1. Mendel
Chapter 14

2. Historical perspective
“Blending hypothesis” – proposes that the genetic material from two parents blend together (like yellow and blue paint to produce green)
This would eventually lead to a uniform population
Gregor Mendel – published his data in1863
“Particulate” theory of inheritance – parents pass on discrete units of heredity that retain their separate identities in offspring
Genes are sorted and passed on generation after generation in undiluted form

3. Mendel’s experiments
Mendel was one of the earliest scientists to apply a quantitative approach to the evaluation of scientific data
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.
Pea plants have several advantages for genetic study
Distinct characteristics
Ability to control mating
Short generation time

4. Use of Pea Plants
Mendel chose only those characteristics that could be described as “etiher – or”
Tall or short
Purple or white
Mendel could use pollen from one plant to fertilize another

5. Experimental Design
Mendel started his experiments with varieties that were true-breeding.
In a typical experiment Mendel would cross-pollinate (hybridize) two contrasting true-breeding pea varieties.
The true-breeding plants are the P generation and the hybrid offspring are the F1 generation
Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation

6. F2 generation
Mendel repeated his experiments multiple times and discovered a consistent ratio of three to one, purple to white flowers in the F2 generation.
Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
Mendel called the purple flower color a dominant trait and the white flower color a recessive trait.
Mendel found the same pattern of inheritance in the other pea characteristics
What Mendel called a “heritable factor’ is what we now call a gene

7. Mendel’s Hypothesis – point 1 & 2
Alternative versions of genes account for variations in inherited characters.
The alternative versions of a gene are called alleles
For each character, an organism inherits two alleles, one from each parent.
Each diploid organism has a pair of homologous chromosomes
The two alleles on the homologous pair of chromosomes may be identical (homozygous) or different (heterozygous)

8. Mendel’s hypothesis – points 3 & 4
If the two alleles at a locus differ, then one, the dominant one, determines the organism’s appearance. The other the recessive allele, has no noticeable effect on the organism’s appearance.
Mendel’s Law of Segregation states that two alleles for a heritable character separate and segregate during gamete production and end up in different gametes
This accounts for the 3:1 ratio that Mendel saw

9. Punnett Squares
A chart used to predict the results of a genetic cross between individuals of known genotype
Genotype = genetic make up (PP or Pp)
Phenotype = physical trait (Purple)
Shows all possible combinations of gametes
Allows for random fertilization

11.  Testcross 
Used to determine the genotype of an organism when it exhibits the dominant phenotype.
The organism in question is bred with an individual exhibiting the recessive phenotype
If any of he offspring display the recessive phenotype, the parent in question must be heterozygous.

13.  The Law of Independent Assortment 
Mendel’s first experiments investigated a single character at a time.
A cross that follows a single character and involves two heterozygote parents is called a monohybrid cross
Mendel then followed two characters at a time.
A cross that follows two characters and involves two parents that are heterozygous for each character is called a dihybrid cross

15. Law of Independent Assortment (continued)
Using a dihybrid cross, Mendel developed the law of independent assortment
This states that each pair of alleles segregates independently of other pairs of alleles during gamete formation.
Genes located near each other on the same chromosome tend to be inherited together.

16. Laws of Probability govern Mendelian inheritance
Mendel’s laws reflect the rules of probability
The probability scale ranges from 0 (an event with no chance of occurring) to 1 (an event that is certain to occur)
The probability of tossing heads with a normal coin is ½
The probability of rolling a 3 with a six-sided die is 1/6; the probability of rolling any other number is 1-1/6 = 5/6
Each event is independent of others
When tossing a coin, the outcome of one toss does not impact on subsequent tosses.

18.  The Rule of Multiplication 
This rule states that the probability that two or more independent events will occur together is the product of their individual probabilities.
Multiply the individual probabilities to obtain the overall probability of these events occurring together.
Determine that event A and event B will occur
AND  Multiply

19. Rule of Multiplication Example
Probability that two coins tossed at the same time will both land on heads is
½ X ½ = ¼
The probability that a heterozygous pea plant (Pp) will self-fertilize to produce a white flowered offspring (pp) is the chance that a sperm with a white allele will fertilize an ovum with a white allele
½ X ½ = 1/4

20. Rule of Multiplication applies to dihybrid and trihybrid crosses
For a heterozygous parent (YyRr) the probability of producing a YR gamete is ½ X ½
We can use this to predict the probability of producing a particular F2 genotype without constructing a Punnett square
The probability that a plant from the F2 generation will have a YYRR genotype is 1/16
(1/4 chance for a YR ovum and ¼ chance for a YR sperm)

21.  Rule of Addition 
This is used to predict the probability of an event that can occur two or more different ways.
Probability of event A or event B
The probability is the sum of the separate probabilities
OR  Add

22. Rule of Addition example
There are two ways that F1 gametes can combine to form a heterozygote.
The dominant allele could come from the sperm and the recessive from the ovum (probability = ¼)
Or the dominant allele could come from the ovum and the recessive from the sperm (probability = ¼)
The probability of a heterozygote is ¼ + ¼ = ½

23. Solving complex Mendelian problems
Determine the probability of an offspring having two recessive phenotypes for at least two of three traits resulting from a trihybrid cross between pea plants that are PpYyRr and Ppyyrr.
Probability of producing a ppyyRr offspring
Probability of producing pp =
Probability of producing yy =
Probability of producing Rr =
Probability of all three being present =
Probability for ppYyrr =
Probability for Ppyyrr =
Probability for PPyyrr =
Prbability for ppyyrr =
Therefore the chance that a given offspring will have at least two recessive traits =

25. For any gene with a dominant allele C and a recessive allele c, what proportions of the offspring from a CC x Cc cross are expected to be homozygous dominant, homozygous recessive and heterozygous.

26.  Beyond Mendelian Genetics 
Alleles show different degrees of dominance
Complete dominance = Mendelian trait
Co-dominance
Two alleles affect phenotype in separate and distinguishable ways.
Both alleles are fully expressed
Example: Blood types M, N, MN
Incomplete dominance
Heterozygotes show a distinct intermediate phenotype not seen in homozygotes
Example: flower color in snap dragons – red x white → pink

28. Dominance/Recessive relationships
The dominant allele does not interfere with the activity of the recessive allele.
Differences between the alleles is due to differences in nucleotide sequences.
The two alleles do not interact with each other
The dominant allele is not necessarily more common in a population than the recessive allele.
Polydactyly results form a dominant allele but is not the most common phenotype

29. Dominance/Recessiveness relationships
The character of these relationships depend on the level at which we examine the phenotype
Example: Individuals with Tay-Sachs disease lack a functioning enzyme to metabolize certain lipids. These lipids accumulate in the brain, damaging brain cells, and ultimately leading to death.
Organism level :Children with the disease have two recessive alleles; complete dominance.
Biochemical level: heterozygotes have reduced activity level of eh enzyme although they do not have symptoms; incomplete dominance.
Molecular level: heterozygotes produce equal numbers of normal and dysfunctional enzyme molecules; co-dominance

30. Dominant/Recessive Autosomal Recessive Autosomal Dominant
Phenylketonuria (PKU)
Cystic fibrosis
Tay-sachs disease
Sickle Cell Disease
Huntington’s disease
Polydactyly
Acondroplasia

31. Beyond Mendel (continued)
Multiple Alleles: more than two alternative forms of alleles
Example: Human ABO blood groups
Individuals with type A blood have type A carbohydrates on the surface of their red blood cells.
Individuals with type B blood have type B carbohydrates on the surface of their red blood cells
Individuals with type AB blood have both A and B carbohydrates on the surface of their red blood cells
Individuals with type O blood have neither A nor B carbohydrates on the surface of their red blood cells
alleles = IA, IB, I
IA and IB are codominant
i = recessive

32. Beyond Mendel (continued)
Pleiotrophy: one gene affects more than one phenotypic character
Example: varied symptoms of cystic fibrosis are due to a single gene.
Epistasis: a gene at one locus alters the phenotypic expression of a gene at a second locus.
Example: coat color in mice depends on two genes.
The epistatic gene determines whether pigment will be deposited in hair or not
The other gene determines whether the pigment to be deposited is black or brown.

34. Beyond Mendel (continued)
Polygenic inheritance: the additive effects of two or more genes on a single phenotypic character.
This results in a population with a range of phenotypic characteristics.
Example: skin color in humans is controlled by at least three independent genes.
Each gene has two alleles, dark and light
The genes are incompletely dominant
AABBCC is very dark; aabbcc is very light.

35. Phenotype depends on the Environment
Multifactorial traits: environment contributes to the phenotype
Example: nutrition influences height in humans
Example: a single tree may have leaves that vary in size, shape, and greenness, depending on exposure to wind and sun.
Example: identical twins accumulate phenotypic differences as a result of their unique experiences.
Many human disorders have multifactorial basis including: heart disease; diabetes; cancer; alcoholism

36. Pedigree Analysis
The distribution of a phenotypic trait is mapped on a family tree.
Phenotypes of family members and knowledge of dominant/recessive relations between alleles allow researchers to predict the genotypes of members of a family.

38. Genetic Counseling
Use of pedigree chart may predict probability of having a child with certain genetic disorders.
Biochemical or genetic tests may be used to determine genotype of parents.
Genetic testing of fetus
Ultrasound allows visual assessment of the fetus in utero
Amniocentsis:
Collects fetal cells from amniotic fluid during 14th -16th weeks of pregnancy
A karyotype is used to identify some disorders.
Chorionic villus sampling:
Allows karyotyping to be performed earlier (8th – 10th weeks)