Download Inheritance (heredity): The transmission of genes from parents to

* Inheritance (heredity):
The transmission of genes from parents to offspring.
* Gene:
- Unit of information about heritable trait passed from parents to
offspring.
- Each gene has a specific location (locus) on a chromosome.
* Alleles:
- Alternate forms of genes.
- Designated by letters.
* Dominant = Capital E, A, S, T, ….
The expression of dominant alleles masks the effect of recessive alleles in
heterozygous individual.
* Recessive = small e, a, s, t, ….
Fully expressed only in homozygous recessive individuals.
* Genotype:
Genes makeup of an individual for a particular traits.
(EE , ee , SS , ss , ….).
* Phenotype:
Visible expression of the genotype or the trait.
(e.g. unattached ear, attached ear, brown eye,….).
* Recessive allele:
Must be received from both parents to bring about recessive phenotype.
* Only one dominant:
Only one parent is enough to express the dominant phenotype.
* Genotype: Heterozygous Ee (2 identical alleles).
* Phenotype: Unattached ear lobe.
----------------------------------------------------------------------------------------* Genotype: Homozygous dominant EE (2 identical alleles).
* Phenotype: Unattached ear lobe.
----------------------------------------------------------------------------------------* Genotype: Homozygous recessive ee (identical alleles).
* Phenotype: Attached ear lobe.
----------------------------------------------------------------------------------------* True breeding lineage:
Offspring of genetic crosses inherits a pair of identical alleles EE
generation after generation.
* Hybrid offspring:
Offspring of a genetic cross inherits a pair of non identical allele for a
trait (Ss, Aa, Ee,….).
(Monohybrid Inheritance)
Cross between 2 parents that are homozygous dominant (EE) &
homozygous recessive (ee) of a single trait.
* In F1, after fertilization (gamete E with e), all individuals will be
heterozygous dominant (Ee).
* In F2,
*Ratio:
75%
:
25%
3
:
1
3/4
:
1/4
Dominant
: Recessive
*Genotype:
EE or ee
:
ee
*Phenotype: unattached ear : attached ear
* E e
E EE Ee
e Ee ee
* Punnet Square:
Used to determine the phenotypic & genotypic ratio among the offspring
when all possible sperms are given an equal chance to fertilize all.
* E e
E EE Ee
e Ee ee
* Test Cross:
* If individual that has dominant phenotype for a definite trait but its
genotype is unknown (EE or Ee).
* Crossed to known homozygous recessive individual (ee).
* If individual was homozygous dominant (EE), the cross with
homozygous recessive individual will give dominant phenotype of the
trait.
* If individual was heterozygous (Ee) then the cross with homozygous
recessive individual will give 50 % dominant phenotype & 50 %
recessive phenotype for this trait.
* Dihybrid inheritance:
Mendle selected true – breeding plants that are different in 2 traits (flower
colour & plant height).
* Purple tall
Dominant
AABB
* White dwarf
Recessive
aabb
F1 offspring inherits 2 gene pairs each consisting of nonidentical alleles
(AaBb).
* The result in F2 will be:
Gametes
AB
Ab
aB
ab
Ratio
9/16
3/16
3/16
1/16
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
Genotype
AABB, AaBB, AABb, AaBb
AAbb, Aabb
aaBB, aaBb
aabb
Phenotype
Purple tall
Purple dwarf
White tall
White dwarf
* Dominance relation:
- Mendle studied traits having clearly dominant or recessive forms.
- Is the expression of all traits like this No.?
1. Incomplete dominance:
- One allele of a pair isn’t fully dominant over its partner.
- Heterozygous phenotype is an intermediate between the 2 homozygous
phenotypes.
e.g. 1: Colour of snapdragon flower..
Red
RR
Pink
RR1
White
R1R1
*
R1
R
R1
R1R1
R1R
R
R1R
RR
Cross between 2 pink result in:
1 Red
2 Pink
1 White
e.g. 2: Hair of caucasians..
Curly hair HH
Straight hair HH1
Wavy hair H1H1
(Multiple alleles)
- The trait is controlled by multiple alleles but individual has only 2
alleles.
- Alleles are equally expressed at the same time in heterozygous
individuals.
e.g. ABO type:
* 3 oeesesi elt res ioes sfs lfrtle res fest rof s le laO type.
* These alleles determine the presence or absence of antigens on the
RBCs.
Genotype
IAIA, IAi
IBIB, IBi
IAIB
ii
Phenotype
Type A
Type B
Type AB
Type O
IA A antigen on the RBCs.
IB B antigen on the RBCs.
i neither A or B antigen on the RBCs.
AB both A & B antigen on the RBCs.
* Alleles IA & IB are dominant over i & are fally expressed when they
present together (codominance).
Mating between AB & AB can produce A,AB & B with a ratio of 1 : 2 : 1
respectively.
* Type AB is known as universal acceptor & type O is known as
universal donor, why?
Rh factor: is inherited separately from ABO type.
- Rh + ve has a particular antigen.
- Rh – ve antigen is absent.
(Continuous Variation)
- e.g. skin colour, height of man, eye colour).
- Skin colour is controlled by poly genes (2 or more alleles) in which all
dominant (capital letters) add equally to the phenotype.
- The individual has a copy of all allelic pair.
- If it is assumed that skin colour is controlled by 2 pairs of alleles & each
capital letter contributes pigments to the skin.
Genotype
AABB
AABb, AaBB
AaBb, AAbb, aaBB
Aabb, aaBb
aabb
Phenotype
Very dark
Dark
Medium brown
Light
Very light
- When very dark person reproduce with very light person, children may
have medium brown skin.
- If 2 people with medium brown skin reproduce together, the children
may range in skin colour from very dark to very light.
Multiple effects of a single genes (Pleiotropy):
Expression of alleles at a single location on a chromosome may have
positive or negative effects on 2 or more traits.
Pleiotropy:
Pleio = more - Tropy = to change.
e.g. sickle - cell anemia.
* HbA: normal haemoglobin molecule normal blood cells.
* Hbs: mutant (abnormal) allele, abnormal Hb molecules, abnormal
RBCs (sickle - shaped).
* HbAHbA: normal.
* HbAHbs: show few symptoms.
* HbsHbs: abnormal sickle - shaped RBCs.
- abnormal sickle - shaped RBCs may lead to rapture of blood
capillaries.
These may result in:
* Oxygen transport affected.
* Cells become starved for oxygen.
* Another phenotypes may appear like mental retardation, kidney
failure, jaundice,….
Sex - linked traits:
Human male & female have 23 pairs of chromosomes, 22 pairs of
autosomal chromosomes (non sex chromosomes) & one pair
allosomes (sex chromosomes).
* Sex chromosomes are xx in female & xy in males.
- Inheritance of sex? Or (sex deter mination).
* Sex - linked traits are found on x chromosome not y.
Very few alleles have been found on y chromosome since it is much
smaller than x chromosome.
e.g. haemophilia & red - green colour blindness.
XB: normal.
Xb: haemophilia.
Genotype
XB XB
XB Xb
Xb Xb
XB Y
XbY
Phenotype
female normal
female normal, carrier
female haemophilic
male normal
male haemophilic
Carrier:
* Individuals appear normal but can pass alleles for genetic disorder.
* If XB Xb mates XB Y the result will be?
- Alleles for sex - linked traits pass from the father to his daughters &
from the mother to her sons.
Chromosomal abnormalities:
Mutation:
A heritable change in the DNA including alternation in chromosome
structure or number & also alternation in a gene due to hange in DNA
composition.
Mutagen:
An environmental agent that increases the change of a mutation.
e.g. radiation, chemicals & viruses.
A. Change in chromosomal structure:
1. Deletion:
Loss of chromosome region one or more genes may be lost.
2. Inversion:
Part of chromosome becomes oriented in the reverse direction without
any loss.
3. Translocation:
Broken part of chromosome become attached to a non homologous
chromosome.
4. Duplication:
Normal chromosome have gene sequences that are repeated several
times.
Deletion of duplication occur between 2 homologous chromosomes.
B. Change in chromosomal number:
This change in number may occur in autosomal or sex chromosomes.
*Occur in the form of polily which may be:
1. Euploidy:
variation in chromosome number by whole set.
- Polyploidy:
individuals have 3 or more of each type of chromosomes Triploidy
3n, tetraploidy 4n.
2. Aneoploidy:
Addition or deletion of one or more chromosome from a diploid set.
Monosomy (2n – 1):
Single copy of one type of chromosome is present.
Trisomy (2n + 1):
3 copies of the same kind of chromosome are present.
* An abnormal chromosomal makeup in an individual can be due to
non disjunction.
1. Non disjunction during meiosis I:
Both members of homologous pairs go into the same gamete.
2. Non disjunction during meiosis II:
Sister chromatids fail to separate.
So, both daughter chromosomes go into the same gamete.
Human autosomal abnormalities:
Down Syndrome:
Chromosome number:
47 chromosomes are present in the cell instead of 46.
Trisomy (2n + 1):
In which chromosome 21 is represented by 3 copies.
Cause:
* Non disjunction during meiosis.
* Egg with 24 chromosome (n + 1) is fertilized with a normal sperm
(n) to give zygote (2n + 1).
Characters:
- Affects males & female.
- Short neck.
- Drooping eyelids.
- Open mouth & protruded tongue.
- Moon - face (similar to Mongolian face). Hence the name
Mongolism.
- Mentally retarded.
Sex - chromosome abnormalities:
Result from non disjunction of sex chromosome.
1. Klinefelter’s Syndrome:
Chromosome number:
Individuals (males) have 47 chromosomes; 44 autosomes + XXY
(44 + XXY).
Cause:
Abnormal egg with 24 chromosomes containing XX is fertilized with
a normal sperm Y.
Characters:
- Affects males only.
- Sterile males.
- Breasts like those of females.
- Tall arm & legs.
- Absence of body hair.
- Some mental retardation.
2. Turner’s Syndrome (44 + X0):
Chromosome number:
Individuals (females) have 45 chromosomes (monosomy); 44
autosomes + X (44 + X0).
Cause:
* Abnormal egg with 22 chromosomes is fertilized with a normal
sperm X.
Characters:
- Affects females only.
- Sterile females.
- Short.
- Poorly developed ovaries & sex organs.
- Broad chest with poorly developed breast.
- Not mentally retarded as other syndromes.