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HUMAN BIOLOGY EDEXCEL INTERNATIONAL GCSE (9-1)
UNIT 4 CHAPTER 12 HEREDITY
CLASS NOTES
The science of heredity is called genetics
...

VARIATION
Sexual reproduction leads to variation in the offspring meaning each individual has
different characteristics
...
No two
offsprings from the same parents are genetically identical
...
Variation increases the chances of survival of species
...
There are two types of variation: continuous and discontinuous
...

Both random assortment of genes and random pairing of gametes may cause
continuous variation
...

Discontinuous variation is the result of inheritance only
...

Examples are blood groups
...

• Genes are the basic units of inheritance
...

• Each gene has 2 alleles, at a particular locus on the chromosome, one on
each of the homologous chromosomes
...

• Alleles are different forms of a gene
...
A dominant allele is one that is expressed in the
phenotype of a person
...
A recessive allele is

•
•
•
•

one that is not expressed in the phenotype when a dominant allele is
present
...

The genotype of an organism is its genetic makeup
...

The phenotype of an organism is its observable characteristic
...
g
...

Homozygous means that the 2 alleles of the gene are the same
...
g
...

Heterozygous means that the 2 alleles of the genes are different
...
g
...


For a particular character, an offspring may inherit 3 possible genotypes:
• 2 dominant alleles, 1 from each parent
...
E
...
, RR
• 2 recessive alleles, 1 from each parent
...
E
...
, rr
• 1 dominant allele and 1 recessive allele
...
E
...
, Rr
The dominant allele always expresses itself in the phenotype
...
Thus, the homozygous dominant (RR) and
heterozygous (Rr) genotypes will give the same phenotype
...
E
...
, the
genotypes controlling the presence or absence of rosettes in guinea pigs
...


Genotype
phenotype

description of genotype

appearance of guinea pig

RR

homozygous dominant

rosettes

rr

homozygous recessive

smooth

Rr

heterozygous

rosettes

When a dominant and a recessive allele is present in the organism, the dominant
character is expressed
...


SELECTION

NATURAL SELECTION
1
...

2
...

3
...

4
...

ARTIFICIAL SELECTION
1
...

2
...

3
...

4
...

EXAMPLES OF NATURAL AND ARTIFICIAL SELECTION
NATURAL SELECTION
• Sickle cell anemia: Common in areas where malaria is endemic
...

• Penicillin resistant bacteria: Mutant allele in bacteria prevents it from
reacting to penicillin
...

• Dark peppered moth: Because of a soot-filled environment from
industrialization, dark moths were able to survive predation while light
moths did not
...

• Outbreeding: Crossing of unrelated species
...

• E
...
, horses, dogs, and cats are bred for racing purposes
...
During growth, cells (diploid, 46 chromosomes) undergo mitosis
...
When sex cells are formed, they are haploid in nature (23 chromosomes)
...
After fertilization, the embryo has a gamete from the father and one from the
mother
...

Differences between Meiosis & Mitosis
Characteristics

Meiosis

Mitosis

No of chromosomes

diploid (2n)

haploid (n)

Purpose

to produce sex cells

growth & repair

No of daughter cells

2 daughter cells

4 daughter cells

Variation

no genetic variation

recombination of chromosomes
gives rise to variation

Location

sex cells

all other plant & animal cells

DOMINANCE

MONOHYBRID CROSS: Monohybrid cross-refers to a genetic cross involving 1

gene
...
There
are 2 types of a monohybrid cross, complete dominance, and codominance
...
g
...
In guinea pigs,
the appearance of rosettes is dominant over smooth appearance
...
Note that the sex of the parents does not matter
...

Phenotypes of parents

rosettes

Genotypes of parents

RR

Gametes

R

The genotype of F1 generation

smooth
rr

Each parent has 2 copies of only 1 allele
...


Rr

The phenotype of F1 generation

Explanation

F1 are all heterozygous

All have rosettes

The R allele is expressed
...


The first generation produced by crossing the parents is called the F1 generation
...

Now, look what happens when 2 of the heterozygous from the F1 generation are crossed
...


Genotypes of F2 generation Rr Rr

Rr

rr

1RR: 2Rr: 1rr

Ratio of the genotypes in the F2 is
result of random fertilization of eggs by
sperms

Phenotypes of F2 generation

3 rosettes:1 smooth

The probability is that the offspring will
be in this ratio

Gametes

R

r

R

RR rosettes

Rr rosettes

r

Rr rosettes

rr rosettes

CODOMINANCE
Sometimes both alleles have an equal effect on the phenotype of an individual
...
For e
...
, if a chestnut
(red-brown) horse is crossed with a white horse, all the foals resulting from the
cross will be an intermediate color called a red roan
...
We can represent the alleles for coat
color with symbols
...

1 chestnut: 2 red roans: 1 white
Crossing red roan horses
Genotypes of parents
Gametes

RW

RW

R&W

R&W

Genotypes of offspring
1RR:2RW:1WW

R

W

R

RR

RW

W

RW

WW

PEDIGREE
A pedigree is a family tree showing the inheritance of a gene
...
It is determined by a dominant allele
...
If we use the symbol D for polydactyly
allele and d for normal number allele, the possible genotypes and phenotypes are:
• DD-person who has polydactyly (has 2 dominant polydactyly alleles)
• Dd- the person has polydactyly (has a dominant polydactyly allele & a
recessive normal allele)
• Dd-person has the normal number of digits (has 2 recessive, normal number
alleles)
A pedigree contains a lot of information
...
Individuals are arranged in 4 horizontal lines
...

• Individual 4 is the firstborn child of 1 & 2 (firstborn child is shown to the
left, then the second born to the right of this, then the third born & so on
...

• Individuals 3 & 4 are fathers & mothers of the same children as are 1 & 2, 6
& 7, 8 & 9, 12 & 13, 14 & 15 (a horizontal line joins them)
...

• All people with the normal number of digits must have genotype dd (if they
had D allele, they would show polydactyly)
...


• The people with a normal number of digits must inherit 1 normal number
allele from each parent, also people with a normal number of digits will pass
1 normal number allele to each of their children
...

• Individuals 1,2,3,16,17 & 18 must be heterozygous
...

• It is impossible to distinguish between DD and Dd
...
There are 3
different alleles involved
...
The blood group of a person is a
result of the presence or absence of 2 antigens, the A antigen and the B antigen on
the surface of red blood cells
...

• IA determines the production of A antigen
• IB determines the production of B antigen
• IO determines that neither antigen is produced
A person inherits 2 alleles of a gene
...
The positive genotypes & phenotypes are shown below:
Genotype

Antigen produced

Blood group

IAIA, IAIO

A

A

IBIB, IBIO

B

B

A& B

AB

IAIB
IOIO

NEITHER

O

Parents who are heterozygous for blood group A and blood group B could produce
4 children each with a different blood group
...
It is determined
by the X & Y chromosomes (sex chromosomes)
...
All cells of males except the sperm contain 1
Y chromosome
...

Phenotypes of parents

male

female

Genotypes of parents

XY

XX

Gametes X &Y

X &Y

X

Female gametes
X
Male gametes

X

XX

Y

XY

Ratio of genotypes

50% XX: 50% XY

Ratio of phenotypes

50% female: 50% male

SEX-LINKED GENES OR SEX-LINKED INHERITANCE
The sex chromosomes do not only determine sex but also carry genes for certain
characteristics
...
Sex-linked inheritance is
characteristic that is associated with the sex chromosomes
...
in humans, the X chromosome
may carry a recessive allele for certain characteristics e
...
, Color blindness,

Hemophilia, etc
...
Females who carry only 1 recessive allele are carriers, but those who carry
a homozygous recessive trait are affected by it
...
This means that for some genes, a
male will only have 1 allele of a pair present on his X chromosome
...


HAEMOPHILIA
• Haemophilia is an inherited bleeding disorder where the blood does not clot
properly
...
A
clotting factor is a protein that controls bleeding
...

• The allele for normal blood clotting is dominant and is given by the symbol
(H)
...

• The genotype for this is shown as XhY
...

• A woman with 2 X chromosomes would need to inherit 2 copies of the
faulty allele, shown as XhXh
...
However, she will be a carrier and can pass the disease
to her children
...
It is
possible for 2 healthy parents to have a son with hemophilia
...


HAEMOPHILIA
Dominant allele XH

Recessive allele Xh

Sex

Genotype

Phenotype

Female

XH XH

normal

XH Xh

carrier

Male

Xh Xh

Haemophiliac

XH Y

normal

Xh Y

Haemophiliac

Inheritance of Haemophilia from a healthy father & an unaffected carrier
mother
Parents

healthy male

Parents genotypes
Gametes

carrier female

XH Y

XH Xh

XH & Y

XH & Xh

XH

Xh

XH

XH XH

XH Xh

Y

XH Y

Xh

Offspring genotypes XH XH

Y

XH Xh : XH Y : Xh Y

Offspring phenotypes 2 healthy females : 1 healthy male
1 haemophiliac male : 1 carrier

Inheritance of Haemophilia from a Haemophiliac father & an unaffected
mother
Parents

Haemophiliac father

Parents’ genotypes
Gametes

Xh Y

XH Xh

Xh & Y

XH & Xh

XH

Offspring genotypes

Carrier mother

Xh

Xh

XH Xh

Xh Xh

Y

XH Y

Xh Y

XH Xh :

Xh Xh : XH Y : Xh Y

Offspring phenotypes 1 healthy : 1 Haemophiliac : 1 healthy : 1 Haemophiliac
Female :

female

: male

:

male

CYSTIC FIBROSIS
Cystic fibrosis is an inherited condition that causes severe damage to the lungs,
digestive system & other organs of the body
...
These secreted fluids are normally thin
& slippery
...
Instead of acting as a lubricant, the secretions
plug the tubes, ducts & passageways, especially in the lungs & pancreas
...
The recessive
allele results in the production of thick, sticky mucus
...

• In the lungs it is not easily removed by the cilia lining the airways
...
Bacteria
trapped in the mucus can cause chest infections
...

• The mucus blocks the pancreatic duct so that enzymes from the pancreas
cannot reach the small intestine
...

• The mucus blocks the tubes leading from the testes so that most males with
cystic fibrosis are infertile
...
Both parents will
be heterozygous for cystic fibrosis
...

N = recessive cystic fibrosis allele resulting in the production of viscous mucus
...

• The normal dominant allele of the gene causes a protein to be produced that
forms the pigment in the cones of the eye that detects green light
...

• People without the normal allele cannot tell the difference between similar
shades of green and red
...
A cross between a man with normal
color vision and a woman who is a carrier of the mutant allele
...
In this cross, there is a 50% probability that a daughter will be a carrier of
the mutant allele
...
During meiosis, the chromosomes separate, and alleles
separate into different gametes
...
The segregation of
alleles of a gene is not influenced by alleles of another gene
...

• This change causes the gene to be coded differently from what it was
supposed to be
...

TYPES OF MUTATIONS
GENE MUTATIONS
They can be inherited
...

They may lead to severe genetic diseases e
...
, Haemophilia
...

Mutations in bacteria & insects usually increase their chances of survival
...

Can happen during meiosis
...

In humans, mutations may lead to harmful characteristics
...
g
...

Humans with Down’s syndrome, have 47 chromosomes instead of 46
...

Characteristics include physical and mental defects
...
It involves artificially inserting genes from one specie to another
...
This is done to produce desirable traits in the production of substances like
herbicides
...
DNA is found in chromosomes
...

These bases are adenine, guanine, cytosine, and thymine
...
A sequence of bases along the DNA will provide instructions to build the
substance e
...
, the gene instructs the hair to grow straight or curly hair
...


Crops and farm animals
...

Corn with resistance to peas
...


Artificial life farm

To create a life form from genes
...

Gene therapy targets only affected tissues
...
g
...


Fingerprinting

In organ donations, transplants & criminal investigations
...

CONTINUOUS VARIATION: when both inherited and environmental factors determine the
characteristics of an individual (e
...
, body mass, height)
...
g
...

GENE: a unit of inheritance, forming part of a chromosome
...

GAMETES: male or female sex cells
...

DOMINANT ALLELES: the presence of a single dominant allele will have the same effect on
the phenotype of an individual as the presence of an identical pair of them
...

GENOTYPE: the genetic combination of an individual
...

HETEROZYGOUS: offspring with one dominant and one recessive allele for a particular
character
...
The homozygous dominant
and heterozygous genotypes give the same phenotype
...

MUTATION: change in genes or chromosomes through environmental forces, or mutagens (e
...
,
X-rays, atomic radiation)
...

COMPLETE DOMINANCE: when the presence of a single allele will have the same effect on
the phenotype of an individual as the presence of an identical pair of alleles
...




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