1. The document discusses genetics concepts including genes, alleles, genotypes, phenotypes, Mendel's laws of inheritance, and models of inheritance. 2. It provides examples of Mendel's experiments with pea plants and dihybrid crosses, demonstrating dominant and recessive traits. 3. Different inheritance patterns are described such as complete dominance, albinism, PKU, and blood types. Pedigree analysis is also discussed.

1. GENETICS

2. 1-Eenjamin A.P.(2003) Genetics A Conceptual
Approach .W.H .Freeman and Com. New York.
P:709.
2-Robert J.B.(2005).Genetics analysis and principles
2nd Ed.McGraw Hill.New York.P:842.
3-Daniel L. Hartl .,and Elizabeth W. Jones.(1998). Genetics: Principles and
Analysis . Fourth Edition. Jones and Bartlett
publishers.USA.pp:1367 .
4- Essentials of Genetics (2011).http://www.nature.com/scitable/ebooks
5-Rooney D. (2001) Human Cytogenetics: Constitutional Analysis: A
Practical
Approach 3rd Edition. 3rd
Edition, Oxford University Press.
6-Peter D., Turnpenny A., Sian E., (2007) Emery's Elements of Medical
Genetics. 14th
Edition, Amsterdam: Elsevier.

3. GENETICS
Genetics: The science of heredity and variation.
Heredity: Resemblance among individuals related by
descent; transmission of traits from parents
to offspring.
Variation: In biology, the occurrence of differences
among the individuals of the same species.
Gene:Basic unit of biological information; specific segments
of DNA composed of distinctive sets of nucleotide pairs
in a discrete region of a chromosome that encodes
a particular protein;a coding locus.
Alleles: Alternative forms of a single gene.

4. Gene for eye color
(blue eyes)
Gene for eye color (brown
eyes)
Homologous pair of
chromosomes
• One pair of Homologous Chromosomes:
Alleles – different genes (possibilities) for the same trait –
ex: blue eyes or brown eyes

5. • Genome:All of the chromosomes and DNA
sequences that an
• organism can possess.(the human
genome size ~3,200,000,000 bp)(No.of genes
~35,000).
• Chromosomes: Nucleoprotein bodies ,which are
dark-staining with basic dyes
.Microscopically observable in the cell
during cell division. They carry the
genes that are arranged in linear order
.Each species has a characteristic
chromosome number.

6. F1: The first filial generation. The first generation
of
descent from a given mating.

7. Recombination: The observed new combinations of
traits different from those combinations
exhibited by the parents.
Character: One of the many details of structure,
form, substance, or function that
make up an individual organism.
Phenotype: Characteristic of an individual observed
or discernible by other means (i.e. color
blindness or blood type in humans).

9. Genotype: The genetic composition of an
individual, especially in terms of the alleles for
particular genes(i.e. Tt or tt) .
• Genetic characters are controlled by unit
factors in pairs.
• In other words, genes are present in two
associated copies in diploid organisms.
• For example, TT plants have two alleles for
tallness, tt plants have two alleles for
dwarfism.

10. MutationMutation::
- Variations in DNA sequence (substitutions,Variations in DNA sequence (substitutions,
deletions, etc) that are present at a frequency lowerdeletions, etc) that are present at a frequency lower
than 1% in a population.than 1% in a population.
- Can produce a gain of function or a loss of function.Can produce a gain of function or a loss of function.
PolymorphismPolymorphism::
- Variations in DNA sequence (substitutions,Variations in DNA sequence (substitutions,
deletions, insertion, etc) that are present at adeletions, insertion, etc) that are present at a
frequency 1% or greater than 1% in a population.frequency 1% or greater than 1% in a population.
- Have a WEAK EFFECT or NO EFFECT at all.Have a WEAK EFFECT or NO EFFECT at all.
A little more basic terminology

11. Mendel’s first law
Mendel found, as one example ,that if a tall peas
Plant crossed with a dwarf plant, then all the
hybrid plants resulting from that cross were tall.
P: TT X tt
Tall dwarf
G: T t
F1: Tall
Mendel allowed the hybrid plant F1 to become
Fertilized with their own pollen, to see if any
dwarf
Plant would reappear in the next generation.

12. F1 X F1
Tt X Tt
Tall Tall
F2 : TT: 2 Tt : tt
Mendel’s experiment showing that a recessive trait dwarf
is not expressed in a hybrid but emerges in
approximately one fourth of the offspring of self-
fertilized hybrids.
Mendel used a capital letter signified a dominant, and
a lower case letter is recessive member of a pair of
alleles.
• Normally, the letters indicates the name of the functionNormally, the letters indicates the name of the function
of the geneof the gene

13. • What would occur in the F3 generation?
• One-third (TT)of the F2 tall plants would
produce
• only tall F3 progeny, whereas two-third (2 Tt)
would produce both tall and dwarf progeny.
• The F2 dwarf plants(tt ) were expected to
produce all dwarf F3 progeny.

14. • One-third F2 (Tall) TT X TT
• T T
• F3 TT All tall plants
two-third F2 (Tall) Tt X Tt
• T,t T,t
• F3 TT:Tt:tt Tall & dwarf plants
• dwarf plants(F2) tt X tt
• t t
• F3 tt All dwarf plants

15. • In other crosses; six pairs of contrasting traits were
studied one member of each pair dominated the other
in the same way as tall pea plant dominated dwarf
pea plant.
• Parents F1 F2
Round x wrinkled Round 2.96:1
seed
Yellow x Green Yellow 3.01:1
Seed
Inflated x wrinkled inflated 2.95:1
Pod
Green x Yellow Green 3.14:1
Pod
Axial x Terminal Axial 3.14:1
Flower
Colored x white Grey-brown 3.15:1
(Grey-brown)
seed coat

16. Wrinkled
P Roundx
F1
All Round
Phenotype
Yet Another Example of Mendel’s Work
F1 x F1 = F2
F2 3
/4 Round
1
/4 Wrinkled
ww WW
Ww
Genotype
Homozygous
Recessive
Homozygous
Dominant
Heterozygous
Wrinkled
ww
Round
Ww
w
Round
Ww
Round
WW
W
wWPunnett Square:
possible
gametes
possible gametes

17. Monohybrid Test Cross
• How can you determine genotype from individual
expressing dominant phenotype? DD or Dd?
• D-?
• Cross individual with dominant phenotype to a
homozygous recessive individual.

18. Unknown Round Wrinkledx
ww
Round
Ww
Round
Ww
W
Round
Ww
Round
Ww
W
ww
possible
gametes
possible gametes
Test Cross:
If Unknown is WW:
Wrinkled
ww
Wrinkled
ww
w
Round
Ww
Round
Ww
W
ww
possible
gametes
possible gametes
If Unknown is Ww:
Test Progeny All Round
Test Progeny Half
Round Half Wrinkled

19. Unknown Yellow Greenx
gg
Yellow
Gg
Yellow
Gg
G
Yellow
Gg
Yellow
Gg
G
gg
possible
gametes
possible gametes
Test Cross:
If Unknown is GG:
Green
gg
Green
gg
g
Yellow
Gg
Yellow
Gg
G
gg
possible
gametes
possible gametes
If Unknown is Gg:
Test Progeny All Yellow
Test Progeny Half Yellow
Half Green

20. • Mendel’s conclusions
• A gene for height has two alleles, one for tall
and one for dwarf. The allele for tall behaves
as dominant, whereas that for dwarf is
recessive. Similarly for the other six traits.
• DominantDominant: An allele which is expressed: An allele which is expressed
(masks the other).(masks the other).
• RecessiveRecessive: An allele which is present but: An allele which is present but
remains unexpressed (masked)remains unexpressed (masked)

21. • *During meiosis the members of each pair of allele
separate and distributed to different sex cells or
gametes, thus they occur in different offspring.
• In other words, when sperm and eggs are
formed, one of each allelic pair is randomly
distributed to each gamete.
• For example, a Tt plant makes pollen or eggs, each
randomly receives either the T allele or the t allele.
• This principle of segregation
*(Mendel’s first law or law of segregation).

22. • Note//
• Zygot or individual organism carrying two units
of allele DD,dd are homozygous(Homo means
the same). Both alleles for a trait are the same.Both alleles for a trait are the same.
• Zygot or individual organism carrying two
alleles Dd are heterozygous (hetero means
different) or the organism's alleles for a trait arethe organism's alleles for a trait are
different.different.

23. • The previous crosses called monohybrid crosses.
• In dihybrid cross:Mendel allowed plants that were
hybrid in two characters to self-pollinate.
• Since yellow and round are dominant,
Let G = yellow, g = green, W = round, w = wrinkled.
P: yellow Round X green wrinkled
seed seed
GGWW ggww
G: GW gw
F1: GgWw
yellow Round
F1 X F1
GgWw X GgWw

24. Dihybrid cross

25. • The result
• Four phenotypes
• Total=556
• (315) 9:G-W- Yellow Round 315/556=9/16
• (108)3: gg W- green Round 108/556=3/16
• (101)3: G-ww Yellow wrinkled 101/556=3/16
• (32) 1: wwgg green wrinkled 32/556=1/16
• This principle of independent assortment or
mendel’s 2nd
law

26. • During gamete formation, segregating pairs of unit
factors assort independently.
• In other words, segregation of 2 alleles at one genetic
locus has no effect on the segregation of 2 alleles at
another locus (unless linked).
• For example, the assortment of yellow and green alleles
has no effect on the assortment of round and wrinkled
alleles.

27. • Mendel recognized this as the result of two
monohybrid crosses ,each expected to result in
a 3:1 ratio, operating together.
• The product of the two monohybrid ratios(3:1)2
• Or(3+1)2
was equal to the dihybrid ratio
• (3+1)2
=9+3+3+1
• This is agree with the law of probability:

28. Dihybrid Testcross:
How to determine the genotype of an individual with
2 traits of dominant phenotype
Let G = yellow, g = green, W = round, w = wrinkled.
All yellow
Mixed All Round

29. • Note//
• Branching method:
• Branching method used to determine the
genotype,phenotype and its ratios in second
filial generation that resultant from matings
between parents differ in numerous pairs of
genes:
• AaBb
• A B AB 1
• b Ab 2
• a B aB 3
• b ab 4

30. AaBbCc:
B C ABC 1
c ABc 2
A
C AbC 3
b
c Abc 4
B C aBC 5
c aBc 6
a
C abC 7
b
c abc 8

31. • AaBbCcDD ????

32. Mothods of genetics study
• 1-Planned breeding: Such as Mendelian’s
• experiments on pea plant
or other genetic experiments to forming a
hybrid by cross-pollination of plants or by
mating animals of different types.
• 2-Pedigree analysis: (A table or diagram
representing the ancestral history of an
individual).
• **In man one cannot make genetic experiments
in the same sense as one can in plants,
insects, or animals.
•

33. Table or diagram are used to study the
inheritance of genes in humans. It is also useful
when studying any population when progeny data
from several generations is limited. Pedigree
analysis is also useful when studying species
with a long generation time.
A series of symbols are used to represent
different aspects of a pedigree.

34. • *** In pedigrees it is conventional to symbolize
females by circles (o ) and males by
• squares(□).
• □ o designates a consanguineous
• Roman numerals to the left or right of the diagram indicate the
generations

36. Once phenotypic data is collected from
several generations and the pedigree is
drawn, careful analysis will allow you to
determine whether the trait is dominant or
recessive.

37. The following is the
pedigree of a trait controlled
by dominant gene action

38. For those traits exhibiting dominant gene action:
affected individuals have at least one affected parent
the phenotype generally appears every generation.
Two unaffected parents only have unaffected offspring
.

39. The following is the
pedigree of a trait contolled
by recessive gene action.

40. unaffected parents can have affected offspring
affected progeny are both male and female

41. Models of inheritance
• The classical Mendelian ratios,such as 3:1
, 9:3:3:1 ,do not by any means occur in all
crosses.
• Phenotype ratios are modified in various
ways, although the fundamental laws of
the transmission of heredity remain the
same.

42. Complete dominance
• In complete dominance the phenotype of
heterozygous organisms are similar to that of
homozygous organisms(AA similar to the Aa in
phenotype) in spite of presence of recessive allele
in heterozygous but its function absent.
• Example:In human teeth,dentinogenesis
imperfecta.
• Dental radiographs of normal teeth compare with
those of a patient with opalescent dentin

43. • *Normal teeth(d)showing normal enamel,dentin,and
patent pulp chambers,and root canals.The teeth are
normal in color on clinical examination.
• *Opalescent teeth(D):The enamel is normal,but the
pulp chambers and root canals in most teeth are
covered with abnormal dentin.There is an increased
constriction at the junction between the crowns and
roots of the molars,the teeth have special opalescent
brown color.
• D=The dominant gene for dentinogenesis imperfecta
• d= normal allele

44. • Affected Dd X Normal dd
• D,d d
Dd : dd
• Affected Normal
• The effect of the dominant gene appears to
mask completely the presence of the recessive
allele.
• Dd X Dd
• DD: 2Dd : dd
• Affected Normal

45. • Albino people (cc) are characterized by a
marked deficiency or complete absence of
pigment in the skin ,hair, and iris of the eyes.
Such people are at risk in sun light because
they have no defense against ultra-violet
radiation. The condition result from a mutation
in the DNA that instruct for the manufacture of
an enzyme needed to form the pigment
melanin.

46. • A mating between two normal people both of
whom are carriers for the allele(c) for albinism, is
• Normal Cc X Normal Cc
• C,c C,c
• C c
C CC Cc
c Cc cc Albino
3.normal:1 albino

47. • Phenyl Keton Urea (PKU)
• It is the result of a mutation in DNA producing a
change in the instructions given to cells.The cells are
not instructed to produce a particular enzyme and if
this enzyme is not present then phenylalanine cannot
be broken down to tyrosine. Instead it accumulated
and affects the brain.
• Normal Male Normal Female
• Pp X Pp
• P p P p
• PP: 2 Pp: pp
• No PKU PKU

48. • The Rh positive genotype dominate over Rh
negative.
• In human the black hair pigment is dominant
over red hair:
• Black-haired father X red-haired mother
• BB homozygous bb homozygous
• B b
• possible offspring Bb black-haired

49. • Black-haired father Red-haired mother
• Bb heterozygous X bb homozygous
• B b b
• Bb :bb
• half with red hair

50. • Black-haired father Black-haired mother
• Bb X Bb
• B b B b
• BB: Bb :bb
• Ratio 3 black hair to 1 red hair

51. Incomplete dominance
• A cross between a red flowered and white flowered
Snapdragon showing absence of dominance in F1 and
all the plants were pink-flowered,and a ratio of ¼
red,1/2 pink,and ¼ white in F2(alleles may produce
the same product but in lesser quantity as compared
with the dominant allele).
• Red white
• RR X rr
• Rr pink
• Rr X Rr
• RR: 2Rr: rr
• Red: pink: white

52. • The alleles R and r produce red and white
colors respectively.Since neither allele is
dominant.
• Also the inheritance of plumage color in
Andalusian fowls crosses of black with white
produce only blue-gray progeny in F1.These
when bred together,produce in F2,1/4 black,1/2
blue-gray and ¼ white color.

53. Codominance
The characteristics of both parents occur
simultaneously in the F1.For example the blood
groups in man show codominant inheritance.
A mating between homozygous IA
IA
and IB
IB
person would result in all heterozygous IA
IB
progeny.
IA
IA
X IB
IB
IA
IB
IA
IB
X IA
IB
IA
IA
: 2 IA
IB
: IB
IB
1 : 2 : 1

54. Overdominance
The phenotype of a heterozygote measured
quantitatively is not always equal or intermediate to
that of the homozygotes.
****The term overdominance has been used for
characteristics concerned with biological”fitness”
such as size,productivity and viability.

55. • Example: in Drosophila the white-eyes
gene (w) in hetrozygous condition (w+
/w)
causes a marked increase in the amount
of certain fluorescent pigments over both
the white and wild-type homozygotes.

56. • Note:
• Dominance of a trait does not imply that its
possessors are healthier or more vigorous than
the recessives. There is no constant relation
between dominance or recessiveness of a
character and its usefulness or harmfulness.
• .

57. • There are many fatal diseases in man and
other organisms are inherited in accordance
with mendel’s laws. Some of these diseases
are due to dominant genes, whereas the
normal ,healthy state is conditioned by the
recessive alleles of these genes
• Again some diseases are due to homozygosis
for recessive genes and the normal state to the
corresponding dominant.Also there is no
relation between dominance of a gene and its
frequency, a gene,dominant or recessive may
have any frequency,from very high to very low ,

58. • for example,that in a population, most
persons have blue eyes but some have
brown eyes. It would not be legitimate to
conclude that blue eye color is dominant
to brown. In fact, the opposite happens to
be the case.

59. Lethal gene
• Genes may affect viability as well as the
visible traits of an organism.
• Some organisms have lower viabilities than
the wild type, and detrimental physiological
effects are apparently associated with the
genes involved.

60. Therefore the genes have such effects (make the
organism is unable to live) called lethal genes. If the
lethal effect is dominant, all individuals carrying the
gene will die and the gene will be lost.
Cuenot noticed that mating between two mice with
yellowish fur produced progenies with yellow
and Agouti in a ratio 2:1 , respectively.

61. • Let
• Ay
= allele for yellow fur
• a= allele for Agouti
• Ay
a X Ay
a
• Yellow yellow
• 1/4 Ay
Ay
: 2/4 Ay
a:1/4 aa
• Die yellow Agouti
• Thus a mouse homozygous for Ay
dies before
• Birth. The hypothesis has been verified by several
investigators who found that some embryos in the
uterus of a yellow female mated to a yellow male die
in an early embryonic stage.

62. Epistasis
• Any gene or gene pair that masks the
expression of another, nonallelic gene is
epistatic to that gene. The phenotype
suppressed is said to be hypostatic.

63. The system of genes that determines skin
colour in man, for example, is independent
of the gene responsible for albinism (lack
of pigment) or the development of skin
colour.
This gene is an epistatic gene. When the
albino condition occurs, the genes that
determine skin colour are present but not
expressed.

64. Different genes contribute to the steps
needed to make P from a precursor
molecule. In order to get to P, all these
steps have to be fully functional. If there is a
mutation in one of these genes, the reaction
cannot take place and the phenotype or P is
affected.

65. Multiple Alleles
• An allele is a specific form or sequence of
nucleotide-pairs of a given gene, many and
possibly all genes can change in several or in
many different ways.
• These changes give rise to several alternative
states or variants of the gene, which are called
multiple alleles.
• (When more than two different forms of a given
gene exist in a species, they are referred to as
multiple alleles)

66. • Multiple Alleles :Three or more
alternative alleles that represent the same
locus in a given pair of chromosomes.

67. The ABO blood system
• This is a controlled by a tri-allelic gene. IA,
IB,
and
Io
.
• The alleles control the production of antigenes
on the surface of the red blood cells.
• Two of the alleles are codominant to one
another and both are dominant over the third.
• AlleleIA
produces antigen A
• AlleleIB
produces antigen B
• AlleleIo
produces no antigen

68. It can generate 6 genotypes.
• IA
IB
heterozygotes have both A and B antigens
• on their red blood cells.
• IA
IA,
IB
IB.
Homozygotes
• Io
Io
homozygotes have no ABO antigens on
• their red blood cells
• IA
Io
and IB
Io
heterozygotes have A and B
• antigens,respectively,on their
• red blood cells.

69. • Genotype phenotype Antigen antibodies
• present present
• IA
IA
A A Anti-B
• IA
Io
• IB
IB
B B Ant-A
• IB
Io
• IA
IB
AB A&B none
• Io
Io
O none Anti-A
• &Anti-B
• Type O blood may be transfused into all the
other types = the universal donor.
• Type AB blood can receive blood from all the
other blood types = the universal recipient.

70. Rh factor Alleles in human
• Rh factor was discovered in 1940 by K.Landsteiner
and A.S.Wiener.
• Individuals whose blood cells react with the Rh-
antibody are termed Rh-positive.
• Individuals whose blood cells not react with the Rh-
antibody are termed Rh-negative.

71. At first a single pair of alleles, R and r,was
postulated to account for the difference between
Rh-positive and Rh-negative individuals. New
soon discovered and additional genes were
postulated to explain the more complicated
situation. This series of multiple alleles as follow:

72. • R1,
R2
,Ro
,Rz
,r,r˝,r ΄,ry.
• The Rh blood type:
• Rh1,
Rh2
,Rho
,Rhz
,rh,rh˝,rh΄,rhy
• Note:
• The number of different genotypes possible in diploid
organisms is ,of course, a function of the number of
alleles that exist for any given gene.
• If n is the number of alleles of a gene, the number of
different genotypes possible is
• [n(n+1)]/2 .Thus with two alleles:
• [2(2+1)]/2=3 or three possible genotypes .
• With four alleles [4(4+1)]/2 =10 or ten possible
genotypes .

73. Pleiotropism
• We have seen from previous studies that
many characters are determined not by single
genes but by cooperation, or interaction, of
several or many genes.
• **In some cases;
• influesnce
• One gene more than one trait
• When an gene cause changes in two or more
parts or characters that are not obviously
related, the gene is called pleiotropic or is said
to have multiple effects.

74. • Even though a structural gene may have
many end effects it has only one primary
function, that of producing one polypeptide,
this polypeptide may give rise to different
expressions at the phenotypic level.

75. • Example:
• The Hbβs
allele provides a classic example of
pleiotropy. It not only causes hemolytic anemia
• (in the homozygous state) but also results in
increased resistance to one type of malaria,that
caused by the parasite plasmodium falciparum.
• Because the increased resistance to falciparum
Malaria occurs in HbβA
Hbβs
heterozygotes,such
heterozygous individuals have a selective
advantage in geographical regions where this
type of malaria is prevalent.

76. • The Sickle-cell allele also has pleiotropic
effects on the development of many tissues
and organs such as bones ,the lungs, the
kidneys, the spleen, and the heart.
• Heterozygotes for the allele for Sickle-cell
anemia are more resistant to falciparum
malaria.
• Sickle-shaped corpuscles that clog the
capillaries thus interfering with circulation and
depriving the cells in the body of oxygen.

77. Sex-linkage in human
• In the previous genetic examples, the sex
of the parents was not specified, because
the previous traits are determined by
genes located on the autosomal
chromosomes (all chromosomes other
than X or Y) .If a trait is determined by a
gene located on the X chromosome is
referred to as X-linked or sex –linked trait.

78. • Example:
• Hemophilia: The blood fails to clot normally
• Lacking a blood clotting factor VIII (antihemophilic globulin, AHG)
• bleeding from even minor cuts.
• Let H=normal gene
• h=hemophilic gene
• ♀ XH
XH
X ♂ Xh
Y
• Normal hemophilic
• XH
Xh ,
Y
•
• ♀ XH
Xh
♂ XH
Y
• Carrier normal
• ♀ XH
Xh
X ♂ XH
Y
• XH
XH ,,
XH
Xh ,
XH
Y, Xh
Y
•
• Normal Normal hemophilic
•
•

79. • Color blindness
• Color vision is produced by specific receptors
called cones in the retina of eyes. (cone: short
light receptors in the retina of vertebrates that
are sensitive to bright light and function in color
vision).
• There are three types of cones that respond to
the colors; Red, green, and blue, respectively.
• This selectivity is due to a slightly different
pigment in each type of cone.

81. • The gene that codes for the pigment in the blue
cones is located on an autosomal chromosome
(chromosome No:7).
• The genes for the red and green cones are
located on the X-chromosome.
• In the most common form of red –green color
blindness ,the gene for green cones is
defective so that green cannot be distinguished
from red.
• In a less-common form of this disorder the
gene for the red cones is defective ,so that red
cannot be distinguished from green.

82. • Lack of both red and green cones in a given
individual would abolish color vision entirely,this
is an extremely rare condition.
• Normal color blind
• P: Xa
Xa X
Xa’
Y
• G: Xa
Xa’
Y
• F1: Xa’
Xa
: Xa
Y
• Xa’
Xa
X Xa
Y
• Xa’ ,
Xa
Xa ,
Y
F2: Xa’
Xa :
Xa
Xa :
Xa’
Y : Xa
Y

83. • Note:
• An inherited characteristic was observed in a
father but not any of his children, either male
or female, and then would reappear in males of
the next generation, this type of inheritance
called crisscross inheritance.

84. Y chromosome linkage in man
• Some genes are located on Y chromosome
such as the gene responsible for hairy pinna
this trait present in male only and inherited from
father to sons directly.

85. Sex-Influenced dominance
• Dominance of alleles may differ in heterozygotes
of the two sexes. Gene products of
heterozygotes in the two sexes may be
influenced differentially by sex hormones.
• Example:
• Autosomal genes responsible for horns in some
breeds of sheep, these genes may behave
differently in the presence of the male and
female sex hormones.

86. • A mong Dorset sheep,both sexes are
horned ,and the gene for the horned condition
is homozygous (h+
h+
).
• In Suffolk sheep,neither sex is horned and the
genotype is (hh).

87. A mong the heterozygous F1 progeny,from
crosses between these two breeds.
Horned males and hornless females are produced.
• Genotype ♀ ♂
• ________ ______ ______
• h+
h+
Horned Horned
• h+
h Hornless Horned
• hh Hornless Hornless
• _______________________________
• Here the gene must behave as a dominant in males
and as a recessive in females ,that is,only one allele
is required for an expression in the male,but the allele
must be homozygous for expression in the female.

88. • h+
h+
X hh
• h+
h horned male or hornless female
• h+
h X h+
h
• Males Females
• h+
h+:2 h+
h : hh h+
h+
: 2h+
h : hh
• Horned hornless horned hornless

89. In humans:
1-white forelock.
2-absence of upper lateral incisor teeth.
3-a particular of enlargement of the terminal
joints
of the fingers.
4-premature pattern baldness.
All these characters have been reported to follow
this mode of inheritance.
b+
dominant in male and responsible for baldness
in case of homozygous b+
b+
or heterozygous b+
b,
whereas it is a recessive in female,and the

90. • Note:
• Frequency of baldness in the infertile
male equal the frequency of baldness
in the female according to the presence
of hormones.

91. • Sex-Limited Gene
• A trait that appears in only one sex is
called sex-limited

92. Sex hormones are apparently limiting
factors in the expressions of some genes.
• Examples are cases of sex limited
expression which might include genes
affecting breast size, and milk
production in mammals, are limited to
females