Transcript Probability and Pedigrees - Biology at Clermont College

Genetics Part II: Probability
and Pedigree
Nestor T. Hilvano, M.D., M.P.H.
(Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain,
Textbook, 2012.)
Learning Objectives
1. Explain the rule of multiplication and the rule of
addition to determine the probability of an
event in monohybrid crosses.
2. Explain how family pedigrees can help
determine the inheritance of human traits.
3. Discuss and provide examples of how
recessive and dominant disorders are
inherited.
4. Describe the inheritance patterns of pleiotropy
and polygenic inheritance.
Laws of Probability Govern Mendelian
Inheritance
• The probability of a specific event is the number
of ways that event can occur out of the total
possible outcomes.
# of actual occurrences
Probability = ______________________
# of possible occurrence
• When tossing a coin, the outcome of one toss
has no impact on the outcome of the next toss
• In the same way, the alleles of one gene
segregate into gametes independently of another
gene’s alleles
• Ex: flipping heads on a coin= ½ (50%)
Rules of Probability
• Independent events – ex.
Tossing of coin
• Rule of Multiplication –
probability of 2 events
occuring together is the
product of the probabilities of
the 2 events occuring apart
• Ex. Probability of recessive
phenotype occuring in
monohybrid? ½ x ½ = ¼
dihybrid? ¼ x ¼ = 1/16
• Rule of Addition –
probabilities are added if
there is more than one way
an outcome can occur, as
in determining the
chances for F2
heterozygous offspring
• Ex. Probability of Bb is
¼+¼=½
Degrees of Dominance
• Complete dominance occurs when
phenotypes of the heterozygote and
dominant homozygote are identical
• Incomplete dominance, the phenotype of
F1 hybrids is somewhere between the
phenotypes of the two parental varieties
• Codominance, two dominant alleles affect
the phenotype in separate, distinguishable
ways; Type AB blood is an example
Fig. 14-10-3
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Incomplete dominance
In snapdragon
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
What is the Phenotype
ratio? _____________
Pedigree Analysis
• Pedigree is a family tree that
describes the interrelationships
of parents and children
• Inheritance patterns of particular
traits can be traced and
described
• can be used to make predictions
of future offspring
• Dominant-recessive inheritance
• Apply Mendelian laws
Figure 9.8A
Dominant Traits Recessive Traits
Freckles
No freckles
Widow’s peak Straight hairline
Free earlobe Attached earlobe
Brachydactyly: dominant condition marked by short/clubbed fingers & toes
Recessive Disorders
• Most common; affected children are
homozygous recessive
• Born to normal parents who are both
heterozygotes (Dd)
• Ex. Deafness, albinism, cystic fibrosis, PKU,
sickle-cell disease, etc.
•
•
•
•
Practice: construct punnett squares
Parents – both Dd (normal but are carriers)
Gametes: D and d
Probability for Hearing (DD, Dd)= ¾ (75%); Deaf (dd)= ¼
(25%); Carriers (Dd)= 2/4 (50%)
Dominant Disorders
• Homozygous dominant causes death
of embryo; only heterozygous have
this disorder ; 50% chance of
passing the condition
• Ex. Achondroplasia (dwarf), extrafingers or web
digits, Huntington’s disease, and
Hypercholesterolemia
Fig. 14-17
Achondroplasia: a dominant trait
disorder
Parents
Dwarf
Dd

Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
d
Dd
Dwarf
Eggs
Normal
dd
Normal
Pleiotrophy and Polygenic
Inheritance
• Pleiotrophy = single gene produce multiple
phenotypic characteristics.
ex: sickle cell disease (ss) – confers phenotypic
traits as anemia, weakness, mental dysfunction,
organ damage, etc.
• Polygenic = 2 or more genes produce a
single phenotypic characteristic.
ex: mixtures of 3 genes (each w/ 2 alleles) for
phenotype of skin color
Crossing Over
• can separate linked alleles
• Producing gametes with recombinant
chromosomes (crossing over of linked
genes)
A
B
a
b
A
b
a
B
A B
a
Tetrad
b
Crossing over
Gametes
Sex-linked Disorders in Humans
• Sex-linked genes determine the sex of individual
and other inheritable characteristics
• Sex-linked disorders are due to recessive alleles
• For a recessive sex-linked trait to be expressed
– A female needs two copies of the allele
– A male needs only one copy of the allele
• Sex-linked recessive disorders are much more
common in males than in females
• ex. Hemophilia (lack protein for blood clotting),
Color blindness (red-green), Duchenne
muscular dystrophy
Homework
1.
2.
3.
4.
Define terms: recessive disorders; dominant disorders;
polygenic inheritance; pleiotrophy; pedigree;
recombinant chromosomes, and sex-linked genes.
Describe the rule of multiplication and rule of addition
in determining the probability of inheritance.
Male with black hair (Bb) married a female with red
hair (bb). Construct a punnett square to show the
location of gametes and offspring. What is the
probability of having a black hair? Probability of red
hair?
Whys is color blindness is more common in males?