Download Patterns of Inheritance

Patterns of Inheritance
• aka Genetics
• Parents pass
heritable traits to
offspring (genes)
• Gregor Mendel
– 1860’s, Austrian monk
experimented with
garden peas
– tracked traits from
parents to subsequent
generations
How?
• Cross fertilization
– Pollen from one plant to
fertilize another plant
• P = Parents
• F1 = 1st generation of
offspring
• Hybrid (cross) = offspring
of two different varieties
– e.g. purple vs. white
flowers
• Mendel tracked 7
characteristics
– Each had two distinct
forms
– Controlled
• Started with pure
varieties
• Monohybrid cross
– Differs by one
characteristic
• No blending (pink)
– What happened to
the white?
• Showed up in F2
Mendel’s 4 hypotheses
1. Alternate versions of each gene = alleles
(Purple or White)
2. Offspring inherit 2 alleles, 1 from each parent
for each characteristic (color)
•
•
Homozygous = each allele is the same (PP or pp)
Heterozygous = different alleles (Pp)
3. Dominant = determines appearance, and
Recessive = not apparent
4. Gametes (sperm or egg) carry only one allele
as a result of pair separation during meiosis =
Law of segregation
• Pure parent plants
– All purple (PP)
– All white (pp)
• Gametes will be either
P or p
• F1 are all purple because
of dominance (Pp)
• F2 results in a
mathematically predictable
3:1 ratio
• Phenotype = expressed
traits
• Genotype = genetic
makeup
• Punnett square
– Hybrid posibilities
• Alleles of a gene reside at the same locus
on homologous chromosomes
– Separated by independent assortment in
meiosis…ultimately within separate gametes
What happens if there are 2 characteristics?
• Are they packaged
together? Or,
independently from one
another?
• Seed characteristics
(alleles)
– Round (R) or wrinkled (r)
– Yellow (Y) or green (y)
• Hypothesis: They are
packaged together.
Hypothesis: Each pair of alleles are independent of one another
• F1 produces equal amounts
of 4 possible genotypes
• F2 reveals even more
genotypic possibilities
(9:3:3:1)
• Dihybrid cross is equivalent
to two monohybrid crosses
(12:4 or 3:1)
• Law of independent
assortment
• Black coat is dominant
(B)
• Chocolate coat is
recessive (b)
• Normal vision (N) vs.
blind (n)
• What are the results of
a dihybrid cross of
heterozygous parents
(black, normal vision) ?
*progressive retinal atrophy (PRA)
causes blindness
• Phenotype of heterozygous
parents?
• Genotype of heterozygous
parents?
• Gamete possibilities
– Sperm?
– Eggs?
• Black coat is dominant
(B)
• Chocolate coat is
recessive (b)
• Normal vision (N) vs.
blind (n)
• What are the results of
a dihybrid cross of
heterozygous parents
(black, normal vision) ?
• Phenotype of heterozygous
parents?
black, normal vision
• Genotype of heterozygous
parents?
BbNn x BbNn
• Gamete possibilities
– Sperm: BN, bN, Bn, bn
– Eggs: BN, bN, Bn, bn
BN
Sperm: BN, bN, Bn, bn
Eggs: BN, bN, Bn, bn
BN
bN
• What is the
genotypic ratio of the
offspring?
• What is the
phenotypic ratio of
the offspring?
Bn
bn
bN
Bn
bn
Sperm: BN, bN, Bn, bn
Eggs: BN, bN, Bn, bn
9
3
3
1
BN
bN
Bn
BN BNBN
BNbN
BNBn
bN bNBN
bNbN
bNBn
Bn BnBN
BnbN
BnBn
Bnbn
bnBN
bnbN
bnBn
bnbn
bn
bn
BNbn
bNbn
• Can you know the
genotype by looking at
the phenotype?
• What are their
genotype possibilities?
• Can you know the
genotype by looking at
the phenotype?
• bb
• What are their
genotype possibilities?
• BB or
Bb
Testcross
• Perform a testcross to
determine an
unknown genotype
Mate an unknown
genotype (black lab)
X
homozygous recessive
(chocolate lab)
Some examples of dominant and recessive
traits in humans (at one gene locus)
Class total
•
•
•
•
•
•
•
•
Bent pinky – dominant (BB or Bb)
Blue eyes – recessive (ee)
No mid-digital hair – recessive (mm)
Tongue rolling – dominant (RR or Rr)
Widow’s Peak – dominant (WW or Ww)
Right over left Thumb – recessive (cc)
Attached ear lobes – recessive (aa)
Hitchhiker’s thumb – recessive (hh)
How do we determine inheritance
of human traits?
• Can’t Testcross
• Pedigree – family tree
of genetic history
• E.g. tracking deafness
(dd) in an isolated
family
– Since deafness
appeared in F1 of
grandparents…carriers
• Abigail? John? Hepzibah?
Other single gene disorders
• Recessive
– Albinism
– Cystic fibrosis
• Thick mucous excretion of
lungs and other organs;
requires life long treatment
– Sickle cell anemia
• Dominant
– Extra fingers, toes
– Webbed fingers, toes
– Dwarfism
• Normal body; short arms
and legs
• DD die, Dd survive, nondwarfs (99.99% pop.) are
dd
David Rappaport
• Probability increases with
less variability
(inbreeding)
• Dominant lethal disorders
less common; usually kill
embryo
• Identifying disorders:
– Amniocentesis is sampling of amniotic fluid
– Chorionic villus sampling (CVS) of placental tissue
– Ultrasound
Traits that don’t conform to Mendel’s laws
• Incomplete dominance
– E.g. snapdragons
– Heterozygotes differ from
homozygotes
– Predictable 1:2:1 ratio
• Different than “blending”
hypothesis
– No testcross necessary
More than two alleles per gene
• 3 alleles in blood type – OAB
– 4 possible phenotypes = O, A, B, AB
– 6 possible genotypes; co-dominance
• Rh factor = “+” and “-” along with O, B, A
– Rhesus incompatibility disease
– 30 combinations; much more complicated
• “O-” was considered universal donor; but
new research shows not always the case
Pleiotropy
• Gene influences
multiple
characteristics
Polygenic Inheritance
• Additive effects of
multiple genes on a
single phenotypic
characteristic
Sex-linked genes
• X-linked recessive
alleles
• More common in
males than females
– Males only needs to
inherit one sex-linked
recessive allele from
mom to be expressed
– Females has to inherit
two sex-linked
recessive alleles for
expression
• Examples
– Red-green color
blindness
– Hemophilia