1. EVIDENCE FROM GENETICS II
Genetics After Mendel

2. 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

3. 1) Concept of Polygene Inheritance
=Traits are determined by several genes at different loci.
e.g. Human height

4. 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

5. 3) Concept of Pleiotropy = Multiple effects caused by a single gene
e.g. Marfan’s syndrome

6. 4) Chromosomes Exist (1875)

7. 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)

8. 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

9. Human Karyotype = Chromosome Pattern

10. 6)Chromosomes Come in Pairs
CELL
Cell Nucleus
6 Chromosomes
3 Pairs

11. Karyotype = Chromosome Pattern

12. Humans Have 46 Chromosomes 23 Pairs
Diploid Number = 46 (2n)
Haploid Number = 23 (n)
Polyploid Numbers = 3n, 4n, 5n,…….etc

13. Humans Have 46 Chromosomes 23 Pairs
One set of 23 from father
One set of 23 from mother 46

14. Humans Have 46 Chromosomes 23 Pairs
22 Pairs are autosomes
1 Pair are sex chromosomes
♀ = xx ♂ = xy

15. Human Karyotype = Chromosome Pattern
Is this from a woman or a man?
Woman
Man
I don’t know

16. 7) Mendel’s Factors (Genes) Occur on Chromosomes
7) Mendel’s Factors (Genes) Occur on Chromosomes in particular places= locus
2 Alleles
Homozygous Dominant T T T t
Heterozygous t t
Homozygous Recessive

17. 8) Many Genes Exist on Each Chromosome
Viruses = 5  10 genes
Bacteria = 1,000  6,000 genes
Human = 25,000 genes

18. 9) Concept of Linkage Genes on the same chromosome are linked Y y R r
Y & R are linked
They do not sort
independently
y & r are linked

19. Do you remember this slide
Do you remember this slide? What if RY are linked together &ry are linked together ?
Non Independent Assortment = REDUCED VARIABILITY

20. 9) Concept of sex-linked traits = Traits carried on the
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.

21. 10) Cell Division Occurs Each arm is called a chromatid
Step 1: The chromosome duplicate
There are two identical arms on each one.
Each arm is called a
chromatid

22. Cell Division in Body Cells
Step 2: The Chromatids Divide Equally During Cell Division
Division in body (somatic) cells 46 92
Mitosis
Daughter cells have
same # chromosomes as parent cell 46 46

23. Mitosis The chromosomes are lined up The chromatids are pulled apart
46 chromosomes in each cell

24. Division in Sex Organs (In the Testes)46 92 23 23 23 23 23 23
4 Sperm

25. Division in Sex Organs (In the Testes)46
Diploid
Meiosis =
Reduction Division
Daughter cells with
½ # Chromosomes
As parent cell 23 23 23 23 23 23
Haploid
4 Sperm

26. 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

27. Division in Sex Organs (In the Ovary)46
Diploid 23
Meiosis 23 23
Egg +
Haploid
3 Polar Bodies

28. Division in Sex Organs (Testes & Ovaries)46 46
Diploid 23 23 23 23 23 23 23 23
Egg +
Haploid
3 Polar Bodies
4 Sperm

29. 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

30. 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

31. The Fate of Genetic Material During Sexual Reproduction
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.

32. Genetic Recombination= Variety
9) Concept of Cross-over =Sister chromosomes swap pieces during meiosis
Genetic Recombination= Variety

33. Now for a Story

34. 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.

35. 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.

36. 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.

37. Breasts and pubic hair: Yes Regular menstrual cycle: Never
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

38. 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.

39. How is Sex Determined in Humans?
sperm
Sex is determined by the sex chromosome carried by the sperm. X Y X XX XY
egg XX XY X
girls
boys

40. Suppose this was Santhi’s Karyotype
Would you
A) Disqualify her
B) Let her keep her medal
C) Do more tests

41. 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.

42. 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.

43. 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.

44. 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

45. Special note:
It may take a little while for the
next slide to load.

47. Sex reversal occurs in 1 in 20,000 births!
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!

48. 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

49. 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

50. 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.

51. 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.