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Human Genetics
Inheritance in humans
Male/female
Multiple Alleles
Inherited diseases
Twins
Sex-linked traits
Single genes (widows peak)
Why is a guy a
guy?
Color blindness is also Sex-Linked
Hemophilia
Inheritance in humans
 A person receives 23 chromosomes from
each parent (46 total)
 1 gene for each particular trait from each
parent
 Genes combine to determine traits
(characteristics)
 Instruct body cells to make certain
chemicals = (proteins)
 For example: special proteins (enzymes) make
pigment in the iris of the eye
Multiple alleles
Who Has Which Blood Type?
TYPES
DISTRIBUTION
RATIOS
O+
1 person in 3
38.4%
O-
1 person in 15
7.7%
A+
1 person in 3
32.3%
A-
1 person in 16
6.5%
B+
1 person in 12
9.4%
B-
1 person in 67
1.7%
AB +
1 person in 29
3.2%
AB -
1 person in 167
0.7%
Multiple alleles
 Co-dominant – when 2 different genes are
both equally dominant–
 4 major blood groups = A, B, AB, O .
 O is recessive, while A and B are
co-dominant
Phenotypes vs. Genotypes
 4 major blood groups = A, B, AB, O




Phenotype
Phenotype
Phenotype
Phenotype
A could be Genotype AA or Ao
B could be Genotype BB or Bo
AB can only be Genotype AB
O can only be Genotype oo
 Allele = each form of a gene
Inheritance
in
humans
 Some genes (traits) have multiple alleles
 Skin color has 3 or more
 Controls amount of melanin
(pigment) in the skin cells
 Any one individual inherits only 2
Genetic disorders
 Non disjunction = failure of a
chromosome pair to separate during
meiosis
 Body cells receive either more or fewer
chromosomes than normal
 May result in certain disorders
 Down’s Syndrome = body cells have
an extra chromosome-21
 Trisomy-21 (3 instead of 2)
 May have various physical problems and
some degree of mental retardation
Genetic disorders
 Down’s Syndrome
 Trisomy-21
 A karyotype (page
117)is a picture of all
the chromosomes in a
dividing cell.
amniocentesis
 Screening for genetic disorders
 Amniocentesis
 Drawing a small amount of amniotic fluid
from the sac surrounding an unborn baby
(fetus)
 Chromosomes of cells from the fluid can be
examined under microscope
twins
 Fraternal twins
(dizygotic)
 2 separate ova are
fertilized at same time
 Can be same or different
sex
 Identical twins
(monozygotic)
 Develop from single
zygote (fertilized ovum)
 Always the same sex
 Can occur up to 12 days
after conception
Identical twins - monozygotic
 One fertilized
egg splits into
two separate
people-with the
same genes-
 A natural clone
Fraternal Twin –Dizygotic
 Two – or more
eggs which are
fertilized by two
or more sperm
cells
 No more alike
than any other
brother or sister
Others (include)




Muscular dystrophy
Huntington disease
Cystic fibrosis
Hemophila (sex linked)
Sex-linked traits
 Traits that are usually carried on X
chromosome
 Females less like to inherit
 Have XX
 X carries other genes besides feminine
traits
 Recessive genes are hidden by dominant
genes of other X
 Males more likely to inherit
 Y carries mostly masculine traits
 Cannot override genes carried on X
Sex Linked traits
Pedigrees – page 119
Inherited diseases
 sickle cell anemia
 Co dominant gene
 Caused by mutation of
gene for hemoglobin (red
pigment in red blood cells
that carries O2
 Over time, affected cells
become rigid & crescentshaped
Sickle cell disease/anemia
Sex Determining Chromosomes
Noted different shapes of
sex chromosomes: X, Y
Female = XX
Male = XY
Male/Female
XX = normal female
 XY = normal male
Sperm determines
sex of the individual
X seems to be
essential for life
No one has been born
without at least
one X chromosome
Abnormalities of inheriting sex chromosomes
X0 = female in appearance
XXY = male in appearance
Both are sterile –
not able to have children
(There are other abnormalities)
(XYY for example)
Glowing Plant: Natural Lighting without Electricity