Transcript Other Laws of Inheritance

Other Laws of Inheritance
• Mendelian Genetics: characteristics
controlled by dominant and recessive
paired alleles
– Many traits follow the patterns outlined by
Mendel’s Laws, many do not.
Incomplete Dominance
• Offspring heterozygous for a trait have an
intermediate appearance.
• Neither allele is completely dominant over the
other
• Ex: Snapdragon flowers
– Homozygous Red x Homozygous White
– Results in all PINK flowers
Showing a Cross for Incomplete Dominance
– Use a capital letter for each allele since each influence trait
equally
• RR (homozygous red) x WW (homozygous white)
• What is phenotype ratio of offspring?
• RW x RW (heterozygous pink)
• What is phenotype ratio of offspring?
– How is this ratio different from the offspring
of Mendel’s Monohybrid cross? (Tt xTt)
Codominance
• If individual is heterozygous, both alleles
for gene are expressed.
• Ex: Chickens
– Black feathers (BB) x White Feathers (WW)
– Offspring (BW) are checkered and have both
black and white feathers
• Try a codominant cross:
• Checkered (BW) x checkered (BW)
• Checkered (BW) x white (WW)
• Checkered (BW) x black (BB)
• Ex: Roan Cattle
– Offspring of pure red and pure white cows
– Have both red and white hair
Multiple Alleles
• A gene that has more than two alleles.
• Each individual can only have two alleles
• Ex: Human Blood Types
Possible Blood Types:
A, B, AB, O
– There are 3 alleles for blood type (A, B, O)
– A and B are codominant alleles (IA, IB)
– O is a recessive allele (i)
Six genotypes are possible for blood:
• IA IA
• IA IB
• IB IB
• IA i
• IB i
•ii
• What blood types would they have?
• Ex: Type A (homozygous) x Type B
• What are the blood types of the offspring?
• Try a Cross:
• What blood types do the offspring have?
• IB IB x IA IA
• IA i x i i
• IA IB x i i
• If a Type A mother and a Type B Father produce a
type O child, what are the parent’s genotypes?
Environmental Influences
on Gene Expression
– Ex: temperature, nutrition, light etc.
• Ex: Coat color in Himalayan rabbits
– Black fur present only on areas of body that are
colder. (ear, nose, feet and tail)
• Ice Pack placed on fur = fur turns black
• If rabbit kept in warm environment, all fur
is white.
Genetic Research and Testing
• Genetic Research:
– To study genes for different traits it is best to
use an organism that grows and reproduces
quickly an produces many offspring.
– Can do controlled breeding experiments
– Can do forced “inbreeding”
• Ex: Drosophila melanogaster (fruit fly)
Genetic Tests
• Test Cross:
– To determine if an individual is homo or
heterozygous for a trait.
– Breed to a homozygous recessive.
– Ex:
• Drosophila: red eyes dominant over white.
• To see if red eyed fly is hetero/homo, breed to
white eyed fly
Sex Determination
• Humans have 23 pairs of chromosomes
• 22 pairs are autosomes
• 1 pair are sex chromosomes
I’m a BOY!
These pictures are
called karyotypes!
I’m a GIRL!
Sex Determination
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•
•
•
XX = female, XY = male
Males can give Y chromosome or X chromosome
Females can only contribute X
There is always a 50% chance of being boy or girl
Sex Linked Traits
• If allele is found on an X or a Y chromosome it is
inherited differently in males and females.
• Ex:
–
–
–
–
Calico Cats
Color Blindness
Hemophilia
Duchenne Muscular Dystrophy
Calico Cats: (Codominant X Linked Trait)
– X chromosome has gene for black or orange
fur (codominant)
– Only females can be “Calico” XBlackXOrange
– Males can only be Black or Orange
they have only one X chromosome and one Y
I’m always a girl!
Colorblindness: (Recessive X linked trait)
• Certain colors cannot be distinguished from
others (usually red or green)
• Most often seen males
• Females are more likely to be “carriers”
• Father cannot pass gene to son, only mother
Hemophilia: (Recessive X-linked trait)
– Blood has trouble clotting
•
•
•
•
•
XHXh
XH Y
XhY
XhXh
XHXH
Carrier female
Normal Male
Affected Male
Affected Female
Normal Female
• Examples of Crosses for Hemophilia
• Queen Victoria of England passed this gene
onto some of the royal family offspring
She must
have been a
“carrier”
Queen Victoria
Chromosomal Disorders
• Due to the presence of absence of an entire
chromosome or part of a chromosome.
• Can be determined by doing a Karyotype
from blood or amniotic fluid
• Amniocentesis:
– Prenatal test
– Grow cells from amniotic
fluid around fetus.
– Helps to rule out major
chromosomal
abnormalities
Nondisjuction
• Nondisjunction:
– homologous
chromosomes fail to
separate properly
during meiosis.
– Results in egg or
sperm with one or
more extra
chromosomes.
Normal Meiosis
Nondisjunction
What is Wrong?
Nondisjunction of Sex Chromosomes
• Klinefelter’s Syndrome (XXY)
– 2 X’s from mother, Y from father
– Often tall, long arms and legs
– May have some gynomastia and small
testicles
• Turner’s Syndrome (XO)
– Offspring has only one X (always female)
– Usually sterile, short, thick neck
Nondisjunction of Autosomes
• Trisomy 21 (Down’s Syndrome) (47, +21)
– Most commonly seen chromosomal disorder
resulting in live birth
– Extra Chromosome 21
• Other trisomys can happen as well
• Babies may make it to term but usually
don’t live very long (nonviable)
– Ex: Trisomy 18, Trisomy 13
Gene Linkage
• When alleles for different traits are located
on the same chromosome
• They tend get inherited together.
– They are “linked”
– They do no “assort independently”
• Ex: Red hair and freckles
Crossing Over and Linked Genes
Crossing Over: homologous chromosomes
line up during synapsis and exchange
material.
– Increases variation in offspring.
Sometimes linked
genes will separate
when crossing over
occurs.
The closer genes are
on the chromosome,
the less chance they
will be separated during
crossing over
Autosomal Genetic Disorders
• Caused by defective alleles on autosomes
• Can be recessive or dominant
Sickle Cell Anemia: (Autosomal recessive)
• Red blood cells have a sickle shape
• Very fragile and break easily
• Clump up on blood vessels (causing pain)
• Reduces oxygen carrying
capacity of red blood cells
• Caused by a single flaw in
gene
• One nitrogenous base in DNA
sequence is different
• Cause wrong amino acid in
protein chain for hemoglobin,
changing it’s shape
• Hemoglobin doesn’t transport
oxygen as well
• Mostly found in people of
African American descent
– (1 in 500 births heterozygous)
• If disorder has negative effects
why so common in population?
– Malaria is a deadly disease
common in Africa
– Heterozygous individuals for
sickle cell have malarial
resistance, more likely to survive
than people who aren’t carriers.
I spread
malaria!
Phenylketonuria (PKU):(Autosomal recessive)
• Enzyme that breaks down amino acid
phenylalanine doesn’t function
• Chemical builds up in system and eventually
forms substances that can damage the brain
and cause mental retardation
• Can test for it at birth and treat with a low
phenylalanine diet
Tay-Sachs Disease:
(Autosomal Recessive)
• Incurable inherited disorder
that damages the brain
• Enzyme doesn’t function that
helps breakdown lipid in
brain.
• Brain tissue deteriorates
• Most often found in eastern
European Jews
• Death occurs several years
after birth
•
http://www.youtube.com/watch?v=SeoPF74
QSms&safe=active
Gene-Chromosome Theory
• Explains the hereditary patterns that
Mendel and others observed.
• The genetic characteristics of an
organism are determined by the genes
for different traits present on their
chromosomes.
Mutations
• Sometimes a genetic disorder is caused by a
mutation of a gene.
• The mutation can only be passed to the next
generation if it happens in a sex cell.
• Mutagenic Agents:
– Exposure to x-rays
– Radiation
– Certain chemical toxins