Download Crossing Over

Chromosomes and Human
Genetics

Mendel was unaware of chromosomes
 The physical structure of genes was unknown

Chromosomal Theory of Inheritance
 Genes
are on the chromosomes
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
1
Homologous Chromosomes

Chromosomes that pair up during meiosis
 Contain

the same genes
 May
have different alleles of these genes
 One
came from each parent
Each is one long DNA molecule
A
gene is a short region of the molecule
 Each
chromosome can have > 1,000 genes
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
2
Homologous
Chromosomes
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
3
Sex Determination

22 pairs of chromosomes are homologous
 Called
autosomes

Females have 2 X chromosomes (XX)

Males have X and Y chromosomes (XY)
Y
is very small
 Has
few genes (~80) mostly dealing with
maleness
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
4
Sex Determination

During reproduction, males determine sex
 If
the sperm has an X, it’s a girl
 If the sperm has a Y, boy
Y chromosome has the SRY gene
 SRY is a master switch that controls
other genes at other loci to develop the
embryo into a boy
 Males only have one X chromosome

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
5
Linkage and Crossing Over

Linkage: 2 genes on the same
chromosome

Do not sort Independently!
 Segregate together on the same chromosome

Exception: Crossing Over

Linked genes should segregate together
 Crossing over causes them to separate
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
6
Linkage and Crossing Over

Test cross of Linkage

Independent Assortment
predicts ¼ of each
phenotype

Results are much
different

These genes are linked
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
7
Linkage and Crossing Over
One would think that linked genes should
always sort together
 One expect to see only two phenotypes

 Both
recessive or both dominant traits
 50% of each as there are 2 chromosomes

Crossing over changes this result
 Results
are usually in between total linkage
and independent assortment.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
8
Crossing Over
Meiosis
Before Meiosis



Gametes produced
Homologous chromosomes line up during meiosis
Parts of maternal and paternal chromosomes exchange
The chance that this crossing over occurs increases with
the distance between the two linked loci.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
9
Human Genetic Variation
F1 aren’t identical to sexually reproducing P
 They differ with respect to their genes
 Variation is important for populations
 Variation is produced in 4 ways:



Mutation (creates new genetic variation)
Independent assortment (sorts preexisting
genes)
 Crossing over (increases the sorting)
 Fertilization (sexual reproduction shuffles the
genes in eggs & sperm & then 2 parents join to
form an offspring)
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
10
Meiosis and Independent
Assortment Generate Variation




Each homologous chromosome independently
sorts
23 pairs of chromosomes
223 or 8,388,608 possible gametes
Fertilization: 223 X 223 possible combinations
 Over 64 trillion possible offspring
 Crossing over increases this!
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
11
Human Genetic Disorders
Many diseases are genetically inherited
 Studied through Pedigree analyses
 Show disease
patterns in
families

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
12
Autosomal Recessive Diseases

Represents several thousand disorders

Can be mild or lethal
Usually both parents are Heterozygous
(Aa) and don’t show the phenotype
 Two carriers have ¼ chance of producing
affected offspring

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
13
Autosomal Dominant Diseases
One copy of this gene causes the disease
 More rare than recessive diseases


Affected individuals may not survive to mate
 Recur due to new mutations

Some diseases express late in life

More common; individuals can reproduce
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
14
Sex-linked
Diseases





Genes on the X
chromosome
Mothers who have a
recessive gene pass it on
50% of sons are affected
(only one X)
Females rarely get these
recessive diseases as
they need to copies
Hemophilia, color blind
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
15
Chromosomal Abnormalities

Some disorders are caused by changes in
chromosomes


Less common – usually lethal
Two main types:

Physical changes in chromosome structure
 Changes in chromosome number
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
16
Changes in Structure

There are four ways the structure can change
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Changes in Chromosomal
Number
Errors in meiosis result in extra
chromosomes
 Most zygotes are not viable
 Down’s syndrome – trisomy 21


An extra copy of chromosome 21
 Live to adulthood, but with developmental
problems
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
18
Sex Chromosomal Number

Extra X and Y chromosomes are possible
 XXY
or XYY = males
 XXX, XXXX, or X0 (have one X) = females

Viable but usually sterile
 Can
have developmental difficulties
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
19
Concept Quiz
A.
B.
C.
D.
Which of the following is not true of
homologous chromosomes?
They contain the same alleles.
They contain the same genes.
One came from each parent.
Each is duplicated during replication.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
20
Concept Quiz
Genes that are linked on the same
chromosome always segregate together.
True or False?
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
21
Concept Quiz
Autosomal dominant disorders
A. Are carried on the X chromosome.
B. Often express late in life.
C. Are spread by carriers (heterozygotes)
mating.
D. Are more common than recessive
disorders.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
22
Cri du Chat


The results of a
chromosomal deletion
Affected individuals have
severe mental retardation
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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