1. Introduction to Genetics
Chapter 11 - Biology

2. Gregor Mendel 1822-1884 - The father of genetics
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
True-breeding peas – self-pollinate
Wanted to stop self pollination he cut away the pollen-bearing male parts and dusted pollen from another plant onto the flower – cross-pollination

3. Mendel’s 7 Traits
Trait – specific characteristic, such as seed color or plant height – makes one different from the other
Mendel crossed plants with each of the 7 traits and studied their offspring (hybrids)
P (parental) – original pair of plants
F1 (first filial) – offspring
Filius and filia are Latin for “son” and “daughter”

4. Mendel’s 7 Traits cont.
He first concluded that biological inheritance is determined by factors that are passed from one generation to the next.
Factors that determine traits are genes
Different forms of a gene are called alleles
His second conclusion: the principle of dominance (some alleles are dominant & others are recessive)

5. Mendel’s 7 Traits cont.
Did the recessive alleles disappear, or were they still present in the F1 plants?
7 kinds of F1 to produce an F2 through self-pollination
Recessive alleles disappeared in F1 and reappeared in F2
Alleles in F1 separated from each other during the formation of sex cells (aka gametes)

6. When similar crosses were repeated, similar results were obtained
ie. Crossed 2 plants that were hybrid for stem height (Tt), about ¾ were tall and ¼ were short
Probability – likelihood that a particular event will occur
This can be used to predict the outcomes of genetic crosses

7. Punnett Square
Chart that shows all the possible combinations of alleles that can result from a genetic cross
used to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome T t TT Tt T Tt tt t

8. Coin Toss Lab Please take out your handout
We will use the remainder of the period to work on and complete this lab

9. How are dimples inherited?
Page 268 in Biology Text Book
1. Write the last 4 digits of your telephone number. These 4 random digits represent the alleles of a gene that determines whether a person will have dimples.
Odd = allele for the dominant trait of dimples
Even = allele for the recessive trait of no dimples
2. First 2 digits represent a certain father’s genotype. Symbols D and d to write his genotype.

10. 3. Use the last 2 digits the same way to find the mother’s genotype.
4. Construct a Punnett square for the cross of these parents. Then, using the Punnett square, determine the probability that their child will have dimples.
5. Determine the class average of the percent of children with dimples.

11. Homozygous – organisms that have 2 identical alleles for a particular trait (ie. TT or tt) – true-breeding.
Heterozygous – organisms that have 2 different alleles for the same trait (ie. Tt) – hybrid.

12. Phenotype – physical characteristics
Things you can see
Genotype – genetic makeup
Things you cannot see – ie. genes (made of DNA) on chromosomes
Mendel performed an experiment to see if seed shape affects seed color – crossed plants and recorded the two traits
One did not effect the other (independent assortment) – genes for different traits can segregate independently during formation of gametes.

13. Summary of Mendel’s Principles
Inheritance of biological characteristics is determined by genes.
When two of more forms of the gene for a single trait exist, some forms may be dominant and others recessive.
In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent. These are segregated from each other when gametes are formed.
Alleles for different genes usually segregate independently of one another.

14. Dominant & Recessive Alleles
Cases in which one allele is not completely dominant over another are called incomplete dominance (heterozygous phenotype is between the two homozygous phenotypes.
Codominance – both alleles contribute to the phenotype. (ie. in chickens, the alleles for black feathers is codominant with the allele for white feathers.

15. Incomplete dominance - neither allele is dominant,
red x white = pink
Codominance - both are expressed in some way,
red x white = white/red spots

16. Many genes have more than two alleles, known as multiple alleles.
Does not mean that an individual can have more than two alleles
Means that more than two possible alleles exist in a population – ie. rabbit coat color
Traits controlled by two or more genes are said to be polygenic traits – “having many genes.”
Often show a wide range of phenotypes
ie. wide range of skin color in humans – more than 4 different genes control this trait

17. Chromosomal Inheritance
Mendel’s principles of genetics requires;
Each organism must inherit a single copy of every gene from each “parent”
When an organism produces its own gametes, those 2 sets of genes must be separated so that each gamete contains just one set of genes
ie. Body cell in an adult fruit fly has 8 chromosomes (4 from male parent, and 4 from female parent) – these two sets of chromosomes are homologous

18. A cell that contains both sets of homologous chromosomes is said to be diploid – 2 sets
Represented by the symbol 2N
ie. For the fruit fly the diploid number is 8 or 2N=8
Diploid cells contain two complete sets of chromosomes and two complete sets of genes
Single set of chromosomes – single set of genes are haploid – 1 set or N=4 for a fruit fly

19. Phases of Meiosis
Meiosis – process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell
Usually involves 2 distinct divisions known as meiosis I and meiosis II

20. Meiosis I Before meiosis I, each chromosome is replicated.
Cells start to divide – 4 chromosomes line up in the middle of the cell and 2 chromatids that make up each chromosome separate
Prophase – each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad
4 chromatids in a tetrad
As tetrads are formed, they exchange portions of their chromatids in a process called crossing-over

21. Meiosis I cont…
Crossing-over, results in the exchange of alleles between homologous chromosomes and produces new combos of alleles
Homologous chromosomes separate, and two new cells are formed
SEE PAGE 276 FIG (MEIOSIS)

22. Meiosis II
2 cells produced by meiosis I now enter a second meiotic division
Each of the cell’s chromosomes has 2 chromatids
Metaphase II - chromosomes line up in the center of each cell
Anaphase II – paired chromatids separate

23. See Page 278 Fig
Meiosis produces four genetically different haploid cells
In males, meiosis results in four equal-sized gametes called sperm
In females, only one large egg cell results from meiosis (the other 3 cells are polar bodies and are not involved in reproduction)

24. Mitosis vs. Meiosis
Mitosis results in the production of two genetically identical diploid cells
Allows an organism’s body to grow and replace cells
In asexual reproduction, a new organism is produced by mitosis of the cell or cells of the parent organism
Meiosis produces four genetically different haploid cells
Begins with a diploid cell but produces four haploid (N) cells
Is how sexually reproducing organisms produce gametes

25. Gene Linkage
Thomas Hunt Morgan researched fruit flies which led to the principle of linkage
Each fruit fly has four pairs of chromosomes – each chromosome is a group of genes
It is the chromosomes that assort independently, not the genes
If two genes are found on the same chromosome they are not linked forever - crossing-over during meiosis sometimes separates genes