to female egg cells. Once fertilized, these eggs will develop into seeds.
The new plants will receive a combination of traits from both parents.

14. 4. Some genes are dominant, while others are recessive.
“Stronger” traits are called dominant.
“Weaker” traits are called recessive.
Geneticists use symbols (letters) to represent the different forms of a gene.

16. Dominant traits are represented by a capital letter.
Yellow seeds are dominant…….. Y

17. Recessive genes (for the same trait) are represented by THE
Recessive genes (for the same trait) are represented by THE SAME lower case letter.
Green seeds are recessive………. y
Dominant (yellow) = Y
Recessive (green) = y

18. In pea plants, tall is dominant over short
In pea plants, tall is dominant over short. The letter used to represent the tall gene is T.
The short gene is represented by t.
Tall =T
Short = t

19. In humans, brown eyes are dominant over blue and green; green is dominant over blue. Amount of pigment in the iris also determines eye color.
Brown = B
Blue = b

20. 5. Dominant genes hide recessive genes when both are inherited by an organism.
Y + y = yellow seeds (yellow is dominant)
T + t = tall plant (tall is dominant)
B+ b = brown eyes (brown is dominant)

21. Homozygous vs. Heterozygous
Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait (TT or tt)
Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt) RR rr Rr

22. A PUREBRED organism has two of the same genes for a trait.
TT = purebred TALL
tt = purebred SHORT

23. A HYBRID organism has two different genes for a trait.
Tt = hybrid TALL (tall is dominant.)
Bb = hybrid for black feathers (black is dominant.)

24. Will a hybrid human with the genes Bb have brown or blue eyes?
Bb = Brown eyes
The dominant gene (brown), will be expressed in a hybrid.

25. Why must all blue eyed people be PUREBRED for that trait?
If the dominant gene is present, it will always be expressed.
The only possible gene combination for blue eyes is bb.

26. Genotype vs. Phenotype
Genotype – The genetic makeup of an organism; The gene (or allele) combination an organism has.
Example: Tt, ss, GG, Ww
Phenotype – The physical characteristics of an organism; The way an organism looks
Example: Curly hair, straight hair, blue eyes, tall, green

27. Homozygous vs. Heterozygous
Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait (TT or tt)
Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt) RR rr Rr

28. Punnett Squares
Punnett Square – Diagram showing the gene combinations that might result from a genetic cross
Used to calculate the probability of inheriting a particular trait
Probability – The chance that a given event will occur

29. Punnett Square
Parent
Offspring
Parent

30. How to Complete a Punnett Square

31. Y-Yellow
y-white
Genotype:
1:2:1
(YY:Yy:yy)
Phenotype:
3 Yellow
1 White

32. You Try It Now!
Give the genotype and phenotype for the following cross: TT x tt (T = Tall and t = Short)

33. TT x tt
Step One: Set Up Punnett Square (put one parent on the top and the other along the side)
T T t

34. TT x tt
Step Two: Complete the Punnett Square
T T t Tt

35. TT x tt Tt t Genotype: 4 - Tt Phenotype: 100% Tall
Step Three: Write the genotype and phenotype
T T t
Genotype:
4 - Tt
Phenotype:
100% Tall Tt
Remember: Each box is 25%

36. You Try It Now!
Give the genotype and phenotype for the following cross: Tt x tt

37. Tt x tt
Step One: Set Up Punnett Square (put one parent on the top and the other along the side)
T t t

38. Tt x tt
Step Two: Complete the Punnett Square
T t t Tt tt

39. Tt x tt Tt tt t Genotype: Tt - 2 (50%) tt - 2 (50%) Phenotype:
Step Two: Complete the Punnett Square
T t t
Genotype:
Tt - 2 (50%)
tt - 2 (50%)
Phenotype:
50% Tall
50% Short Tt tt
Remember: Each box is 25%

40. Some Terminology P1 – Original parents F1 – First generation
F2 – Second generation
P1 X P1 = F1
F1 X F1 = F2

41. 6. Some genes are neither dominant nor recessive
6. Some genes are neither dominant nor recessive. These genes show incomplete dominance.
If an organism shows INCOMPLETE DOMINANCE for a trait, the hybrid will show a blend of the two traits.
In humans, hair texture is such a trait.
Because neither curly or straight hair is recessive, no lower case letters are used.
CC = curly hair
CS = wavy hair
SS = straight hair

42. These basic laws of inheritance control the traits of all living organisms on Earth.