A. Rules of probability

1. Rule of multiplication

a. how do we determine the chance that two or more independent events will occur together in a specific combination?

b. compute probability for each event then multiply independent probabililities to obtain the overall probability

2. Rule of addition

a. a. used to find the probabilility of an event that can occur in two or more different ways is the sum of the separate probabilities of those ways

Example: TtRr X TtRr

• The probability of getting a tall offspring is ¾.

• The probability of getting a red offspring is ¾.

• The probability of getting a tall red offspring is ¾ x ¾ = 9/16

Comment

• Use the Product Rule to calculate the results of complex crosses rather than work out the Punnett Squares.

• Ex: TtrrGG X TtRrgg

• What is the probability that you would get a Tall plant with Wrinkled (dom) Green seeds?

Solution

What is the probability that you would get a Tall plant with Wrinkled (dom) Green seeds?

“T’s” = Tt X Tt =

“R’s” = rr X Rr =

“G’s” = GG x gg =

Product is:

3. Rules can be combined

a. Trihybrid cross

PpYyRr x Ppyyrr

What is the probability that offspring will exhibit recessive phenotypes for at least two of the 3 traits?

Possibilities:

ppyyRr, ppYyrr, Ppyyrr, PPyyrr, ppyyrr,

B. Extending Mendelian Genetics

1. Incomplete Dominance

2. Codominance

3. Multiple alleles

4. Pleiotropy – “pleion” (Greek for “more”)

a. one gene affecting many phenotypes

5. Epistasis – (Greek for “standing upon”)

a. a gene at one locus alters phenotype of a gene at a second locus

b. Mice coat color and pigment deposit (color) or not (albino)

6. Polygenic Inheritance

a. an additive effect of 2 or more genes on a single phenotypic trait (converse to pleiotropy)

b. skin pigmentation (3 separately inherited genes)

Comment

• Rh blood factor is a separate factor from the ABO blood group.

• Rh+ = dominant

• Rh- = recessive

• A+ blood = dihybrid trait

Skin Pigmentation

Polygenic Inheritance

Quantitative character

Summary for chapter 14• Know the Mendelian crosses and their

patterns.

• Be able to work simple genetic problems (practice).

• Watch genetic vocabulary.

• Be able to read pedigree charts.

• Understand “carriers” and be able to work problems with these.

• Know Huntington disease and sickle cell

Chapter 15 Highlights

• Sex linked traits

• Duchenne Muscular dystrophy

• Hemophilia

• Color blindness

• Seen more in males…why?

X inactivation in Females

• One of the X chromosomes is inactivated in each cell of a female during embryonic development.

• Inactive X is condensed and called a “Barr body”…lies along the inside of the nuclear envelope.

• Occurs randomly and independently (females consist of a “mosaic” of 2 types of cells)

• Example: Tortoiseshell cat and Calico cats• How is the X chromosome inactivated”

– Modification of the DNA (attachment of a methyl group to nucleotides) chapter 18

– XIST gene

Mapping the distance btw genes using recombination data

• Recombination frequencies depend on the distance between genes on a chromosome– Can create a “linkage map” from this data– Distances btw genes are “map units” – A map unit is 1% of recombination frequency

• Genes are linked if they are on the same chromosome

How you tell if genes are linked

A Test Cross to determine linkage and if linkage, what is the percentage of recombination so you can “map” the genes.

They don’t look like the parents

1. Abnormal # of Chromosomes: Nondisjunction

a. members of a chromosome pair fail to separate

b. Meiosis I

c. Meiosis II

d. Monosomic vs. Trisomic

e. Abnormal # of sex chromosomes (“aneuploidy”)

f. polyploidy (triploidy 3n or tetraploidy 4n)

2. Alterations in chromosome structure

a. Deletion

b. Duplication

c. Inversion

d. Translocation

Know the following disorders and causes for the disorders:• down syndrome• Klinefelter syndrome•Turner syndrome• cri du chat

Genomic Imprinting• The differential expression of genetic material depending

on whether it is inherited from the male or female parent• Occurs during meiosis and results in the silencing of one

allele of certain genes. • Example: mouse gene Igf2…only the paternal gene is

expressed (it had methyl groups attached to cytosine nucleotides) exception to the rule that methylated DNA is silenced (not expressed)

• Most imprinted genes are critical for embryonic development. (mouse experiment)…normal development requires embryonic cells have exactly one copy of certain genes.

Organelle genes

• Cytoplasmic genes = genes outside of the nucleus

• Mitochondria, chloroplasts, other plant plastids

• Contain small circular piece of DNA• Distributed to offspring from the maternal

parent (egg)• Mitochondrial diseases (Myopathy, Leber’s

hereditary optic neuropathy)