1. Intro to Genetics and Mendel Honors Biology Ms. Kim

2. Transmission (passing down) of Traits
How?
One possible explanation of heredity is a “blending” hypothesis
genetic material contributed by two parents mixes

3. Another Hypothesis
An alternative to the blending model is the hypothesis of inheritance (genes)
Parents pass on discrete heritable units (factors) called genes
The Novelty Gene Video

4. Gregor Johann Mendel (1843)
Austrian Monk- “Father of Modern Genetics”
Documented a mechanism of inheritance through his experiments with garden peas
The scientific study of heredity is called GENETICS!
Worked with pea plants in his monastery
Correctly believed that heritable factors (genes) retain their individuality from generation to generation
i.e. – marbles (no blending of colors!)
Figure 14.1

5. Gregor Johann Mendel
Mendel used the scientific method to identify two laws of inheritance
Mendel discovered the basic principles of heredity
By breeding garden peas in carefully planned (CONTROLLED) experiments

6. Mendel’s Experimental Method
Why did Mendel choose pea plants?
available in many varieties of traits
They have seven distinct & observable traits
easy to get
he could strictly control which plants mated with which
Grow quickly
They reproduce quickly & have a short life cycle
They produce many offspring in one cross

7. Mendel’s Experimental Cross
Purebred white and purple flowers
All Purple
All White
Both White/Purple
Offspring were allowed to self pollinate
White flowers reappear in some offspring
What did Mendel notice? Did the trait for white flowers disappear in F1 generation?

8. Some genetic vocabulary
Character: a heritable feature, such as flower color
Trait: a variant of a character, such as purple or white flowers

9. Mendel observed the same pattern
In many other pea plant characters

10. Pea Plant Fertilization
Self fertilization : mate with self  produce identical offspring
TRUE or PURE breeds
Cross fertilization : mate with another  can produce different offspring
HYBRIDS
Colored Cotton Video

11. Mendel’s Experiment Mendel only looked at “either-or” characters
Ex: Purple OR white flowers
Mendel started his experiments with “true-breeding”
Made through self fertilization so plants are “TRUE” for only 1 trait
Known as HOMOZYGOUS for trait

12. What was Mendel’s Procedure?
1. He made 14 “TRUE BREEDS”
1 for EACH trait he looked at
These are the original parents
Are called the P generation

13. What was Mendel’s Procedure?
2. He used cross fertilization to mate 2 true breeds for same gene
Ex: Purple vs white flower color
3. He collected the offspring (progeny)
The hybrid (mixed) offspring of the P generation
Are called the F1 generation

14. What was Mendel’s Procedure?
4. He crossed (using cross fertilization) male and female from F1 progeny
When F1 individuals are mated together
The F2 generation is produced

15. What did Mendel Discover?
P  F1  F2
A 3:1 ratio, purple to white flowers, in the F2 generation
P Generation
(true-breeding
parents)
Purple
flowers
White 
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation
Where did the white color go?

17. Genetics Vocab
Mendel worked with his pea plants until he was sure that all were true- breeding varieties (pure bred)
P generation  parental generation have offspring called the F1 generation (hybrids)
Hybrid (F1) the offspring of two true breeding varieties
If F1 generations self-fertilize/cross, their offspring are called the F2 generation

18. What are Mendel’s factors?
Mendel’s “factors” are now called alleles
Alternative version or form of a gene
Figure 14.4
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers F f

19. Mendel’s Experiments After studying pea plants, Mendel concluded that:
Traits are passed from one generation to the next through genes.
Each trait is controlled by a different form of a gene called an allele
Some alleles are dominant to others called recessive traits
New question: Have the recessive alleles disappeared or are they still present in the parents?
Genes are the different parts of the DNA that decide the genetic traits a person is going to have. Alleles are the different sequences on the DNA-they determine a single characteristic in an individual.

20. What did Mendel Conclude?
Mendel reasoned that
In the F1 plants, only 1 factor (ex: purple flower) was affecting physical outcome color in hybrids
This factor was dominant and the hidden factor was recessive

21. Recessive is…
Represented by a lowercase letter (it is NOT the letter itself, though)
an allele that does NOT produce a characteristic effect when present with a dominant allele
only expressed when present with another (identical) recessive allele
This is known as the homozygous condition
aa or hh

22. Dominant is… Represented by a uppercase letter
an allele that produces the same trait whether inherited with a another dominant allele (homozygous) or with a recessive allele (heterozygous)
Aa or AA
The allele that is expressed if present

23. Frequency of Dominant Alleles
Not necessarily better, stronger, etc. than recessive alleles
Ex: Polydactyl

24. Mendel’s Experiments
Mendel crossed the first generation and saw that the recessive trait showed up in about 1 of 4 plants.
Conclusion: Law of Segregation!
What is the Law of Segregation?!
Organisms inherit two copies of each gene
(one from each parent)
Organisms donate only one copy of each
gene in their gametes (sex cells)
Therefore, the two copies of each gene
segregate (separate) during gamete
formation (meiosis)
Segregation (means separation): Alleles for the SAME trait (like height) separate (segregate )into different gametes; i.e., one sperm gets a "t" and the other gets a "T" (that's it !)
Segregation of the alleles happens during the formation of gametes
Each gamete will carry one form of the allele

25. The Law of Segregation: Mendel’s 1st Law
Each gamete ONLY gets 1 allele

26. More about Alleles… located on homologous chromosomes
Each individual has 2 alleles for the same gene
located on homologous chromosomes
Each parent passes 1 allele for each gene to his/her offspring
In sperm or egg
What stage of meiosis are alleles segregated into gametes?
Meiosis Anaphase I

27. a A

28. Mendel’s Observations
Used pea plants to see patterns in the way various traits were inherited
Using his data, he saw that different traits are inherited separately
Example: Green pea color isn’t always inherited with wrinkled pea shape
Green peas can be smooth and round too!
This is called the Law of Independent Assortment!

29. What is the Law of Independent Assortment?
Allele pairs (traits) separate independently of each other during gamete formation (meiosis)
Different traits are inherited separately
Example – peas can be green and wrinkled OR green and round
This explains genetic diversity among organisms
Presence of one trait does not affect the presence of another trait! Law of Independent Assortment – each pair of alleles for different traits segregate independently of other pairs of alleles during gamete formation

30. Law of Independent Assortment: Mendel’s 2nd Law
Says genes are inherited independently of other genes
Genes are not linked unless on the same chromosome!
Mendel assumed traits occur on different chromosomes!
Occurs during Metaphase I

31. Useful Genetic Vocabulary
Homozygous
A pair of IDENTICAL (same) alleles for that gene
Exhibits true-breeding
aa = homozygous recessive (or just recessive)
HH = homozygous dominant
Heterozygous
Pair of alleles that are different for that gene
Aa or Hh

32. More Genetic Vocabulary
An organism’s genotype
Is its genetic (DNA) makeup
A.k.a.-the allele combination (includes 2 alleles)
An organism’s phenotype
Is its physical outcome of the genotype
Ex: blue eyes or AB blood type

33. Mendelian Genetics… aka- COMPLETE DOMINANCE
If an organism is heterozygous (Hh),
The effect of the recessive allele is HIDDEN
Heterozygous and homozygous dominant have SAME phenotype
The 1st allele is “completely dominant” over the 2nd allele

34. Phenotype Genotype 3 Purple White Ratio 3:1 Ratio 1:2:1 1 Purple 2
Figure 14.6 3
Phenotype
Purple
White
Genotype PP
(homozygous) Pp
(heterozygous) pp
Ratio 3:1
Ratio 1:2:1 1
Purple 2 Pp
(heterozygous)
Purple 1 1

35. Why Did Mendel Keep Getting the SAME results?
We can answer this question using a Punnett square
a diagram (box) used to predict probabilities of possible outcomes for offspring that will result from a cross between 2 parents
SHOWS EXPECTED RESULTS (not necessarily actual)

36. Practice Which flower color is recessive? White Purple Neither Both
PURPLE X WHITE PURPLE PURPLE & WHITE
Which flower color is recessive?
White
Purple
Neither
Both

37. Practice Which flower color is recessive?
PURPLE X WHITE PURPLE PURPLE & WHITE
Which flower color is recessive?
WHITE
What would the genotype be for the recessive flower?
A. PP homozygous dominant
B. pp homozygous recessive
C. Pp Heterozygous

38. Practice Which flower color is recessive?
PURPLE X WHITE PURPLE PURPLE & WHITE
Which flower color is recessive?
WHITE
What would the genotype be for the recessive flower?
pp homozygous recessive
Which flower color is dominant?
White
Purple
Neither
Both

39. Practice Which flower color is recessive?
WHITE
What would the genotype be for the recessive flower?
pp homozygous recessive
Which flower color is dominant?
PURPLE
What would the genotype be for the dominant flower color?
A. PP homozygous dominant
B. Pp heterozygous
C. pp homozygous recessive
D. Both A & B
PURPLE X WHITE PURPLE PURPLE & WHITE

40. Genetics Vocab (pt 2)
Monohybrid cross  cross where parents differ in only one trait (Rr x rr)
Dihybrid cross  cross where parents differ in two traits (RrHh x rrHH)
Punnett square – a diagram that shows the gene combinations that might result from a genetic cross of two parents