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
=Traits are determined by several genes at different loci. e.g. Human height 1) Concept of Polygene Inheritance
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
e.g. Marfan’s syndrome 3) Concept of Pleiotropy= Multiple effects caused by a single gene
When do you see chromosomes? 0 • 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
6)Chromosomes Come in Pairs CELL Cell Nucleus 6 Chromosomes 3 Pairs
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 46
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? • Woman • Man • I don’t know
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
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 Y y R r Y & R are linked They do not sort independently y & r 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 Each arm is called a chromatid Step 1: The chromosome duplicate There are two identical arms on each one.
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
Mitosis The chromosomes are lined up The chromatids are pulled apart 46 chromosomes in each cell
Division in Sex Organs (In the Testes) 46 92 23 23 23 23 23 23 4 Sperm
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
Stages of Meiosis 0 • 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 Diploid 23 Meiosis 23 23 Egg + Haploid 3 Polar Bodies
Division in Sex Organs (Testes & Ovaries) 46 46 Diploid 23 23 23 23 23 23 23 23 Egg + Haploid 3 Polar Bodies 4 Sperm
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
0 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 swappieces during meiosis Genetic Recombination= Variety
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/how-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? sperm • Sex is determined by the sex chromosome carried by the sperm. X Y X XX XY egg XX XY X girls boys
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 overcan 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
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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.