sex- linked inheritance

SEX- LINKED INHERITANCE

Lateral view of a male Fruit Fly (Drosophila melanogaster) showing its red wild type eye. LM

P: FEMALE DROSOPHILA X MALE DROSOPHILA

RED- EYED ♀ WHITE-EYED ♂

F1: ALL RED-EYED OFFSPRING

CONCLUSION: RED-EYE GENE IS DOMINANT

P (FROM F1) RED- EYED ♀ X RED-EYED ♂

F2: 3/4 RED-EYED ; 1/4 WHITE-EYED

♀ AND ♂ ♂ !

P: XR XR X Xr Y

G: 1/1 X 1/2 , 1/2

F1: 1/2 X R X r ; 1/2XR Y

CONCLUSION: ALL RED-EYED OFFSPRING

P (FROM F1): XR Xr X XR Y

G: 1/2 XR , 1/2 Xr x 1/2 XR , 1/2 Y

F2: 1/4XR XR ; 1/4 XRXr ; 1/4 XR Y ; 1/4 Xr Y *

XRXr

Y

¾ RED-EYED ♀ AND ♂ ¼ WHITE-EYED ♂

P: RED EYED FEMALE FROM F1 x ORIGINAL WHITE-EYED MALE

F1: Female offspring ; male offspring

(50% red; 50% white) ( 50%red; 50% white)

Conclusion: Eye-color gene is found on the X chromosome and there is no corresponding allele on the Y chromosome

P : XR Xr X Xr Y

G: 1/2 XR , 1/2 Xr X 1/2 Xr , 1/2 Y

F1: 1/4XR Xr ; 1/4 XrXr ; 1/4 XR Y ; 1/4 Xr Y

RED-EYED ♀ WHITE-EYED ♀ RED-EYED ♂ WHITE -EYED ♂

TEST CROSS

SEX- LINKED INHERITANCE IN HUMANS

X-LINKED TRAITS IN HUMAN(HEMOPHILIA,

DUCHENNE MUSCULAR DYSTROPHY, COLORBLINDNESS)

HEMOPHILIA

(a)

P: Homozygous normal female X hemophiliac male

X H X H X Xh Y

G: 1/1 XH 1/2 Xh ; 1/2 Y

F1 : 1/2 XH X h ; 1/2 XH Y

(carrier – female) (normal - male)

H: DOMINANT GENE FOR HEMOPHILIA

h: RECESSIVE GENE FOR HEMOPHILIA

(b)

P: Heterozygous (carier) female X normal male

X H X h X XH Y

G: 1/2 XH , 1/2 Xh 1/2 XH , 1/2 Y

F1 : 1/4 XH X H ; 1/4 X H X h ; 1/4 XH Y ; 1/4 Xh Y

Homozygous carrier ♀ normal hemophiliac

normal ♀

Normal phenotyped ♀

(c)

P: Heterozygous (carrier) female X hemophiliac male

X H X h X Xh Y

G: 1/2 XH , 1/2 Xh 1/2 Xh , 1/2 Y

F1 : 1/4 XH X h ; 1/4 X h X h ; 1/4 XH Y ; 1/4 Xh Y

carrier ♀ hemophiliac ♀ normal hemophiliac

DUCHENNE MUSCULAR DYSTROPHY

COLORBLINDNESS

Normal ♀ color blind♂ carrier♀

XR XR Xr Y XR Xr

Normal ♂ colorblind ♀

XR Y X r Xr

The pattern of inheritance of X-linked traits;

• A man inherits his X chromosome and all his X-linked traits from his mother

• A woman inherits one X chromosome from her mother and one from her father

• A defective X chromosome in a man is always passed to each daughter but never to a son

• A defective X chromosome in a woman has a 50% chance of being passed to each son and daughter

Y-LINKED TRAITS IN HUMAN

• Ichtyosis hystrix (balık pulluluk)

• Hair on the rim of the ears

• joined 2nd and 3rd toes

A pedigree chart: Inheritance of Y-linked trait in a family

Ichtyosis hystrix : balık pulluluk

Name: Radhakant Bajpai, India.Length: 13.2cm (5.2 inches)

Hair on the rim of the ears

A map of the human X and Y chromosomes, showing the locations of some of the genes that cause disease

Duchenne muscular dystrophy

Hemophilia AColorblindness Hemophilia BDiabetes insipidus

Genes for testes

Ichtyosis hystrixHair on the rim of ear

Joined 2nd and 3rd toes

DNA that is not organized into genes

SEX-INFLUENCED TRAITS

P: Heterozygous female X heterozygous male

Bb X Bb

G: 1/2 B , 1/2 b 1/2 B , 1/2 b

F1 :

Inheritance of pattern baldness in humans

♀ BB Bb bb

Bald normal

♂ BB Bb bb

Bald normal

Other examples of sex- influenced traits include;

-Development of mammary glands in human

-Coat patterns and milk production in cattle

-Egg production in birds

NON-DISJUNCTION

P: Vermillion- eyed female X red eyed male

6 + X r X r X 6+ XR Y

G: 1/1(3+ Xr) 1/2(3+ XR) , 1/2(3+ Y)

F1 : 1/2(6 + XR X r ) ; 1/2 (6+ Xr Y)

R= dominant red-eye gene

r = vermillion (recessive) gene

P: Vermillion-eyed female X red eyed male

6 + X r X r X 6+ XR Y

G: (3+ Xr Xr ) (3+0) (3+ XR) , (3+ Y)

F1 : (6 + XR Xr Xr ) (6+ XR 0) ; (6+Xr Xr Y) (6+ Y0)

R= dominant red-eye gene

r = vermillion (recessive) gene

Nondisjunction explains exceptions in the sex–linked inheritance of eye color in Drosophila:

nondisjunction

♂ Red (sterile) ♀Vermillic dies

Primary nondisjunction Secondary nondisjunction

ANEUPLOIDY: When an organism gains or loses one or more chromosome but not a complete set

MONOSOMY

The loss of a single chromosome

TRISOMY

The gain of one chromosome

EUPLOIDY: The condition in which complete haploid sets of chromosomes are found. If there are three or more sets, the condition is called polyploidy.(3n, 4n, 5n, 6n, 7n etc.)

Egg (n-1) + sperm (n) (2n -1)

ex: Turner (XO)

Egg (n+1) + sperm (n) (2n + 1)

ex: Down Syndrome (45 +XX or 45 + XY)

GENE LINKAGEGenes linked on the same chromosome are generally inherited together; they move together during meiosis (does not obey Mendel’s Principle of Independents Assortment unless crossing over occurs)

F1: All purple-flowered and elongated pollen-grained offspring(purple-flowered and elongated pollen grain genes are dominant)

P : x Pure purple-flowered and elongated pollen-grained pea plant

Pure red-flowered and elongated pollen-grained pea plant

PPEE ppee

P from F1: Dihybrid purple-flowered and elongated pollen-grained parents

PpEe PpEeX

F2: ¾ purple-flowered and elongated pollen-grained offspring

: ¼ purple-flowered and elongated pollen-grained offspring

• Expected phenotypic ratio was 9:3:3:1

• But the actual ratio was found to be 3:1

Conclusion:

• The genes for flower color and pollen grain shape are linked

• The inheritance of two linked traits produced the same genotype and phenotype ratio in F2 as in monohybrid cross, (1:2:1) and (3:1) respectively.

• Gene linkage reduces variety

P: PPEE x ppee

G: PE pe

F1: PpEe (purple elongated pollen *dominant traits)

P (from F1): PpEe x PpEe

G: ½ PE , ½ pe ½ PE , ½ pe

F2: ¼ PPEE, 2/4 PpEe, ¼ ppee

are linked

P

E

p

e

¾ purple elongated ; ¼ red-round

3:1 phenotypic ratio

1:2:1 genotypic ratio

Test cross of F1

P: PpEe X ppee PpEe X ppee

G: 1/2 PE , 1/2 pe 1/1 pe ¼ PE, ¼ Pe, ¼ pE, ¼pe X 1/1 pe

F1 : 1/2 PpEe ; 1/2 ppee ¼ PpEe; ¼ Ppee; ¼ppEe; ¼ ppee

Expected phenotypic ratio: 1:1

Actual phenotypic ratio: 7 : 1 : 1 : 7

Recombinants due to the crossing over

Gene linkage

CROSSING OVER AND RECOMBINATION

Ex: In Drosophila the genes for gray body (G) and normal wing (N) are dominant over the genes for black body (g) and vestigial wing (n)

In a Drosophila of genotype GgNn, 12% of the primary sex cells undergo crossing-over.

a) What are the types of gametes formed?

b) What are the expected percentages of the types of gametes formed?

Ex 2: A Drosophila dihybrid forbody color and wing shape (GgNn) is tes crossed and F1 is as follows:

Gray – normal : 965 Gray – vestigial: 185

Black – vestigial: 944 Black – normail: 206

What is the crossing over value (recombination frequency)?

Chromosome Mapping

•Genes that are close together rarely cross-over

•Genes that are farther apart will cross-over more often

•Cross-over studies and cross-over values have been used to construct chromosome maps

• Ex: Determine the order (sequence) of genes along a chromosome based on the following cross-over (recombination) frequincies.

A-B: 8 map units (m.u) A-C: 12 m.u A-D: 25 m.u

B-D: 33 m.u D-C: 13 m.u B-C: 20 m.u