SEX- LINKED INHERITANCE
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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
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