1. Introduction to Genetics

2. Some Vocabulary Words to Get You Started…
Heredity—the passing on of characteristics from parents to offspring
Genetics—the branch of biology that studies heredity
Trait—inherited characteristic
Gene—section of DNA that determines a trait

3. Who was Gregor Mendel? Austrian Monk
Taught Math and Biology to High School Students
First person to succeed in predicting how traits are transferred from one generation to the next
“Father of Genetics”

4. Monohybrid Crosses
Mendel conducted his experiments on the garden pea plants at the monastery.
His first experiments are called monohybrid crosses because the two parent plants differed by a single trait.

5. P1, F1, F2 P1=the parent generation; the original cross
F1=the offspring of the parents; “filial” 1
F2=the offspring of F1; “filial” 2

6. Two Chromosomes, Two Genes
Recall that each organism has two sets of chromosomes.
Chromosomes contain genes.
Therefore, each organism has two genes for each trait. Ex. A
Different forms of a gene are called alleles. Ex. A or a

7. Homozygous and Heterozygous
HOMOZYGOUS=the two alleles for a trait are the same Ex. TT or dd
HETEROZYGOUS=the two alleles for a trait are different Ex. Yy
Rr ww Xx
HH Ss Ll

8. Purebred or Hybrid?
Homozygous individuals are also known as purebred or “true-breeding.”
Heterozygous individuals are also known as hybrids.

9. Phenotypes and Genotypes
PHENOTYPE=the way an organism looks Ex. Tall or short
GENOTYPE=the gene combination an organism contains Ex. Aa
red hair YY green eyes
Rr ww blue skin

10. The Rule of Dominance
Some alleles are dominant, and others are recessive.
Dominant alleles are always seen when they are present and have the ability to “hide” recessive genes.
Recessive alleles are the traits that seemed to disappear.

11. More on Dominance
When recording the results of crosses, it is customary to use the same letter for different alleles of the same gene.
An uppercase letter is used for the dominant allele, and a lowercase letter is used for the recessive allele.
Dominant alleles are always written first.
Ex. Y=yellow y=green Yy

12. Crossing the F1 Generation

13. The Law of Segregation
To explain how a trait could disappear and then reappear, Mendel proposed the law of segregation.
He stated that the two alleles for each trait must separate when gametes are formed.
A parent passes on only one allele for each trait to each offspring.

14. Reginald Punnett In 1905, Reginald Punnett, an English
Biologist, came up with
a way of predicting the
outcomes of a cross
when genotypes are
known. It is called the
Punnett Square.

15. A Cross of Yy and Yy

16. A Cross of RR and rr

17. A Bit About Probability
Punnett Squares show all the possible combinations of gametes and the likelihood that each will occur.
In reality, you don’t always get the exact ratio shown in the square.
The larger the number of offspring you look at, the more closely your results will mirror the predicted ratios.

18. Dihybrid Crosses
Mendel later crossed pea plants that differed in two different traits instead of just one.
Example: Yellow Seeds=dominant
Green Seeds=recessive
AND Round Seeds=dominant
Wrinkled Seeds=recessive

19. Dihybrid Genotypes T=tall, t=short, B=blue, b=white
The genotype of a homozygous tall and white individual would be TTbb.
What would be the genotype of a heterozygous tall and blue individual?
TtBb
What would a ttBB individual look like?
Short and blue

20. Forming Gametes
The genotype of a plant that is heterozygous for both round and yellow seeds would be RrYy.
To determine the gametes this parent would produce, use the foil method:
First letters of each trait: RY
Outer letters: Ry
Inner letters: rY
Last letters of each trait: ry
The four gametes produced: RY, Ry, rY and ry

21. Dihybrid Crosses A=axial A=terminal P=purple P=white
Cross two plants heterozygous for axial and purple flowers.

22. Cross for RrYy and RrYy

23. As you might expect, some of the F2 plants had yellow round seeds and some had green wrinkled seeds.
BUT some also had yellow wrinkled seeds and round green seeds.
This occurred in a predictable 9:3:3:1 ratio.

24. The Law of Independent Assortment
Mendel’s second law states that genes for different traits are inherited independently of one another.
In other words, the genes for seed shape and seed color are not attached to each other.
This occurs only if the genes are located on different chromosomes.

25. Mendel’s Laws Mendel proposed 3 genetic laws:
law of segregation - as the gametes mature, homologous chromosomes separate from each other and pass into different gametes
law of dominance - some genes are dominant to others, which are then called recessive
law of independent assortment - traits controlled by different genes pass to offspring independently of each other.

26. The Story So Far….
The traits we have studied have been inherited by simple Mendelian Inheritance.
Alleles are either dominant or recessive, and dominant covers up recessive.

27. Exceptions
But, not all traits are inherited in the same way……

28. Incomplete Dominance
One allele is not completely dominant over another.
The heterozygous phenotype is somewhere in between the two homozygous phenotypes.

30. Codominance Both alleles contribute to the phenotype.
No blending; both appear.
Ex: Speckled or roan colors, cholesterol- controlling proteins in human blood

32. Multiple Alleles
A gene with more than two alleles (forms) is said to be controlled by multiple alleles.
An individual can still only have two alleles.
Ex. Rabbit coat color and human ABO blood types

34. Polygenic Traits
Traits that are produced by the interaction of several genes
A wide variety of phenotypes results
Ex. Skin color and height in humans

37. Genetics and the Environment
The characteristics of an organism are not determined solely by the genes it inherits.
Characteristics are determined by interactions between genes and the environment.
Environmental factors include climate, soil conditions, availability of water and food, hormone levels, air quality, etc.

38. This unit made possible by the work of Gregor Mendel.