1. Cells and Heredity Chapter 3: Genetics – The Science of Heredity
Lesson 1: What is Heredity?

2. Gregor Mendel Priest in the mid 1800’s
Studied heredity by investigating the inheritance of traits among the pea plants.
Heredity: the passing of physical characteristics from parents to offspring.
Trait: specific characteristic.
Genetics: the study of heredity

3. Gregor Mendel
Mendel began with purebred plants, meaning they came from generations of the same trait (for example, tall plants vs. short plants).
The P (parent) generation consisted of a purebred tall plant and a purebred short plant.

4. Gregor Mendel All of the offspring of the P generation were tall.
They are called the F1 generation.
When Mendel then crossed two of these F1 plants together, he found that about ¾ (or what percent?) were tall, while about ¼ were short.
75%
They were called the F2 generation.

5. Gregor Mendel
Mendel repeated this with several traits, and consistently found that 100% of the F1 generation showed one trait.
He also found that in the F2 generation, the same trait showed up about 75% of the time, with the “lost” trait returning about 25% of the time.

6. Alleles and Inheritance
Gene: factors that control a trait.
Alleles: different forms of a gene (example: height – short or tall).
An organism receives one allele from its mother and one allele from its father.
The traits are controlled by the combination of alleles.

7. Dominant vs. Recessive
Dominant: trait always shows up in the organism when the allele is present (only needs to have one copy).
Represented with a capital letter
Recessive: trait only shows up in the organism when two copies of the allele are present.
Represented with a lowercase letter

8. Dominant vs. Recessive
When there are two dominant alleles present, the trait is said to be homozygous dominant.
Ex: the purebred tall pea plant in the P generation. (TT)
When there are two recessive alleles present, the trait is said to be homozygous recessive.
Ex: the purebred short pea plant in the P generation. (tt)
When there is one of each allele present, the trait is said to be hybrid, or heterozygous.
Ex: all of the pea plant F1 generation (Tt)

9. Cells and Heredity Chapter 3: Genetics – The Science of Heredity
Lesson 2: Probability and Heredity

10. What is Probability?
Probability: a number that describes how likely it is that an event will occur.
Think of a coin – there is a 50% chance of heads and 50% chance of tails.
Can be said “1 in 2” chance
Can be a fraction: ½
Can be a percent: 50%

11. Punnett Squares
Shows the combination of alleles that parents can pass on to their offspring based on probability.
Follow the steps to create a Punnett Square. Our example will have two heterozygous parents.

12. Step 1:
Draw a box and divide it into four equal squares.

13. Step 2:
Put the male parent’s alleles along the top, and the female parent’s alleles along the side. T t T t

14. Step 3: Copy the female’s alleles into the boxes to the right. T T t t

15. Step 4: Copy the male’s alleles into the boxes below. TT Tt tT tt T t

16. Reading the Punnett Square
In this case (and all cases of two heterozygous parents), the offspring have a 25% chance of being HD, a 25% chance of being HR, and a 50% chance of being heterozygous.
TT Tt
tT tt T t T t

17. Phenotype and Genotype
The phenotype is an organism’s physical appearance, or visible traits.
The genotype is an organism’s actual genetic makeup, or the way their alleles are.
Remember, we know that only one copy of a dominant allele needs to be present for that trait to show.

18. What does this mean? This is our example from yesterday.
How many of the offspring have a “T”?
How many of the offspring have “tt”?
TT Tt
tT tt T t T t

19. What does this mean?
Since 3 of the boxes have a “T”, we can say that ¾ or about 75% of the offspring will show the dominant allele.
Since 1 of the boxes has “tt”, we can say that ¼ or about 25% of the offspring will show the recessive allele.
TT Tt
tT tt T t T t

20. The genotype vs. phenotype
Genotypes: ¼ HD, ½ heterozygous, ¼ HR
Phenotypes: ¾ tall, ¼ short
TT Tt
tT tt T t T t

21. Cells and Heredity Chapter 3: Genetics – The Science of Heredity
Lesson 3: Patterns of Inheritance

22. Inheriting Traits
The examples we have seen so far only two options, like tall or short.
This is not how most traits are inherited; they are usually the result of a very complex pattern in which there are more than two options.

23. Incomplete Dominance
Occurs when one allele is only partially dominant.
This means that there could be some sort of third option when the trait is heterozygous.
For example, a red snapdragon mixed with a white snapdragon results in a pink snapdragon.

24. Codominance Occurs when both alleles for a gene are expressed equally.
For example, a black hen and a white rooster result in offspring that have both black and white feathers.

25. Multiple Alleles
This is when three or more possible alleles determine the trait.
For example, rabbit fur color has more than three options.
The color of the rabbit’s fur depends only on two alleles, but the variety of colors is more than that.

26. Polygenic Inheritance
Occurs when more than one gene affects a trait.
Human height is an example of this. You do inherit genes from your parents to help determine your height, but other factors, such as diet, exercise, sleep, and drug use can affect height as well.

27. Genes and the Environment
An inherited trait is something that is passed on to you from your parents.
A learned trait, or acquired trait, is something that you learn how to do over time.
Environmental factors, as mentioned on the last slide, can greatly impact how genes are expressed.

28. Cells and Heredity Chapter 3: Genetics – The Science of Heredity
Lesson 4: Chromosomes and Inheritance

29. Chromosomes and Inheritance
Walter Sutton used Mendel’s work to find the chromosome theory of inheritance: genes pass from parents to their offspring on chromosomes.
One allele comes from the mom, one allele comes from the dad, and together they make a chromosomal pair.

30. Human Chromosomes Humans contain 46 chromosomes.
You receives 23 chromosomes from your mom and 23 chromosomes from your dad.
Each chromosome matches with another chromosome, forming 23 pairs.
However, there are 20,000-25,000 traits found in these 23 pairs.

31. Meiosis
The process by which the number of chromosomes is halved as sex cells, or gametes, form.
In meiosis, a cell begins by duplicating its DNA. Then, it divides normally into two cells.
These two cells have a full set of chromosomes (called diploid). Without duplicating their DNA, they divide again into 4 daughter cells.
These daughter cells only have half of the chromosomes, and are called haploid.

32. Meiosis 1 Prophase 1: chromosomes pair up in doubled homologs.
Metaphase 1: each set lines up in the center of the cell.

33. Meiosis 1 Anaphase 1: the homologs separate into two matching sets.
Telophase 1: a new cell membrane forms and the parent cell forms two daughter cells that are 2n.

34. Meiosis 2 Prophase 2: the two copies of each homolog remain attached.
Metaphase 2: the chromosomes line up in the middle.

35. Meiosis 2
Anaphase 2: chromosomes split and one copy of each homolog is moved to the opposite ends.
Telophase 2: both daughter cells divide into 4 haploid cells (1n).

36. Meiosis

37. Mitosis vs. Meiosis Gametes undergo meiosis.
2 cell divisions in meiosis, but DNA is copied only once.
DNA is copied with every division in mitosis.
Meiosis creates 4 haploid cells with half of the genetic info; Mitosis creates 2 diploid cells with exact genetic info.