1. Introduction to Genetics
Chapter 11

2. How can two brown rabbits have a white offspring?
If two white rabbits mated what color would their offspring be?

3. Gregor Mendel The work of Gregor Mendel
Austrian monk who is the father of Genetics
Genetics – the scientific study of heredity
Mendel worked with pea plants
Spent 14 years researching pea plants and how they reproduced

5. Gregor Mendel
Fertilization – when male and female reproductive cells join
Male – pollen or sperm
Female – eggs
Gamete – the individual egg or sperm
Seed – fertilized egg or new cell (zygote)
Fertilization results in a new cell which will form an embryo
Pea plants are normally self pollinating – sperm and egg come from same flower
This means that the seeds produced have the same characteristics as the plant they came from

6. Gregor Mendel
True-breeding – if they were allowed to self-breed they would have identical offspring.
Experiment
Cross-pollination
Mendel took sperm from one plant and fertilized eggs from other plants
Mendel was interested in what would happen if different ones were crossed

7. Gregor Mendel Traits – a specific characteristic (ex. Brown hair)
Genes and Dominance
Traits – a specific characteristic (ex. Brown hair)
Seed color
Plant height
Parental generation (P) – Original pair of plants
Filial generation (F1) – offspring, progeny
Hybrids – offspring of crosses between parents with different traits
Mendel studied seven different pea plant traits – shape, seed color, seed coat shape, pod shape, pod color, flower position, plant height
Each trait had two distinctive charateristics
Mendel crossed each of the 7 contrasting plants and observed the outcome

9. Gregor Mendel
Genes – chemical factors that determine traits (Hair color)
Alleles – different forms of traits (Brown vs. blond hair, brown vs. blue eyes)
What do we think happened – blending?
Each one had a character of only one parent – the other characteristic seemed to vanish
Two conclusions
Inheritance is determined by factors passed from generation to generation – Genes!

10. Gregor Mendel 2 Principles
Principle of Inheritance – factors are passed from one generation to the next.
Principle of Dominance – some alleles are dominant and other are recessive
Dominant traits will always show over recessive traits
Second conclusion – Principle of dominance.

11. Gregor Mendel Segregation F1 generation self-pollinated F1 X F1 = F2
F1 Cross
¼ of the F2 plants now show the recessive traits
Did the recessive gene really disappear? (in the F1 generation)
Self pollinated the F1 generation to produce F2
Recessive trait reappeared

12. Gregor Mendel Explaining the F1 Cross
When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene.
Therefore, each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness.
Segregation -separation of alleles during gamete formation
Mendel now proposed the allele for short had seperated from the allele for tall.

13. Probability and Punnett Square
Probability in Genetics
Probability – the likelihood that a particular event will occur
Coin flip
½ or 50 %
Chance that you’ll end up with heads 3 times in a row.
½ X ½ X ½ = 1/8
Past outcomes do not affect future ones
With the coin flip there are two possible outcomes – head or tail
Chance for heads – 1 out of two
Now flip it three times – the chance of getting heads is still 1 out of two, but it applies to each of the three times – 1 out of 8
Previous flips do not affect the current.
We can use probability to predict genetic cross outcomes.

14. Probability and Punnett Square
A diagram showing the gene combinations that might result from a genetic cross
Letters represent each allele
Top and left letters are the parents genes
The four boxes show each possible gene combination
Punnett squares can be used to predict, not determine, the results of a genetic cross.
Go back to Mendel’s F1 cross – this will show the chances of possible results for the F2 generation.

15. Probability and Punnett Square
Homozygous-- two identical alleles for a particular trait.
Heterozygous – two different alleles for a particular trait

16. Probability and Punnett Square
GG, Gg, gg are all of the possible combinations of genes
GG means homozygous dominant
gg means homozygous recessive
Gg means heterozygous
Homozygous organisms are true breeding for a particular trait.
Heterozygous organisms are hybrids

17. Probability and Punnett Square
Phenotype
Physical characteristics
Tall or short
Rolling the tongue
Genotype
Genetic makeup
DD, Dd, or dd

18. Probability and Punnett Square
Probability and segregation
Each parent only donates one of their two alleles to each offspring
Mendel had made the assumption that each parent was able to donate only half of their genetic information to their offspring.

19. Probability and Punnett Square
Probabilities predict averages, not exact outcomes
Probability is more accurate when you have more chances
Just as when you flip a coin twice, you may get one head and one tail OR you may get two heads OR two tails
To get the expected 50:50 ratio you would have to flip the coin many times.
This true for genetics – the more offspring you have, the closer to the expected values you will be.

20. REVIEW What is a gamete? What is a zygote? What is the genotype?
Sperm or egg cell
What is a zygote?
Fertilized egg cell
What is the genotype?
Genetic make-up
What is the phenotype?
Trait that shows
What is heterozygous?
2 different alleles
What is homozygous?
2 identical alleles

21. 11-3 Exploring Mendelian Genetics
Two-factor cross : F1
Crossing true-breeding organisms does not answer this question
They do produce hybrid offspring used for the next test
RrYy
Mendel then wondered if the genes were seperating, was there a factor in how they separated?

22. Exploring Mendelian Genetics
Two-factor cross : F2
9:3:3:1 ratio
Mendel did an experiment called the two-factor cross
First he crossed true breeding round yellow peas (RRYY) with palnts that produced wrinkled green peas (rryy) – All the F1 offspring produced round yellow peas
This showed that the alleles for round and yellow are dominant – the genotype produced was RrYy – This cross provided the plants needed for the second part of the experiment

23. Exploring Mendelian Genetics
Independent assortment
States that genes for different traits can segregate independently during the formation of gametes.
Accounts for the many genetic variations observed in plants, animals, and other organisms.
Mendel knew that F1 was all heterozygous – he then self pollinated the F1 generation to produce the F2 generation. He wondered if the two dominant genes would stay together. Based on his results (315 round and yellow, 32 wrinkled and green; these were parental phenotypes; 209 were not like the parents of the F1 generation thus demonstrating that the alleles segregate independently).

24. Exploring Mendelian Genetics
Mendel’s Principles
The inheritance of biological characteristics is determined by individual units known as genes. Genes are passed from parents to their offspring.
In cases in which two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.
In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed.
The alleles for different genes usually segregate independently of one another.

25. Exploring Mendelian Genetics
Beyond dominant and recessive alleles
Incomplete dominance – alleles are not completely dominant
Even though Mendel made incredible discoveries, there are exceptions to the rule.
Not all genes show simple patterns of dominance and recessiveness.
Most genes have more than two alleles.
Some traits are even controlled by more than one gene
Some alleles are neither dominant or recessive, and many traits are controlled by multiple alleles or multiple genes. – Height, skin color, eye color
Incomplete dominance is a particular situation where no allele wants to take over – therefore, you get a blending of the two alleles.

26. Exploring Mendelian Genetics
Codominance – both alleles contribute to the phenotype
Codominance is the opposite of Incomplete dominance – both traits want to take over and compete, and thus contribute to the appearance of the individual.
Instead of blending, you get variations in patterns.

27. Exploring Mendelian Genetics
Multiple allele – more than two alleles
Polygenic trait
Two or more genes control one allele
Blood type is controlled by one gene with three alleles A, B, and O
As we said height, skin color, eye color are all controlled by more than one gene.

28. Exploring Mendelian Genetics
Mendel’s Principles
Apply to animals as well as plants
More importantly to humans
Ex. – Fruit fly – good because they reproduce large numbers and can reproduce quickly

29. Exploring Mendelian Genetics
Genetics and the Environment
Environment affects how genes are displayed

30. 11-4 Meiosis Meiosis Chromosome number
Human
Body cell – 46 Chromosomes
23 from mom
23 from dad
Homologous – the 2 sets of 23 chromosomes.
Mendel did not know where these genes were located in the cell, he just knew they were there.
It didn’t take long for scientists to figure out they were in the nucleus.
Each of the 4 from mom has a corresponding 4 from dad

31. 11-4 Meiosis
Diploid – cell that has both sets of homologous chromosomes 2N
Body cells
Haploid – cells that have just one set of chromosomes
1 N
Sex cells

32. 11-4 Meiosis Phases of meiosis
A process of reduction and division in which the number or chromosomes per cell is cut in half through the separation of homologous chromosome in a diploid cell
Meiosis is a process by which diploid cells can produce haploid cells used for reproduction.
Meiosis is essentially doing mitosis twice with a few differences.

33. 11-4 Meiosis
Meiosis I
Each chromosome lines up with its corresponding homologous chromosome making a tetrad
They exchange genetic information called crossing-over
Homologous chromosomes separate and form two new cells with different chromosome and alleles
A tetrad is essentially 4 chromatids lined up together – this occurs after the chromosome is replicated.
In mitosis, the two sister chromatids line up on the equator and then are pulled apart to opposite poles.
In meiosis, crossing over allows for exchanges of information and production of new combinations of alleles.

34. 11-4 Meiosis Meiosis II Two new cells divide
Neither cell makes a copy of the chromosomes
All four new cells have one set of chromosomes
The two new cells from Meiosis I can now go through Meiosis II, EXCEPT they do NOT replicate – this would defeat the purpose of making haploid cells.

35. 11-4 Meiosis Gamete formation Haploid cells Sperm or pollen Eggs
For males, there are 4 gametes produced, but females only get to keep one of the four. The remaining cells will die.

36. 11-4 Meiosis Mitosis vs. Meiosis
Mitosis results in two genetically identical diploid cells
Meiosis results in four genetically different haploid cells

37. 11-5 Linkage and Gene Maps Gene linkage
Chromosomes assort independently, not individual genes
Genes are located on chromosomes so why don’t all the genes located on a particular chromosome get inherited together? – YES – There are certain genes that will be inherited together.

38. 11-5 Linkage and Gene Maps Gene maps
Shows the exact location of each known gene on one chromosome
So all genes linked on the same chromosome will be inherited together – right? Wrong!
Remember, crossing over can shuffle the genes on a chromosome.
Sturtevant hypothesized that the further two genes were apart on a chromosome, then the more likely they would be separated by crossing over. The rate at which these genes separate and recombine could produce a map of the distances between genes.
This process has been use to produce maps of types of organisms chromosomes.