Patterns of Inheritance: Human Genetics
Introduction
nGregor Mendel
¨Father of Genetics
¨Austrian monk who developed
the basic rules of inheritance
¨Based on experiments with
garden peas
Introduction
nMendel
¨Called genes “factors of
heredity”
¨Developed the Law of
Segregation – when gametes form in parents, genes separate so the sperm and
egg only get one unit of each pair
¨Developed the Law of
independent assortment – genes for different traits sort independently of
each other
Alleles
nWhat is genetics?
nGenetics –
the study of genes and their transmission from one generation to the next
nInheritance –
something (in this case DNA) received from an ancestor
Alleles
nAutosomes – 22 pairs of chromosomes that account for
all cells in the body except for the pair of Sex chromosomes – X and Y
nInherit one
pair of chromosomes from each parent which look alike
nAlleles
– are different structural and functional variations of homologous genes
Alleles
nThe
alteration will affect how the protein functions
¨For example; one allele of a
gene for hair pattern produces a widow’s peak whereas the other allele produces
a straight hair line.
nHomozygous –
both alleles for a certain gene are identical
nHeterozygous
– the alleles are different
Alleles
nSometimes
there are multiple alleles for a particular gene
¨For example; alleles for blood
type or A,B or O
nHow did we
get different alleles?
Alleles
nMay have resulted from
millions and millions of years of millions of mutations
n*Not as a result of a mutation,
more like a result of cut and paste applications!
Alleles
nAll
of the alleles for all human genes are collectively referred to as the human
gene pool
nYour
complete set of genes is called your Genotype
nPhenotype
– is the observed physical and functional traits that characterize us
Genetic Inheritance
nPunnett
square – predicts patterns of inheritance for a single pair of alleles that a
particular genotype will be inherited
Dominate and Recessive
nGenotype and
environment affect phenotype
nDominate –
the alleles is dominate to the other allele
nRecessive –
this alleles is being dominate to the other allele
Dominate and Recessive
nComplete
dominance - when the presence of just one dominate allele is all that is
necessary for the dominate phenotype to be expressed
nFor example;
widow’s peak Ww or WW
but an individual ww will not have a widow’s
peak
Dominate and Recessive
nIncomplete dominance –
the heterozygous genotype results in a phenotype that is intermediate between
the two homozygous condition
nFor example; the color
palomino in horses
Dominate and Recessive
nCodominance – the heterozygote exhibits the
phenotype of both alleles equally, rather than an intermediate phenotype
nFor example;
the relationship between the A and B alleles for blood type, a person can
inherit two of them resulting in 6 possible genotypes, but only 4 phenotypes
Dominate and Recessive
nSickle-cell Anemia – a
disease caused by one of the two codominant alleles
which are involved in he production of hemoglobin for
red blood cells.
nOne codes for an altered
hemoglobin molecule
nHbsHbs
= sickle cell HbAHbA and HbAHbs
= trait only not disease
nHomozygous Recessive
Phenotypes
nPolygenetic
inheritance – inheritance of phenotypic traits that depend on many genes
nDistributed in the population as a continuous range of
values, with more people in the middle and fewer at the extremes
nFor example;
eye color, height, body size,
Phenotypes
nBoth genotype
and environment may affect phenotype
nFor example
the effect of diet on height and body size
nOur genotype
is not the sole determinate in whether or not we develop an inheritable disease
Genetic Inheritance :linked genes
nLinked genes – genes for
different traits that are located on the same chromosome
nOften inherited together, the
closer they are the higher the probability they are inherited together
nBut not always, because of
crossing over, which partially reshuffles the homologues pair during Meiosis
nThe more crossing over the
lower the chance of them being inherited together
Sex-linked Inheritance
nSex chromosomes determine an
individuals sex
nHow do sex chromosomes
determine gender?
nThe sperm (or Males determine
the gender in humans) because it has and X and Y chromosome
Sex-linked Inheritance
nA single gene located on the Y
chromosome is activated for a short period of time
nThis activation leads to
development of testes and the production of the hormone testosterone
nIn the absence of the Y
chromosome or the absence of Y chromosome activation the embryo develops female
traits.
Sex-Linked Inheritance: X and Y Chromosomes
nSex-linked
inheritance – the pattern s that depend on genes
located on the sex chromosomes.
nMay be
Y-linked or X-linked
Sex-Linked Inheritance: X and Y Chromosomes
nX-linked
¨Hemophilia – well documented
X-linked disease
¨Person’s with this disease
lack a blood-clotting factor that is controlled by an X-linked gene with two
alleles
¨Xh
is the mutant that cannot produce the clotting factor where XH can
produce the clotting factor
Sex-Linked Inheritance: X and Y Chromosomes
nIndividuals
will inherit this disease if they do NOT have at least one normal dominant
allele
nXh XH or females and Xh
Y in males.
nMore males
than females have the disease
Sex-Linked Inheritance: X and Y Chromosomes
nThe disease
is passed to the sons solely through the mothers
nStatistically,
half the sons will have the disease and half the daughters will be carriers
nFathers
cannot pass the disease to their sons but all of their daughters will be
carriers
Sex-Linked Inheritance: X and Y Chromosomes
nRed-Green
color blindness and muscular dystrophy are also X-linked recessive conditions
that follow the same pattern as hemophelia
Sex influenced Traits
nSex
influenced traits: affected by presence of testosterone, estrogen
nFor example
male pattern baldness
nThe allele
can be present in both men an women, however the allele is recessive in women
Chromosomes maybe altered in number and structure
nNondisjunction
– failure of homologous sister chromatids to separate
properly
nMost serious are those that
occur in meiosis
nMost error of this type are never seen because so many of the genes on the
chromosomes are needed for embryonic growth and the embryo with error is
unlikely to survive. Usually before we
are even aware of their presence.
Chromosomes maybe altered in number and structure
nDown syndrome
¨Trisomy
21 – the 21st chromosome fails to separate and results in 3
chromosomes instead of two at chromosome 21
¨Affects 1 in 800 births in the
¨The risk of having a child
with down syndrome increase with a woman's age
Chromosomes maybe altered in number and structure
¨People with Down’s have
distinctive physical traits. Most are
very affectionate. They generally are
slow to develop mentally and are prone to respiratory complication or hear
defects
¨Less common are Edwards
syndrome (trisomy 18) and Patau
syndrome (trisomy 13)
Nondisjunction of the number of
sex chromosomes
nIndividuals
with one Y chromosome will be a phenotypic male and individuals lacking a Y
chromosome will exhibit the female phenotype.
Nondisjunction of the number of
sex chromosomes
nXYY Double Y
syndrome – individuals are males, tend to be tall and
otherwise normal. Some show impaired
mental function
Nondisjunction of the number of
sex chromosomes
nXXY Klinefelter
syndrome – tall, male phenotype, are sterile, may show slight mental
development and may develop large breast
Nondisjunction of the number of
sex chromosomes
nXXX Trisomy X syndrome – female phenotype, typically normal,
except some have slight mental impairment
Nondisjunction of the number of
sex chromosomes
nXO Turners syndrome –
Individuals with only one X chromosome are phenotypic ally female. They tend to be short with slightly altered
body form and small breast. Most are
mentally impaired. They are
sterile. Turners is rare because most
the embryo with only one X is more likely to spontaneously abort
Deletions and Translocations: alter chromosome structure
nDeletion – occurs when a piece
of a chromosome breaks off and is lost
nSome result in a live birth – Cri-du-chat syndrome - deletion in chromosome 5. Have mental
and physical impairment
nTranslocation – a piece of a
chromosome breaks off but reattaches at another site, either on the same
chromosome or on another
Inherited Genetic disorders usually involve recessive
alleles
nMeaning only
a person who has inherited two defective alleles will exhibit the disorder
nFor example: Phenylketonurid, Tay-Sachs
Disease
nDominant
lethal allele:
¨Always expressed, though at
midlife
¨Always lethal
Genes and Behavior
nMechanism
¨Product from gene-specific
proteins
¨Proteins have specific
functions leading to phenotypes
nProtein
functions: hormones, enzymes, structural, neurotransmitters