Transcript Human Genetics - Kentucky Department of Education

Complex Patterns of
Heredity
Chapter 12
Human Inheritance
• Humans have 23 pairs of chromosomes
• These are made of about 100,000 genes
• Scientists usually study disease causing
genes because they can easily be traced
• They often prepare a Pedigree – a family
record that shows how a trait is inherited
over several generations.
Pedigree
• Carriers – usually, Heterozygous; they do
not express the recessive allele, but they
pass it along to their offspring.
Square = Male
Circle = Female
No shading = normal
Shaded = displays trait
Half/Half = Carrier
Pedigree
A Pedigree of Hemophilia in the Royal Families of Europe
Simple Dominant Heredity
• Many traits are inherited just as the rule of
dominance predicts.
• Remember that in Mendelian inheritance, a
single dominant allele inherited from one
parent is all that is needed for a person to
show the dominant trait.
Patterns of Inheritance
• Phenotypes repeated in a predictable pattern
from generation to generation
• Genetic Disorders – diseases or debilitating
conditions that have a genetic basis
Patterns of Inheritance
• Traits controlled by a Single Allele
– More than 200 human traits are determined by a
single dominant allele
• Ex. Huntington’s Disease
– More than 250 other traits are determined by
homozygous recessive alleles
• Both parents must have this allele in order for their
offspring to have the disease.
• Ex. Cystic Fibrosis & Sickle cell anemia
Inheritance Patterns
• Recessive Allele Inheritance Patterns
– Unaffected parents can have affected offspring
– The phenotype can skip a generation
– Individuals with no signs of the allele can be
carriers
• Dominant Allele Inheritance Patterns
– Affected offspring must have at least one affected
parent
– The phenotype appears in every generation without
skipping
– Two unaffected parents have no affected offspring
Sometimes Heredity Follows
Different Rules
• Incomplete Dominance: Appearance of a
third phenotype
• Codominance: Expression of both alleles
• Multiple phenotypes from multiple alleles
• Sex determination
• Sex-linked inheritance
• Polygenic Inheritance
Incomplete Dominance
• With incomplete dominance, a
cross between organisms with
two different phenotypes
produces offspring with a third
phenotype that is a blending of
the parental traits.
– RED Flower x WHITE Flower --> PINK Flower
• R = allele for red flowers
W = allele for white flowers
red x white ---> pink
RR x WW ---> 100% RW
Codominance
• The genetic gist to codominance is pretty
much the same as incomplete dominance. A
hybrid organism shows a third phenotype --not the usual "dominant" one & not the
"recessive" one ... but a third, different
phenotype.
• In COdominance, the "recessive" &
"dominant" traits appear together in the
phenotype of hybrid organisms.
Codominance
• Example
red x white ---> red & white spotted
With codominance, a cross between
organisms with two different phenotypes
produces offspring with a third phenotype
in which both of the parental traits appear
together.
• R = allele for red flowers
W = allele for white flowers
red x white ---> red & white
spotted
RR x WW ---> 100% RW
Examples of Codominance
• A very very very very very common
phenotype used in questions about
codominance is roan fur in cattle. Cattle
can be red (RR = all red hairs), white (WW
= all white hairs), or roan (RW = red &
white hairs together). A good example of
codominance.
Another example of codominance is human
blood type AB, in which two types of
protein ("A" & "B") appear together on the
surface of blood cells.
Codominance and Blood Types
• Blood transfusion can only take place
between two people who have
compatible types of blood.
• Human blood is separated into
different classifications because of the
varying proteins on the surface of
blood cells.
• These proteins are there to identify
whether or not the blood in the
individual's body is it's own and not
something the immunity system should
destroy.
Patterns of Inheritance
• Traits controlled by Multiple Alleles
– Controlled by 3 or more alleles of the same
gene that code for a single trait
– Ex. Blood types
• A & B are codominant – both are expressed when
together; and both are dominant to O.
• A person can only have type O blood if they receive
the “O” allele from both parents.
Human Blood Types
ABO Blood type and genetics
• The protein's structure is controlled by three alleles;
• i, IA and IB.
–
–
–
–
i, the recessive of the three,
IA and IB are both codominant when paired together.
If the recessive allele i is paired with IB or IA, it's expression is hidden and is not shown.
When the IB and IA are together in a pair, both proteins A and B are present and
expressed.
• The individual's blood type is determined by which combination
of alleles he/she has.
– There are four possible blood types in order from most common to most rare: O, A, B
and AB.
– O blood type represents an individual who is homozygous recessive (ii) and does not
have an allele for A or B.
• Blood types A and B are codominant alleles.
– Codominant alleles are expressed even if only one is present. The recessive allele i for
blood type O is only expressed when two recessive alleles are present.
– Blood type O is not apparent if the individual has an allele for A or B.
– Individuals who have blood type A have a genotype of IAIA or IAi and those with blood
type B, IBIB or IBi,
– An individual who is IAIB has blood type AB.
Blood type Chart
Blood type practice
Use a Punnett Square!
1. A woman has type
A blood. Her father
has type O blood.
The woman marries
a man with type O
blood. What is the
chance that they
will have a child
with type A blood?
2. What is the chance
that the couple
from question 1 will
have a child with
type AB blood?
*Show me your
answers when you
are finished. Keep
these in your notes!
Multiple Phenotypes from Multiple Alleles
• Although each trait that we have studied so far
only has two alleles, it is common for more that
two alleles to control a trait in a population
• For instance, Pigeons – three colors possible (red,
blue, chocolate)
• However, each pigeon can have only two of these
alleles
• Complete P.S. Lab 12.2 to observe multiple alleles
in how coat color in rabbits is inherited.
Sex determination
• Remember that in humans the diploid number of
chromosomes is 46, or 23 pairs.
• There are 22 matching pairs of homologous
chromosomes called autosomes.
• The 23rd pair differs in males and females, they
determine the sex of an individual (sex
chromosomes)
– X females (XX)
– Y males (XY)
Complete a punnett square to determine the expected
ratio of males to females produced given their
possible gamete contribution
Sex-linked inheritance
• Traits controlled by genes located on sex
chromosomes are called sex-linked traits
• Read about Thomas Hunt Morgan’s
research with fruit flies on pg 325.
Complete a punnett square to show how the
allele for red eye color is a sex-linked trait.
Patterns of Inheritance
• Polygenic Traits
– Most human characteristics are controlled by
several genes (2 or more)
– Ex. Skin color – 3 to 6 genes
– Ex. Eye color
– Some are also affected by the environment
• Ex. Height – nutrition and disease
Eye Color Activity
• http://www.athro.com/evo/gen/genefr2.html
Patterns of Inheritance
• Sex-Linked Traits
– Are found only on the X chromosome
– Ex. Colorblindness (recessive)
– Ex. Hemophilia (recessive)
Patterns of Inheritance
• Sex-Influenced Traits
• Influenced by male or female sex hormones
• Ex. Patterned Baldness
– Homozygous baldness-both will lose hair
– Heterozygous-men will lose hair but women
will not
Nondisjunction
• Nondisjunction is the failure of
homologous chromosome pairs to separate
properly during meiosis. The result of this
error is a cell with an abnormal (too few or
too many) number of chromosomes.
Nondisjunction
Patterns of Inheritance
• Disorders due to Nondisjunction
– Monosomy (45 Chromosomes)
– Trisomy (47 Chromosomes)
• Trisomy-21 (Down’s Syndrome)
– Klinefelter’s (XXY)-male w/ some female traits
– Turner’s (XO)-female appearance
– Single Y chromosome do not survive
Environmental Effects
• Genes are inherited from parents, but
sometimes their expression is modified by
environmental factors.
• An example is the snowshoe hare we
discussed earlier in the year-these hares
have dark fur in the summer and white fur
in the winter.
Snowshoe Hare
Detecting Human
Genetic Disorders
• Genetic Screening – examination of
genetic makeup
– Karyotype: a picture of chromosomes
grouped in pairs and arranged in
sequence.
– Screening of Blood: look for certain
proteins
• Genetic Counseling-medical guidance
informing of problems that could affect their
offspring.
Human Karyotype
Detecting Human
Genetic Disorders
– Amniocentesis: removal of small amount
of amnionic fluid surrounding the fetus
– Chorionic Villi Sampling: tissue that
grows between the mother’s uterus and
the placenta (between the 8th and 10th
week)
– Screening Immediately after Birth:
• Ex PKU (Phenylketonuria)-body cannot
metabolize the amino acid phenylalanine
– Special diet lacking phenylalanine
Karyotyping
• http://www.biology.arizona.edu/human_bio/
activities/karyotyping/karyotyping2.html