Transcript Chromosomes - s3.amazonaws.com

EVIDENCE FROM
GENETICS II
Genetics After Mendel
Are all traits this simple?
 Some phenotypes are
due to the action of a
single gene
 MOST, however, are
polygenic
 Height
 Skin color
 Disease predisposition




Heart disease
High blood pressure
Diabetes
Cancer
1) Concept of Polygene
Inheritance
 =Traits are
determined by
several genes at
different loci.
 e.g. Human height
2) Concept of Multiple Alleles
 Many traits have multiple alleles not just two
such as T or t.
e.g. ABO blood type there are 3 alleles = A, B, O
So:
AA, AB, AO, BB, BO, OO = 6 Genotypes
3) Concept of Pleiotropy
= Multiple effects caused by
a single gene
 e.g. Marfan’s
syndrome
4) Chromosomes Exist
(1875)
When do you see
chromosomes?
 First of all you have to
use stains for DNA
 Chromosomes only in
condensed form when
preparing for nuclear
division
 Otherwise present as
chromatin (unraveled)
5) Chromosome Numbers
Vary Among Species







Ant
2
Mosquito
6
Orange
18, 27, 36
Frog
26
Coffee
44
Spanish Butterfly 380
Stalked adders tongue 1020
Human Karyotype
= Chromosome Pattern
6)Chromosomes
Come in Pairs
CELL
6 Chromosomes
3 Pairs
Cell Nucleus
Karyotype
= Chromosome Pattern
Humans Have 46 Chromosomes
23 Pairs
 Diploid Number = 46 (2n)
 Haploid Number = 23 (n)
 Polyploid Numbers = 3n, 4n, 5n,…….etc
Humans Have 46 Chromosomes
23 Pairs
 One set of 23 from father
 One set of 23 from mother
Humans Have 46 Chromosomes
23 Pairs
 22 Pairs are autosomes
 1 Pair are sex chromosomes
♀ = xx
♂ = xy
Human Karyotype
= Chromosome Pattern

Is this from a woman or a man?
A) Woman
B) Man
C) I don’t know
7) Mendel’s Factors (Genes)
Occur on Chromosomes
in particular places= locus
2 Alleles
Homozygous Dominant
Heterozygous
Homozygous Recessive
8) Many Genes Exist on
Each Chromosome
 Viruses = 5  10 genes
 Bacteria = 1,000  6,000 genes
 Human = 25,000 genes
9) Concept of Linkage
 Genes on the same chromosome are linked
Do you remember this slide?
What if RY are linked together &ry are linked
together ?
Non Independent Assortment = REDUCED VARIABILITY
9) Concept of sex-linked
traits = Traits carried on the
sex chromosomes





Muscular dystrophy
Color blindness
Pattern baldness
Hemophilia
All are carried on the X chromosome
and are displayed in males
Because ♂ only have one X chromosome.
So if they have the allele for the trait,
they will show it.
10) Cell Division Occurs
Step 1: The chromosome duplicate
There are two identical arms on each one.
 Each arm is called a
chromatid
Cell Division in Body Cells
 Step 2: The Chromatids Divide Equally
During Cell Division
 Division in body (somatic) cells
92
46
46
46
The chromosomes are lined up
The chromatids are pulled apart
46 chromosomes in each cell
Division in Sex Organs
(In the Testes)
46
23
23 23
92
23
23
4 Sperm
23
Division in Sex Organs
(In the Testes)
46
23
Meiosis =
Reduction
Division
23
Daughter cells with
½ # Chromosomes
23 23
23
4 Sperm
23
As parent cell
Stages of
Meiosis
 Meiosis (in animals)
produces 4 haploid
gametes from 1
diploid cell
 At the end of the first
division (Meiosis I)
the 2 cells are
already haploid
 The second division
(Meiosis II) splits the
2 chromatids
Division in Sex Organs
(In the Ovary)
46
Meiosis
23
23
Egg +
23
3 Polar
Bodies
Division in Sex Organs
(Testes & Ovaries)
46
46
23
23 23
23
23
4 Sperm
23
23
23
Egg +
3 Polar
Bodies
Distribution of
Chromosomes in gametes
The gametes end up with only one set of
chromosomes. It is random which
chromosome of a pair goes into which
gamete.
 SO, each gamete has a mixture of the
mother & father’s chromosomes.
 This produces enormous gamete variability
How many combinations of maternal
chromosomes are possible in a
human egg due to independent
assortment during meiosis?
A: 23 combinations
B: 46 combinations
C: 232 = 529 combinations
D: 223 = ~ 8 million combinations
The Fate of Genetic Material
During Sexual Reproduction
 You contain
genetic material
from both your
Mom and Dad.
 You have 46
chromosomes,
and so will your
children.
9) Concept of Cross-over
=Sister chromosomes swap
pieces during meiosis
Now for a Story
Santhi Soundarajan
Santhi Soundararajan won
the silver medal for 800
meters at the 2006 Asian
Games in Doha, Qatar.
Following her silver medal
performance, her sex
was officially questioned.
http://www.ibnlive.com/videos/28851/h
ow-are-athletes-gender-tested.html
18 Dec. 2006: Indian athlete Santhi
Soundararajan fails gender test
 Sports writer KP Mohan said that a team of
doctors, including a gynecologist,
endocrinologist and psychologist, normally
examines athletes, and puts them through
physical and clinical examinations during a
gender test.
 Santhi Soundararajan's test was done soon after
Soundararajan came second in the women's
800m race on 9 December but is not clear how
she failed the test at the Asian Games in Doha.
How is sex determined in
humans?
 Anatomy: Primary and secondary sexual
characteristics - genitalia, body hair, pelvis, etc.
 Physiology: Function and interaction of the sex
organs including concentrations of sex
hormones such as estrogen, progesterone, and
testosterone.
Imagine that you are a member of the
committee assigned to determine whether
Santhi is female. Here are possible results of
the initial tests (we don’t know the real results):
Female genitalia: Yes
Breasts and pubic hair: Yes
Regular menstrual cycle: Never
From this information, you conclude
that Santhi is
A: Male
B: Female
Is there another way that sex is
characterized in humans?
 Chromosomes:
 Females possess two X chromosomes in
each of their cells, whereas males have one X
and one Y chromosome.
How is Sex Determined in
Humans?

Sex is determined by
the sex chromosome
carried by the sperm.
sperm
X
X
Y
XX
XY
XX
XY
girls
boys
egg
X
Suppose this was Santhi’s
Karyotype




Would you
A) Disqualify her
B) Let her keep her medal
C) Do more tests
How is Sex Characterized in
Humans?
In addition to anatomy, physiology, and
chromosomes, there is a 4th answer:
Genes: Specific genes determine
whether an embryo will develop as a
male or female.
SRY (Sex- Determining Region
of the Y chromosome) Gene
 Early in development, the
immature gonads of males and
females are indistinguishable.
 Males: In the 7th week of
development, the SRY gene on the
Y chromosome activates a number
of genes, and the gonads develop
as testes.
 Females: With no SRY gene,
gonads develop as ovaries by
default.
Could Santhi have an
XX karyotype and be
male?
A: No, an XX individual is always female.
B: Yes, this is common.
C: Yes, if a male-determining control gene is
carried on one of her X chromosomes.
What about crossing over ?
Crossing over can occur anywhere along
the autosomes.
In males, the sex chromosomes (X and Y
chromosomes) normally cross over only
at their tips.
BUT sometimes the SRY gene is involved
Special note:
It may take a little while for the
next slide to load.
http://www.hhmi.org/biointeractive/media/meiosis-lg.mov
Can you have an XX male or
XY female?
 What sex is XX (SRY+)?
 What sex is XY (SRY-)?
Sex reversal occurs in 1 in 20,000 births!
If you were a member of the Asian
Games medal committee, and
Santhi’s karyotype revealed that she
is XY and SRY-, what would you do?
A: She has female genitalia, allow her to
keep her medal
B: She is genetically male, take her
medal away
C: She has male genitalia , take
her medal away
What do you think about requiring
gender testing for female athletes
in international competition?
A: It should be banned because gender
determination is so complex
B: It is necessary to ensure an even playing field
C: It is necessary but needs to include a large
number of genetic tests to ensure fairness
D: It should be required for all athletes, both male
and female
What about Santhi
Soundararajan?
Santhi was stripped
of her silver medal.
An anonymous official in
the know has reported that
Ms. Soundarajan's tests
revealed more Y
chromosomes than are
usually present in the
genetic make-up of a
female. No official
statement has been made.
What about Santhi
Soundararajan?
In September 2007, Santhi
Soundarajan was reported
to have attempted suicide,
reportedly by consuming
pesticide at her residence.
The attempt was blamed on
gender, economic, and
sports pressure in India.