This document discusses several key concepts in human genetics and evolution: 1. It defines important genetic terms like allele, dominant and recessive alleles, genotype and phenotype. 2. It describes types of chromosomes including autosomes, sex chromosomes, and karyotypes. 3. It summarizes human evolution from early primates to Homo sapiens, noting changes in the skull, vertebral column, teeth and limbs that enabled bipedalism.

1. Yapa Wijeratne
Human Genetics
Allele- alternative forms of a gene, which occur at the same locus on homologous
chromosomes.(If the maternal & paternal alleles are identical, person is homologous )
Dominant allele- an allele that exerts its phenotypic effect in the heterozygous; it hides the
expression of the recessive allele.
Recessive allele- an allele that exerts its phenotypic effect only in the homozygous; its
expression is masked by a dominant allele.
Gene locus- the specific location of a particular gene on homologous chromosomes.
Genotype- the genes of an organism for a particular trait or traits; e.g. BB , Aa
Phenotype- the visible expression of a genotype. e.g. brown eyes, attached earlobes
Heterozygous- possessing unlike alleles for a particular trait.
Homozygous- possessing 2 identical alleles for a particular trait.
Autosome- any chromosome other than a sex chromosome.
Mutation- an alteration in chromosome structure or number & also an alteration in a gene
due to a change in DNA composition.
Polyploidy (polyploid)- a condition in which an organism has more than 2 complete sets of
chromosomes.
Sex chromosome- a chromosome that determines the sex of an individual ; in animals,
females have 2 X chromosomes & males have an X & Y chromosome.
X-linked gene- gene located on the X chromosomes that does not control a sexual feature
of the organism.
Karyotype- chromosomes arranged by pairs according to their size, shape & general
appearance in mitotic metaphase.
Nondisjunction- the failure of homologous chromosomes or daughter chromosomes to
separate during meiosis I & meiosis II respectively.
Anticodon-3 nucleotides on a tRNA molecule attached to a complementary codon on
mRNA.
Codon- 3 nucleotides of DNA or RNA; it codes for a particular AA or termination of
translation.
Exon- in a gene, the portion of the DNA code that is expressed as the result of polypeptide
formation.
Intron- non-coding segment of DNA that are transcribed but removed before mRNA leaves
the nucleus.
Transcription- the process whereby a DNA strand serves as a template for the formation of
mRNA.
Translation- the process whereby the sequence of codons in mRNA determines (is
translated into) the sequence of AAs in a polypeptide.
Double helix of DNA- maintain stability of structure.
DNA contains the information needed to encode proteins. 64 combinations are possible
within 4 bases (20 AA). 3 bases represent a 1 AA.
Most proteins have more than one complementary codon.-Redundancy of AA.
Gene- sequence of DNA which codes for one polypeptide.
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Replacing / removal/ addition of nucleotides may lead to mutations. If the replaced AA has
same chemical properties of the previous AA the resulting protein may function normally.
Replacing may sometimes lead to enhance the function of a protein.
Polymorphism
Difference in DNA sequence among individuals, groups or populations that gives rise to
different forms.
e.g. blood group formation.
Producing a protein or a characteristic of the organism (trait) is the expression of a gene.
In the heterozygous state the dominant allele (dominant trait) expresses itself while the
recessive allele (recessive trait) does not.
House keeping genes are the genes which produce structurally & functionally vital proteins
for the normal integrity of a cell. Mutations of these genes will produce proteins which are not
capable of doing particular functions.
75% of the genes can not afford even a subtle change in their structure. (sequence of AA)
Miscarriages – (Abortions- particularly within 1st 3 months of pregnancy) will result, if
mutations occur in house keeping genes.
Other 25% of genes (polymorphic genes) can afford subtle mutations but it may reduce or
enhance its expression.
Gene pool- all the genetic material in a given population.
If a gene has a frequency over 1% in a gene pool, that variant of the gene is accepted as a
polymorphic form of that particular gene.
Polymorphic forms of a morphogene give rise to morphological variations of individual
species. Variations are vital to withstand environmental changes.
e.g. Defective Hb (sickle cell) prevents African people from infecting with malaria. Having
sickle cell Hb is now vital for the continuity of African people.
Heterozygous advantage - In the heterozygous state if the recessive gene is defective, it is
covered by the normal gene which is dominant.
In Heterozygous state both dominant & recessive genes are expressed but dom. gene
overrides. There are various mechanisms to prevent the expression of certain genes.
e.g. Attaching to the nuclear membrane but still a small portion of the inactivated X
chromosome [Barr body] is necessary for the secondary sexual characteristics of a female.
So having a 1X chromosome will result defects. This inactivation is the reason for the
females to become carriers of various chromosomal abnormalities.
Genetic load-amount of hidden inactive lethal mutations.
There is a high chance of these genes to express in marriages between relatives. That is
why 1st cousin marriages are discouraged.
Chromosomes
Centromere is an adenine-thymine (A-T) rich region ranging in size from 10 2-106 base pairs.
It binds several proteins with high affinity. This complex- Kinetochore provides an anchor
for the mitotic spindle.
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3. Yapa Wijeratne
The ends of each chromosome contain structures -> Telomeres
Telomeres consist of short, repeat TG-rich sequences. Proteins.
Telomeres prevent joining free 3’-->5’ ends of DNA strand.
DNA in eukaryotic genome unique/ non-repetitive sequence
repetitive sequence moderately repetitive
highly repetitive
Repetitive sequence is found in telomeres.
Telomerase – is the enzyme responsible for telomere synthesis & thus for maintaining the
length of the telomere.
After each division telomere shortening Aging(when telomere cannot loss,
sequence stops)
Malignant transformation
Cancer cells do not depend on telomeric sequence. Telomerase restores/ replaces lost
parts, telomeric sequence. So cell division doesn’t stop.
So Cancer chemotherapy
Drug development Telomerase
The basic number of chromosomes in the somatic cells of an individual or a species-
Somatic number- is designated 2n.
2n --> homologous chromosomes for each given chromosomes.
Karyotype
Characteristic chromosome complement of a eukaryotic species.
The preparation & study of Karyotype is part of cytogenetics - Karyotype analysis.
Human Karyotype preparation
1. Blood cells are centrifuged. WBC & RBC are separated.
2. Induce the cells to divide.-chemicals
3. Colchicine is added to stop division of WBC.-to arrest the division in metaphase
(usually 48-72 hrs after begin)
4. Break he nuclear membrane.
5. Hanging drop method- to divide each cell’s nuclear material.
6. Slide is prepared. Sample is fixed & stained (Giemsa, immunofluorecent stains).
7. Slide is examined for cells about to divide.
8. Chromosomes are photographed, enlarged & then cut apart.
Position of centromere
Banding appearance can identify homologous chromosomes.
Reverse banding appearance -> different banding pattern -> but same areas can be
identified.
Metacentric chromosomes- centromeres in the middle.
Submetacentric chromosomes- centromeres intermediate position/ towards the end.
Acrocentric chromosomes- centromeres close to one end /end.
Chromosomal aberrations
1.Numerical
Polyploidy
Aneuploidy-missing or having an extra chromosome.
Monosomies or trisomies
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Remove 1 chromosome for certain chromosomes.
Loss or extra chromosome-if in germ cells, not compatible for life. definitely
cause problem (brain development & morphology) ->syndromes
e.g. monosomic for <number> chromosome
Syndrome Sex Chromosomes
Down M/F Trisomy 21
Patau M/F Trisomy 13
Edward M/F Trisomy 18
Turner F XO
Metafemale F XXX (or XXXX)
Klinefelter M XXY (or XXXY)
XYY M XYY
O-sex chromosome is missing.
Nondisjunction-occurs when either homologous fail to separate during anaphase I of meiosis
Or sister chromatids fail to separate during anaphase II.
Can happen when people are old as many cells are not functioning properly.
Nondisjunction is more common during meiosis I than during meiosis II. It can also occur
during mitosis.
In animals , autosomal monosomies & trisomies are generally lethal.
e.g. Down syndrome
o Aneuploidy situation
o Alter morphological & IQ
o Space between eyes is large
o lowstep ears
Klinefelter-aberrations in sex chromosome-usually IQ is OK.
2.Structural
Chromosomal mutations
1. Inversion
2. Translocation
3. Deletion
4. Duplication
Inversion
When a segment of a chromosome is turned around 180 0.
New position might lead to altered gene activity.
Translocation
Movement of a chromosomal segment from one chromosome to another, non-
homologous chromosome.
Usually have reduced fertility due to production of abnormal gametes.
When translocations occur Down syndrome->Translocation Down syndrome
2 types. i. Reciprocal- heterologous chromosomes
ii.Robertsonian
Deletion
When an end of a chromosome breaks off or when two simultaneous breaks lead to
the loss of a segment.
e.g. cat’s cry syndrome
small head mentally retarded facial abnormalities
abnormal development of the glottis & larynx
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Duplication
Doubling of a chromosomal segment.
Fragile X syndrome
Isochromosomes
Ringchromosomes
Gene mutations
1.Frame shift mutations
Drastic.
1 or more nucleotides are inserted or deleted from DNA. Then “reading frame” shifts
& sequence of codons is altered.
2.Point mutations
A change in a single nucleotide & therefore a change in a specific codon. When one
base is substituted for another, the results can be variable.
Can be drastic.
e.g.
Tyr codon
Additional Tyr codon Stop codon His codon
Normal protein Incomplete protein Faulty protein
Silent mutation Nonsense mutation Missense mutation
3.Mosaicism
Chromosomal aberrations
1. Numerical
a. Polyploidy
b. Aneuploidy
c. Monosomies or trisomies
2. Structural
a. Chromosomal mutations
i. Inversion
ii. Translocation
iii. Deletion
iv. Duplication
b. Gene mutations
i. Frame shift mutations
ii. Point mutations
3. Mosaicism
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6. Yapa Wijeratne
Human Evolution
Primates ( chimpanzees, orangutan, gorilla, human)
One cell tube birds bat
(Unicellular) (coelenterate) amphibians
Primates-most of them are mammals
4 legged 2 legged Homo erectus Homo sapiens
Changes in vertebral column
4 legged -> brain & spinal cord in one plane
2 legged -> Angulation between brain & spinal cord
Skull
Cranium (Cranial cavity) small Brain is small
Increase in the size of Cranial cavity
Sagittal crest (top of the skull) -> for prominent muscle attachment ( in low primates)
No Sagittal crest -> in Homo erectus & Homo sapiens
Supra ciliary ridges(eye brows area) prominent in low primates.
Vision- position of eye- binocular vision is absent in lower animal
Facial skeleton- long snout –jaws show Prognathism – because nasal cavity is large
Humans -> Prognathism is absent
Teeth – lower animals
1. Herbivore-leaves/vegetarian diet-more prominent grinding teeth-Molar teeth
2. Carnivore-meat eating-Canine (means “dog”) sharpness is reduced.
3. Omnivore-both
Hand- prominent & opposable thumb
Legs- mobility of the big toe is reduced.
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