1. Mendel and the Gene Idea
Chapter 14

2. You Must Know
Terms associated with genetics problems: P, F1, F2, dominant, recessive, homozygous, heterozygous, phenotype, genotype
How to derive the proper gametes when working a genetics problem
The difference between an allele and a gene
How to read a pedigree
How to use data sets to determine Mendelian patterns of inheritance

3. Mendel’s Model
1st: alternative versions of genes account for variations.
2nd: for each character, an organism inherits two copes of a gene, one from each parent.
3rd: if the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
4th: The law of segregation, states that the two alleles for a heritable character segregates during gamete formation and end up in different gametes.

4. True-breeding parents: over many generations of self-pollination, offspring is always the same variety.
P Generation: Parental generation
F1 Generation: first filial generation
F2 Generation: second filial generation.
Hybrids: mating, or crossing, of two true-breeding parents.

5. The Law of Segregation
The two alleles for each trait separate during gamete production. (meiosis)
If a parent is homozygous (PP or pp), all offspring will get that allele, but if the parent is heterozygous (Pp), each offspring has a 50% chance of getting either allele.

6. Alleles
Alternative versions of genes (alleles) account for variation in inherited characteristics among offspring.
If alleles at a locus differ, the dominant allele determines the appearance of the organism.
Ex. Two alleles for flower color – both are the flower color gene, found at the same locus, but one version codes for purple flowers while the other codes for white. PP pp

7. For each characteristic, every organism inherits one allele from each parent.

8. If the two alleles are different, then the dominant allele (shown as a capital letter) will be expressed in offspring.
The recessive allele (shown as a lower case letter) will have no noticeable effect on the offspring.

9. Law of Independent Assortment
Alleles on different chromosomes separate independently of each other.

10. Phenotype vs. Genotype
Phenotype: an organism’s physical expression of a trait.
Genotype: an organism’s genetic combination of alleles.

11. How do you know if an organism is heterozygous or homozygous?
Performing a testcross: which breeds a homozygous recessive organism with the unknown genotype.
It will reveal the genotype by looking at the offspring that is produced.
NOTE: Really only practical in species that produce large numbers of offspring.

12. Monohybrid Cross vs. Dihybrid Cross
Monohybrid Cross- examines one trait at a time.
Dihybrid Cross- examines two traits together.

13. Laws of Probability
Multiplication Rule: states that to determine the probability, we multiply of one event by the probability of the other event.
Ex. The flipping of a coin twice and getting tails each time.
½ x ½ = ¼
Or the chance of drawing an ace of spades from a deck.
1/52

14. Practice
If you cross two individuals that both have genotype of AaBbCc, what is the probability of an offspring being AaBBcc?

16. Addition Rule: the probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities.
Ex. If two parents are crossed that are AaBb, what are the chances of an offspring being either AABB or aabb?

17. Codominance vs Incomplete Dominance
Codominance: Two alleles are both dominant and are both fully expressed in the phenotype.
Incomplete Dominance: Hybrids are a blend of the two traits.

18. Multiple Alleles A gene that has more than 2 alleles.
Note: Use superscripts to denote different alleles.
* Blood is also an example of codominance.

19. Pleiotrophy One gene that has multiple phenotypic effects.
These genes are responsible for the multiple symptoms associated with certain hereditary diseases.

20. Epistasis
A gene at one locus alters the effects of a gene at another locus.
Bb- codes for black
Bb- codes for brown
A second gene (Ee) determines whether or not pigment will be deposited in the hair.
Bbee- will be a yellow lab, because ee is recessive and pigment will not be deposited in the hair.

21. Polygenic Inheritance
Two or more genes have an additive effect in a single characteristic.
These are quantitative characters, expressed phenotypically in all degrees of variation between one often indefinite extreme and another.

22. Pedigrees
Diagram that shows the relationship between parents and offspring across generations.
Which is recessive and which is dominant?

23. Recessively inherited disorders
Cystic fibrosis- defective cell membrane protein for movement of chloride ions leading to high levels of extracellular chloride which causes thickening of mucus which causes organ malfunction and chronic infections.
The most common lethal genetic disease in the U.S.
1 out of 2,500 people of European descent, rarer in other groups.
1 out of 25 are carriers of the cystic fibrosis allele.

24. High among Jewish people whose ancestors lived in central Europe.
Tay-Sachs codes for a dysfunctional enzyme that does not properly breakdown lipids in the brain. Lipids accumulate causing blindness, seizures, loss of brain function, death by 5.
High among Jewish people whose ancestors lived in central Europe.
1 out of 3,600 births

25. About 1 in 10 African-Americans have sickle-cell.
Sickle-cell disease: Hemoglobin gene is mutated, resulting in sickle shaped blood cells.
About 1 in 10 African-Americans have sickle-cell.
Why hasn’t the dominant gene eliminated this?
Reduces the chance of getting and the severity of malaria.

26. Lethal Dominant Alleles
Requires only one copy of the gene to be expressed.
Usually symptoms are not present until later in life.
Huntington’s disease- degenerative disease of the nervous system, onset usually in 40’s.

27. Prenatal Genetic Testing
Amniocentesis- removal of amniotic fluid on generation of a karyotype
Chorionic villus sampling- suction of a small sample of portion placenta that contains fetal cells and generation of a karyotype.