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SEX DETERMINATION from these studies concluded that genes are on

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Presentation on theme: "SEX DETERMINATION from these studies concluded that genes are on"— Presentation transcript:

1 SEX DETERMINATION from these studies concluded that genes are on
chromosomes and specialized chromosomes determine sex

2 Modes of Sex Determination
XX/XY (ZZ/ZW) (Lygaeus) - heteromorphic sex chromosomes XX/XO (Protenor)

3 heteromorphic chromosomes
- a chromosome pair with some homology, but differing in size, shape or staining properties heterogametic sex (e.g. XY OR ZW) - the sex with different sex chromosomes; the gametes have different sex chromosomes …..the opposite of homogametic sex

4 Human Males have Heteromorphic Sex Chromosomes

5 XY Method of Sex Determination
XX XY XY method common in higher organisms Y-chromosome evolved to give X a pairing partner A = autosome, any non sex-chromosome

6 Sex Determination in Humans
Primary non-disjunction XXX XXY XO YO

7 Aneuploid Offspring XXX -- Trisomy-X XXY -- Klinefelter XO -- Turner
YO -- Dies aneuploidy involving the sex chromosomes is much better tolerated than autosomal aneuploidy

8 XXX Syndrome 47, XXX female (also see XXXX and XXXXX)
1/1200 female live births Variable phenotype, frequently appear normal, sterile, may show mental retardation. Under-developed secondary sex characteristics

9 Klinefelter Syndrome 47, XXY male (XXXY, XXXXY, XXYY)
1/500 male live births Male genitalia, testes undeveloped, fail to produce sperm Female secondary-sex characteristics

10 Turner Syndrome 45, XO female 1/2000 female live births
Ovaries rudimentary, sterile Short stature, shield-like chest, webbed neck Normal intelligence Many XO fetuses miscarry

11 XYY Condition 47, XYY male 1/1000 male live births
Above-average height, 2% in maximum security prisons are XYY, personality disorders, subnormal intelligence Fertile Most XYY males socially normal

12 Human Sex-Determination Mechanism
XY XO XXY XX XYY XXX XXYY XXXX XXXY XXXXX XXXXY CONCLUSION: Presence of Y is male-determining Lack of Y is female-determining Y produces testis-determining factor (TDF)

13 crossing over between X and Y during
meiosis occurs in pseudoautosomal region (PAR) ……necessary for normal disjunction PAR homologous to region of X-chromosome non-recombining region of Y (NRY) unique to Y chromosome human Y chromosome contains gene SRY (sex-determining region Y) which makes TDF XY females deleted for SRY region are observed and XX males with TDF gene translocated to X chromosome

14 hemizygous gene a gene present in only one copy in a diploid organism e.g. any gene on the X or Y (NRY) chromosome in human males

15 Primary and Secondary Sex-Ratio
Maturity 1 : 1 Old Age 1 : <1 Explanation for primary sex-ratio: Y sperm more able to fertilize egg, or, better survival in reproductive tract Y lacks most genes on X, any subvital/ lethal genes uncovered in hemizygous males, higher lethality

16 Autosomal genes are inherited in equal frequencies by both sexes
Aa x Aa 1AA : 2Aa : 1aa (if allele a is recessive equal numbers of females and males should be affected)

17 Y-Chromosome Linkage in Man Holandric Inheritance
Hairy Pinna Y always inherited father to son Gene found only on Y-chromosome

18 Inheritance of Sex (X)-Linked Traits Color Blindness
= = colorblind cc CY Color-blindness is a sex-linked recessive trait CC cY Cc cY Cc CY CY Cc Cc Cc or CC CY Cc or CC Cc or CC cY trait more common in males because of hemizygosity

19 Dosage Compensation in Mammals
Observation -- Trisomys- XXX, nearly normal XXXY looks fairly normal 21/21/21 Down’s quite abnormal 13/13/13 Severely affected Why can individuals tolerate additional X-chromosomes but not additional autosomes? How is X-chromosome gene expression controlled?

20 Dosage Compensation Background
X-chromosome inactivation in mammals 1. Barr (1949) -- Observed chromatin bodies in the nerve cells of female cats. Somatic cells only. 2. Ohno -- Proposed Barr body is X-chromosome 3. The # of chromatin bodies is one less than the # of X-chromosomes. Restricted to somatic cells. Interphase Cells of Cells of XXX = XXXX = XY = XXY =

21 Dosage Compensation Explained
Lyon Hypothesis (1962) (English Mouse Geneticist) 1. The chromatin body is an X-chromosome One X-chromosome is inactivated - visible as a differentially stained Barr body during interphase. Genes inactivated. 3. The inactive X can be either maternal or paternal in origin. Inactive X-chromosome is late-replicating. It lags on the metaphase plate during mitosis. 5. X-inactivation in humans begins about 16 days into development. 6. Once an X-chromosome becomes inactive it will always be inactive in all cells derived from that cell.

22 mosaic / chimera - an individual composed of two or more genetically different cell types

23 Dosage Compensation (cont.)
B B b b B B b b B B B B b b b b

24 Dosage Compensation Example Calico Cat
Black BB BY Calico Bb ---- Yellow bb bY Exception: XXY Klinefelter male calico cat

25 Bridges Sex-Balance Theory Drosophila melanogaster
The white eye gene in Drosophila was the first gene to be located on a chromosome. a) Crossed XW XW (x) X+ Y XW X+ Y XW X+ XW Y Wild Type White (red eye) Exceptional offspring occurred at low frequency 1. White 2. Wild type (sterile) } Primary Exceptions

26 Hypothesis XW XW (x) X+ Y [Primary non-disjunction] XW XW X+ Y XW XWX+
X+ O XW XWY OY Fertile Sterile Dies Metafemale dies

27 Chromosomes of Drosophila
centromere X II III IV X II III IV X & Y = Sex-chromosomes Autosomes = XXAA II + III + IV = XYAA = 1 Set Autosomes (A) Autosome = any chromosome that is not a sex-chromosome

28 Non-disjunction shown for X-chromosome and complete sets of autosomes.
Mutation of a third chromosome gene, C3G, leads to a high degree of non-disjunction. Non-disjunction shown for X-chromosome and complete sets of autosomes. XXXAAA (X) XYAA Chromosome Segregation during Meiosis X XX X XX XA XXAA XAA XXA A AA AA A

29 XA XXAA XAA XXA (Triploid ) XXXAAA (Intersex) XXAAA (Meta ) XXXAA XA
XXYAAA (Meta ) XYAAA XXYAA YA XYAA CONCLUSIONS: Female and male determining factors on X-chromosome and autosomes Y-chromosome carries male fertility genes X:A ratio determines sex

30 .5 .67 1 Intersex XXAA = 2/2 = 1 = XYAA = 1/2 = .5 = XXXAAA = 3/3 = 1 = XXXAA = 3/2 = 1.5 = Meta XYAAA = 1/3 = .33 = Meta XXYAA = 2/2 = 1 = XXAAA = 2/3 = .67 = Intersex XXYAAA = 2/3 = .67 = Intersex

31 External Environment and Sex Determination
Bonellia Worm -- Marine Worm 1. and have same chromosomes. 2. If young worm is raised from an isolated larva it becomes a female. 3. Drop larva in water with females. If larva comes in contact with female then larva turns into male and eventually migrates to female reproductive tract. 4. Possibly a hormone is secreted from the proboscis of female to initiate male sexual differentiation.

32 Sex Determination in Turtles all males at 25oC all females at 30.5oC

33 Sex Determination in Alligators and Crocodiles
all females at 31oC; all males at 33oC; at 35oC temperature affects levels of male and female sex hormones

34 Solving Pedigrees 5 basic types - autosomal recessive - autosomal dominant - sex-linked recessive - sex-linked dominant - Hollandric inheritance 1. Recessive characters often skip generations 2. Dominant characters are usually observed in every generation. 3. Autosomal characters are inherited equally by females and males. 4. Recessive sex-linked characters are inherited at a much higher frequency by males. 5. Y-linked characters are only inherited by males.

35 Hints: - when determining inheritance of a rare mutation - if an individual marries into the family assume that the individual is homozygous for the wild-type allele (non-disease allele) unless there is evidence suggesting otherwise - dominant mutations are usually lethal when homozygous; assume that individuals carrying dominant mutations are heterozygotes - when determining the mode of inheritance in a pedigree be sure to rule out other possibilities

36 Duchenne Muscular Dystrophy : X - linked dominant inheritance

37 Rare autosomal recessive kidney disease
What is the probability that individuals 1 and 2 will have an affected child?


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