Download meiosis mitosis Independent orientation of chromosomes in meiosis

mitosis
meiosis
• A form of asexual reproduction
• Daughter cells are genetic copies of
the parent and of each other
• Sexual reproduction
• Creates a variety of offspring
LM 340!
Independent orientation of chromosomes in meiosis and
random fertilization lead to varied offspring
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1 Combination 2
Combination 3 Combination 4
How crossing over leads to genetic variation
Coat-color Eye-color
genes
genes
E
C
c
e
C
E
c
e
C
E
c
e
1
2
3
C
C
E
e
c
c
E
e
Parental type of chromosome
C
E
Recombinant chromosome
Recombinant chromosome
C
e
c
E
Parental type of chromosome
c
e
4
Figure 8.18B
Accidents during meiosis can alter chromosome
number
– Abnormal chromosome count is a result of nondisjunction
• The failure of homologous pairs to separate during meiosis
I
• The failure of sister chromatids to separate during meiosis
II
Egg cell
n+1
Zygote
2n + 1
Sperm cell
n (normal)
Accidents during meiosis can alter chromosome
number
Nondisjunction
in meiosis I
Normal
meiosis I
Normal
meiosis II
Nondisjunctio
n in meiosis II
Gametes
Gametes
n+1
n+1
n+1
n "1
n "1
Number of chromosomes
n
Number of chromosomes
Figure 8.21B
Figure 8.21A
Down syndrome is caused by trisomy 21
5,000!
• An extra copy of chromosome 21
Figure 8.20A
n "1
Figure 8.20B
n
The chance of having a Down syndrome child
• Goes up with maternal age
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
Figure 8.20C
Poor beard
growth
Breast
Development
Under-developed
testes
XXY
25
30
35
40
Age of mother
Characteristic facial
features
Web of skin
Constriction of
aorta
Poor breast
development
Under developed
ovaries
45
50
X
Genetics
Mendels Laws
Law of Segregation
1.
Alternative versions of genes account for variations in inherited characters.
2.
For each characteristic, an organism inherits two alleles, one from each parent.
3.
If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's
appearance; the other, the recessive allele, has no noticeable effect on the organism's
appearance.
4.
The two alleles for each characteristic segregate during gamete production.
Law of Independent Assortment
1.
The emergence of one trait will not affect the emergence of
another. This is actually only true for genes that are not linked
to each other.
Geneticists use test crosses to determine unknown genotypes
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
RRYY
P generation
rryy
Gametes RY
RRYY
ry
rryy
Gametes RY
ry
!
RrYy
RrYy
F1 generation
Sperm
Sperm
1
2 RY
1
2
ry
1
RY
2
F2 generation
Eggs
1
2
1
4
1
4
1
4
Eggs
ry
1
4
Actual results
contradict hypothesis
1
4
1
4
RY
ry
1
RY
4
1
4
ry
RY
RRYY
RrYY
RrYY
rrYY
RRYy
RrYy
ry
Ry
RRYy
RrYy
RrYy
rrYy
RrYy
RRyy
rrYy
9
16
Rryy
ry
Rryy
Actual results
support hypothesis
rryy
3
16
3
16
1
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
Mendel’s laws reflect the rules of probability
– Inheritance follows the rules of probability
– The rule of multiplication calculates the probability of two independent events
Law of Independent Assortment
1.
The emergence of one trait will not affect the emergence of
another. This is actually only true for genes that are not linked
to each other. Thomas Hunt Morgan
Law of Independent Assortment
1.
The emergence of one trait will not affect the emergence of
another. This is actually only true for genes that are not linked
to each other. Thomas Hunt Morgan
Genetic traits in humans can be tracked through family
pedigrees
– The inheritance of many human traits
• Follows Mendel’s laws
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Figure 9.8 A
Sex-linked genes exhibit a unique pattern of inheritance
– All genes on the sex chromosomes
•Are said to be sex-linked
– In many organisms
•The X chromosome carries many genes unrelated to sex
Sex-linked disorders affect mostly males
Most sex-linked human disorders
•Are due to recessive alleles
•Are mostly seen in males
Pedigree analysis
•Dominant
•Recessive
•X-linked