Transcript LEQ: How do the events of meiosis account for Mendel`s laws?

LEQ: How do the events
of meiosis account for
Mendel’s laws?
9.18
Chromosome Theory of
Inheritance
Genes occupy a specific location on a chromosome
Chromosomes undergo segregation and independent
assortment
* Segregation occurs when homologous
chromosomes are separated during anaphase I.
* Independent Assortment occurs when
homologous chromosomes line up together
during metaphase I.
LEQ: How are linked
genes inherited?
9.19 and 9.20
Linked Genes
 Genes found on the same
chromosome
 Linked genes do NOT
independently assort
 Look and behave like a
monohybrid Punnett
square
Thomas Hunt Morgan
 American embryologist –
early 1900’s, studied fruit
flies, identified the process
of “crossing over” by
studying linked genes
 Why is Drosophila
melanogaster a good
organism to study?
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
Few chromosomes
Small
Easy to care for
Lots of offspring
Quick generation time
How do you get genetic
diversity with linked genes?
Crossing over occurs. Chromatids with new gene combinations are
referred to as recombinant chromatids.
Recombination Frequency
The percentage of recombinants from a cross. In this example Morgan
observed 17% recombination frequency.
LEQ: How is gender
determined?
9.22
Sex Determination Systems
 X-Y: humans & fruit flies; XX
= female; XY = male; sperm
determines sex
 X-O: insects (grasshoppers &
roaches) O = absence of a sex
chromosome; XX = female;
X- = male; sperm determines
sex by having an X or having
no sex chromosome
 Z-W: fish, butterflies, &
birds; ZZ = male; ZW =
female; egg determines sex
 Number: ants & bees;
females are diploid –
fertilized eggs; males are
haploid – unfertilized eggs
LEQ: How are some traits
linked to our gender?
9.23
Sex Linked Genes
 Genes that are located on
sex chromosomes
 Thomas Hunt Morgan
identified sex linked traits
by studying eye color in
fruit flies
 He noticed that white eye
color is more common in
males than females
Fruit Fly Eye Color
Figure 15.7
XNXN
Sperm Xn
XNXn
XnY
Sperm XN
Y
XNY
XNXn
Sperm Xn
Y
XnY
Y
Eggs XN
XNXn XNY
Eggs XN
XNXN XNY
Eggs XN
XNXn XNY
XN
XNXn XNY
Xn
XNXn XnY
Xn
XnXn XnY
(a)
(b)
(c)
 Some disorders caused by recessive alleles on the X
chromosome in humans
 Color blindness (mostly X-linked)
 Duchenne muscular dystrophy
 Hemophilia
© 2011 Pearson Education, Inc.
X Inactivation in Female
Mammals
 In mammalian females, one of the two X chromosomes
in each cell is randomly inactivated during embryonic
development
 The inactive X condenses into a Barr body
 If a female is heterozygous for a particular gene located
on the X chromosome, she will be a mosaic for that
character
© 2011 Pearson Education, Inc.
X chromosomes
Early embryo:
Two cell
populations
in adult cat:
Allele for
orange fur
Allele for
black fur
Cell division and
X chromosome
inactivation
Active X
Inactive X
Active X
Black fur
Orange fur