University of South Florida
Instructor: Dr. Stephen Karl

GENERAL GENETICS
FALL 1998

I) Review from last lecture

 

A) Cell cycle - all cells have a "life cycle" consisting of 5 phases (18 - 24 hours needed for the entire cycle)

Figure 3.2

1) G0 - stationary, normal activity phase of the cell

2) G1 - beginning phase of cell replication

3) S - DNA synthesis phase where chromosomes are replicated

4) G2 - short phase after S but before beginning to divide

5) M - mitosis or division phase

B) Mitosis - One cell is replicated giving rise to two nearly identical daughter cells.

-----------o---------- This is my diagram for a stylized chromosome

G0, G1, S and G2 phases happen during Interphase of the cell

Figure 3.3

1) Prophase

2) Metaphase

3) Anaphase

4) Telophase

II) Continue with genes and chromosomes

D) Mitosis - production of gametes. A single diploid cell goes through what is essentially two rounds of mitosis to produce a haploid cell (i.e., only a single copy of each chromosome in each gamete)

Figure 3.7

1) Meiosis I - Considered the reduction division because the number of chromosomes is cut in half (as counted by the centromeres - COUNT THE SISTER CHROMATIDS AS A SINGLE CHROMOSOME BECAUSE THEY ARE COPIES OF EACH OTHER)

a) Prophase I

i) chromosomes condense

ii) nuclear envelope disassembles (by end)

iii) homologous chromosomes pair (called synapsis - remember the 2 copies of the same chromosome in a diploid individual are the homologs)

---------------O------------- This is a Bivalent
---------------O-------------

iv) chromosomes continue to condense so that now the bivalent above can be see as 4 chromatids (each line above becomes a pair of sister chromatids). This is the tetrad

v) crossing over occurs - during synapsis there can be exchange of pieces of chromosomes between non-sister chromatids of a bivalent.

vi) synapsed chromosomes are being pushed apart

vii) where crossing over is occurring, chiasmata can be seen

Figure 3.9

b) Metaphase I - similar to mitosis except for the arrangement of the chromosomes. Now the bivalents are lined up on the metaphase plate. This means that pairs of homologous chromosomes are together whereas in mitosis they were not.

 

Figure 3.11

1) all alignments are equally likely and produce gametes of all types in a 1:1:1:1 ratio (considering the two chromosomes in the figure).

This is independent assortment of genes on different chromosomes that Mendel found

c) Anaphase I - homologous chromosomes (consisting of two, sister chromatids) are separated therefore the number of chromosomes in the daughter cells is reduced by 1/2.

This is random segregation of chromosomes

d) Teolphase - similar to mitosis but can be skipped in some organisms.

2) Meiosis II - Considered the equatorial division - chromosome number stays the same.

a) Prophase II, Metaphase Ii, Anaphase II are essentially similar to mitosis

i) anaphase II - chromatids are separated

ii) remember chromatids are not identical because of crossing over during Prophase I

b) Telophase II - nuclear envelopes form and cytokinesis begins

E) Generalizations and Exceptions

1) The process of reduction, crossing over, and equatorial division of chromosomes happens to all chromosomes in the cell (e.g., humans have 23 pairs of chromosomes so there are basically 23 bivalents at the metaphase plate of metaphase II)

2) The exception to this is the sex chromosomes - in many organism the sex of the individual is controlled by a single, specific chromosome. All the other chromosomes are called autosomes

a) in humans and many mammals there is chromosome called the X chromosome that consists of a large number of genes. These genes, however, are not involved in the regulation (or determination) of sex. There is a second chromosome called the Y chromosome that consists of very few genes. The genes that are start the processes of differentiation the fetus into male and female. The main gene for this is the Testis Determining Factor (TDF). XY (heterogametic) individuals have TDF and become male. XX (homogametic) individuals do not have TDF and become female.

b) some birds, reptiles, fish and moths the order is reversed and the females are the heterogametic individuals. Many reptiles lack sex chromosomes and have Temperature Dependent Sex determination (TDS).

 

3) Example 1: red-green color blindness (RGCB) is sex linked (i.e., the gene for seeing red and green is on the X chromosome)

 

a) XB indicates an X chromosome with the normal allele for RGCB

b) Xb indicates an X chromosome with the allele causing RG color blindness

P1:       XBXb    X       XBY

      normal, carrier       normal

 

F1:

XB

Y

XB

XBXB

XBY

Xb

XbXB

XbY

 

c) 1/2 are female and 1/2 are male

d) all females and 1/2 of the males are normal vision

e) 1/2 males are color blind

4) Example 2:

P1:      XBXb       X       XbY

     normal carrier           color blind

 F1:

Xb

Y

XB

XBXb

XBY

Xb

XbXb

XbY

 

c) 1/2 are female and 1/2 are male

d) 1/2 of the males and 1/2 of the females are normal vision

e) 1/2 males and 1/2 of the females are color blind

 

F) Probability and Testing - much of genetics relies on calculating the probability of an event and testing the observed data to the conformance to the expected probability.

1) Binomial Theorem - used to calculate the probability of events.

a) e.g., Prob{girl} = 0.5 = variable p

Prob{boy} = 0.5 = variable q

What is the Prob{2 girls and 1 boy}?

Three ways to get a family of three with 2 girls and 1 boy.

GIRL, GIRL, BOY

GIRL, BOY, GIRL

BOY, GIRL, GIRL

since these are essentially simultaneous, mutually exclusive events (i.e., either boy or girl, three times) the combined probability is:

Prob{girl} * Prob{girl} * Prob{boy} = 0.5 * 0.5 * 0.5 = 0.125

Prob{girl} * Prob{boy} * Prob{girl} = 0.5 * 0.5 * 0.5 = 0.125

Prob{boy} * Prob{girl} * Prob{girl} = 0.5 * 0.5 * 0.5 = 0.125

 

The Prob{2 girl and 1 boy} is the sum of these three orders or:

0.125 + 0.125 + 0.125 = 0.375

b) in general this can be addressed by solving a binomial with the following format: (p + q)n where p = probability of event 1, q = probability of event 2 and n = number of combined events (or p = Prob{girl}, q = Prob{boy}, n = number of children in family

The above example is then: {p + q)3 =

1p3 + 3p2q + 3pq2 + 1q3

1p3 corresponds to the probability of a 3 child family that is all girls (GGG)
3p2q corresponds to the probability of a 3 child family that has 2 girls and 1 boy (GGB, GBG, BGG).
3pq2 corresponds to the probability of a 3 child family that has 1 girl and 2 boys (GBB, BGB, BBG).
1q3 corresponds to the probability of a 3 child family that is all boys (BBB).

c) First easy trick for any (p+q)n:

powers are in the form:

pnq0 + pn-1q1 + pn-2q2 + pn-3q3 + .... + p0qn

coefficients can be found from PascalŐs Triangle

Figure 3.19

Combine the coefficients and powers into a single formula. Each term in the formula corresponds to a specific single condition (i.e., 2 girls and 1 boy or 3 girls and 0 boys etc.)