8/9/2019 304 Lec3.pdf

1/25

12/24/14

1

Biol 304 General Genetics

Lec 3: Basic Principles of Heredity II

Sex Chromosomes

Heteromorphic sex chromosomes - different between thesexes.

All mammals have the XY-systemFemales: XX = homogametic sexMales: XY = heterogametic sex

Human Male: 46, XYHuman Female: 46, XX

Karyotype - the chromosome complement of a cell or anindividual. Often used to refer to the arrangement ofchromosomes in a sequence according to size.

8/9/2019 304 Lec3.pdf

2/25

12/24/14

2

~Equal numbers of X- and Y-bearingsperm are produced

We expect 1:1 (Female:male) sex ratio

At birth, ~1:1.05

~age 20 and 25, ratio is close to 1:1

Ratio decreases with age (females>males)

Sex Ratios

8/9/2019 304 Lec3.pdf

3/25

12/24/14

3

Egg with Xchromosome

Sperm with Ychromosome

Fertilized by Fertilized bySperm with Xchromosome

Male FemaleEmbryo with XY sex

chromosomesChromosomal

sexEmbryo with XX sex

chromosomes

Gonads

Ducts

Sex-determining region ofthe Y chromosome ( SRY )

brings about developmentof undi ff erentiated

gonads to form testes

No Y chromosome, sono SRY . With no

masculinizing in fl uence,undi ff erentiated gonads

develop into ovaries

Gonadalsex

Testes Ovaries

1

2

3

1

2

3

Testes secretemasculinizing hormones,including testosterone, a

potent androgen

No androgens secreted

In presence of testicularhormones,

undi ff erentiatedreproductive tract and

external genitalia developalong male lines

With no masculinizinghormones, undifferentiated

reproductive tract andexternal genitalia develop

along female lines

Phenotypicsex

UterusOvary

VaginaPenis

Testis

4

5

6

4

5

6

8/9/2019 304 Lec3.pdf

4/25

12/24/14

4

Aberrations of the XY SystemDue to SRY Gene

XX - sterile male - part of SRY geneattached to end of X during recombination(rare).

XY - sterile female - Y chromosome lost

crucial part of SRY gene

8/9/2019 304 Lec3.pdf

5/25

12/24/14

5

Complete Androgen Insensitivity(CAI)

Maria Patinos disorder

Mutation of androgen receptor ( AR )gene on the X chromosome

Lack receptors for testosterone or

hormones derived from testosterone

Cells in the gonad cannot respond totestosterone.

Complete Androgen Insensitivity(CAI)

! Female duct system and external genitalsdevelop

! Individuals are chromosomal males (XY)! Physically appear to be females with well-

developed breasts but limited pubic hair! Do not menstruate, infertile, and have

undescended testes in their abdomens notovaries

8/9/2019 304 Lec3.pdf

6/25

12/24/14

6

Aberrant Sex ChromosomeCompositions

Usually result from nondisjunction - thefailure of a pair of homologouschromosomes to separate during meiosis

47, XXY (Klinefelter Syndrome; extra Xs) 45, X (Turner Syndrome)

47, XXX (Triplo X) 47, XYY (Jacobs Syndrome)

Klinefelter syndrome

8/9/2019 304 Lec3.pdf

7/25

12/24/14

7

Aberrant Sex ChromosomeCompositions

47, XXY - Klinefelter Syndrome (extra X) 1-2 in 1000 births Possess male genitalia Testes don t produce sperm Tall, long arms Slight enlargement of breasts / rounded hips

! Features do not develop until puberty, usuallysterile, can have learning disabilities

Aberrant Sex ChromosomeCompositions 45, X - Turner Syndrome 1 in ~3,000 births; phenotypic female Possess female genitalia Undeveloped ovaries and breasts Short (under 5ft tall)

Skin aps on back of neck ~Normal intelligence

8/9/2019 304 Lec3.pdf

8/25

12/24/14

8

Aberrant Sex ChromosomeCompositions

47, XXX 1 in 1200 births; phenotypic female Normal appearance Possess female genitalia Underdeveloped sex characteristics Sterile Mentally retarded

XYY karyotype

8/9/2019 304 Lec3.pdf

9/25

12/24/14

9

Aberrant Sex ChromosomeCompositions

47, XYY 1 in 1500 births; phenotypic male Tall (over 6ft) 1965 study by Patricia Jacobs

9 of 315 males in maximum security prisonwere XYY

Predisposition to behavioral problemsmaybe high, but not constant correlation

Barr Bodies / X Inactivation

Genetic mechanism in mammals that compensatesfor X chromosome dosage disparities. Females 2 X s; Males 1 X

Inactivated X chromosome lies against nuclearenvelope of cells

# of Barr Bodies = N-1 where N = the totalnumber of X chromosomes.

Usually same X is inactivated in all somaticcells

8/9/2019 304 Lec3.pdf

10/25

12/24/14

10

Barr Body in the Nucleus

8/9/2019 304 Lec3.pdf

11/25

12/24/14

11

X inactivation and the tortoiseshell cat

8/9/2019 304 Lec3.pdf

12/25

12/24/14

12

Other sex determination systems

Sex in the isopod Armadillidium vulgare is usually determined by sex

chromosomes, but genetic males may be converted into functionalfemales by the presence of infecting bacteria.

8/9/2019 304 Lec3.pdf

13/25

12/24/14

13

In Crepidula fornicata, the common slipper limpet, sex is determined

by an environmental factor: the limpets position in a stack of limpets.

XO Method of Sex Determination

Some insects (no sex specic chromosome Females: XX Males: X0 (where 0 indicates lack of 2nd X)

8/9/2019 304 Lec3.pdf

14/25

12/24/14

14

ZW System Some insects Some birds (chickens) Some sh Same as XY except female is heterogametic The use of ZW is arbitrary and for distinction

between XY only, but is commonly used intextbooks

ZZ = male ZW = female

Haplodiploidy

8/9/2019 304 Lec3.pdf

15/25

12/24/14

15

8/9/2019 304 Lec3.pdf

16/25

12/24/14

16

Temperature-Dependent Sex Determinationcommon in some reptiles

Sex-Linked Traits

Many genes and the respective traitscontrolled by them are recognized as beinglinked to the sex chromosomes

Males receive single X-chromosome frommother are hemizygous (pseudo-dominance)

Traits can be easily identied in pedigrees.

8/9/2019 304 Lec3.pdf

17/25

12/24/14

17

Chi-Square ( X 2) Analysis Statistical test used to determine whether

experimentally obtained data constitute agood t to a theoretical expected ratio

In other words, X 2 enables us to determinewhether it is reasonable to attributedeviations from a perfect t to chance

If expected = 10 and observed = 10 then X 2= 0

Equation: X 2 = (O E) 2 E

Chi-Square ( X 2) Analysis

X 2 = (O E) 2

E

In a cross of tall (TT) tomato plants to dwarfones (tt), the F1 consisted entirely of tall(Tt) plants and the F2 consisted of 102 tall(T_) and 44 dwarf (tt) plants. Do these datat an expected 3:1 phenotypic ratio?

8/9/2019 304 Lec3.pdf

18/25

12/24/14

18

X 2 = (O E) 2

E

Tall (T_) Dwarf (tt) Total

Observed 102 44 146

Expected 109.5 36.5 146

O-E -7.5 7.5

O-E 2 56.25 56.25

O-E 2 /E 0.5137 1.5411 X 2 = 2.0548

Chi-square Analysis

Are the observed deviations within the limits expected by chance?

Chi-Square ( X 2) Analysis

Generally, statisticians have agreed on thearbitrary limits of odds of 1 chance in 20(probability = 0.05 ) for drawing the linebetween acceptance and rejection of thehypothesis as a satisfactory explanation ofthe data tested.

degrees of freedom = one less than thenumber of terms in the ratio. Since we havetwo terms (i.e., phenotypes) our df = 2-1 = 1

8/9/2019 304 Lec3.pdf

19/25

12/24/14

19

Chi-Square ( X 2) Analysis

With our calculated X 2 = 2.05 With df = 1 and a probability of 0.05 the

theoretical X 2 = 3.841 Since our calculated X 2 = 2.05 < the

theoretical X 2 = 3.841, we conclude thatthere is not a statistically signicantdifference between our observed data andthe expected ratios.

8/9/2019 304 Lec3.pdf

20/25

12/24/14

20

Chi-Square Analysis 1:1 Ratio

Human Pedigree Analysis Analysis of family trees to determine the inheritancpatterns of selected traits.

8/9/2019 304 Lec3.pdf

21/25

12/24/14

21

Autosomal Recessive Traits AA=normal, Aa=normal, aa=affected 90% of all autosomal genetic defects Trait not carried on sex chromosomes, but on

other 22 pairs (autosomes) Many arise from relatives mating - increased

chance for recessive alleles to pair Two affected parents will always produce affected

offspring

Parents can be carriers (Aa) and have a 25%chance of producing affected offspring Trait often skips a generation Males and females affected equally

8/9/2019 304 Lec3.pdf

22/25

12/24/14

22

Autosomal Dominant Traits AA=affected, Aa=affected, aa=normal ~10% of autosomal genetic defects (chin dimple) Not carried on sex chromosomes Two normal parents can only have normal

offspring Two affected parents who are heterozygotes (Aa)

have a 25% chance of producing normal offspring An affected offspring must have at least one

affected parent Traits will not skip a generation Trait should appear in almost equal numbers When an affected person mates with a normal

person, 50% of the offspring are expected to be

8/9/2019 304 Lec3.pdf

23/25

12/24/14

23

X-Linked Recessive Traits XB=normal, X b=affected Affects more males (50% chance of

inheriting recessive allele from mother =hemizygous )

Traits can skip a generation Affected mother produces affected sons Normal mother produces normal daughter,

unless mother is a carrier and father isaffected then daughter could be affected Two affected parents will have affected

children

8/9/2019 304 Lec3.pdf

24/25

12/24/14

24

X-Linked Dominant Traits XB=affected, X b=normal Affects more females. Traits does not skip a generation Affected males must come from affected

mothers Two normal parents will have normal

children All the daughters, but none of the sons, of

an affected father are affected Approximately 50% of the children of an

affected heterozygous female are affected

Y-linked Traits Only males affected If one male in pedigree is affected then all

related males must be affected If one male is normal then all related males

are normal

8/9/2019 304 Lec3.pdf

25/25

12/24/14