Download Chapter 8 - Napa Valley College

Sexual reproduction and heredity
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Ploidy
•  In several organisms, somatic cells have paired sets of chromosomes,
meaning these cells are diploid or 2n
•  Germline cells are the spermatozoa and ova or egg also known as gametes . They are haploid or n cells with only one copy of each
chromosome
n
n
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Sexual Reproduction
ü Meiosis
ü Gametes
ü Fertilization
ü Genetic variation
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Homologous chromosomes
• same shape
• same size
• centromere in the same position
• genes for the same traits
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Meiosis scrambles the specific forms of each gene
that each sex cell (egg or sperm) receives. This makes for a lot of genetic
diversity. This trick is
accomplished through crossingover.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Allele: One of multiple alternative forms of a single gene.
Different
alleles produce
variation
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Independent Assortment
Independent Assortment:
One way meiosis makes lots of
different sex cells (Gametes) Independent assortment
produces 2n distinct gametes,
where n = the number of
unique chromosomes. In beans, n = 22
≈ 8.4 x 106 distinct gametes.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Between independent assortment and crossing-over, no
two gametes are identical.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Mendel’s laws
Principle of segregation
Individuals carry pairs of genes
for each trait and these pairs
separate (segregated) during
meiosis.
Principle of independent
assortment
The alleles of a gene segregate
independently of the alleles of
the other genes.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Linkage
•  Not all characteristics
are Mendelian –  Quantitative traits:
controlled by more
than one gene
•  Mendel s laws only
work perfectly for
unlinked traits
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
UNLINKED: like Mendel
LINKED
A
a
B
b
VS.
A
a
B
b
Makes AB, Ab, aB, ab
Only makes AB or ab
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Quantitative
traits
These traits are controlled by multiple genes and they are
affected by the environment.
The area of genetics that studies their mode of inheritance is
called quantitative genetics.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Linkage
① 
② 
③ 
④ 
⑤ 
⑥ 
Photosynthetic ability
Crop yield
Some plant disease resistances
Variation for flower diameter
Number of flower parts Color of the flower Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Transposable elements
Transposable elements are the primary hitchhikers of the cell
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Transposable elements
Comprise the bulk of higher plant genomes, ranging from 15% of the
nuclear DNA in Arabidopsis thaliana to more than 90% in some
Liliaceae.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Mechanisms for transposition
1.- The element itself moves from the donor site into the target site
2.-The element moves a copy of itself to a new site via a DNA intermediate Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Mechanisms for transposition
'Copy-and-Paste' mechanism 3.- The element makes an RNA copy of itself which is reversed-transcribed into
a DNA copy (cDNA) which is then inserted
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
It might seem unfair to reward a person for having so much pleasure
over the years, asking the maize plant to solve specific problems and then
watching its responses Barbara McClintock
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Transposable elements
Retrotransposons life cycles are inherently error-prone and mutagenic. Once a copy
is replicated and inserted, it likely displays neutral or nearly neutral rates of decay
over time as a component of the genome.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Transposable elements
Most of the plant retrotransposons produce larger pools of transcripts in
response to stress, biotic as well as abiotic
• chilling
• infection
• mechanical damage
• in vitro regeneration
• hybridization. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.