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Genetics


INTRODUCTION TO GENETICS

  • Genetics is the study of heredity, that is, how characteristics such as eye colour are inherited from parents to offspring.
  • Genes are the chemicals in the nuclei of cells that determine the characteristics that are inherited. Each human cell has thousands of genes in the nucleus. Genes are made of DNA (deoxyribonucleic acid).
  • Chromosomes are fine thread-like structures in the nucleus of all body cells. Genes are situated on chromosomes.
  • Numbers of Human Chromosomes - Human body cells have 23 pairs or 46 chromosomes. Human gametes (sperm and egg) have only 23 chromosomes.
  • Human Sex Chromosomes - Of the 46 chromosomes in human body cells, 1 pair or 2 sex chromosomes only determine whether a person is male or female. Human females have 2 X-shaped sex chromosomes (XX). Human males have 1 X-shaped and 1 Y-shaped chromosome (XY).
  • Autosomes are the other 22 pairs or 44 chromosomes in human body cells.

ALLELES

  • Genes are made of DNA. Genes help us to make proteins (e.g. muscle protein, hormones, enzymes, antibodies, skin collagen, hair keratin) that will determine certain characteristics we inherit.
  • For a specific characteristic (e.g. eye colour), there may be one or more types (e.g. blue, brown) that may be determined by slightly different variations of the DNA in the gene. These are called alleles. For example, in eye colour, there may be brown eye colour determined by a gene or allele B, or for blue eye colour, it is determined by a slightly different gene or allele b.

MULTIPLE ALLELES

  • For a particular characteristic, there may be more than 2 alleles. For example, in Blood Type, there are 3 alleles - A, B and O.)

GENOTYPES AND PHENOTYPES

  • Each characteristic (e.g. eye colour) is determined by a pair of genes/alleles. One of those genes came from the father via sperm, and the other came from the mother via egg.
  • Genotype is the pair of genes/alleles written in letter form (e.g. BB).
  • Phenotype is the characteristic that appears as a result of that genotype (e.g. brown eyes).
  • Example 1 - Eye Colour in Humans

GENOTYPE

PHENOTYPE

BB

Brown-eyed person

Bb

Brown-eyed person

bb

Blue-eyed person
  • Example 2 - Height in Pea Plants

GENOTYPE

PHENOTYPE

TT

Tall pea plant

Tt

Tall pea plant

tt

Short pea plant
  • Homozygous (or Purebred) - The genotype for the characteristic has the same genes/alleles ( e.g. BB, bb).
  • Heterozygous (or Hybrid) - The genotype for the characteristic has different genes/alleles (e.g.Bb).
  • Wild-type - The most common phenotypes in a population are called the wild-type. These may be dominant (e.g. brown eye colour) or recessive (e.g. green pea colour).

PEDIGREES (OR FAMILY TREES)

  • Pedigrees are drawn to examine a characteristic being studied (e.g. eye colour, haemophilia).
  • Symbols used are:

Genetics

Male without characteristic being studied
Female without characteristic being studied
Male with characteristic being studied
Female with characteristic being studied
  • Example of Pedigree

Genetics


SEX DETERMINATION

  • Because the sperm or egg will contain only ½ of the genes and chromosomes of a person, then each sperm could contain either an X or a Y chromosome. Each egg contains either an X or the other X chromosome.
  • To have a daughter, the sperm from the father and the egg from the mother must both contain X chromosomes.
  • To have a son, the father's sperm must have a Y chromosome to join with the mother's egg that has an X chromosome.
  • It is the father who determines the sex of a child.

DOMINANT-RECESSIVE INHERITANCE

  • Of the pair of genes/alleles for a characteristic, one may be dominant (or more strongly inherited in the offspring), and the other may be recessive (or less strongly inherited in the offspring).
  • Dominant genes/alleles are shown by capital letters (e.g. B, T).
  • Recessive genes/alleles are shown by small letters (e.g. b, t).
  • Example 1 - Eye Colour

Dad has purebred brown eyes (BB) and Mum has purebred blue eyes (bb). The Punnett Square below shows the possible eye colours inherited by the children.

 

B

B

b

Bb

Bb

b

Bb

Bb

Possible genotypes of children = all Bb
Possible phenotypes of children = all brown-eyed children
  • Example 2 - Eye Colour

Dad has heterozygous brown eyes (Bb) and Mum has blue eyes (bb). The possible eye colours of the children will be ...

 

B

b

b

Bb

bb

b

Bb

bb

Possible genotypes = 2Bb : 2bb
= 1Bb : 1bb
Possible phenotypes = 2 brown : 2 blue
= 1 brown : 1 blue
This means that about ½ of the children will be brown-eyed and the other ½ will be blue-eyed.

TEST-CROSS

  • If an individual has a dominant phenotype, it is not known what the exact genotype is. For example with the phenotype of brown eye colour, the genotype could be BB or Bb.
  • To find out the genotype of a dominant phenotype, one must cross the individual with the dominant phenotype (e.g. BB or Bb) with an individual with the recessive phenotype (e.g. bb). If the offspring all have the dominant phenotype (e.g. brown eyes), then the parent was pure-bred (e.g. BB). If the offspring have any with the recessive phenotype, then the parent was hybrid (e.g. Bb).

MONOHYBRID CROSS

  • If both parents are hybrid or heterozygous for brown eye colour (both are Bb), the possible eye colours of the children are ...
 

B

b

B

BB

Bb

b

Bb

bb

Possible genotypes = 1 BB : 2 Bb : 1 bb
Possible phenotypes = 3 brown : 1 blue
This means that ¾ of the children will be brown-eyed, and ¼ will be blue-eyed.

INCOMPLETE DOMINANCE (OR CO-DOMINANCE)

  • Incomplete Dominance occurs where both genes/alleles are incompletely expressed in the phenotype.
  • Example - 4 O'Clock Flowering Plants

GENOTYPE

PHENOTYPE

RR

Red flowers

WW

White flowers

RW

Pink flowers

If a red-flowering plant produced pollen that fertilised a white-flowering plant's egg, the possible offspring would be ...

 

R

R

W

RW

RW

W

RW

RW

Possible genotypes = all RW
Possible phenotypes = all pink-flowering plants

CO-DOMINANCE AND MULTIPLE ALLELES
(AN EXAMPLE OF ABO BLOOD TYPES)

  • Co-Dominance occurs when both genes/alleles in the genotype are equally dominant.
  • Multiple Alleles occurs when more than 2 genes/alleles determine a characteristic, such as in ABO blood groups.
  • Example of ABO Blood Types

There are 4 different blood types - A, B, AB and O.

BLOOD
TYPE
OR
PHENOTYPE

GENOTYPE

ANTIGEN
PRESENT

ANTIBODY
PRODUCED

A

AA or AO

A

Anti-B

B

BB or BO

B

Anti-A

AB

AB

A and B

none

O

OO

none

Anti-A and Anti-B

  • Example 1 - Blood Types

Mum has blood type AB and Dad has blood type O. The possible blood types of the children are ...

 

A

B

O

AO

BO

O

AO

BO

Possible genotypes = 1 AO : 1 BO
Possible phenotypes = 1 A : 1 B
½ the children will be A blood type, and the other 1/2 will be B blood type.
  • Example 2 - Blood Types

Mum has A blood type and Dad has AB blood type. The possible children's blood types are ...

First Possibility

 

A

A

A

AA

AA

B

AB

AB

Possible genotypes = 1 AA : 1 AB
Possible phenotypes = 1 A : 1 AB
½ the children will have blood type A, and the other ½ will have blood type AB.

Second Possibility

 

A

O

A

AA

AO

B

AB

BO

Possible genotypes =1AA:1AO:1AB:1BO
Possible phenotypes = 2 A : 1 AB : 1 B
½ will have blood type A, ¼ will have blood type AB, and ¼ will have blood type B.

ANTIGENS AND ANTIBODIES IN ABO BLOOD TYPES

  • The blood type is so-called because the blood contains particular antigens - A, B, both A and B, or neither A nor B.
  • The body produces antibodies to neutralise any particle (e.g. bacteria, dust, foreign blood in transfusions) that it recognises as foreign. For example, if blood type A contains Antigen A, then it will produce antibodies against B blood type (Anti-B Antibody), because B antigens are foreign. Similarly, if blood type O contains neither antigens A nor B, then a person with blood type O would produce anti-A and anti-B antibodies.
  • Agglutination or 'Clumping' - If antigen-A came in contact with the antibody against it (Anti-A), then the blood would clump or clot. This could occur in an incorrect blood transfusion.
  • Universal Recipient - This is a person with blood type AB who can receive a blood transfusion from any of the other blood types.
  • Universal Donor - This is a person with blood type O who can donate blood to any other blood type.

COMPLETE DOMINANCE OR DOMINANT-RECESSIVE INHERITANCE
(AN EXAMPLE OF RHESUS FACTOR IN ABO BLOOD TYPES)

  • The ABO blood types are sub-divided into positive and negative types also, depending on whether that blood type does or does not contain the Rhesus Factor.
  • If the Rhesus Factor is present, the genotype contains one or two R genes/alleles. If the Rhesus Factor is absent, the genotype is rr.
Blood Type ABOAntigens
Present
Rhesus
Antigens
Present
ABO
Genotype
Rhesus
Genotype

A+

A

yes

Aa or AO

RR or Rr

A-

A

no

AA or AO

rr

B+

B

yes

BB or BO

RR or Rr

B-

B

no

BB or BO

rr

AB+

A and B

yes

AB

RR or Rr

AB-

A and B

no

AB

rr

O+

none

yes

OO

RR or Rr

O-

none

no

OO

rr


SEX-LINKED INHERITANCE

  • This is a form of inheritance where the gene/allele for the characteristic being studied is on the X chromosome.
  • Diseases such as colour-blindness and haemophilia are inherited this way, and are more common in males than females.
  • Alleles, Genotypes and Phenotypes for Haemophilia (Blood-Clotting Inability)
XH - allele for normal blood clotting
Xh - allele for haemophilia
 
XHXH - genotype of normal female
XhXh - genotype of haemophiliac female
XHXh - genotype of carrier female (with normal blood-clotting ability, but who can pass the defective gene to her children)
 
XHY - genotype of normal male
XhY - genotype of haemophiliac male
  • Example 1 - Haemophilia

A man with normal blood-clotting ability (XHY) marries a woman who is a haemophiliac (XhXh). The possible phenotypes of their children are ...

 

XH

Y

Xh

XHXh

XhY

Xh

XHXh

XhY

Possible genotypes = 1 XHXh : 1 XhY
Possible phenotypes
= 1 carrier female : 1 haemophiliac male
  • Example 2 - Haemophilia

A normal woman (XHXH) marries a haemophiliac man (XhY). The possible genotypes and phenotypes of the children are...

 

XH

XH

Xh

XHXh

XHXh

Y

XHY

XHY

Possible genotypes = 1 XHXh : 1XHY
Possible phenotypes
= 1 carrier female : 1 normal male
  • Alleles, Genotypes and Phenotypes for Colour-blindness
Xc - allele for normal colour vision
Xc - allele for colour-blindness
 
Xc Xc - genotype of normal female
Xc Xc - genotype of colourblind female
Xc Xc - genotype of carrier female (with normal colour vision, but who can pass the defective gene to her children)
 
Xc Y - genotype of normal male
Xc Y - genotype of colourblind male
  • Example 3 - Colour Blindness

A male with normal vision (XcY) and a colourblind female (Xc Xc) have children. The possible genotypes and phenotypes of the children are ...

 

Xc

Y

Xc

XcXc

XcY

Xc

XcXc

XcY

Possible genotypes = 1 Xc Xc : 1 Xc Y
Possible phenotype = 1 carrier female : 1 colourblind male

  • Example 4 - Colour Blindness

A carrier female (Xc Xc ) marries a normal-visioned male (Xc Y). The possible genotypes and phenotypes of the children are...

 

Xc

Xc

Xc

XcXc

XcXc

Y

XcY

XcY

Possible genotypes
= 1 XcXc: 1 XcXc: 1 XcY: 1 XcY
Possible phenotypes
=1 normal female: 1 carrier female: 1 normal male: 1 colourblind male